# Best inflation pressure of car tire in the rain- higher, same, lower than normal?

If I am driving a car in the rain, and want to increase the available traction, should I:

1. Increase pressure in the tires
2. Decrease pressure
3. Leave the pressure set to optimum dry pressure
4. Increase or decrease just the front or back tires

Does the answer change if the surface has no standing water (i.e. is merely wet)?

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Increased tire pressure certainly reduces the tendency of the vehicle to hydroplane, that is, to ride on a layer of water. That's not the same thing as contact friction with the road. Once the tire is actually contacting the road and the pressure of the water layer pushing up is not so significant, then pressure does not appreciably affect traction.

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At the physics-101 level of analysis, this is correct: the traction is independent of the pressure. The static friction force (i.e., the traction) is simply $\mu_s N$, and both the coefficient of friction $\mu_s$ and the normal force $N$ are independent of pressure. Reducing the pressure increases the contact area but does not affect the traction. But rolling friction, I've always heard, is not well-modeled by this sort of analysis. How sure are you that your answer is correct? Can you supply a bit more detail? –  Ted Bunn Mar 2 '11 at 14:58