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How air pressure reacts in a car tyre.

  1. Apr 2, 2013 #1
    hello to everyone, i'm sorry to bother you with what may seen a very easy question but i'm hoping someone can help me with the correct scientific answer.

    now there has been a debate regarding tyre pressures in car tyres corresponding with there size and shape, a lot of people say that the pressure should stay the same regardless of shape or size (within reason) as the same pressure of air inside the tyre is holding up the same weight, but i beg to differ as the amount of room to be compressed as in the tyres profile will be reduced or increased, now i know that if you really got deep into it you'd have to take into account tyre wall thickness, temperature plus other things, but for example say you have a tyre of 205/55/16 and a tyre of 205/40/18 same width, nearly the same radius but less volume of air inside the 205/40/18, i think the pressure should be raised to allow the tyre to be the correct shape.

    i'm sorry to have littered your forum with such a question but i thought if anyone would know the answer then you guys would, any help on the matter is appreciated, thankyou.
  2. jcsd
  3. Apr 2, 2013 #2


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    Staff: Mentor

    You are right in principle but wrong "within reason", because the change in volume as the load varies is very small, and therefore the pressure barely changes. Even a very small change in tire volume will produce a large increase in the contact patch area - and load carried by the tire is given by the product of the area and the pressure.
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