Tire psi effects on over/understeer...(long)
10-24-2000, 05:39 AM
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Tire psi effects on over/understeer...(long)
I responded to WClark yesterday who had a problem with his car's understeer.
He said that when he pumped up his rear tires to match the fronts (40 psi), he still had a nagging push problem.
I commented that he was going in the wrong direction: lower rear pressure = less understeer.
Well, all respondents disagreed with me.
The physics:
A stiffer rear roll bar = less understeer because the weight is shifted more to the outside rear tire. The outside rear tire reaches its limit and slips. Pretty simple.
Less pressure in the rear tires has the same effect: The tire rolls onto its shoulder and the contact patch is diminished. Thus, rear traction is diminished, and oversteer is the result.
Is there some confusion regarding rear roll bars ans rear tires?
He said that when he pumped up his rear tires to match the fronts (40 psi), he still had a nagging push problem.
I commented that he was going in the wrong direction: lower rear pressure = less understeer.
Well, all respondents disagreed with me.
The physics:
A stiffer rear roll bar = less understeer because the weight is shifted more to the outside rear tire. The outside rear tire reaches its limit and slips. Pretty simple.
Less pressure in the rear tires has the same effect: The tire rolls onto its shoulder and the contact patch is diminished. Thus, rear traction is diminished, and oversteer is the result.
Is there some confusion regarding rear roll bars ans rear tires?
10-24-2000, 06:12 AM
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Re: Tire psi effects on over/understeer...(long)
If you're talking about two *flat* rear tires, you'd likely see the effect you're talking about.
However, if you're talking about varying the PSI in the tire's normal operating range, having a lower pressure increases the size of the contact patch. The results are slightly more grip and slightly less responsive handling (especially when lowering the pressure of the front tires). The tire should not be able to distend so much that it collapses onto its sidewall (at least not any more easily than it normally does).
Dan
However, if you're talking about varying the PSI in the tire's normal operating range, having a lower pressure increases the size of the contact patch. The results are slightly more grip and slightly less responsive handling (especially when lowering the pressure of the front tires). The tire should not be able to distend so much that it collapses onto its sidewall (at least not any more easily than it normally does).
Dan
10-24-2000, 06:28 AM
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And a good example of the dynamics is the older model 911
Notorious for oversteer, due to the engine hanging out past the rear axle. Major rear weight shift in the turns, about it's centroid in the horizontal plane.
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10-24-2000, 06:36 AM
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I'm agreeing with you. At the bottom of the chart, they say to
increase oversteer (or decrease understeer), shift the weight back. That was the issue with the 911's. They made the rear tires wider to counteract it, as it was TOO much for the average driver that's not used to oversteering cars, and would break away too suddenly.
We agree.
We agree.
10-24-2000, 06:51 AM
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??? What was the circumstances? What were your reactions?
Braking at the wrong time can induce understeer, as hitting the gas increase oversteer. What happened? I didn't see the old post.