So I go and get the paper yesterday from the shop 400 yds away.

30 mins and many miles later I'm back - with a silly grin of course.

I then check the tyre pressures - 40psi F, 35 psi R. Am amazed - they were 34/31 last week...and 34/31 this morning as well

Obviously(?) as temperature goes up pressure goes up - but I never thought it would have that much effect.

<anorak on>
Is this difference expected - can't remember my O level Gas Laws too well (http://www.pmel.org/HandBook/HBpage20.htm seems to be a good start though)

And if people talk about running 34/31 I presume they mean cold - if I'd have let the air out to give 34/31 when warm they'd be on the flat side of flat this morning.

Even saying '34/31 cold' would seem to be inexact - how cold is cold? Ice on the windows cold or just the car sitting unused for a day in 18C ambient temp? What pressure would you set at a service station after 50 miles of high speed (no more than 70mph of course) driving?

...and Murray/Martin said at the GP yesterday that F1 teams fill tyres with helium (or was it nitrogen?) to avoid having any water vapour in the tyre so that the pressure isn't affected by differences in temperature

</anorak on>

I'll get my anorak...

martin
well it all depens on what put the grin on your face , driving hard is going to increse the temp of tyres and 6 psi isn't much to expect,also size http://bbs.scoobynet.co.uk/smile.gif matters low profiles will increase more.
airplane tyes are also filled with nitroge?

i'm allready wearing my anorak.
john

Martin,
Yeah that is normal and I would say drop the tyre pressures a couple of psi to compensate not only for the scorching days, but also for generating extra heat from friction by driving enthusiastically.

I went out for a drive the other day and found my rear end was all over the place because the tyres had overheated like a balloon. Dont take measurements from warm tyres as accurate though, always make sure that when taking readings, car has been idle in the shade for at least an hour.
Cem

I am sure im not an anorak though.
http://bbs.scoobynet.co.uk/smile.gif

Thanks guys

So many coats, the BBS needs a coat rack!

yep, tyre presures go up a graet deal in hot weather, especialy if you set them when your car has been garaged overnight.

You will get much higher presure increases if the compresor you use to pump up the tyres is generating a lot of water vapour because of poor/no water filter being fitted.

F1 cars used nitrogen to try and keep the water vapour to a minimum and therefore presure change to a minimum.

Ill get me coat