"Thanks to that differential between your axles, an AWD car will send your engine’s power down the path of least resistance—the wheel with the least grip. Where a two-wheel drive car can only choose between two wheels, an AWD system looks for that least resistance across all four wheels."
"4WD works by locking the front and rear axles together, splitting torque 50:50 between them. This provides great traction, but a vehicle locked in 4WD cannot safely be operated on dry pavement because its front and rear axles are forced to rotate at the same speeds."
I remember being scared when I bought my first 4wd vehicle, a '91 Toyota pickup. I knew I shouldn't have the wheels locked in 4wd on dry pavement, and I was worried about going from snowy roads to dry pavement. When I moved on to an Outback (AWD, not 4wd) I wondered why I didn't have to worry about transitioning between slippery roads and dry pavement. Now I understand.
There's also a great video from 1937(!) that shows why we need differentials, and how they work. Here's a direct link to the video if you're going to skip the article:
And in the next paragraph "To counteract this [sending of power to the wheel of least resistance] , the better AWD cars are fitted with a center differential that contains a clutch or viscous drive unit. This splits torque front-to-rear, directing it away from the spinning wheel. "
So, a well engineered AWD system actually sends power away from the slipping wheel(s) and towards the wheels with traction, which makes more sense.
>> "...sends power away from the slipping wheel(s)..."
Exactly this. The best ones are entirely mechanical, without any fancy computers or even viscous couplings. Audi sold (sells?) IMHO the most sophisticated AWD system on the market, which uses a system of crossed helical gears to continuously and instantaneously redistribute torque proportionally to wheel traction, up to a roughly 80-20 split. There are no computers and the whole thing is mechanical. The differential is a minor mechanical miracle sold under the trade name Torsen, short for "torque sensing." Despite Audi's German-engineering themed marketing, the differential was actually designed by an American.
In the 80's, the station wagons even had a manually-actuated center locker, which was eventually replaced by a servomoter driven locker. I don't know what the new ones have.
Audi Quattro is great, although IIRC, it also eats a bit tires on tight turns like anything but an open diff would. I also like the viscous clutch on the jeep, although it definitely is less on-road friendly (A tight 90 degree turn is enough to hear the tires, 180 and you can almost feel the car get lower). My worst system was my 98 explorer - it engages the clutch completely when front axle falls behind, waits a second, disengages to check if axle is still behind, if so, reengages... It's jumpy as hell when turning, and sucked no matter what. You could mod it for pure 2WD operation by switch, but stock was awful.
However, I don't think you should underestimate electric systems. I mean, I hate car software with a passion, but when they work, they actually have really good characteristics. Things like LSD's where the pump is electrically enabled give quite smooth transitions, and can simulate the response of both on and offroad mechanic systems. Response time is also nearly instant. If car computers weren't bloody black boxes (or at least leave only important things to the black box!), these systems often end up being much simpler than their purely mechanic black voodoo counterparts.
Well both tactics work in different scenarios. When power goes to the slipping wheels, it helps keep the car drive strait. For example, in the rain or light snow. If one wheel slips, then suddenly power goes to the wheel with traction, you'll veer off-course.
When it goes to the wheels with traction, it helps get the car un-stuck. This is usually what you want when you're doing serious off-roading, stuck in snow, etc... Also, in this scenario you're usually going very slowly/not moving, so the sudden change of direction won't be a problem.
Modern AWD does a bit of both - it'll send power to wheels with traction, but will also compensate by sending power to a wheel on the other side of the car. For example, if there's no traction on your left front tire, it'll send power to your right front tire and left rear tire, and reduce power on your right rear tire to keep you going strait.
Everyone's focusing on the technology involved, but that's kind of pointless because AWD isn't a technical term. It's a term to distinguish an on-road car with four driven wheels from an all-terrain car, because when the first WRXes and such came out, people saw 4WD and thought that meant they could be used to go bush bashing.
That's right. A poorly-engineered 4WD system means you are stuck on a hill because one wheel is on slippery wet leaves. AWD should mean you can get up that hill with three wheels on slippery wet leaves, and one with grip. That's the simplest explanation.
I had an old Toyota Tercel 4WD station wagon and it was very difficult to steer sharply at low speed with 4WD engaged because of the lack of a differential. It would lurch and I had to press harder of the accelerator to get straightened out and pop it back into 2WD. It was my first car so I didn't know any better. Fun fact: In 4WD, you got an 'extra low' gear below 1st and that car could very nearly climb trees. The gear ratio was insane. No ground clearance but I used to scare the crap out of friends by going up very steep inclines in extra low gear with 4WD engaged. They would have to park their pickup trucks at the bottom of the hill and ride with me, and this caused them some embarrassment. Good times.
> That's right. A poorly-engineered 4WD system means you are stuck on a hill because one wheel is on slippery wet leaves. AWD should mean you can get up that hill with three wheels on slippery wet leaves, and one with grip. That's the simplest explanation.
I think you just said the opposite of the guys above you while saying you agree. 4WD should not have a problem on slippery terrain.
It actually has nothing to do with 4WD or AWD, the article is poorly written and confuses several terms. At the surface 4WD is manually selected and AWD is 4WD all the time. Thats it they are the same thing, it's just 4WD means you opt to put the vehicle in 4WD.
The term the are talking about when traction is applied to the wheel with the most binding to the ground and away from the one that has the least traction is called a Limited Slip Differential and by virtue of the slipping of the low traction wheel it will spool the high traction wheel and bind it transferring torque to it. This can also be done by what is called a locker, either electronic or manual but manual lockers can be dangerous at high speed if you are not used to their manor. You generally only see manual lockers and welded differentials in offroad trucks. Limited Slip differentials and electronic lockers are available in both 4WD and AWD vehicles.
The term they are talking about to apply power to both the front and rear wheels is not a differential but rather a transfer case and it is not always at a 50:50 ration man times they run 60:40 or 70:30. Both 4WD and AWD have a transfer case, 4WD's allow you to select whether they are engaged or not.
The term they are talking about when they talk about adjusting power front to back is called a Traction Control Unit and is handled by a Body Control Module that may or may not also communicate with the Engine Control Unit or the Transmission Control Unit to defuel or detorque the powertrain, as well as adjust torque via electronic lockers in the differential to the wheels that need it and away from the ones that do not. As well as engage or disengage front and back axles via the transfer case. Many newer AWD systems have this but it is also available in most newer 4WD vehicles.
TLDR is 4WD and AWD can be virtually identical depending on the subsystems but the article confuses a lot of those subsystems as being unique to one or the other. Though the upper end of 4wd's have more "hardcore" options not generally found or offered in AWD vehicles.
I found the article very good explaining the basics and history of awd vs 4wd.
What you explain is the status of today's 4wd vs awd, where indeed the features in awd vs 4wd cars can overlap a lot and because of the big variety of implementations quite hard to understand by regular Joe
A typical 4WD without locking diffs will still spin wheels. You have to have differentials at each end to go around corners on pavement.
An AWD car has _3_ diffs, one in the middle that splits power F/R, and then one at each axle that splits it L/R.
Highend 4wd setups can lock the diffs so all wheels spin at the same speed all the time - essentially on slippery ground, but very counterproductive when you actually have traction.
That's the nature of 4wd -- take a modern jeep, but it in 4wd on dry pavement and feel it lurch and moan because it can't slip. Put it in 4lo and on dry pavement and it'll lurch so hard your face will go through the windshield.
4wd without lockers means you can still get stuck (been there) because front and rear each have their own diffs. One wheel in each diff will be spinning and then you need to get a tug from a friend. The rule of thumb is always have 3 points of contact if you don't have a locker -- then one diff can spin while the other pulls you along.
Yeah, it probably has full-time 4WD with a viscous clutch at center - when you turn, your axles lock, and 3 out of 4 wheels will have to slip to let you get anywhere. This will cause jumping and tire noise. Engine torque won't change much.
LSD's front and rear makes this worse, but center is the most important when it comes to on-road maneuverability.
I loved my old Tercel wagon. Didn't use that low gear all that often but fun to have. CV joints were always my problem, ended up having to replace them quite often.
