Stuck accelerator and antilock brakes?

[Kristofor]

My airplane weighs 3100 pounds fully loaded and has two disc brakes. While standing on the brakes, I can apply full throttle (235 HP) and not worry about having the airplane move. Airliners can do the same thing. The brakes have plenty of stopping and holding power.

Sure, but locking your brakes on a car doesn't help you stop, it just ruins your tires and leaves black marks on the road. In normal cases the ability of the brakes to stop the wheels from spinning is not the limiting factor in how quickly the vehicle will stop. That's determined by the amount of friction at the tire/surface interface.

A car with a stuck accelerator pedal however is one of the non-normal cases. In this case you would need to know how much surplus force the engine can generate after accounting for wind resistance/drag and the various internal/rolling friction losses. If that quantity is greater than the force the brakes can apply to the wheels the car will accelerate, otherwise it will slow down.

I'm guessing that Jim is right and most street vehicles have more braking power while moving than engine power but I've never tested it on my cars, and in any case Loring is right when answering the OP that ABS doesn't come into the equation....

[cgallery]

and in any case Loring is right when answering the OP that ABS doesn't come into the equation....

Well, what about this scenario...

You are driving along on the expressway at 60-MPH and notice traffic slowing/stopping ahead, so you take your foot off the gas and apply it to the brake. But you notice you're not decelerating very quickly.

You instantly realize the accelerator is stuck and you're coming up on a logjam of traffic pretty fast.

So you press on the brakes with all your might.

Now, if Alex Franke's post is correct, your brakes sense the locked up wheels and start pulsing. But for every millisecond the brakes aren't engaged, the engine is driving the wheels.

So wouldn't be possible that stopping distance would INCREASE with anti-lock brakes in this scenario?

[woodturner]

So wouldn't be possible that stopping distance would INCREASE with anti-lock brakes in this scenario?

One of the sometimes-debated issues with antilock brakes is that they increase the stopping distance for an "expert" driver. An "expert" driver, in theory, can maintain the brakes just short of the lockup point and thus stop quicker. ABS, on the other hand, releases the brakes, then reapplies them. As a result the stopping distance is greater than if the brakes were applied constantly at nearly the lockup point.

The reality for most of us, though, is that in a panic stop situation, we would not have the presence of mind to demonstrate "expert" driving techniques, even if we had the training. So for most people, ABS is safer.

One other aspect of ABS that is sometimes mentioned in reference to accidents is the absence of skid marks. In the past, skid marks were an indication of an attempt to stop, and the absence of skid marks suggested no attempt to stop. Measurement of the skid marks provided a means to estimate vehicle speed as well. With ABS, there are usually no skid marks and thus no evidence to suggest the driver tried to stop, or not.

[Alex Franke]

But for every millisecond the brakes aren't engaged, the engine is driving the wheels.

Wow, that's an interesting observation.

My understanding (from reading my post, :lol is that the brakes would let up until the wheel started accelerating again (due to the floored gas pedal or to regaining traction), and then be reapplied until it starts slowing down again. I think this would be just a matter of milliseconds, though, so I can't imagine it would have that big of an impact on braking distance.

One other aspect of ABS that is sometimes mentioned in reference to accidents is the absence of skid marks. In the past, skid marks were an indication of an attempt to stop, and the absence of skid marks suggested no attempt to stop. Measurement of the skid marks provided a means to estimate vehicle speed as well. With ABS, there are usually no skid marks and thus no evidence to suggest the driver tried to stop, or not.

Except the cars now record all of that information and hang onto it. There was something about that in they my car's manual (yes, I actually read the manual!) -- something like "We're recording lots of stuff about your driving for the most recent X minutes. Some of it we use to tell you cool things about your driving (estimated MPG, etc), but all of it can be used against you if you screw up and kill someone."

That was paraphrased, of course.

[master53yoda]

My understanding of ABS is that it pulses the pressure to the wheel that is sliding. It is using sensors on the wheel that are measuring speed. The brakes if applied while cool while stop the car under full power, although needing more time (seconds). Most of the computers in anything in the last 15 years will limit engine RPM similar to a governor, by restricting fuel injection. The accelerator in the current autos mechanically open the air flow to the intake manifold. there is a mass air flow sensor in the intake that measures the available air and adjust fuel injection and timing according to engine RPM change etc. the oxy sensor also affects the above.

If it was me I would use hard braking until stopped and then shut off the engine.

Hot brakes will fail so it is best in any emergency type situation to get off the road and stop as quickly as possible. (that means don't run in to anyone trying to get of the road.) I broke an oil pump rod in my truck once in the outside lane of a freeway. I ran that engine without an oil pump for about 2 minutes before i could get to the shoulder of the freeway. I spent the next 3 days rebuilding an engine. but it wasn't worth taking a chance of an accident on the freeway.

[JimD]

There seems to be some debate about the second and third steps to take if your car is accelerating and you are not pushing on the accelerator. There does not seem to be any debate on the first step - hit the brakes as hard as you can. You will stop (unless there is a mechanical defect in your brakes which is highly unlikely). After you get on the brakes, the question is whether you should do anything else. Popular Mechanics has a spot on their website which recommends shifting an automatic into neutral and letting the rev limiter limit damage to the engine. They further recommend turning the engine off but I think that is after you stop. Some people question the logic of shifting into neutral or stopping the engine (requires a 3 sec push of the start/stop button if you have one). If you have plenty of space to stop, leaving the engine running may make sense. If you do not have the space, why worry about possible engine damage? If you turn the engine off, you may loose power assist on the brakes. But that should not affect you if you leave the brake pedal down until you stop.

