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ABS systems work by allowing a tire to continue to rotate rather than "locking-up" (stop turning) due to loss of traction with the road surface. A rotating tire can influence the direction of the car, a "locked-up" tire has no directional influence.

This also gives the tire more traction than a fully "locked-up" tire.

As a car enters a turn, angular momentum causes weight to shift to the tires on the outside of the turn, reducing weight, and thus traction, on the inside front tire.

My question is: does ABS's effect of increasing traction on the inside front tire cause a force on the car that causes it to go more straight (resist turning) compared to a car that has no ABS and whose inside front tire is allowed to stop (or mostly stop) turning?

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If the extra traction on the inside front tire is used for braking, shouldn't that help the car turn? (In the limit, I'm thinking of the inside front tire "planting" firmly, and the car pivoting around it.) –  coneslayer Feb 25 '11 at 20:01

1 Answer 1

In terms of forces exerted by the road and the tire on each other, ABS increases the force by keeping your tires in the "static friction" regime instead of the "kinetic friction". That is, objects tend to have higher friction before they begin to slip then they do after slipping. So when the system detects that a wheel has slipped, it reduces the brake pressure to the point where the wheel returns to the static friction case. (This is said to not work so well on deep gravel, but it's suitable for typical pavement.)

The other reason ABS is useful is because it allows the driver to control the direction of the force. That is, it helps you use your steering wheel to control the direction that the car moves. So one of the primary purposes of ABS is to help you corner.

If you slam on your brakes without ABS, while going around a corner, you will slide in the direction you're traveling. Since the maximum of rolling friction is larger than sliding friction slamming on your brakes will take you longer to stop. In addition, you will have no control on your direction and you will likely run off the road.

When you're stopping, your front tires take more of the weight of the car than the rear so I would think that you would lose traction on your rear wheels first. This would definitely make it more difficult to steer and more difficult to stay on the road.

But if, for one reason or another, only your inside front tire lost traction (without ABS), this would decrease the force on the tire and that would likely make it more difficult for you to turn. That is, you'd have more difficulty getting the car turned without ABS than with.

Interesting question. I look forward to see what others have to say about this.

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Car brakes are biased so the the front tires always lose traction first to prevent directionless rear tires from overtaking the front. :) Virtually always the inside front tire loses traction first because there is less weight on it than the outside tire. Does increasing traction on that tire create additional turning moment that is resisting the car from turning into the corner? –  james creasy Mar 2 '11 at 2:23
    
I'm convinced it does. –  Carl Brannen Mar 2 '11 at 3:10
    
"On an infinite surface, this action will bring you to a halt faster than ABS, but only in the direction that you're already moving in." Not true. A rolling tire has more grip than a sliding tire, infinite surface or not. The only caveat is that, because ABS allows you to turn as well as stop, you have the option of using some of that grip do do something other than stop. In a straight line though, ABS will stop you faster than a wheels-locked skid, but slightly slower than a professional driver who can keep the brakes right at the limit of locking up, without needing to "pump" them. –  Colin K Jul 2 '11 at 2:05

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