One time in college I was turning too sharply while parking and the axle actually popped out of the right front. Loose axle would just spin in 2WD but switching to 4WD enabled me to make it to the nearby Mech E. building and get help. :)
I gave a 89 Toyota that kinda took care of, with 240,000 miles. She still has 125 psi in the cylinders. Changed the oil when I felt like it. Changed the oil filter only sometimes. Never changed the air filter. Never repaired a tapping valve. In other words, didn't take care of her like other vechicles I owned.
One day, I think, even 80's and 90's Toyotas will be collectors items. I know some of them are collectors vechicles now, but I feel the 80's to 90's vechicles will see a huge boost in interest.
I had an 87 pickup and 87 tercel. The engines will last forever, but rust is the killer on the truck. Currently have a 2005 Tacoma and it is rust free (plastic wheel wells) but even it has a recall for rusty suspension.
Well, not quite; the viscous drive unit really just ensures that every path has some resistance floor, and thus that not all torque goes to one wheel.
At one point I remember there was a workaround mentioned by the Hummer engineers (I believe) to apply the brake simultaneously with the gas if you ran into the "one wheel spinning in the air" problem. This is a simply a manual application of the same physical solution of creating at least some resistance everywhere.
The trick with the hummers was slightly different.
Hummers have torsen diffs, which have a pair of worm gears where a conventional open diff would have single star gear.
As a result, the torsen diff sends some fixed multiple (changes with model) of the torque used by the easier to spin side, to the harder to spin side.
Of course if one wheel is spinning in air, the easy side uses effectively 0 torque, and 0 times anything is still 0, hence the brakes.
Applying the brakes increases the torque requirement of all wheels by a fixed amount, but the hard to spin wheels get More than that amount of extra torque.
This isn't always true either. 4WD can mean that you send equal amounts of power to each axle, but at that point there are things that can change the amount of power going to each wheel within the center differential casing. If you have a limited slip differential (why some 4WD vehicles can drive perfectly fine on pavement) it'll basically send a 50/50 split (lock the axles together when going straight) but allow the axles to turn at different rates when making turns (otherwise bad things would happen). Some cheaper 4WD vehicles (and many 2WD vehicles) have open differentials. Open differentials send the power to the path of least resistance (bad for traction, and also why if you're in gravel the loose wheel usually just spins while the other one doesn't). There's also a fully locking differential that literally locks the axles no matter what. These cannot be used on pavement for obvious reasons (well, think about turning a corner, the outside wheel will have to turn faster than the inside wheel). AWD vehicles have a center diff typically and also often employ limited slip differentials (but sometimes again, they have open and/or auto-locking diffs...ARB made air locking diffs at one point, not sure if they still do) at each axle as well which enables a mechanical distribution of power. Some newer vehicles have replaced the axle differential gears with anti-lock breaking systems that provide resistance to open diffs to regulate wheel spin (artificially add resistance to the spinning wheel).....sorry, before I became a computer geek I built motors and off-roaded in my 81 CJ-5, after that I drag raced for fun on the weekends at the local track. I had a limited slip in the mustang I raced and a fully locking one in my Jeep. Jeeps (and older 4WD) are interesting b/c many come with what are called locking hubs ( https://en.wikipedia.org/wiki/Locking_hubs ) which are not all that common today. Even if you do find them, they're usually auto-engaging vs. the ones you have to get out an manually turn to engage the front drive shaft.
I once had a Jeep Grand Cherokee that had a "full time 4WD" system. Basically you could choose between 4WD low (traditional 4WD with front and rear diffs locked to each other, in a low gearing for massive torque), 4WD high (traditional 4WD as above, in normal gearing), 2WD (front diff unlocked and spinning freely as if it were a regular 2WD car) and full time 4WD (front wheels would automatically lock together only if the system detected slippage in the rear, and would receive about 35% of the engine's total power output).
The funny thing was, even though you were supposedly able to drive in full time 4WD on dry pavement, it would sometimes engage the front diff in parking lots and when taking tight turns at low speed, causing the front wheels to hop. I ended up leaving it in 2WD unless I was on a dirt road or it was raining, figuring full time 4WD was a marketing gimmick to compete with Subarus.
From what I remember when I had to replace the transfer case a year after I bought it (it was a 1996 that I bought in 2006 and the previous owner abused it), it was more or less a "traditional" Jeep 4WD setup but depended on the transfer case to keep the front and rear axles independent until slippage was detected. I think it was pretty much fully mechanical, except for the auto locking front hubs. I believe Jeep's marketing term for it was Select Trac or something like that.
I think the issue was with the auto locking hubs; when it malfunctioned it would lock the front hubs together and in turn the transfer case would lock the front and rear diffs together, causing the wheel hop. This defect probably contributed to the death of the original transfer case, now that I think about it.
Locking the center differential in a 4WD vehicle is fine on pavement as long as you don't turn, so forgetting for a small straight of pavement in the middle of the trail is no big deal. Some trails are half dirt road, half long-forgotten pavement, so constantly reaching down can get tiring if one follows that rule verbatim. Snow transitions are similar; your truck would have been fine and I'd just reach down for a dry corner if you can disengage in motion. If not, yeah, bigger concern.
Front axle locker is similar, but with that you feel how difficult it is to steer as soon as you lock it. (In my Wrangler if I'm finding that I need the fronts we are in some serious shit anyway.)
Well, yeah. Mine will bark pretty loudly if you do that but it did survive the one time it happened. Said experience is also why nobody but me sits left seat.
I'd have to find my paperwork but I think such maneuvers are cause to void drivetrain warranty. Chaining the tires is, too, for unrelated reasons.
woooow! Re: youtube video - Go to market in 1930s was actually how to go to market. When you think that Chevrolet really got going in 1920, 17 years later their marketing was super on point (here we are still watching it today, content marketing at it's best).
I would add that in addition to locking between the front and rear axles, many 4x4s allow for the wheels to be locked across single axles. Right and left tires then turn at the same rate. This is horrific on dry pavement as one wheel will drag in every corner, but an enormous advantage in rough terrain.
Some of these can be locked manually/physically, but more often these days they are controlled electronically. Of course this defeats any traction control systems, normally leading to a warning light telling you that you are on your own in that regard. Antilocks also often disabled in this mode.
Wow, that video does a really great job building up the explanation. I already knew how a differential worked (thanks to Legos), but I had never thought about it that way before.
There are lots of variations on this theme. There are off-road differentials which limit the speed difference between the output shafts to a fixed maximum ratio, usually in the 2:1 to 4:1 range.
It's possible to do some or all of this in software. Tesla has an ordinary open differential but will apply the brake on an overspeeding wheel. Tesla's all wheel drive system has a separate motor for the front and rear wheels. Power distribution between the two during acceleration is mostly equal, but once speed stabilizes, the rear powertrain takes most of the load. Off-road slip handling doesn't seem to be well documented, but Tesla cars aren't intended for off-roading.
The most advanced systems are seen on locomotives. Modern heavy locomotives (GE Evolution series) use three-phase AC synchronous motors driven by IGBT inverters from power from a Diesel generator. Each axle has its own motor. All axles are normally locked together electronically by the control software, so no wheel can slip ahead of the others. Locomotives can be cabled together so that the axles on multiple locomotives synchronize. Now that's all-wheel drive. This is a huge win when starting a heavy freight train. Locomotive axles that are slipping provide little pulling power and damage the tires and rails.
"It's possible to do some or all of this in software. Tesla has an ordinary open differential but will apply the brake on an overspeeding wheel. Tesla's all wheel drive system has a separate motor for the front and rear wheels. Power distribution between the two during acceleration is mostly equal, but once speed stabilizes, the rear powertrain takes most of the load. Off-road slip handling doesn't seem to be well documented, but Tesla cars aren't intended for off-roading."
I don't think about this a lot, but I assume that all of the interesting ways to distribute power from a single engine to 1-4 wheels are, all of them, inferior to having a dedicated motor for each.
That is, I assume that no matter how fancy you made your differentials, two motors is better than one and four motors (one for each wheel) is better than two (all else being equal).
