Anti-lock Brakes & Tire Pressure

As we all know - I think - increasing tire pressure will cause a wheel to be more likely to bounce during an emergency stop. And, as we all know - I think - a car with bouncing tires requires a greater distance to stop than a car with tires that are rolling.

Anti-lock brakes should substantially eliminate the chance that an over-inflated tire will bounce in an emergency stop - I think. Can anyone muse or,perhaps even pontificate on why this statement is wrong?

There is support for the position that over-inflating can decrease stopping distance.

If the tire is bouncing, while stopping you have far more problems than ABS can solve. For the tire to bounce, the dampers are essentially not functional. The oil has leaked out and the shock cannot control the unsprung mass. It isn’t 1965 so shock damping is not chosen to be so underdamped that cars float and tires bounce.

Anti-lock brakes (ABS) cannot control up-and-down movement of the tire. The springs and shocks do that. The ABS is releasing the apply pressure to the caliper and then re-applying pressure to control the rotational speed of the wheel and tire.

Bouncing of the tire off the pavement can lengthen the stopping distance with ABS since as the tire’s contact patch decreases in a bounce the tire tends to stop rotating “fooling” the ABS into releasing the brake pressure. When the tire is back in contact with the ground on the down cycle, the ABS can apply more pressure. The frequency of the tire bounce - about 10 to 16 cycles per second - can hit a harmonic of the ABS and cause the ABS to essentially turn off the brakes.

GM trucks and SUV’s had this problem in the 1990s when equipped with Kelsey Hayes ABS units. Bumpy roads, tall SUV tires, and poor damping control would conspire to allow the tires to bounce and the trucks to fail to stop at the most inopportune times.

The data shows no clear evidence of that. It is completely situational and doesn’t include ABS in the testing at all so no conclusions about ABS can be made.


Okay, I just skimmed the report. As far as I can tell it only dealt with UNDERinflation.
Just stay with OEM size and pressure specifications.

ABS is not a force field. If the tire is in the air, nothing is going to have braking effect except the aerodynamic forces from wind resistance.

Your ABS will probably cycle when your tire leaves the ground, but that’s because it has no way of knowing that the tire is off the ground - it just sees that the tire has slowed/stopped much more than the other wheels and therefore assumes you’re sliding, not flying.

Over-inflating a tire reduces the area it contacts the road, which is going to adversely affect stopping distance. The abs won’t be able to correct for that, but if the tire bounces on a small bump and intermittently loses contact with the road it would probably help in that situation.

First, the study cited doesn’t deal with bouncing tires.

Second, the study cited doesn’t have consistent results. (Note: This is consistent with what I know about traction testing of tires - that is, it is highly variable. In order to get better data, more data is needed.)

Third, they really didn’t explore inflation pressure very much - and certainly didn’t explore over inflated tires. (Note: The test was conducted at 75% of the rated load at 35 psi, which is in the vicinity of what a passenger car tire would normally experience.) If anything they explored under inflated tires and only got single digit percentage reductions in traction. I think we can agree that that is fairly small - certainly not the difference one experiences between dry roads and wet ones! Not to mention ice covered roads!

I wonder about this. If the coefficient of friction between the tire and the road stays constant, that reduction in contact patch will be offset by an increase in the normal force per area, ergo no change in stopping distance.

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If that worked, then drag cars would use thin tires on the back wheels as well as the front.

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Except that Amontons’ and Coulomb’s Law don’t apply to tires, because the rubber interacts with the macrotexture of the pavement - ergo a larger contact patch results in more grip.

An interesting tidbit: A tire gets its maximum grip when slipping about 10% to 15%!


And that “slipping” isn’t actually slipping! It is compression of the rubber with load. The tire is still in contact.

Sorry, but the tire is actually slipping relative to the pavement. That’s why race tires frequently have stripes painted on them. The slippage can actually be measured when viewing slow motion videos. Youtube is your friend!

Very true but “slip” doesn’t always involve slipping. You can develop percent slip below that 10-15% where actual slipping does not occur. Same for generating a slip angle when cornering. Slip angles are developed well below peak lateral adhesion without actual slipping occurring. The compression of the rubber in each case decreases the linear or lateral distance of the rubber against the road that snaps back as it rolls.