Griff Handling
Discussion
And air is mostly made up of nitrogen anyway (78%)so in terms of the broad pressure characteristics it won't make a lot of difference. The advantages are in the details.
One reason that nitrogen is used in high performance tyre situations on its own is because it leaks less than the other contituents of air like oxygen. "Constant pressure" is maintained in the sense that the tyre deflates much more slowly over time and in a more predictable way, which I guess is a good thing when the tyre is heating and cooling to extremes. The pressure still rises when the tyre gets hot, so maybe "constant inflation" is a better description.
The changes in the performance properties of the tyre during use are also more predictable when there is only one pure gas inside, especially if variable factors like air humidity etc. can be eliminated. It is this factor that makes the most difference to the rate at which the pressure increases with heat. Damp air does expand more under heating than dry nitrogen.
All this must be very helpful if you are trying to maintain lap times in a well set up car driven on the limit, but I'm not sure how it would help you on the road, especially since all the advantage is lost as soon as you top up your tyres at the petrol station.
BTW, if you don't believe that rubber leaks some gases faster than others, think about how long a balloon filled with helium (a very small atom) stays inflated compared to one with air.
One reason that nitrogen is used in high performance tyre situations on its own is because it leaks less than the other contituents of air like oxygen. "Constant pressure" is maintained in the sense that the tyre deflates much more slowly over time and in a more predictable way, which I guess is a good thing when the tyre is heating and cooling to extremes. The pressure still rises when the tyre gets hot, so maybe "constant inflation" is a better description.
The changes in the performance properties of the tyre during use are also more predictable when there is only one pure gas inside, especially if variable factors like air humidity etc. can be eliminated. It is this factor that makes the most difference to the rate at which the pressure increases with heat. Damp air does expand more under heating than dry nitrogen.
All this must be very helpful if you are trying to maintain lap times in a well set up car driven on the limit, but I'm not sure how it would help you on the road, especially since all the advantage is lost as soon as you top up your tyres at the petrol station.
BTW, if you don't believe that rubber leaks some gases faster than others, think about how long a balloon filled with helium (a very small atom) stays inflated compared to one with air.
19560 says set the pressures (22F 24R) when cold, Guillotine says do it when hot. Or is that only for trackdays? Do I conclude 'cold' for normal on-road driving? Strange that handbooks never seem to mention this. Luckily there's a garage with an air jobbie only 1/2 a mile away, so at least I have the option to do either.
simpo two said:
Strange that handbooks never seem to mention this. Luckily there's a garage with an air jobbie only 1/2 a mile away, so at least I have the option to do either.
Cold is easier especially as with most road driving the tyres don't get that hot and setting them hot can cause problems in that the pressures will drop when cold.
For track days I tend to measure when cold and then ignore the hot pressure but adjust relatively. I might put 28 in cold and then change it by 1 or 2 pounds irrespective of what the hot temp is. Easier and simply to do especially when there is a cool down lap.
Each technique works and each has plusses and minuses - use whichever one makes sense.
As for garage tyre pressure equipment.... Ask Santa for a decent mechanical guage. Most garage tyre pressure kit is hopelessly inaccurate. Could be 20-30% wrong. The Sykes Pickavent guage from Halfords is pretty good for about £10-11. Each of my Tivs has one in the boot and they are pretty accurate. Compared with a very expensive race version.
Steve
www.tvrbooks.co.uk
I measure my tyre pressures when cold but cold varies throughout the year. During the onset of winter, I find myself having to add 1/2-1 pound every fortnight and come summer, I take have to let them down by a similar amount. I presume this is all down to air temperature inside the tyres.
Ian A.
Ian A.
shpub said:
The Sykes Pickavent guage from Halfords is pretty good for about £10-11.
Thank shpub, it all makes sense. I haven't looked at standalone pressure gauges so I suppose you have to put the gauge on, measure the pressure, take the gauge off, put some air in or out to suit, guessing how much, then put the gauge on again to see if you guessed right, then repeat until correct.
Wonder if anyone makes a standalone gauge that connects between the tyre and the compressor so you can take accurate measurements *and* adjust at the same time? (If not, consider it hereby patented and my roylaty fee is 10%!)
My experience is that the simple mechanical (lolipop stick) pressure guages are much more accurate than the cheap electronic versions. The elctronic effort I had claimed accuracy to +/- 1 PSI but in fact read about 4 PSI too low. A certain gentleman at The Garage, Compton politely told me I was driving around on baloons. I invested in one of these fuddy duddy old stylee ones and the measurement agreed precisely with what David Batty had set.
Quick thought on the cold/hot discussion. As has been pointed out, from the ideal gas law (PV=nRT) pressure in a tyre will increase more or less directly in proportion to the increase in temperature (ignoring the tyre's volume increasing a little). The T in that forumla is an absolute temperature, i.e. T=0 is absolute zero. So if you want to calculate the % increase in pressure you just divide the Hot temp by the Cold Temp and subtract 1. Easiest to use the Kelvin temperature scale, so just add 273 to the temp measured in centrigrade. As an example lets say the air in the tyre starts at 10 degrees centigrade and rises to 25 degrees during the drive. Pressure increase would be:-
Pressure Inc = (273 + 25) / (273 + 10) - 1 = 0.053 or 5.3%
So if the tyre started at 24PSI it would have risen to 25.3PSI
I would guess from normal road driving that those numbers are a reasonable estimate, and you can thus see that the pressure variations are small enough not to worry about.
Formula 1 tyres are designed to operate at around 100C. If you managed to get your road tyre up to that temperature, the forumla above implies your pressure would go up 8 or 9 PSI.
Quick thought on the cold/hot discussion. As has been pointed out, from the ideal gas law (PV=nRT) pressure in a tyre will increase more or less directly in proportion to the increase in temperature (ignoring the tyre's volume increasing a little). The T in that forumla is an absolute temperature, i.e. T=0 is absolute zero. So if you want to calculate the % increase in pressure you just divide the Hot temp by the Cold Temp and subtract 1. Easiest to use the Kelvin temperature scale, so just add 273 to the temp measured in centrigrade. As an example lets say the air in the tyre starts at 10 degrees centigrade and rises to 25 degrees during the drive. Pressure increase would be:-
Pressure Inc = (273 + 25) / (273 + 10) - 1 = 0.053 or 5.3%
So if the tyre started at 24PSI it would have risen to 25.3PSI
I would guess from normal road driving that those numbers are a reasonable estimate, and you can thus see that the pressure variations are small enough not to worry about.
Formula 1 tyres are designed to operate at around 100C. If you managed to get your road tyre up to that temperature, the forumla above implies your pressure would go up 8 or 9 PSI.
Good post ATG!
Seeings as my front tyres on a track day seem to rise from 22psi to 29psi (which from the formula looks like my tyres are at 100degC!!) the effect of water/water vapour in the tyres must be quite significant. The tyres are very hot to the touch but I would think some way off 100degC.
Seeings as my front tyres on a track day seem to rise from 22psi to 29psi (which from the formula looks like my tyres are at 100degC!!) the effect of water/water vapour in the tyres must be quite significant. The tyres are very hot to the touch but I would think some way off 100degC.
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