Is that correct ? Or is there some scenario(s) wherein mechanical distribution of torque (with differentials) is superior to (again, all else being equal) a dedicated motor on that wheel ?
...
If I try to answer my own question, all I can come up with is:
1. If you have a motor for each wheel, the max output on that wheel is the max output of that motor, and theoretically, you can distribute more than 1/4 of the single engine to that wheel with differentials, so ... maybe that's a very big deal ? Do 4WD vehicles often send 70-80-90% of output to one wheel ?
2. Sending power to a specific wheel via the path of least resistance takes zero time - it's instantaneous - whereas deciding what to do with each of the four wheels (in software, presumably) might have a lag ... although that sort of breaks my "all else being equal" tag, above ...
> If you have a motor for each wheel, the max output on that wheel is the max output of that motor, and theoretically, you can distribute more than 1/4 of the single engine to that wheel with differentials, so ... maybe that's a very big deal ? Do 4WD vehicles often send 70-80-90% of output to one wheel ?
The situation that leaps to mind would be an especially muddy, low-speed, off-road terrain where the differential is keeping three wheels from spinning. The neat thing about that situation is that, in spite of having at most 1/4 of the vehicle's power available, electric motors are still probably a big win, even if you're towing something.
That's because with electric motors you've got almost all the motor's torque available at low RPM. With a gasoline or diesel you've got to get RPM up a bit before getting the power you need. Getting the vehicle moving and balancing RPM and wheelspin and everything is a goofy exercise that would be seem to be made a lot easier by just being able to just gradually bring up the throttle, which you can't always do with a ICE motor.
And more to the point, 1/4 of the total available torque is still more than you're likely to get with even a good diesel and a good automatic transmission that deals with everything gracefully at low-speed, low-RPM. Maybe I'm wrong about that last part: all the off-roading I did was in a primitive Jeep with a manual transmission.
> I don't think about this a lot, but I assume that all of the interesting ways to distribute power from a single engine to 1-4 wheels are, all of them, inferior to having a dedicated motor for each.
While off-roading it's not uncommon to completely loose traction on two wheels (search for cross-axle articulation for a way to mitigate this). I've also lost traction on three wheels during steep hill climbs, rutted, muddy traverses, and on sand.
In situations like that locked differentials are vastly preferred as you can transfer the majority of the car's torque to 1-2 wheels. A motor for each wheel (or each set of wheels) would not be preferred as 1/4 or 1/2 of the vehicle's torque is often not enough to maintain forward momentum.
The challenge is that the axles of many stock vehicles are not designed to withstand all that torque, so breaking your vehicle's axle when all the torque is transferred to one wheel is a real risk.
One more thing to think about, on this line of questioning, is efficiency in "normal" driving situations.
If you're driving in a straight line, with good traction on all wheels, will a single larger motor be more efficient than two smaller ones on each wheel? For many people, I think that's 99% of their driving, so it makes sense to optimize. Especially on electric vehicles, where increasing battery capacity is much harder than just adding an extra gas can.
Maybe in that case the complexity of AWD/4WD/Traction control makes sense in that case.
One motor per wheel, in the wheel, is an old idea. It's been used most notably on LeTourneau heavy equipment. The usual problem is too much unspring weight, not a problem for giant earthmovers but bad for fast cars.
Michelin was pushing it for cars, with their "Active Wheel" concept, from about 2003 to 2012.[1] That seems to have disappeared. Siemens has demoed a motor-in-wheel unit, and Volvo and Nissan have fooled around with this. Protean, in Shanghai, is trying to sell their wheel motor. There are others. Nobody has shipped production cars yet, though.
"One motor per wheel, in the wheel, is an old idea. It's been used most notably on LeTourneau heavy equipment. The usual problem is too much unspring weight, not a problem for giant earthmovers but bad for fast cars."
hmmm ... I wasn't thinking about motors in the wheel, although I am familiar with that concept.
I was thinking about a more pedestrian motor per wheel configuration wherein the motor is just behind the spring ... and is thus, sprung weight ... is there even space for that ?
Separate motors for front and rear wheels is far less complex than adding more moving parts.
For AWD, you need a driveshaft and a differential. The differential has a lot of parts, there are a lot of bearings that sap power, and it adds a lot of weight.
Using separate electric motors for front and rear instead increases the available power, enables unlimited torque ratios from all-forward to all-rear, and has many fewer moving parts.
Thanks. But isn't coordinating the motors complex? If the power delivered to the wheels is out of sync for a moment, I would imagine it could be catastrophic, and the system has to deal with variables of turns, traction, acceleration/braking, etc.
It's not like trying to coordinate rowers in crew; you have full, absolute, instantaneously-responding control over the motors, as well as information on exactly how fast they're going, how much force they're applying, how fast each individual wheel is turning, what the steering angle is, etc. And then what's controlling it is a computer, not a human.
Stability control, which already exists in every car, deals with all of those variables and is able to correct for driving with judicious braking (or release of the brake) on individual wheels, even when some or all may be slipping or locked up, the car going sideways, on varied surfaces.
Adding more things for a computer to control makes it easier, if anything.
Keeping the front and rear motors in sync is a cake walk compared to controlling the brakes for traction control.
"If the power delivered to the wheels is out of sync for a moment, I would imagine it could be catastrophic"
The control software is probably updating every 3-15 milliseconds, so even if the software had a massive hiccup, it would have to last for multiple frames for it to actually get translated to the pavement. The drivetrains of cars are actually pretty mushy. Everything's mounted on bushings, there's backlash in the gears, and tires are rubber. All of this adds together to buffer out any kind of spikes. Much like a capacitor and resistor can be used to buffer electricity. So if, say, the front wheel was 5% underpowered for a frame, then all that would happen is the gears in the front differential would go slack for a few milliseconds. If on the next frame, the power was corrected, the differential would go back to being taut. If it was just 5% underpowered, you likely wouldn't even feel this. If the front motor totally locked up for one frame, you'd still probably only feel a little jitter.
You have 4WD if you have 4 driven wheels (and unless you count the spare, that's all my wheels). Then, you can have a multitude of differential configurations on the 3 differentials (open, lsd, locker, diff-less viscous clutch, diff-less locker).
A Jeep WK uses a locking differential center, and electric engaging LSD's front and rear. By that article, the car is AWD on road, and sorta 4WD when off. The Jeep WJ is either controlled by a viscous clutch (Quadra-Drive), or has manually selectable 2WD, 4WD open ("AWD") and 4WD locked ("4WD") modes. A 98 Ford Explorer pulses an electric clutch to go between 2WD and 4WD locked. These cars all behave very differently both on and off road, and they don't fit well in the authors categories.
If you're buying or owning a car that drives all wheels, the only useful information is the diffs. Labels like "4x4", "4WD", "AWD", "Quadra-Trac", and "Quattro" mean nothing.
Most new 4WD's use open diffs with electrically controlled lockers or LSD's, as that gives the best combined on/offroad experience. Permanent LSD's/viscous clutches (Jeep WJ, for example) is a bit more responsive in surprise low-traction conditions, but it eats your tires when you make tight u-turns, as the turn will progressively engage a complete diff lock. Center lock without diff works like a regular 2WD until you lock, but remember that if you have ANY type of locker on ANY axle (not LSD), then engaging in high traction conditions might snap your axels in a very loud and explosive manner, even if you think you're driving straight.
In real life, in most situations, ground clearance, good tires, traction control and ABS make more difference than driving all four wheels, most of the time. Personally, I'm more concerned with stopping than starting, and that's where ABS and good tires matter and what kind of transmission you have doesn't.
This is a sales pitch video, but it is interesting to illustrate the differences in AWD approaches, with different vehicles on rollers:
https://www.youtube.com/watch?v=9cuZYTQLfA0
I spent a lot of time looking at this several years ago when my family was planning on taking a vacation to the Outer Banks of North Carolina where you have to drive on the sand. We ended up renting a Ford Explorer that had a "sand mode" in it's electronically controlled awd setup. Worked great.
Have been a driver for only 2 years but I can confirm that. I live in Eastern Europe where we tend to have at least one or two snow storms per winter, and, indeed, having winter tires and ABS makes a ton of difference, even on a 1.4-liter FWD vehicle like mine.
