To what degree does the ambient temperature affect tyre pressures, because when i checked mine last week with the dash display showing -1c they were all at 1.9 Bar when they should be 2.2 Bar which they had been a month earlier when the weather was milder ?
I duly inflated them back up to 2.2 Bar but the ride is now best described as "over firm", so i am now thinking that i should have left them as they were. Does freezing weather = lower tyre pressures or had mine just equally deflated ?
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The pressure in the tyre is proportional to temperature. But you need to convert both to absolute units, i.e. temperature in degrees Kelvin so add 273, and add 1 bar to the pressure (the 1.9 bar you measure is the differential between the pressure in the tyre and the 1 bar approx of atmospheric pressure).
So, using the 1.9bar we can calculate the temperature at which the pressure would have been 2.2 bar:
=(-1+273)*(2.2+1)/(1.9+1)-273 = 27C.
This is probably a but higher than you expected - but maybe the tyres were a bit warmer, or there's a bit of rounding error.
For practical purposes every 10c difference will have about 0.1 bar effect if your pressures are in the 2 bar region.
The tyre flexing /expanding will reduce the variation a bit in theory, but I can't imagine it has much of an effect.
This doesn't really answer the question of whether you should bother adjusting pressures as the temperature goes up and down. Personally I don't bother - I think they can stand a bit less when it's cold, and a bit more when it's hot - and in any case the tyres warm up when you drive.
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if you fill the tyres with nitrogen the pressures dont alter with temp differences, according to martin brundle but why he contradicts himself during a race by announcing that they have to get some heat in the tyres to get operating tempretures up baffles me
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I don't see why nitrogen should behave differently from air, except that ambient air usually has water vapour in which might condense/evaporate more or less and mess up the pressure/temperature relationship by making the variations bigger.
Dry gas (whether air or nitrogen) might well be more predictable in its effect.
My tyres have Costco nitrogen in and the pressure certainly went down with the cold weather ;-)
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nitrogens inert, unlike the stuff you get at the airline at the garage...i think thats something to do with it, same reason they use nitrogen in jet aircraft tyres... at high altitude the tyres would explode if they had normal air in them
Edited by zookeeper on 02/01/2010 at 15:13
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nitrogens inert unlike the stuff you get at the airline at the garage...i think thats something to do with it same reason they use nitrogen in jet aircraft tyres... at high altitude the tyres would explode if they had normal air in them
Which chemistry book did this come from?
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Which chemistry book did this come from?
dunno mate...but instead of being sarcastic why dont you help answering the OP,s question
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nitrogens inert unlike the stuff you get at the airline at the garage...i think thats something to do with it same reason they use nitrogen in jet aircraft tyres... at high altitude the tyres would explode if they had normal air in them
This isnt quite right. Nitrogen is inert when it comes to burning and exploding etc, but the laws governing expansion (Boyles law I think ??), due to temperature change apply just the same as with air, which is mostly nitrogen anyway. Aircraft tyres are designed to prevent explosion in this way, which is why they are tested albeit very dangerously at the factory to extreme limits.
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just the same as with air which is mostly nitrogen anyway. Aircraft tyres are designed to prevent explosion in this way which is why they are tested albeit very dangerously at the factory to extreme limits.
The main reason for using nitrogen in a/c tyres is the one manatee mentioned ? predictable pressure vs. temperature behaviour without interference from water vapour. They have to set the pressures at departure so they'll be correct for the ambient temp and pressure at the destination airport for the more critical landing. If there's a brake fire that gets hot enough to pop the tyres' thermal fuses, it's good to have them filled with something inert though, as you say :-)
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if you fill the tyres with nitrogen the pressures dont alter with temp differences according to martin brundle but why he contradicts himself during a race by announcing that they have to get some heat in the tyres to get operating tempretures up baffles me
Getting heat into the rubber is a different matter to the effect of temp on the pressure.
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if you fill the tyres with nitrogen the pressures dont alter with temp differences according to martin brundle but why he contradicts himself during a race by announcing that they have to get some heat in the tyres to get operating tempretures up baffles me
That's because the rubber needs to be hot and become "sticky" to make it grip the track surface properly, not to get pressures right.
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That's also the difference between "winter" tyres used in some countries and standard tyres-they stay soft in the cold to get more grip unlike standard that go hard below 0 deg.C.
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Thought I was having deja vu (again).
This came up almost exactly a year ago - could have cut and pasted!
www.honestjohn.co.uk/forum/post/index.htm?t=70548
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