What temp. should tyre be when you take pressure reading?
 

It was about -1C in my garage today when I checked tyre pressures. 25psi front 29psi back. Is that normal(re Charles' Law) for a dead cold tyre?

JohnH

The combined gas laws would apply and as the volume of an inflated tyre is effectively constant then pressure is directly proportional to temperature.

So this would result in a perfect non leaking tyre seeing about a 2-3 lb loss as a result of a change in ambient temperature form a nice 16 to a chilly -1.


So you would appear to have lost a tad more. Add about 5lb only as once the tyre gets up to full working temperature you may find it otherwise over inflated.

Thanks Prof. :shock:

recommended pressures are for cold tires

But what is a cold tyre?

A cold tyre in Tangiers might be 20degree C and in Glesga a cold tyre could be -5'c

Yes, tyre pressure must always be checked on a cold tyre.

I was simply making the point that the pressure will change as a result of a major change in ambient temperature. Checking the pressure first thing in the morning during a frost is perhaps not a good time to do it if the temperature during the day will rise by about 8 degrees celcius.

As I said the combined gas law applies. there pv/t =c and assuming there is no change in volume then


We need to use absolute (Kelvin) temperature
P1/t1 = p2/t2. So assuming a starting -1 Celsius and an initial pressure of 36 psi.
p2= (36/271.15) * 279.15)

Then by the ambient temperature rising by 8 degrees during the day. a tyre having been inflated to 36 psi would become 37psi.

I know it is not alot. of change. However if the tyre is inflated in the depth of winter without any loss it may become seriously over inflated in the height of summer.

O dear, enough of the respect.

During a ride a tyre will achieve a temperature that is probably independent of the ambient. When Suzuki or mr rubber company give a recommended tyre pressure it is worked out on what the warmed up tyre pressure is likely to be.

What is the temperature at which the Suzuki given recommended pressures apply? Someone said 'cold'. What exactly is 'cold'?

Since pressure for a gas in a fixed volume is directly proportional to temperature it is surely obvious that for a given volume of air, a tyre will read a higher pressure at 16C than at 2C.

Does anybody know what the 'standard' temperature is? No more blinding(no information) science please! :o)

Happy new Year

JohnH

Tyre pressures are cold check.

As the temp change takes place over a very long period (in math/ engineering terms)

Assume the tyre volume is constant.

in short. the -1 deg. c doesn't really matter.

Cheers

i really must pay more attention to my tyres :roll: if there pumped up around 35ish then that ok :oops:

I had to check out the combined gas law on the net for this:
Assuming that the advised pressures are give for a 'cool' tyre (ie at 16C) then the pressure at -1C should be approximately 2 lbs less.

ie combined gas law is P1.V1/T1 = P2.V2/T2. Volume is constant so P/T is constant. T is kelvin so P2 = P1.T2/T1. So if front tyre pressure at 16C is 30psi then front tyre pressure at -1C = 30x272/289 = 28psi. Probably doesn't matter a toss.

John

what a way to spend Christmas!