And, even more important, you have to keep a level head when you see that there's snow on the road and not try to do anything crazy, like making sudden left or right wheel movements. Also try to keep a safe distance from the vehicle in front of you, and, before you venture on a trip outside of the city, just check the weather forecasts: if there's a "red code" storm announced you'd better postpone your journey, only a tank can get you out from something like this: https://www.youtube.com/watch?v=Frf2lV77fl0
Bah, you can still see the road in that video. :-)Not to one-up you, but rather to point out what's possible, your video is a not-uncommon regular winter storm in the midwestern U. S. I grew up driving 2WD RWD vehicles in that stuff, usually got home just fine. I did land my parent's full-sized van in the ditch once. Should have put 25kg bags of salt in the back end to add weight, but might have used said for the water softener, don't recall.
So you don't need 4WD (let alone a tank) if you have some sense. Which is why a lot of people buy 4WD vehicles, I guess.
It really depends on the amount of snow and type of snow. Where I live in Canada, we get enough snow that AWD > snow tires. Given enough snow, or conditions, even snow tires will get stuck. Having power on all 4 wheels really does give superior traction in those situations.
Definitely a good point. For reference, I should have mentioned that I'm in the middle of Wisconsin. We actually had very little snow over the past two years, and I'm hoping for more this winter.
Great! Finally somebody explained this to me, I thought it was just a marketing term related to different patents of similar ideas.
I was puzzled by this as I was stopped by a ranger coming out of Waipio Valley on Hawai'i, where there was a sign "4WD only" and my rental car had AWD. I thought they were the same, just different names, so I argued with the ranger that I do in fact have 4WD, which is in my case just called AWD and in other cases 4x4 as well. Well, he was still pretty upset but the "I am a European and we have different cars" finally disarmed him...
Generally a sign saying you need 4wd in the US really means you need a lot of ground clearance. Your car probably would have high-centered, that is stuck resting on the middle of the car with the tires in the air.
The annoyed ranger was probably just tired of pulling stuck tourists out.
Normally I would 2nd this, but it does not apply on this particular 4x4 road. It really sucks when opposing traffic appears as it's barely wide enough for 1 vehicle in most parts:
The road gains 800 vertical feet (243.84 m) in 0.6 miles (0.9 km) at a 25% average grade, with steeper grades in sections. This is a paved public road but it is open only to 4 wheel drive vehicles. It is the steepest road of its length in the United States.
That doesn't seem that bad for normal traffic, conditions permitting - unless the steeper gradients are markedly so. I'm sure they have their reasons for 4wd only though - picking too many tourists out of the ditches or whatever.
There's quite a few roads in the UK with 1:4 gradient, and one just down the road from me at 1:3 whose only restriction is no trucks and vans. Going downhill on the motorbike makes it feel like you're falling off! :)
There's an annual charity event that involves the rolling of over 30,000 Jaffas (spherical confectionery-coated chocolate confectionery) down the hill.
There are roads in Spain with 1:4 gradient, too. 200 of the world's best cyclists rode up some of them in today's stage of the Vuelta a Espana. I'm no slouch cycling up hills but my jaw dropped seeing today's footage...
That footage is from a rally but every weekend, fwd cars with 800/1000cc engines and a peak torque of about 70 Nm do it just fine. Most of the cars have 2-5 occupants decreasing the already low torque/kg, and almost the whole climb is done in a bumper to bumper traffic(no momentum for the climb).
The Waipio Valley road, as I understand it, is mostly paved but extremely steep and narrow/twisty. So the 4x4 requirement is probably more about being able to control speed without burning up brakes and dealing with often wet conditions.
However, in the general case throughout most of the US West, "4WD roads" do indeed tend to be as much about having high clearance as about having 4WD per se. It's also just a way of saying that, if you just have a random rental car and aren't used to driving very rough and often narrow/exposed roads, this is probably not a good idea.
In Death Valley you rather need more spare tires (if you go e.g. to Racetrack; seen a car that got 2 punctures at different locations). Otherwise high clearance is needed only if you go off-the-beaten track; even Titus Canyon is fine on a normal road vehicle.
I've always wondered why this matters. I've got car insurance, I've got additional rental coverage on my credit card, and there are plenty of other vendors from whom to rent cars if you want to blacklist me.
Having violated this all the time I've been to Hawai'i I can understand why do they do that - for example, if you continue on the road to Hāna/Maui beyond, down to the southern side of Haleakalā the road becomes really dodgy, later turns into gravel and there is no water/gas for xy miles available, so for casual American tourists it might be pretty scary. Also, driving the north-western part of Maui is on a very narrow road over cliffs and fallen rocks (not as bad as parts of Tenerife though). I actually had an encounter on a pretty tall cliff where I was driving Subaru Forrester and some US female celebrity named Kim was driving the opposite way in even a larger car; we drove past each other with smiles in about 1mph to avoid collision and she must have been right on the edge with her car while I was almost scratching the right side rocks. Even Saddle Road between Maunas on the Big Island could be pretty demanding to casual tourists in fog and really slippery when descending down to Hilo in strong rain, which is like always (saw a crash once). There are also some scary roads on Kaua'i and O'ahu. In US it's so much easier to get a driving license than in EU, so your casual drivers are not used to fast and safe driving like in Germany, many off-road segments like in Spain or navigating chaotic situations as in 3rd world countries, and your society is generally on the safe side (even excessively so).
They probably don't want you taking a road you shouldn't, getting hurt, and then suing them because they didn't warn you about the dangers. Or they get tired of having all their cars smashed up. Insurance covers it, but it's still annoying.
Well, they still have trouble with you if you get stuck or wreck the car, so it is easier for them to just list the "forbidden" roads. Not sure, but your insurance might be void if you crash the car where you were not supposed to drive.
Also 4WD cars generally have a super low gear for crawling at low speeds & high torque whereas AWD cars usually don't have a similar gear. Low gear is critical on steep grades, particularly descending them so you can go down safely and slowly without riding your brakes and overheating them.
It was some US-made SUV (Ford or GM, can't recall which one) with fairly high clearance - I specifically booked it so that I can go everywhere (Waipio, Mauna Kea etc.). The road to valley was fine, it was also dry and the only possible issue could have been crossing a few streams at the bottom of the valley, but SUV had no problem there as well (clearance was sufficient). I experienced far worse roads on Tenerife while blindly following a route computed by then NOKIA maps that took me to super steep straight-up roads where my car ended up spinning all wheels unable to move up ;-)
>Generally a sign saying you need 4wd in the US really means you need a lot of ground clearance
The Subaru Outback (AWD) has 8.7" of ground clearance. The Ford Explorer ("Intelligent" 4WD) has a ground clearance of 7.8".
If "Intelligent 4WD" means some kind of differential, then it really is just a marketing term at this point, though I learned about the distinction between 4WD and AWD when I bought a Subaru.
AWD is meant for adverse conditions on a road and will probably help on a unpaved road. Think rain, ice, snow. It will keep you on the road.
4WD is meant for off road conditions. You get more torque so you'll be able to get over rocks and out of ditches. The downside is you shouldn't use it over 10mph or so.
I can't think of any recent car that is 4WD. It doesn't make sense since you'll probably never be "off road". Cars don't have proper ground clearance.
> The downside is you shouldn't use it over 10mph or so.
That's not strictly true, and varies between implementations/transfer case designs. In Jeep Wranglers, for example, which have both high and low gearing options in 4WD, the owner's manual will indicate that you can drive at any speed (on loose surfaces such as sand or dirt) in 4HI and 0–25 MPH in 4LO.
You right that's why I said 10 or so. Try taking a turn at 25 in 4LO. I'm not sure of the use case for 25 in LO. My Grand Cherokee can operate the same way but I doubt I would go into LO at those speeds. It also will switch to 4HI when it detects wheel slip.
A friend of mine had a old truck that would randomly switch to 4LO on the highway. It's pretty scary driving 60 and having 4LO switch on.
More often than not, even more so than slip, the outer wheel will "hop". This is especially true, if you've done something potentially unintelligent like welding the spider gears together.
