Chain reaction car crash *Video* (1 Viewer)

[Philip_G]

This explains pretty plainly why I don't think the ABS system cares if you're on ice, nor if all 4 wheels lock simultaneously.

http://auto.howstuffworks.com/anti-lock-brake2.htm

 

[Chuck Mullen]

I spent many years in the car business, and if I recall correctly airbags are designed to not deploy unless your speed is greater than 30 or 35 MPH.

 

[Philip_G]

as I think I mentioned on the first page, I slid into a car at a speed similar to that of the cars in the video and both my bags popped, go figure.

 

[BrianW]

Thanks for the reference, Philip. HowStuffWorks.com is a great site. I would agree with it in that it looks for sudden deceleration (in addition to grossly assymetrical wheel rotation), but when you're going only 5mph, it may not trigger, since a complete stop (and subsequent lockup) can be achieved in a split second on dry pavement. (The example given was 60mph.) But the following quoteAgain, this is just plain wrong.

 

[Philip_G]

I think that if you compare stopping distances you'll find that ABS cars can indeed stop quicker. A skidding car doesn't have much friction. Next time it's icy in TX click off or pull your abs fuse and slam on the brakes, you'll find that while skidding you decelerate very slowly.

As the article mentions there are several different types of ABS systems, the one they describe being the simplest, but I still feel the "brain" is smart enough to realize a car can't stop from even 5mph in .1 seconds. I've activated the abs on my car pulling into a parking spot while playing around with them.

 

[BrianW]

(I tried to find something from the American Automobile Association about ABS, but all I could find were generic statements like, “ABS is good.” Besides, Canadians are Americans, too, eh?)

 

[Keith Mickunas]

I don't know if ABS will shorten stopping distance, but it will allow you to maintain control, which skidding often will not. However as I've said before ice is another matter. Philip, you can deny it all you want, but look at that video, you can lock up on ice with ABS. How many cars made in the past few years don't have ABS? Not many. Especially cars like Mercedes and Jags. So you've seen it with your own eyes. On really slick ice cars with ABS can still lock up the tires. I have felt it myself, and it's not fun.

 

[Scott L]

My car has ABS and it's not uncommon for my tires to lock up on ice.

 

[Philip_G]

Watch the video again. There's a white mercedes M class that skids into another car, it isn't even ON the brakes and the wheels aren't spinning. Possible it's slick enough that the wheels won't spin after you skid/let off the brakes? Dunno.

 

[Chris Farmer]

Skidding is less efficient because the coefficient of friction for a rolling tire is higher then that of a skidding tire. If you remember back to college physics, any given surface has two coefficients of friction. One that must be overcome to begin sliding an object across the surface, then a second, lower one which must be overcome to maintain that motion. A rolling tire is NOT moving against the surface, once the tire rolls into position, it will contact that exact spot on the surface for the entire time until it rolls back up again. For an easier-to-envision example of this, think of a tank tread. It comes down the front, stays on the same spot of ground, but additional treads come down in front of it, causing the overall take to move, but the tread itself remains stationary until the tank has moved sufficiently far enough to raise the tread back off the ground.

When you're braking the car, you have two surfaces in play. the first is the friction of the brake pads against the wheel, and the second is the friction of the tires against the ground. Energy can be dissipated in both these surfaces. Brakes work because these two forces are in opposite directions. Brakes try to stop the wheels rotating, and the tire meeting the road wants to keep them moving. These opposing forces then dissipate the kinetic energy of motion as heat and slow the car to a stop. When in these conditions, there are two possible combinations. The first, more common one, is where the tires keep turning and the brakes slide across the wheel. This uses the stationary coefficient for where the rubber meets the road, and the sliding coefficient for where the brake pad contacts the disc. The second is when your breaks lock up. In this case, you have a sliding coefficient where the tires are contacting the ice, but the brake pads have locked up and stopped the wheels rotating. That first combination can dissipate far more energy, because a sliding tire across an icy surface doesn't have that much force trying to induce the wheels to turn, and without that, the brakes themselves can dissipate no energy. When the wheels are still turning however, they use the second, higher coefficient that tries to keep the wheel rotating. The brake pads then counter that force, allowing you to stop. Even though the brake pads are actually using the lower sliding coefficient as well, they are designed to still have a very high coefficient in either situation, so a rotating tire and a sliding brake pad is the idea stopping situation.

To demonstrate this easily, grab your bike and find a steep hill. Get going at a pretty good speed, say 15 MPH or so, jam on the rear brake, and lock the tire. Note how long it takes you to stop. Then go back to the top of the hill, get going the same speed, and hit the rear brake as hard as you can without locking it up. You'll find that you stop in a significantly shorter distance.

 

[John Alvarez]

But do it on ice and let the fun begin...

 

[MarkHastings]

But on the other hand, what difference would it have made for these people if they stopped short? The car behind THEM would have just ramed them and pushed them into the car in front of them.

 

[Graham Perks]

You mean, "I would have aimed for the cameraman"

 

[MarkHastings]

or Cats and children.

 

[BrianW]

Chris, I originally dismissed the effects of the differences between static and kinetic coefficients of friction because if you're not sliding on one surface, you're sliding on the other. However, I'll concede that the difference between the two coefficients on the brake rotor surfaces may not be as great as the difference between the two coefficients on ice. Thanks for the analysis.

And to Philip, I'll concede that not only should it be possible to design an ABS so that it can't be fooled into thinking the car's stopped when it isn't, but that it's likely such systems already exist. It's just ones and zeros, after all.