Tyre Pressures

[desertrat]

Why does a car tyre which has a far greater volume than a bicycle tyre need a lower pressure compared to a bicycle to maintain its firmness?

[druiaghtagh]

It has a greater surface area.

[sddsddean]

It is also to do with the volume and load carried. For instance, my Daf 85 8 wheel tipper has 95psi in the rear tyres(which are doubled up) and 120psi in the front tyres. You may have seen truck trailers that have had blow outs and there are wings and lights etc missing. This is because the big super single tyres on a trailer normally run at 125psi. Don't forget, these are all cold temperatures. As the tyres heat up, so does the air inside and the pressure on a super single can go up to 140psi. So try not to stand next to one when they pop!! They make a hell of a mess and can do your ears in too!

[Squirrel]

It's probably due to car tyres being so obviously thicker, stronger and sturdier than bike tyres that causes them keep their circular shape at a lower pressure. I don't know what's inside a car tyre to make it stay rigid, but I know that bike ones are just flimsy rubbery stuff, and as a result they need a lot of "help" to stay the right shape. So, you have to pump the tyre tubes up to a greater pressure than the car ones, and the air provides the pushing force from the inside that makes the tyres harder to squash. I'm not sure how this makes for an easier ride on the bike, though - probably preserves the tyres or something.

[roomby]

is it something to do with the fact that pressure is measures in psi, or per square inch? Bicycle tyres are smaller and therefore require a higher amount of pressure per square inch to achieve the same hardness as a larger car tyre. Tell me if I'm being crazy - I'm no physicist!