Any new Land Rover or Range Rover? They come with 2 locking differentials, with 3rd being an option that can be added extra. Same with Mercedes G-class - they come with 3 locks as standard. And I've no idea what you mean by "cars don't have proper ground clearance" - just buy a one that does?
I don't think you understand what I mean. When I say car it doesn't include SUVs/trucks. The difference used to be cars (unibody) vs SUVs/trucks (body on frame). A lot of SUVs are now unibody including my Grand Cherokee.
There are many types of SUVs and trucks with 4wd in the US. I guess in Europe you only have Land Rovers or G-wagons. I don't think GLs or MLs are common in EU but you can get beefed up off road packages in the US.
As I explained in my other comment - I forgot that in US, "car" and "suv" are two different things. You could have a Ford F150 or a Chevy Escalade here and it would still be a "car" , just like a car without roof is still a car but of a subtype "cabriolet".
>Any new Land Rover or Range Rover? They come with 2 locking differentials, with 3rd being an option that can be added extra.
Source? Center locking diffs are more or less standard, rear lockers are options in LR3/4/post 2004 Disco's, Sports and Range Rovers (although standard in some, I think the Supercharged and possibly the SVRs, but I'd need to check) Front lockers are not factory options and I don't ever recall seeing after market lockers for these models advertised.
Defenders, Discos upto 2004, Range Rover classics (and probably P38s) and all series vehicles have the standard after market options for front and rear lockers.
I had a 2005 Discovery LR3 V8(US Spec, imported to EU) and I could swear it had 3 diffs - at least the 4x4 Info screen would always show 3 diffs working independently when I was driving it offroad. Of course I might be remembering it incorrectly, but a quick search on google shows "genuine Land Rover Discovey 3 front differential" part on ebay, so I guess it must exist?
They have differentials on the front, but they're not "locking" differentials, they're "open" differentials, no locking capability at all.
Center diffs are for the most part (in Land Rovers) locking differentials, though some Discovery IIs had open center diffs and things like Freelanders use a viscous coupling in place of a mechanical diff lock.
Newer models have an electronically controlled locking diff in the gearbox (center) and an optional electronically controlled locking differential at the rear.
This distinction isn't really understood in the UK either, at least not as far as I've ever seen. If your car drives all 4 wheels, it has 4 wheel drive - no, seriously! - and you just have to know what its limitations might be. AWD vs 4WD seems to be one of those distinctions that Americans have, like drink vs beverage, that British English lacks completely.
He actually stopped me after climbing up back to the viewing platform. The car was some US-made SUV with high clearance (don't remember if it was Ford or GM), just there was a sign AWD in the back (I specifically asked for a 4x4 rental vehicle). The road was fine, I think the issue might have been a few streams you had to cross at the bottom of the valley, but SUV had no problem with them. Ranger wanted to fine me but after arguing about 4WD, AWD, 4x4 he dropped it, still a bit angry.
This article is good but goes overboard in explanation. AWD and 4wd by all accounts are just buzz words for having front and rear differentials. In actual use, a 4x4 (a term I mean to refer to both 4x4s and awds as its just a buzz word) has three differentials. A center diff splits power from the transmission out to the other diffs, which split power to the wheels. There are three general types of diffs, whether centered or front or back:
Open (just gears, sends power the the least resistant path)
Locked (a spindle connects the input directly to the output and all three spin proportionally)
Posi (multiple ways of actuating this, can be viscous clutch, electric or air powered actuation, or mecanical geared torsion style - regardless the point is to keep power at all wheels, some with variable speeds allowing for turns - others act like a spindle)
Most "awd" cars have a open gear in all three diffs. My jeep has a spindle in the front and center and an air locker in the rear (not the stock design). My old BMW 325ix has a viscous clutch in the center and in the back, and open in the front. My dad's audi has open gears all around and uses the brakes to actuate were the power is delivered. At the end of the day tho, its three differentials.
There is a non-buzzword practical difference; one system cannot be driven on dry pavement (part-time 4x4). One system can be driven on dry pavement (full-time AWD).
It'll be interesting to see what happens when off-road vehicles start becoming electrified. Separate front/rear motors eliminate the need for that middle differential or transfer case, and if we eventually transition to a separate motor for each wheel, we'll be rid of all of the differentials and have much better ground clearance as well as better traction.
For now, I suppose electric trucks/jeeps are more affected by "range anxiety" than daily-driver sedans, since people tend to imagine themselves driving to some remote location when they buy them. But as technology improves, I would love to see something like a modern reincarnation of a 1960's era CJ-5 (but with better safety features) -- simple, easy to modify, nothing that doesn't need to be there, and reasonably priced.
The army gets interested in electric vehicles for troop transport because of their range. It happens that 1. A hybrid can basically provide the same range, 2. A hybrid can regenerate energy when the driver slows down, providing much better range, and 3. They can approach a target in silence for the last 15 minutes, thanks to the electric motor.
The drawback is, they struggle with on-field repairs.
The video from the 30s was fascinating. It gave a perfect progression of complexity while explaining the concept of a differential. I'd like to see more of those kind of videos (from now our from clear back then).
You might like Sheaffer's "The Twenty-Six Old Characters", c. 1947. It touches a bit on the progression of language, but it's really about writing instruments. It culminates in explaining Sheaffer's "Snorkel" mechanism, which was probably the most novel and complex pen system ever designed. I have several and love them :)
The performance of either system is completely dominated by the traction aid at the axles. The article is otherwise good because most AWD vs 4WD stuff completely igores the axles (because it's a heck of a lot to explain if you really want to compare the various systems) but kinda misses the main point.
Open, bad traction control (cuts throttle, applies a little brake), good traction control (all brake no throttle), the many varieties of LSDs, automatic lockers, manual lockers, Lincoln lockers and spools all have a huge affect on traction.
Lincoln lock the diffs of any AWD crossover, screw with some wiring so it's full time and you'll have something that can out wheel a $50k Jeep but trades off a tiny bit of grocery getting capability.
4WD/AWD are almost never necessary in USA and Europe today, to the extent that, if you buy 4WD/AWD you're likely suboptimizing and even taking a risk (for your drivers who aren't familiar with the quirks of 4WD/AWD). As Saab has shown for decades, FWD will get you almost almost anywhere a 4WD/AWD will in bad weather.
But sales of 4WD and AWD cars have been a boondoggle for the auto manufacturers. Cost, complexity and maintenance are significantly higher: often you must "fiddle" with them, woe to you should you damage them due to bad judgement. They reduce mileage, increase car weight, and reduce reliability.
I prefer a RWD car for most driving, since it handles better and I'm never climbing bare hills or plunging through swamps, but I was raised on oversteering go-karts.
> 4wd/AWD are almost never necessary in USA and europe today...
I'd have disagreed a few years ago, then I moved to Norway. Snow is normal.
And the average car is simply a FWD. Most folks tend to have cars or station wagons instead of trucks and SUV's, and most of those are FWD.
The biggest things that help in snow?
Snow tires and snow socks for the tires, and chains when necessary. My spouse thought it was nuts that people didn't change tires in the winter, and average travel speeds on roads is slightly higher on snow and ice. Most folks will take their cars lightly offroad without incident (prinmitive vacation houses are often on dirt roads with weedy grass parking). In cities, at least, it is also pretty common to have bins with grit and I think folks need to keep safety items (a shovel) in their cars.
"I'd have disagreed a few years ago, then I moved to Norway. Snow is normal.
And the average car is simply a FWD. Most folks tend to have cars or station wagons instead of trucks and SUV's, and most of those are FWD."
This is true.
I live in the US and am guilty as charged with over-provisioning vehicles. It's what we do.
Meanwhile, last time I was in Denmark, I was amused to see a decent sized horse trailer being towed by ... an Audi A8.
I live in Utah where we regularly get snow measuring feet deep. Driving up the canyon to go skiing, or doing anything in the state during winter or in all the wilderness, it is required by law to have 4WD.
All over the west there are roads that are impassable without 4WD. I use it all the time.
The parent and other people commenting about trashing 4WD systems are from Europe. I'm not sure if there are as many remote places in Europe anymore especially compared to Utah.
