# Friction and Tire Pressure

DeepSeeded
Everywhere I read on tire pressure for bikes I hear that lower pressure gives you increased control and climbing traction. Lower tire pressure gives you more surface area between your tires and the ground.

Whatever the frictional coefficent may be of the surface you are riding on (and your tires), the frictional force (traction) is independent of the surface area, correct?

Your tires should slip under the same force no matter what the tire pressure is according to classical physics right? (This of course is not intuitive and I am not sure I believe it)

This is because the Normal force increases on each individual particle as surface area is reduced and decreases on each particle as surface area is increased. So the product of force and particles is always the same for almost all material.

Gold Member
Try looking at it from an extreme perspective. Say that you could increase the pressure so much that your contact area on the tire was only one molecule wide. If the vehicle weight remained the same, how much traction do you think you could get?

Homework Helper

DeepSeeded
Try looking at it from an extreme perspective. Say that you could increase the pressure so much that your contact area on the tire was only one molecule wide. If the vehicle weight remained the same, how much traction do you think you could get?

The physics still hold, you would get the same traction because the Normal force on that line of molecules would increase by a huge amount. This is what the study of friction shows, 1 atom wide by 1 atom thick is no different than 10 miles wide by 10 miles thick.

I am going to check these links the other guy posted.

DeepSeeded
Hi DeepSeeded!

See

and

That is interesting and explains well why you can go faster when you increase your tire pressure, less energy is lost to heat because there is less deformation and recovry or:

That doesn't seem to explain the decrease in friction as you increase tire pressure however, I am still not convinced.

dst
Perhaps an energy perspective?

Friction is caused by molecular adhesion on the surface - one atom by one atom will take far less energy to break any bonds that form than anything normal/extreme.

Homework Helper
The relationship bewteen normal force and maximum static friction is not linear in the case of tires. The coefficient of friction decreases as the normal force increases. More downforce results in more grip, but not by the same amount, for example, doubling the downforce will result in less than double the grip. Because of this property of tires, a larger contact patch provides more grip because there is less normal force per unit area of the contact patch. In addition, a lager contact patch will handle irregulaties and contamination between surfaces better.

Another reason for a larger contact patch is the reduction of heat produced per unit area of the tire, allowing for more cooling, although this is acheived by using a wider tire, since lowering pressure results in more deformation at or near the contact patch, which results in more heat.

pantaz
Everywhere I read on tire pressure for bikes I hear that lower pressure gives you increased control and climbing traction.

Based on this portion of your post, I believe you are talking about off-road use. In that context, you must also consider the physical deformation of the tire over an irregular surface. Extreme examples can be seen in "observed trials" (motorcycles) and "rock crawling" four wheel drive vehicles.

Gold Member
Gotta love 'Extreme 4x4' on Spike's Power Block.
My everyday vehicle is a '76 El Camino frame-stacked on a '74 Jimmy chassis with 33" Trailblazers and a 445 Olds Rocket motor. I'll tell you right now, the worst mistake that urbanites make is in thinking that four wheel drive makes you immune to physics. Some idiot buys a Hummer, thinks 'I can do anything now', and proceeds to wrap himself around a tree.
My Camino has about as much psi on the ground as a Harley fat-boy; stopping on anything slick is out of the question. My only advantage is that I can go off-road if necessary. I'd rather hit the ditch than another car.

kamerling
If there is a bump in the road, the tires with higher pressure will more easily lose contact with the road entirely. The average friction is the same for both tires.