Checking tyre pressures - when?

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Thu 22 Jun 2006 17:20
Checking tyre pressures - when? - edisdead {P}
We all know that tyre pressures should be checked when tyres are cold, but I would like to know how long it takes for my tyres to cool down after a 50 mile motorway run (175/70R13 tyres on a Honda Civic)
Any ideas?

Many thanks,
Ed.
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Thu 22 Jun 2006 17:25
Checking tyre pressures - when? - patently
No idea!

How about an experiment? Do the motorway run, then measure the pressure of one of the tyres. Measure it again every 5 or 10 minutes, making sure you stick to the same tyre. Keep going until the pressures don't change. Then you'll know for the future.

And do let us know - it's potentially useful information. Maybe we could repeat it on different tyres and see if there is a relationship with tyre size, profile etc.
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Thu 22 Jun 2006 17:28
Checking tyre pressures - when? - Chris S
How about an experiment? Do the motorway run, then measure
the pressure of one of the tyres. Measure it again
every 5 or 10 minutes, making sure you stick to the
same tyre.

Slight problem - every time you measure the type pressure you reduce it slightly by letting some air out.

I just check my tyre pressures on a Sunday before I've driven the car.
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Thu 22 Jun 2006 17:30
Checking tyre pressures - when? - Lud
Feel the tyre with your hand. If it feels cold, it is.
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Thu 22 Jun 2006 17:41
Checking tyre pressures - when? - patently
Slight problem - every time you measure the type pressure you
reduce it slightly by letting some air out.


It is possible, with practice, to do it with only minimal loss even on a low profile tyre.

A valid point, though. Obviously a control is needed - repeat the same process on a stone cold tyre and observe the loss rate due to the measurement. Then, you just need to correct the figures on the "hot" test ... and bingo.

Or you could just feel the tyre and see if it's hot or cold!

(although surely it's the temperature of the air in the tyre that matters ... not the rubber. Might the air still be warm after the tyre has cooled to about ambient temperature?)
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Thu 22 Jun 2006 17:45
Checking tyre pressures - when? - edisdead {P}
...blackcircles.com says wait for an hour after driving, sounds reasonable. I'm too lazy to be scientific about it!
cheers,
Ed.
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Thu 22 Jun 2006 19:42
Checking tyre pressures - when? - local yokel
Universal gas law says that Pressure x Volume = nR x Temp (in Kelvin). Volume, n and R remain constant in this situation.

So to raise the pressure by 20% the temp would have to be about 75 C in the tyre (75/(273+75) against the pressure measured at 0 C.
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Thu 22 Jun 2006 22:08
Checking tyre pressures - when? - mike hannon
Why not just do it first thing in the morning when the ambient temperature is at it's lowest for the day and the car hasn't moved?
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Thu 22 Jun 2006 22:23
Checking tyre pressures - when? - kithmo
Why not just do it first thing in the morning when
the ambient temperature is at it's lowest for the day and
the car hasn't moved?

Would that be in winter or summer, check them at -2 deg C and they will be different to checking them at 20 deg C
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Fri 23 Jun 2006 12:40
Checking tyre pressures - when? - Andrew-T
"So to raise the pressure by 20% .."

I am well aware of the gas laws, but am still rather surprised by how much my tyre pressure varies with changes in the ambient temperature (i.e. 'cold' tyres). After setting them during a cold spell, the pressure can rise by 2 psi when the weather warms up.
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