Tyre Pressures.

[Bramleyboy]

This has bugged me for years! How can the pressure in car tyres be the same in the spare in the boot as when it is put on the wheel? Surely, the weight of the car pushes the air out and this would give a greater tyre pressure?I'm really perplexed.

[SirOracle]

How is the air pushed out Bramley ? Assuming there is no leakage in the tyre or valve.

[Sipowicz]

Young Jim'll be along in a minute to explain! ;-)

[RATTER15]

The weight of the car just redistributes the weight, the bottom of the tyre when on the car has less air and it has been moved around to other parts, so it all balances out to the pressure.

[RATTER15]

^^//The weight of the car just redistributes the air//

[Old_Geezer]

My guess is that the weight of the car distorts the tyre but the volume taken up by the air inside the tyre remains much the same, in which case the pressure would be unchanged also ?

[bhg481]

OG is correct.

[1ozzy]

Ratter15 and AOG are both "spot on".

The more under inflated your tyre is the more distortion you will see, but the pressure does not change. It is very important to have the correct pressure otherwise your tyre is distorting a lot with every rotation. This results in excess heat building up in the tyre and can result in tyre failure. Also incorrect tyre pressure will affect the way your car handles and affect fuel consumption as well as the mileage you will get from the tyre.

[1ozzy]

My apologies, OG, not AOG.

[Bramleyboy]

Thank you! I can see OG reasoning and understand it as I am an OG as well! Thanks.

[Clanad]

It should be noted, however, that an increase in outside air temperatures will increase tire (spelled as such here in the U.S.) pressure. A good rule of thumb is 1 psi per 10 degrees increase (or decrease) of outside air temps. When originally filled by the car manufacturer (or the tire replacement service if changed) the tires should have been filled at fairly cool temps for accuracy's sake...

[Old_Geezer]

http://en.wikipedia.org/wiki/Gas_laws ;-)

[Hopkirk]

Surely the force of gravity acting on the car must squash the tyre, thus increasing the tyre pressure when the spare is fitted.

[TWR]

Does this still stand with 4 very obese person's in the car?

[Hopkirk]

Absolutely

[TWR]

A good while back hop I picked up a Container from Felixstowe only to find the Unit / Trailer was responding & handling badly, it turned out I was well over weight, my reason for a reply, Obese.