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Tire Grip

Tire Grip

Tire Grip

Does anyone have any input on the physics of tire grip and what is really happening at the contact patch (CP)? I have read Chuck Hullman's SAE paper and his theory of delayed localized heating at the contact patch. How does CP area relate to overall laterial grip since friction is the normal load*mu? Would not an increase in air presssure reduce the slip angle by stiffening the torsional component of the tire? How does air pressure affect overall grip? Is it the tire spring rate change or a change in the contact patch size?

RE: Tire Grip

Tyres are a mystery to me and most of the tyre manufacturers as well.

"Would not an increase in air presssure reduce the slip angle by stiffening the
                    torsional component of the tire?"

Increasing the tyre pressure increases the sidethrust for a given slip angle, but the increase is non linear even over a small change eg 1.8 bar, 2.7 kN, 2.5 bar, 3 kN, 3.2 bar 3.55 kN.

"How does air pressure affect overall grip?"

It generally improves lateral force for a given slip angle  (not always) but may increase or reduce the longitudinal force for a given slip ratio.

 Is it the tire spring
                    rate change or a change in the contact patch size?


Greg Locock

RE: Tire Grip


Thanks for the input.  I know it is impossible to get tire data from the mfg's so I spend alot of time collecting vehicle performance data and trying to make sense of it.
You confirm what I have found-a small increase in air pressure (1 to 2 psi)increased lateral grip (approx 0.1 G) and the driver comments supported the results.  This is opposite to the beliefs of people in my industry who are considered experts. They believe a reduction in A/P will improve lateral grip.  My testing shows otherwise.


RE: Tire Grip

a reduction in ap will increase grip, if your tires are overinflated.
 if you reduce tire pressure excessively (15 to 20 psi, I like 18 if im stuck) you can get better traction on a loose suface due to the greatly increased contact patch, but this does not translate to pavement, the oppisite happens at that pressure, you have no useable traction on pavement (5000lb truck)

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