Some military trucks are designed the opposite way on purpose - because they’d rather have no brakes and be able to move than be stuck on the battlefield.

> Some military trucks are designed the opposite way on purpose - because they’d rather have no brakes and be able to move than be stuck on the battlefield.

Do you have any details on that? Or a source I could read? I'm pretty curious now.

I drove trucks in the military - specifically, FMTV, HEMTT, HMMWV, and 5 tons. None of them had the reverse brakes you describe.

The deuce/half air-over-hydraulic brakes fail open (loss of air pressure leaves you with hydraulic brakes alone, loss of hydraulic leaves you with no brakes at all). However, there is a failure mode that leaves the brakes full on if the air cylinder jams.

At the time it was an acceptable trade-off; later military vehicles have more in common with trucks than tanks, I suspect.

Losing brake boost in a failure condition is the typical failure mode in most vehicles with hydraulic brakes. A hydroboost failure in modern medium trucks reduces braking performance just as much as an air pack failure in the deuce and is just as scary..

The brakes, unboosted, are supposed to still be capable of stopping the vehicle. This is also why hybrid and electric vehicles with regenerative braking, that normally use brake-by-wire with a pedal feel simulator, fail through to using the PFS as a direct unboosted hydraulic cylinder.

Losing all brakes in a hydraulic circuit failure is just because those trucks are so old they only have single circuit hydraulic brakes. Part of the A3 upgrades was changing over to a modern dual circuit hydraulic system.

So basically, the issue is that medium trucks are in a weird in-between. Not heavy enough to require full air brakes, but heavy enough that some of the safety assumptions we make about light vehicles are potentially less true.

They have a way to change tire inflation to suit the terrain. I wonder if they can change the failsafe direction of the brakes to suit the situation.

That’s a great example of how you always have to design for a context.

Yeah, any design is going to be various trade-offs, and you have to understand them (and how they may relate to your mission) when determining what to do and what would be a bad decision.

For example, cars have "fail open" brakes but have independent cylinders so that it is relatively hard to have all four wheels fail at the same time (older cars had single master cylinders) - and one of the tradeoffs is that people want cars to get going immediately and not wait for brake cylinders to "charge up".

As more and more things get computerized, you have more options for "intelligent failover" - anyone who used to drive an old manual car knew you could slow down using the engine - but now the computer could detect brake failure and do things for you to help recover.

And as seen from other comments, the modern US military prioritizes "on-road safety" over "under ongoing attack ability" - partially because of changing strategies I suspect, but also from the realization that the US military kills more members of the US military each year via accidents, etc, than anything else.

Pretty much every car made since the 1980s has had dual-circuit brakes with usually one front and one rear wheel cylinder on each circuit. Volvo being Volvo took it further with their 2-series cars in the 1970s and 1980s, where the front discs had four-pot calipers configured as two sets of two-pot calipers. Each braking circuit had one half of each front caliper and one rear caliper, so in the event of one half of the brakes failing it would still stop in a straight line.

You can use engine braking in automatic vehicles too, by simply changing to a lower gear.

There is one subtle difference between automatic transmissions and manual ones and that is the manual one will allow a shift into a lower gear even when engine damage is likely.

An automatic one will not, and will hold the current gear until car speed falls within shifting range.

Ask me how I know :)

Yeah, newer ones can be too smart for your own good (but not theirs) - older ones had no problem with you slamming it into reverse or park at freeway speeds - ask me how I know!

I am reminded of a friend who changed his steering wheel plastic trim, in the process he bumped the wheel position sensor out of calibration. No worries, trigger a recalibration. Uh oh, won't recalibrate.

This of course meant the entire cars safety and stability control systems, ABS and traction control were all disabled. It drove like a tank.

The fix, obviously, was to replace the brake booster primer pump (hybrid car). It had worn enough to no longer recalibrate. Still perfectly functional, and would have kept working for a long time, bit it wouldn't quite meet the factory expected parameters so it failed it's test.

According to the other guy that founded the Range Rover forum I run, in the 1990s when they were getting P38s as traffic cars the training for "really really stop right now" was to throw it into reverse at 70mph and plant the throttle.

It appears that ZF 4HP20/4HP22 gearboxes can do this an unlimited number of times without apparently being damaged, but I'm not trying it on either of mine.

I have always wanted to try that! I'm not going to try it on any car I own either. It is all just super high risk, and I just don't have a spare car laying around that has the beans to pull that off.

Ahhh, the classic money shift.

Depends on the gearbox. Really modern ones won't, anything more than about ten years old will downshift if you select a lower gear.

Well, I have a 2000 Honda that won't do it. And I had an 03 Kia that also won't do it.

Yeah - but as we saw with the floormat/unintended acceleration, many people don't think of it in an emergency (all the fatalities could have been avoided by shifting into neutral in those cases, might have blown up the engine but wouldn't have crashed/killed).

It's something you're taught to do as part of your driving test, or at least you were when I sat mine.

> For example, cars have "fail open" brakes but have independent cylinders so that it is relatively hard to have all four wheels fail at the same time (older cars had single master cylinders) - and one of the tradeoffs is that people want cars to get going immediately and not wait for brake cylinders to "charge up".

And emergency brake that's wholly separate circuit.

https://en.m.wikipedia.org/wiki/Battleshort Might interest you!

Not in the US. US trucks are built using standard truck parts, including off-the-shelf air brakes that fail safe.

they didn't always fail safe.

I remember a house at the bottom of a steep mountain grade that got plowed into on a regular basis especially during that time.

EDIT: hmm... seems freezing might still be a way to cause failure?

Failure at the bottom of a steep hill is a different type of failure caused by brake fade due to the brake drum swelling with heat as the truck goes down the hill.

But I believe at some point air brakes had to be reversed by law.

In other words, the brakes would apply on loss of pressure, instead of via pressure.