# How does tyre rotation increase air pressure?

1. Oct 18, 2012

### polka129

the pressure of air inside a car tyre increases when the car is travelled at HIGH SPEEDS?why is this so?my guess is that at high speeds,the friction between the tyre and road surface increases..so kinetic energy is dissipated as heat energy due to friction.this heat is transfers to the inside of the tyre RADIALLY by conduction and consequently the temperature of air increases thereby increasing its pressure because the volume is constant..

am i correct?.

2. Oct 18, 2012

### AlephZero

Describing it as "friction" is a bit misleading. There is always a "flattened" area of the tire in contact with the road (because the total contact area, times the air pressure, is what supports the weight of the car.) As the wheel turns, each point of the tire has to bend from "round" to "flattened" and back to "round" again. The material of the tire is not perfectly elastic, and the bending generates heat in the tire material. The same amount of heat is generated in each revolution of the wheel, so heat generated per second is proportional to the wheel RPM, or the car's speed.

Some of the heat generated is conducted through the tire to heat up the air inside, increasing the pressure, and the rest is transferred to the air outside the tire.

3. Oct 23, 2012

### polka129

thank you for your reply sir .quite enlightening.however they stimulate me with some more queries,

"There is always a "flattened" area of the tire in contact with the road (because the total contact area, times the air pressure, is what supports the weight of the car.)"-->does the total flat contact area make it necessary for the contacting area of tyre to be FLAT?

and you mean that as the number of RPM increase the temperature increases because the round section that comes in contact with the road EVERY second increases?

4. Oct 24, 2012

### Bandit127

Like bending a piece of wire.... The more often you bend it, the more heat you get.

An under inflated tyre will also have a larger difference (angle) between round and flat. This creates more heat and is the reason an underinflated tyre has a higher probability of a blow out.