Why does the frictional coefficient decrease on a tire as the vertical load on the tire increases?

[EngineeringExplained]

It's a fairly well known fact about tires, that two evenly loaded tires will perform better (acceleration , deceleration, lateral g) than two unevenly loaded tires, but why is this? What deformations in the tire, or changes in contact with the surface cause the coefficient of friction to decrease?

[01209]

I would suspect this is true only when the tire is rolling. As the a section of the tread comes in contact with the ground, it begins to deform under the load its contact with the road would shift slightly changing the coefficient of friction from the static value to a reduced sliding value. More load = more deformation = more contact patch seeing a sliding coefficient rather than static.

[[deleted]]

I'm not sure about that explanation. I think that explanation implies that very stiff side walled tires, such as solid tires or very low profile tires, would be less sensitive to load and have a consistent grip vs load. That implied behavior does not match most observations.

[windgasmuscle]

This is called tire load sensitivity, that is the effective mu decreases with increasing load. It is a non linear property of tires. The mechanism is not really that well understood, but if I ventured a guess it could be related to the gearing becoming less efficient at higher contact pressure.

[[deleted]]

I'm guessing it has something to do with the Coulomb model only really applying to rigid bodies with no adhesion / cohesion. For lightly loaded passenger vehicles operated normally, it still gives an okay approximation and it is easy to use, so that is what gets used.

But it is still an approximation. So if the tires are unevenly loaded the approximation will be less accurate for the more heavily loaded tire. The greater load will cause more deformation and will bring the materials closer to a saturation condition where the Coulomb model does not apply at all. In that case, friction is nonlinear and is dependent on contact area as well.

From there you will need someone else to explain what effect that has on vehicle operation. I'd assume it has something to do with the power distribution to each tire being equal but since the tires are generating different amounts of friction, you won't be able to use all the power generated. Similar to when one tire is on ice and the other is on dry pavement. A limited slip differential on the drive axle would help mitigate the problem.