Tyre pressures how to check?
Discussion
Does anyone have a ready reckoner for tyre pressures that takes into account ambient temperature? The manual talks about pressures when cold as being at 20 deg C. So do I have to wait to next summer? Is it a function of wheel size or can you can assume that the volume of air in the tyre remains constant?
If we treat air as an ideal gas and assume that the tyres are sufficiently stiff not to change volume significantly then does the equation simplify to P1/T1=P2/T2
So if the pressure at 20 degrees C is suppposed to be 2.3 bar then at 10 degrees C it would be 2.3x283/293 where temperatures are in degrees Kelvin. This would suggest that if the ambient temperature were 10 degrees C the pressure you would set the tyre to would be 2.22bar.
Anyone got at different answer?
So if the pressure at 20 degrees C is suppposed to be 2.3 bar then at 10 degrees C it would be 2.3x283/293 where temperatures are in degrees Kelvin. This would suggest that if the ambient temperature were 10 degrees C the pressure you would set the tyre to would be 2.22bar.
Anyone got at different answer?
lowndes said:
So do I have to wait to next summer?
Tyre pressures are usually quoted at ambiant temperatures for road cars, but remember that it is the pressure at normal operating temperature which matters. In the winter your tyres will run colder, so you will need slightly more air in. Setting the pressures at a colder ambiant temperature is one way to achieve this. It's not perfect, but we're only talking about a psi or two either way. Getting the pressures around the car consistent with each other and knowing whether you have any leaks is far more important than getting the pressure absolutely right.
lowndes said:
If we treat air as an ideal gas and assume that the tyres are sufficiently stiff not to change volume significantly then does the equation simplify to P1/T1=P2/T2
So if the pressure at 20 degrees C is suppposed to be 2.3 bar then at 10 degrees C it would be 2.3x283/293 where temperatures are in degrees Kelvin. This would suggest that if the ambient temperature were 10 degrees C the pressure you would set the tyre to would be 2.22bar.
Anyone got at different answer?
So if the pressure at 20 degrees C is suppposed to be 2.3 bar then at 10 degrees C it would be 2.3x283/293 where temperatures are in degrees Kelvin. This would suggest that if the ambient temperature were 10 degrees C the pressure you would set the tyre to would be 2.22bar.
Anyone got at different answer?
Air contains a fair bit of moisture so that equation doesn't really work. If you filled your tyres with nitrogen then I think that formula would be very close and that's indeed one advantage to using nitrogen: less pressure variation with temperature changes.
Brett928S2 said:
Hi 
I use an Infrared Laser thermometer...
Get tyres up to NORMAL temp....put red dot on EDGE, MIDDLE, EDGE...
They should all be same temp...if not adjust up or down until they are...
You can get the thermometers from Ebay for about £20....
All the best Brett
I use an Infrared Laser thermometer...
Get tyres up to NORMAL temp....put red dot on EDGE, MIDDLE, EDGE...
They should all be same temp...if not adjust up or down until they are...
You can get the thermometers from Ebay for about £20....
All the best Brett
That's fine so long as you stay within perhaps 10-20% of the factory recommended inflation pressure. But, for example, if you're running a degree or two of negative camber and then try to use tyre pressure to even the temps you'll probably drastically overinflate them.
Brett928S2 said:
Hi
I use an Infrared Laser thermometer...
Get tyres up to NORMAL temp....put red dot on EDGE, MIDDLE, EDGE...
They should all be same temp...if not adjust up or down until they are...
I use an Infrared Laser thermometer...
Get tyres up to NORMAL temp....put red dot on EDGE, MIDDLE, EDGE...
They should all be same temp...if not adjust up or down until they are...
That might work in some specific cases, I don't know. But as a general guide, that's not the way to set tyre pressures. You need to use a pyrometer to measure the temperature inside the tread not just the surface temperature (which will be a lot lower and more variable), and you should aim to get the temperature at the middle of the tread to be roughly the average of the inner and outer edge temperatures, but you aren't aimig to get the inner and outer temperatures equal. Varying the inner/outer temperatures is done by altering the geometry and typically the optimum is with the outer edge running hotter than the inner. But if you're going to play this sort of game you need to be considering lap times as a whole and not just measuring tyre temperatures.
For a road car I would suggest setting the tyre pressures when the tyres are cold. Don't concern yourself with air temperature as 1 or 2lb ain't going to make any difference. After all you are assuming that your gauge is accurately calibrated. I believe that it is more imprtant to have the tyres pressures balanced across the car. GreenV8s is correct in saying that for race cars on slicks getting an even spread of temperature across the tyre is important by adjusting camber and toe and gives you an indication of how effectively you are using the overall grip level that the tyre has to offer. You would not want to be overheating the outside of the tyre whilst the inside remains cold. In effect you would want to increase the negative camber so that you equal the temperature between the outside and inside of the tyre.
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