Tyre Pressures

[Lancerlot]

Moisture and experience.

Regards,

Clive.

 

[tscaptain]

ORIGINAL: RJKflyer



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.

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?

 

[RJKflyer]

ORIGINAL: tscaptain

ORIGINAL: RJKflyer



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.

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![]

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..

 

[RJKflyer]

OK, OK, having done the math, and under the most adverse of conditions (e.g. filling with saturated air at 20 degC on a really humid day to 40psi, followed by it cooling to near freezing, followed by it being topped up again when cold to 40psi, and then being run on the track) I can see that a rear tyre might show between 1 and 2psi MORE rise if filled with air versus Nitrogen.

Hope you'll all let me off now...

 

[tscaptain]

See my edit above.
Air holds water vapour and the warmer the air the more it can hold before it becomes saturated and falls out of the air as a liquid. Rising air cools and the vapour condenses and when there is enough of it we get wet! So it doesn't have to exist as visible liquid water in the tyre to be a villain. The increase in water vapour pressure with temperature rises is not linear. Water vapour pressure just above freezing is negligible and a lot at 100C! See table below.

Edit - Don't you just love this homespun physics![]

 

[RJKflyer]

The increase in water vapour pressure with temperature rises is not linear. Water vapour pressure just above freezing is negligible and a lot at 100C! See table below

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)

 

[Motorhead]

Sorry......just wandered in here.

Is this part of some sort of SATS test?

Jeff

 

[RJKflyer]

ORIGINAL: Motorhead


Sorry......just wandered in here.

Is this part of some sort of SATS test?

Jeff

Hope you've got your copy of 'SI Units' and a British Thornton slide rule...

 

[tscaptain]

ORIGINAL: RJKflyer


I think you may find that air's ability to absorb water is reasonably linear with temp.

I think you may find that it isn't!

http://www.engineeringtoolbox.com/maximum-moisture-content-air-d_1403.html

 

[RJKflyer]

ORIGINAL: tscaptain

ORIGINAL: RJKflyer


I think you may find that air's ability to absorb water is reasonably linear with temp.

I think you may find that it isn't!

http://www.engineeringtoolbox.com/maximum-moisture-content-air-d_1403.html

Yes, sorry, you're right on that.

 

[RJKflyer]

So how do get all the air out before refilling with N2 ?

Else you'll have 40-ish. % worth of wet air and 60% of N2. Obviously better than none, but not what you want.

 

[tscaptain]

ORIGINAL: RJKflyer

So how do get all the air out before refilling with N2 ?

Else you'll have 40-ish. % worth of wet air and 60% of N2. Obviously better than none, but not what you want.

Like this....
https://www.youtube.com/watch?v=w8zzuXbsgRo