Tyre pressures
- Thread starter M.newsome
- Start date
If you have TPMS then follow its guidance: it's clever in that when you go to the 'info' mode in the instrument display it will show you the air to add/remove to ensure the right pressures at the 20 deg C 'standard'. In other words it know the wheel temp and adjusts accordingly. 'Best' pressures are what the manual says. (2.3/2.7 bar or 34/40psi - with 2 people). These are at 20 degC, and so if you need to correct manually use about 4% per 10 degC temp diff, BUT if you've been driving it, it will be the wheel temp you need to know, not the ambient temp. Even for a modest track day you don't really need to adjust as these tyres (with N-rating) are designed with stiffer sidewalls (rear notably) to prevent any creep off of the rim - you often hear of people putting extra air in for track days to prevent tyres rolling off of the rims but not necessary here. No doubt experience track day-ers will chip in with better knowledge for harder use.
How do you work that out Clive? Any two gases exposed to the same temperature increase pressure by the same amount... The only extremely minor factor is that if you use atmospheric air, it may contain a certain amount of water vapour which when it cools may conceivably condense a small proportion, which in turn may cause a very very slightly greater pressure drop than if it had been either dry air or nitrogen. Before someone asks "why do aircraft use nitrogen?", the answer is (an Airworthiness Directive) "To eliminate the possibility of a chemical reaction between atmospheric oxygen and volatile gases from the tire inner liner producing a tire explosion" and also to prevent the air in the tyre from fuelling any sort of combustion, especially given that a wheel bay fire is one of your worst conceivable nightmares.ORIGINAL: Lancerlot and get your tyres inflated using nitrogen as this doesn't cause as much pressure variation when the tyres get hot. [] Regards, Clive.
I do the exact opposite. I let air out. After a quick session on track the 34/40 goes up to around 42/48 and it's like driving a spacehopper. So I then drop them back down to 34/40 hot, which seems to work ok. Of course at the end of the day I have to put air back in when they have cooled for the drive home. BTW, my experience too is that the pressure rise is less with N2. Air/O2/N2 are already in the gaseous state - not so the water. The water is the villain of the piece, as I understand it which, given the temperature of rims, must be turning to steam - and we know what that does to pressure in a confined space. That's my take on it anyway![
] Just as a edit to the above, water vapour pressure at 20C is 2.3 kPa and at 100C it is 101 kPa. At 40C it only rises by 3x, at 60C 10x, 80C 20x and at 100C 50x. So I figure a small amount of water has a big effect at higher temps?
Yes I've heard that re letting air out on these cars for hard track work. The 'water' is only the villain of the piece if it genuinely is sitting as water in the tyre (and then vaporises as they heat up) which it only can be doing if your tyre/wheel temp goes below that of the air that you originally filled it with. So if you top up in the summer, and then check the pressures in the winter I concede fully that there will be moisture that has dropped out inside the tyre. Then you top up again and when the wheel heats up, the moisture (water droplets) vaporises and i agree you get an unexpected rise in press which does not correspond to the temp change. BTW, I'm certainly not arguing with anything that you have experienced - I'm afraid I just like to work out how and why. Cos I'm a bit OCD... Which is possibly why I bought the Porsche, not an AM..ORIGINAL: tscaptainI do the exact opposite. I let air out. After a quick session on track the 34/40 goes up to around 42/48 and it's like driving a spacehopper. So I then drop them back down to 34/40 hot, which seems to work ok. Of course at the end of the day I have to put air back in when they have cooled for the drive home. BTW, my experience too is that the pressure rise is less with N2. Air/O2/N2 are already in the gaseous state - not so the water. The water is the villain of the piece, as I understand it which, given the temperature of rims, must be turning to steam - and we know what that does to pressure in a confined space. That's my take on it anyway
Edit - Don't you just love this homespun physics![]
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Isn't that chart for saturated water and steam? (not saturated air) I think you may find that air's ability to absorb water is reasonably linear with temp. The pressure the water vapour exerts as temperature rises is no different to any other gas (bar compressibility effects which are irrelevant at these pressures)
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