I go trout fishing a lot in North Georgia and many places I go would be accessible in a small fwd car. In fact if people saw you driving an incapable vehicle they would stop and warm you about proceeding further.
Try Norway for size. I live in a rural area and the everyday car is a Land Cruiser - in summer, just about anything goes - but come winter, and big, studded tyres, low range and diff locks all over (well, center and rear in my case) are what gets you from A to B and back.
Anecdotal evidence makes me claim that electronic traction control systems are excellent, but still no match for good ol' manual systems - probably in part because the automagic systems are easily taken for granted, making drivers forget what icy conditions are like - and using the electronic systems to drive faster, not safer.
Odd, here in Washington, 4WD/AWD get exemptionss[0]. However, my dad, who drives a 4WD, bought chains because he was told 4WD do not get an exemption, so there seems to be some confusion. The WSDOT site clearly says both are exempt though.
AFAICT you're required to have the chains available, so they're usually in the trunk, unless you're in a situation where they're required. Of course if you buy chains you should immediately put them on your vehicle, if only to ensure that later, when you need them, they will fit. Once you've done that, then you can put them in the trunk.
I disagree with your interpretation of the law: dot.ca.gov simply state what they require and say nothing about which is superior. And note that 4WD/FWD vehicles _also_ must carry chains in the R2 & R3 areas (from the URL you posted http://www.dot.ca.gov/hq/roadinfo/chcontrl.htm):
"(NOTE: Four wheel/all wheel drive vehicles must carry traction devices in chain control areas.)"
Unfortunately for an AWD/4WD vehicle, following this provision costs twice as much as for a FWD car and will take twice the time to install (not an insignificant factor in freezing rain) and, should you fail to install on both front and rear properly, or should they suffer mechanical failure, may put one in a bad situation.
The cost of chains is negligible compared to the cost of the car. The advantage of not having to get out of your car in the snow and put on chains at a chain control checkpoint is huge though.
AWD absolutely gives you more control of your vehicle in icy and snowy conditions. No, they don't help you stop more quickly, but they do aid in preventing fishtailing, and in being able to accelerate when you choose to, and maintain control of your car. Dedicated snow tires are a bigger factor, but the best option is to have both AWD and snow tires if you will be driving in snowy conditions on challenging roads. All of this make you more able to safely control and operate your car. Also you're far less likely to get stuck or slide from a stop.
I don't know what your definition of USA is, but here in eastern Massachusetts I have had spin outs on regular roads while driving FWD cars in snow. Ever since I started driving an AWD vehicle, I had zero skids/spin outs in similar conditions.
So, my personal experience is quite contrary to your opinion. In fact, we are buying a second vehicle now and one of the critical requirements is 4WD/AWD, esp since we don't know how much snow the coming winter will bring.
That's probably a difference in suspension/tires/balance rather than drivetrain. With front biased AWD (very common setup) you have to apply power to maintain attitude in a skid, just like FWD.
4WD/AWD definitely helps in some day-to-day driving cases. One example is parking in a foot of snow in NYC. Otherwise you have to spend an hour digging. Another example is getting out of a driveway in the morning when you are late after a snowstorm.
> That’s why AWD ... helps you safely navigate both inclement weather
This is certainly how AWD/4WD etc is marketed to consumers, but it's worth remembering that every car has four wheel stop. Better acceleration really just helps you hit that snow bank first.
(Also this article loved to misuse the word "torque" when they really meant rotation or energy. Sigh.)
It's not just "better acceleration", it's more consistent and predictable acceleration. I find it makes driving backroads on a dark stormy night quite a pleasant experience.
Well we can't really compare subjective experiences. Driving backroads on dark stormy night can be fun regardless.
If you're driving in weather, acceleration should be a low priority (barring edge cases, like steep driveways). Contrary to Fast and Furious movies, accidents aren't generally avoided via acceleration. I'd rather be reminded that traction is spotty when attempting to accelerate on two wheels than abruptly finding out when I need to brake on all four.
You know what really sucks? Discovering that you have food poisoning while you are out and about, and pulling off to the side of the road to throw up. Then realizing you pulled a little too far off onto wet grass and mud, and need the kind property owner's help to push your truck back onto the road.
Or going to the SO's cousin's for a family thing, parking in the yard like everyone else, getting rained on, and needing their tractor to pull you out.
Now, I'm fine with my 2WD truck. But if I had to pull those kind of antics more than once a year, I would seriously consider 4WD.
The problem of the outer wheel going farther during a turn also occurs with trains, and trains do not have differentials to let the wheels have different angular velocity. On a train, the inner and outer wheels have the same angular velocity regardless of turns.
I'm linking to Jalopnik, which embeds Richard Feynman's video on train turning, rather than directly to the video because there are some interesting comments on the site. In particular, see the comment from railroad employee "Gonemad" about how they have a system that automatically greases the wheels before turns, which seems rather counterintuitive at first.
Kind of an odd post to see on hacker news. As somewhat of a car buff my take away is that this article gives an adequate high level explanation but leaves out a lot of useful details, especially to anyone evaluating 4WD/AWD vehicles, like the tremendous differences between modern AWD systems.
One thing that this article only touches on briefly is that more and more AWD systems today are just open differentials plus the TCS instead of using more expensive and heavier limited slip differentials. These modern systems are lighter and cheaper but they have disadvantages; not least of which is that using the brakes to force power to move around wastes torque, and it's only going to be as good as the sensors. If the ABS/TCS system has any issues your AWD goes out too.
I've owned cars with every type of AWD system (Audi, Subaru and currently a Honda) and day to day they all work just as well. Given that, I suppose the manufacturers are ultimately right to go with the cheaper and weight saving system.
The brake based system is great if the traction control is programmed for "get me up this snowbank" and not "prevent me from being an idiot"
There was an episode of Dirt Every Day where Fred and co. took a Jeep Renegade(?), or whichever the small one is, off-roading there were some great scenes of the traction control in action. Basically by spinning the dial to rocks the traction control knows to use ABS to approximate how one would work the brakes with a torsen.
The Honda VTM-4 system is quite good in it's intended operating environment - which is operation during winter weather. It uses electromagnetic clutches in the rear differential housing to direct torque to the rear wheels with left/right independence. This gives you the effect of a locking differential, up to the friction limit of the clutch material. There's no center differential in the system, and the front differential is open (but Honda includes a TCS, which is cheap because once you have the ABS sensors & pump, it's just software).
Even during good weather on dry roads, the system has benefits by engaging the clutches when accelerating from a stop, so all four wheels are driven when you need to get going. The clutches are disengaged after about 20 mph so you get the fuel economy of a FWD vehicle.
The newer Jeeps actually have both! It's an improvement over AWD and they call it "Active Drive".
It engages/disengages the center differential (4WD) as needed automatically, but can also transfer power to be greater in the front or rear as desired - with presets for 60/40 and 40/60 rear/front power. (Sport mode, Mud/Snow)
It's also got traction control w/ ABS, and it uses the brake locking differential technique on each axle as the article describes in advanced AWD systems.
Of course they have different variations, one of which also has locking differentials on top of everything else.
It's amazing to drive. It's all fully automatic too.
No mention of the "Detroit Locker", IMHO one of the more clever automatic-locking differential designs --- it uses a pair of dog clutches which are normally engaged, one for each side, and the wheel that spins faster is allowed to. It's self-contained and requires no additional controls, just like a regular differential. I think the only reasons why they didn't become more popular are due to their cost and the noise they make --- a very noticeable clunking on tight turns as the outer wheel tries to go faster and unlocks, then locks again as the vehicle straightens out.
Detroit locker is effective, but I wouldn't really call it clever. For that I think of Torsen, the Torque Sensing differential that uses planetary gears. The problem with the Detroit is primarily noise, as you mention. The problem with Torsen is that it doesn't work in the presence of near 0 traction on one of the outputs, so people will drag the hand brake in that case.
Of course, this only matters when you are driving four wheels with one engine. Now that we're going electric you can have one engine per wheel, or one front engine and one back engine and use ABS across single axles. It's kind of amazing how much progress we have made with simple internal combustion engines.
