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Old 9 Jun 2008, 10:44 (Ref:2223548)   #1
NPP
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Tyre pressure and temperature in road cars

We all know about the importance of correct tyre pressure and of warming up the tyres in motor racing. I am wondering about the importance of these effects in normal road use. Of course, I don't want to sugggest that you warm up your tyres Alonso-style after leaving your driveway - rather, I am interested in the following real-life scenario:

The cheapest petrol station in my area is about 20 kms of motorway into my commute. That means that if I have a chance to check tyre pressures, it will be after travelling some time at about 150 km/h, mostly in a straigt line, with no hard cornering involved. How will this affect the temperature of the air in the tyres, and tyre pressure? Will the pressure be significantly higher than measured if cold? How would pressure and temperature be affected if it were 20 kms of twisty B-road, driven quickly, but within the limits imposed by law and by safety considerations? Will I have to put in an extra stop on cold tyres when I want to check tyre pressures?

Thanks for your thoughts!

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Old 9 Jun 2008, 11:13 (Ref:2223577)   #2
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It would still be higher due to the friction with the road

on a twistier road the pressure would be higher

Its best to check tyre pressures after being parked a while or if you do them before you leave
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Old 9 Jun 2008, 13:06 (Ref:2223695)   #3
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Road car tyres should be checked cold. The ambient temperature will affect the pressure, and if standing in the sun can easily alter 5psi or more depending on the size and will alter from side to side. Race tyres on my car can alter by 10psi during a long race on normal air.
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Old 10 Jun 2008, 06:43 (Ref:2224415)   #4
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Thank you. It'll have to be an extra stop to check tyre pressure then.

5 psi just by having the car in the sun sounds quite a lot (0.34 bar), and a change of 10 psi / 6.8 bar over a race distance really requires some planning and calculating beforehand ...

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Old 10 Jun 2008, 06:57 (Ref:2224431)   #5
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I shouldn't get too paranoid about checking your tyre pressures.
Once a week when cool will be fine .
You would be surprised at a lot of cars on the road as the drivers NEVER check at all !

Last edited by GORDON STREETER; 10 Jun 2008 at 07:01.
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Old 10 Jun 2008, 10:19 (Ref:2224626)   #6
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The best idea would be to buy yourself a gauge so you can check your pressures before you set out. That would also add consistency to the checking, using the same gauge, and eliminate the possibility of striking an inaccurate gauge at a petrol station.

After 150km the warming of the tyre would be significant (not huge) due to flexing of the tread as the tyre rotates, and it becomes a guessing game as to what the "real" cold pressure is.
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Old 11 Jun 2008, 08:48 (Ref:2225509)   #7
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Originally Posted by johnh875
The best idea would be to buy yourself a gauge so you can check your pressures before you set out. That would also add consistency to the checking, using the same gauge, and eliminate the possibility of striking an inaccurate gauge at a petrol station.
obviously a good idea - how stupid I never thought of that in 20 years of driving!

While we're at it - if you only ever use your car for a 450 km non-stop autobahn trip twice a week (at 160 kph where traffic and the law allow it) and let it sit in the driveway for the rest of the time, would you chose a different "base" pressure from when you do only shorter trips?

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Old 11 Jun 2008, 09:47 (Ref:2225556)   #8
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Originally Posted by NPP
While we're at it - if you only ever use your car for a 450 km non-stop autobahn trip twice a week (at 160 kph where traffic and the law allow it) and let it sit in the driveway for the rest of the time, would you chose a different "base" pressure from when you do only shorter trips?
Read your car's handbook! Some manufacturers recommend higher pressures for high-speed driving, some don't.
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Old 11 Jun 2008, 23:22 (Ref:2226518)   #9
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Agreed, a good idea to adjust pressure to suit conditions. I don't adhere to the figures in the handbook as they are normally biased to a comfortable ride above grip, rolling resistance etc, I am normally a few psi above that. One thing too look at is the wear pattern across the tread face of the tyre.
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Old 12 Jun 2008, 08:01 (Ref:2226686)   #10
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As I said its no good getting paranoid about pressures for road cars.
As long as they are checked frequently with a consistent gauge.
I often find when servicing vehicles (especially people movers ) some have terrible inconsistencies (as much as + or -30psi) and the drivers (normally women) adapt to driving around the ill handling problem. They will admit to never checking tyre pressures as forecourts don't have anyone to help them anymore, also they don't have the time, as they are too busy taking the kids to school !!!
And before I get the flack from the female fraternity, I only said that because most of the "little diamonds" are taken to school by women.
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Old 12 Jun 2008, 08:20 (Ref:2226694)   #11
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Originally Posted by GORDON STREETER
As I said its no good getting paranoid about pressures for road cars.
don't worry My last question was asked out of curiosity rather than for any practical reason. I check pressure every now and then, take a close look at the wear pattern when switching between winter and summer tyres, and that's about it. Thanks for your help!

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Old 13 Jun 2008, 01:39 (Ref:2227442)   #12
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A side note - the accepted method of road train drivers in the outback for checking their tyre pressures is to walk around the rig with a length of pipe thumping each tyre - the "boing" sound gives an indication of pressure. When they have over 60 tyres to check (not including spares) you can understand why a quicker if slightly less precise method is used! (this is for a truck with 3 'A' trailers, some have more)
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