# Tire pressure physics

I understand that with less tire pressure you get more surface area on the pavement and more friction which gives more rolling resistance, but does having less air mean that there’s more weight per square inch of the tires surface area? It makes sense to me since there’s less air in the tire to divide the load.

Rolling resistance would be a measure of how much work it takes to roll the car forward on its wheels. Notice that when you roll the car, the bit of rubber tire contacting the ground at any given moment is stationary.
You can think of rolling resistance as how far the car can coast on flat ground. Whether or not the effect of air flowing around the body of the car should be included in that is probably not relevant to your question.

With less air in the tires you'll get higher rolling resistance for three reasons.

• The air inside a tire has to move around as the tire deforms. More deformation means more internal air turbulence. I don't imagine this effect is even measurable.
• The rubber of the tire is being stretched, sheered, and compressed (hopefully within its elastic range) as the tire deforms. These movements have hysteresis. More deformation means more hysteresis loss.
• The swatch of rubber in contact with the ground isn't perfectly stationary. Because it's deformed, the leading and trailing edges have a slight movement relative to the ground, which they are touching. This movement happens in spite of friction. Higher tire pressure means a smaller contact swatch, which means that the edges of the contact swatch have less such motion.

Breaking resistance, or stopping power, or more generally traction is how much lateral force the car can exert on the ground by applying torque to the wheels. Note that if the wheels are spinning freely then this is irrelevant.

With less air in the tires the contact area with the ground is larger, but this doesn't affect the net traction because, as you've noted, the pressure per area decreases proportionately. In other words, the normal force doesn't depend on the tire pressure, and the contact area doesn't affect the traction.

Technically, the point about normal force apples to dry friction. On the one hand, dry friction is preferred over alternatives (such as hydroplaning) for driving a car. On the other hand, we want our cars to work on a wide variety of surfaces, so other kinds of resistance besides dry friction should be considered.

P.S: Bicycles are awesome and cars are lame.

The deflecting and flexing back of every segment of tire, about 10 times a second, produces heat, and this is the lost energy that you have to overcome and experiënce as resistence. So also the sidewal bending gives that energy loss , mayby even most of the part. If rubber would be totaly flexible,so no energy loss when flexing back, rolling-resistence would be zero( for the part the tire gives as rolling resistence wich is most).

To react about aswer of Andrew about little movement to the ground, I daubt, if that would be the tire would wear out much faster.