I don't really buy this article. The terms AWD and 4WD don't really define a set of functions, but the article says they do then waves hands at all the exceptions.
Really, from what I've seen, current usage is AWD is a 4WD system marketed for on road use, and 4WD is an AWD system targeted to off road use. The biggest indicator seems to be if a vehicle has a low range transfer case, it will likely be marketed as 4WD. Also, if it is marketed as 4WD, HOPEFULLY it will have under body armor.
Remember, not all AWD/4WD systems are equal. Last winter our neighbor's Subaru Impreza with Symmetric AWD (which Subaru claims is super good) couldn't make it up a small incline by our houses. Both my Audi and Land Cruiser on all season tires had no problem.
I owned a WRX that I never got stuck in snow. I once came up to a hill with a Sabb and Civic both spinning; and a semi-truck that couldn't really move when they were in the way. I had to wait for both of them to give up, left off my clutch, and crawled up the hill.
There's a lot of blurring and different labeling of 4WD systems based on the market segment.
Generally it's more helpful to think of 4WD/AWD as fitting into three categories:
1: A system with a transfer case that predominantly drives one axle (almost always the rear, and almost always labeled "4WD"). This is your typical pickup truck 4wd system. When it is engaged, the two axles are locked together and may as well be welded and they'll turn at the same speed.
2: A system with a center differential, in which both axles are driven but may turn at different speeds (Almost always labeled AWD). The differential may allow full or progressive locking and/or torque vectoring. It's rare to find one of these that can fully lock (Subaru WRX STI is the only one I can think of), but it's common to see ones that favor one axle (Audi, BMW, Mercedes) or that can partially lock in response to slipping of one axle (often with the brand name Torsen). These are almost always found on passenger cars, and with few exceptions they're found on cars with longitudinally mounted engines (Mitsubishi EVO being the only transverse layout car with a center diff I can think of), that means they're usually found on higher end cars that are based on front-engine rear-wheel drive models.
3: A system with a power-take-off unit (often brand name Haldex), in which one axle is driven and a clutch pack can partially or fully engage the other axle. (this is often labeled AWD when put on a passenger car, and 4WD when placed on a "truck") The axles can operate at different speeds but can also fully lock in the way a transfer case can. Typically you find these on "AWD" cars that are based on transverse-mounted front-drive models as it's easier to add such a system and get partial engagement of the rear axle, but you'll also find them on performance AWD applications based on rear-drive mid/rear-engine models like Porsche 911s or the Buggatti Veyron. You'll even find it labeled as a 4WD system on "trucks" like Honda Ridgeline or Pilot, or on the new Ford Raptor. These tend to operate more like an automatic transfer case, as the only way to partially engage the secondary axle is to slip the clutch, which you don't want to do 100% of the time. (Ford actually calls this system a transfer case on the Raptor).
You'll see all sorts of spurious claims about systems "sending 100% of the torque to the rear axle" (often just means that the system can fully lock, and 100% torque to the rears means the fronts are in the air). You'll also see things like "rear biased", which sometimes means you have a center differential that overdrives the rear axle by default, which is great, but is often misleadingly applied to PTO/Haldex systems where the rears are also overdriven, but only during the rare times when the system is engaged.
Basically if you're looking for traction in low speed settings where you might not have grip at all on one axle, like offroading or being stuck in the snow, you want as many of your axles locked as possible, which technically you can get with any of these systems, but is usually seen on transfer cases or haldex/PTO systems.
If you're driving quickly on pavement with some traction, you usually want something more like a center differential, preferably one that can partially or entirely lock when you lose some grip at one end.
Right. I've got a Golf R with Haldex; it allows 30+mpg under cruising conditions but can "lock 100%" in the sense that the fronts and rears are locked together. It's created a lot of forum wars because some people read the marketing materials saying "100% of torque to the rear wheels" or "max 50/50 split" and don't understand the difference. If it's fully locked, on dry pavement, of course you're getting 50/50. If the front wheels are on a magic 0 friction surface, of course, all of your torque is going to the rears because the fronts have nothing to grip on, but they're all spinning at the same speed (theoretically).
I wonder why car manufacturers still use direct force transmission with hugely complex (and in case of ignoring the instructions, outright dangerous) like 4WD/AWD instead of diesel-electric or diesel-hydraulic systems like on locomotives.
A computer could instantly shift torque with both of the hybrid solutions, you wouldn't need a clutch or a gearbox any more, not to mention cheap all-wheel-drive including the trailer in trucks. And a fuel-electric drivetrain could easily plug in a battery or supercaps for braking energy recuperation, and the fuel engine could always run at peak efficiency/exhaust gas condition...
The diesel-electric/hydraulic systems are used in situations where the gearing ratio would have to be absurdly high, they're not used because they're efficient or because they have good torque vectoring capabilities. The electric motor can produce nearly 100% of torque at zero RPM, which helps locomotives get several million pounds of train moving. In your car, truck, or even semi, first gear plus some clutch/torque converter slippage is plenty of mechanical advantage to get going, and once you're underway, a lockup torque converter or a fully engaged clutch is more efficient than the combination of an electric generator and a motor.
There's also the matter of power distribution, if you have four independent electric motors, you can only ever send 25% of available torque to one wheel, if you have front, rear, and center lockers, you can send 100% of torque to one wheel.
> a lockup torque converter or a fully engaged clutch is more efficient than the combination of an electric generator and a motor.
I doubt it, generators and motors operate at 99% efficiency. Do you have any reference?
> if you have four independent electric motors, you can only ever send 25% of available torque to one wheel, if you have front, rear, and center lockers, you can send 100% of torque to one wheel.
That assumes that engines are so small that they operate at 100% under normal conditions. This doesn't have to be the case, electric engines are so small, you can have bigger-sized engines where one single engine can use all the power. The nature of electric engine is such that the loss of efficiency by doing this is minimal. Plus you need to do this only fractions of the time, so the engine doesn't have to be designed to operate in high power mode at full duty cycle.
> I doubt it, generators and motors operate at 99% efficiency. Do you have any reference?
A locked up torque converter or an engaged clutch is basically a solid steel bar. You're the one that is going to need to provide sources that show an electric generator and motor, operating in series, are 99% efficient across a wide operating range. (you won't find that that's true).
>That assumes that engines are so small that they operate at 100% under normal conditions. This doesn't have to be the case, electric engines are so small, you can have bigger-sized engines where one single engine can use all the power. The nature of electric engine is such that the loss of efficiency by doing this is minimal. Plus you need to do this only fractions of the time, so the engine doesn't have to be designed to operate in high power mode at full duty cycle.
Look up the efficiency of electric motors running outside of their optimum power/rpm ranges. Also, you basically just said that you can get around a percentage problem by increasing the coefficient, which kind of misses the point. Also, if you're going to put four motors, you're probably doing that because you want to put them in the hubs, (if you're putting them inboard, you'd just use one or two motors and put a differential in there, like Tesla does), if you're putting motors in your hubs you want the smallest motors you can get away with, because larger motors increase unsprung weight.
Mechanical transmissions are still more efficient, at least for cars, than spinning a generator that then powers an electric motor.
Even hybrids usually have a way to get the engine power directly to the wheel. In the Prius's system, it uses a planetary gear with a pair of electric motor/generators which allow it to split power between the mechanical and electrical paths. In the Volt, there are clutches which allow the car to transition between parallel and serial hybrid modes.
1) That's not a "jet engine". It's a turboshaft. A turboshaft is a gas turbine producing its power mechanically at the shaft. A turbojet is a gas turbine producing its power in the form of reaction thrust out the tailpipe.
2) Gas turbines scaled down to small size (let alone "micro" size) generally have dismal thermal efficiency. All the most efficient gas turbines are gigantic (tens of thousands of hp).
I've never heard that jet engines are less efficient. From what I remember piston engines are 33% thermally efficient, diesels 36% and turbine engines are upwards of 60%+. Of course there are many types of jet engines and ways to get power off of them.