I think anybody short of a professional race car driver should be happy to have anti lock brakes under all circumstances. The only time they take pressure off the brakes is when they would otherwise stop rotation of the tire. Skidding is much worse for stopping distance than a little reduction in pressure on the brakes. If you have stability control too, you may also have it interrupt your braking, but only if you would otherwise go out of control.

Jim

[jackellis]

Popular Mechanics has a spot on their website which recommends shifting an automatic into neutral and letting the rev limiter limit damage to the engine.

Sacrifice the engine. Absolutely. You can always replace or repair it.

ABS might increase the stopping distance a bit, but more than likely with a stuck throttle it won't make any difference. If you lock up a wheel, it'll take you longer to stop.

I like ABS because it's got enough intelligence to save you but it's not so smart that the complexity can kill you. What bothers me about a connection between brake and throttle is that it's likely to be software driven, and the software could be wrong or it could react in a way that's counterintuitive.

[Kristofor]

Well, what about this scenario...

You are driving along on the expressway at 60-MPH and notice traffic slowing/stopping ahead, so you take your foot off the gas and apply it to the brake. But you notice you're not decelerating very quickly.

You instantly realize the accelerator is stuck and you're coming up on a logjam of traffic pretty fast.

So you press on the brakes with all your might.

Now, if Alex Franke's post is correct, your brakes sense the locked up wheels and start pulsing. But for every millisecond the brakes aren't engaged, the engine is driving the wheels.

So wouldn't be possible that stopping distance would INCREASE with anti-lock brakes in this scenario?

I suppose it's possible.

In that scenario where the ABS isn't released until the wheel is locked rather than as it's slowing there would be some slight loss of performance when the next pulse hits to re-engage the brake to slow the wheel down. However, there would also be some performance benefit of the engine power getting the wheel out of a skid condition and back up to rotational speed faster between pulses and back to a static friction state. I don't know which would predominate, I think you'd need actual values to calculate it for a given scenario, and the impact would be small in either direction...

[cabinetman]

I've conducted driving events that included braking exercises, and skid pad exercises both wet and dry. ABS in general terms prevents wheel lockup under hard braking allowing the wheel to rotate which aids car control.

Firm or hard braking may not actuate ABS. It takes a one time hard mash of the brake pedal and holding it down. The pulsating replicates what a driver would be doing in lieu of ABS, which is modulating the brakes to maintain rolling control without locking them up. Personally, I prefer the manual method, as in a crisis, there may be timing involved where the modulation interval and degree of braking can be used to manipulate the dynamics of the car, as far as over steering or under steering.

In addressing the conditions of braking under an accelerating, the speed can be defeated by braking, and shifting to neutral. If the condition is a straight line, no power would be needed. If the condition exists in a turn or an upcoming turn, some power may be needed to maintain control.
.

[JimD]

I found this after my earlier posts. I think the data from their tests is very good proof that brakes overpower engines, at least if you are driving less than 100mph. It also says which types of vehicles have a circuit to turn the throttle off if you are on the brakes - which they think Toyota will be adding.

http://www.caranddriver.com/features...tion-tech_dept

Jim

[cgallery]

It also says which types of vehicles have a circuit to turn the throttle off if you are on the brakes - which they think Toyota will be adding.

It is interesting that they give accolades to the Infiniti for having a throttle cut-out when you step on the brakes, but the test results don't show much better results than the Toyota. From 70-MPH at least, I guess at 100-MPH the Infiniti did significantly better. Although I'd like to think I'd become aware of the problem and start working on a solution (slam on brakes and switch to neutral) before I hit 100-MPH.

BTW, many of the links I've read indicate that the computers in these cars have rev limiters, so switching to neutral when your accelerator is stuck won't cause any damage to the engine.

Has anyone ever tried this? Where is the limit set, to the red line or something?

[Alex Franke]

Although I'd like to think I'd become aware of the problem and start working on a solution (slam on brakes and switch to neutral) before I hit 100-MPH.

I dunno -- the speed gets up there pretty quickly especially if you're flooring it for the purpose of passing. You lose a little bit of time in the initial "WTF" moment when you're trying to figure out why the car isn't behaving the way you expect it to behave. Then you think, "Holy cr*p I'm still speeding up," then you hit the brakes and start working on what might be going wrong.

My guess is that the limiter is set right before redline. I think I did trip the limiter because I remember that slamming it into neutral didn't behave quite the way I expected it to either -- there what a moment of thinking "What's the engine doing?" Of course that's a bit of a red herring when you're trying to figure out that the gas pedal might be stuck.

[cgallery]

Interesting. I will have to apologize for my amazement at the entire concept of anti-lock brakes as this is my first car w/ them and each time it snows I'm intrigued by their operation.

Sometimes the pedal pulsates when I'd least suspect it.

It is kinda telling me that my perception of how slippery the pavement is may be all wrong.

[JimD]

I took delivery of my most recent vehicle, a BMW 128i convertible, at their Performance Center which is about 1.5 hours from my house. They put you up the night before at the Marriott and give you dinner and breakfast there plus lunch at the Performance Center. The morning is 3 driving exercises, one of which is for the ABS. In the ABS exercise, they set up a series of cones in a J shape where you enter at the top of the J. They set the length so you can stop in the straight part if you hit the braking point at 40 mph and get on the brakes hard enough to activate the ABS. Then you do it at 45 and 50 mph. That makes you steer into the curved part of the J while you are at maximum breaking. The only thing they yelled at us about in this exercise (you have a radio in the car) is if you didn't hit the breaks hard enough to activate the ABS. The whole day was fun and I liked the little autocross exercise more but this was useful.

But in my BMW, the brake pedal does not pulsate that I can detect. The wheel motion is jerky so you can see from outside the ABS is working but I could not tell it from inside the car. Maybe it was just me.

The skid pad exercise to show the stability control was the third exercise. 360s are fun!!

Jim