You heard wrong. Diesels under optimum conditions can easily exceed 45%, and state of the art is right around 50%. Gasoline engines can reach 40% or better (again, under optimum conditions). Turboshafts are generally around 35-40% efficiency at best-economy conditions, and dismal under low-load conditions. The very largest turboshafts can reach 40-45%, but come nowhere near to diesel efficiency at low load.
In all cases, you can increase efficiency somewhat by going to the complication of compound cycle, where you harness some of the wasted heat in the exhaust by making steam.
Large, high bypass turbofan jet engines are very efficient from an overall propulsive efficiency standpoint, but you aren't measuring the same thing (shaft power times time, divided by energy consumption).
Gas turbines operate efficiently at constant output, which a hybrid-drive system with storage can manage. They only need to kick out enough power, on average, to keep the storage system fully charged. Batteries and eletric motors can handle both high-demand accelleration, and regenerative braking. Since the turbines are running at a very constant range, and can be rated to average out power load inclusive of idle times, they can be designed for optimum efficiency within that range. Or that's the theory.
There've been other gas-turbine automobiles, including some prototypes built in the 1960s. A friend of mine test drove one, claimed it could lay a patch (spin the tires) at highway speeds. Though my understanding is that in direct-drive applications (such as that), turbine lag is an issue.
The exhaust also runs quite hot, and turbines typically emit a lot of NOx (nitrous oxides), what you get when you run atmospheric nitrogen through a high-temperature field.
It sounded like a gigantic loud vacuum cleaner even just idling, and gave the impression of straining mightily to achieve even mild acceleration. Fuel consumption was TERRIBLE.
Locomotives are used on railroad tracks. Adding weight does not matter.
Cars generally suffer quite a lot if you add significant weight: both performance and fuel economy will be hit. That is why locomotive solutions are not very suitable for cars.
Alright, time to bust a myth. As an AWD owner, every time I have a tire issue, they tell me I need to replace all 4 even if the others are good ... because AWD. "Won't someone think of the tires ... the tires need to to be the same size so they can rotate at the same rate."
Clearly, with AWD the car is designed to allow wheels to rotate at different rates (hence the diffs). Otherwise you would't be able to make a turn. Once you dispel that myth, they then tell you to replace the pair ...
Why do dealerships, repair shops and tire places keep insisting on replacing all 4? ... good for tire sales I guess.
That makes no sense to me. Unless you are driving in straight lines, the 4 wheels will always be turning at different rates (and consequently, the front and back axles will be rotating at different rates).
Subaru's higher end AWD system has separate output shafts from the engine for the front and the rear, with electronically-controlled clutches modifying the torque. Default is 60/40 front biased, but it can send 100% of the torque to either end of the car. Excellent performance with no manual mode switching needed, also manages good fuel economy at the same time. Best system in the business.
Not quite. All AWD Subarus (i.e. except the BRZ) have one output shaft out of the engine, 3 out of the transmission/transaxle (2 to the front tires, 1 to the rear diff), and 2 out of the rear diff. Torque split varies between models and years with most near the 60/40 you quote, but the only one (in the US, anyway) that can send 100% torque to either end is the STI with DCCD on full lock.
However, if you don't have locking differentials you can still get into the situation where one wheel slips and the wheel that grips doesn't get any power - as shown by the Subaru getting stuck on the hill in this video: https://youtu.be/hrdb_UVTa20?t=543
There is a trick that seems idiotic but if you have a mechanical hand brake you can stop the wheel from free spinning. I'm not sure how well it would work for a front wheel, maybe you could hold the brake slightly while on the throttle. Again, seems ass backwards but if you think about what you're doing you're basically "locking" the open diff.
To add to confusion, Subaru sells several very different systems and calls all of them the same "all time all-wheel drive". There is viscous coupling center differential that goes with manual transmissions and nearly always splits torque 50/50 between front and rear, computer-controlled limited-slip clutch pack that comes with most automatics and changes torque split with speed, and some newer systems that do even more.
All this talk of traction and moving forward is great but one of the most important things to remember about 4WD or AWD especially in snow and ice is that is doesn't enhance your ability to brake. It is easy to feel fearless being able to accelerate on a slippery road but a bummer to discover you can't stop (yes, I get that engine braking plays a role...).
And then there are the dual motor cars like Teslas and Toyotas that have no mechanical link between front and rear axle at all. As you'd imagine, this can allow for a heck of a lot of flexibility.
Can't we do away with all the complexity by just having 4 electric motors and a gas engine as a generator? Then each wheel can be computer controlled independently.
>Torque goes to whichever of the four wheels has the least grip.
That should be the tire with the most grip. If torque goes to the wheel where the tire on it has the least grip the tire still has no traction torque doesn't make grip appear out of nowhere.
In four-wheel or all-wheel drive one wheel where a tire has no grip the other three wheel with tire that have grip move the vehicle isn't that the entire point of 4WD and AWD?
Mechanically a drivetrain has no means of differentiating between a tire that is slipping vs a tire at that's on the outside of a turn. A 4wd car with three open differentials will absolutely send all of the engine power to one slipping wheel.
The author is describing the tricks used to get around that fact. Namely locking differentials or limited slip differentials. The end result is a fraction of the torque makes it to other Wheels depending on the design of the drivetrain. The author spent a lot of time on locking differentials when those are rare. Limited slip differentials are more common. Most awd cars will have a LSD in the center diff.
There are torsen diffs which do what you describe but they are rare.
Even a torsen diff, or similar helical/gear type limited slip diffs are "torque multiplying", they need a load on one wheel in order to send the "multiplied" load to the wheel with traction. Modern stability/traction systems equipped with helical diffs will apply the brake to the "free" wheel in order to provide a load to the mechanical diff. You can also apply the brakes yourself if you have a torsen diff but no electronics.
A clutch-based diff or some other type of locking diff would otherwise be required to solve the "wheel in the air" problem without applying an artificial load to the free wheel.
This article is careless with physical terms. What it means to say here is that rotation (or energy) goes to whichever of the four wheels has the least grip. The torque of all the wheels is basically equal [0]. The ones that are gripping are creating linear force, but not enough to move the car. The wheel that is spinning freely has its torque going into dynamic friction and rotational acceleration. When that wheel has accelerated to as fast as possible (which happens awfully quickly because a wheel isn't that massive), the engine's rev limiter kicks in and cuts the total torque down to 4x the dynamic friction.
>To counteract this, the better AWD cars are fitted with a center differential that contains a clutch or viscous drive unit. This splits torque front-to-rear, directing it away from the spinning wheel.
The differential contains a nifty little clutch. When the vehicle detects no traction, it engages and sends the torque to the correct location. Otherwise you'd end up in a situation similar to engaged cruise control with no traction. Limited slip differentials and locked differentials cause much confusion.
This is a discussion site, meaning we discuss things, telling us a potential problem with the source is a great discussion topic. Taking unqualified potshots at the article doesn't provide a jump-off point to any discuss worthwhile.
To user: phpnode that replied to me. I can't reply directly to you but that's a fair enough point.
I was avoiding doing so before because I didn't (and still don't) think this article is even deserving of being picked apart.
I would be happy to do so for the sole benefit of users here though. Just give me a little time to get back from doing some errands and I promise I'll clarify my grievances with the information in the article.
"Thanks to that differential between your axles, an AWD car will send your engine’s power down the path of least resistance—the wheel with the least grip. Where a two-wheel drive car can only choose between two wheels, an AWD system looks for that least resistance across all four wheels."
"4WD works by locking the front and rear axles together, splitting torque 50:50 between them. This provides great traction, but a vehicle locked in 4WD cannot safely be operated on dry pavement because its front and rear axles are forced to rotate at the same speeds."
I remember being scared when I bought my first 4wd vehicle, a '91 Toyota pickup. I knew I shouldn't have the wheels locked in 4wd on dry pavement, and I was worried about going from snowy roads to dry pavement. When I moved on to an Outback (AWD, not 4wd) I wondered why I didn't have to worry about transitioning between slippery roads and dry pavement. Now I understand.
There's also a great video from 1937(!) that shows why we need differentials, and how they work. Here's a direct link to the video if you're going to skip the article:
https://www.youtube.com/watch?v=yYAw79386WI