Tyre Pressure changes from cold to hot
Discussion
Just got a new set of Toyos on the back of my Chim. Initial pressure set at 26. Yesterday checked 'em cold before I set out and found one to be at 26 the other at 22 so headed to the Petrol station to top up. When I got there (went the long way 
) the one which had been at 22 was at 25 and the one at 26 was at 29. 3 psi increase seems a lot to me. Would be interested to hear whether this is experienced by others here. After setting both at 29 hot drove home and have to say they felt a bit less grippy.
Just generally wonder what people's thoughts are.
) the one which had been at 22 was at 25 and the one at 26 was at 29. 3 psi increase seems a lot to me. Would be interested to hear whether this is experienced by others here. After setting both at 29 hot drove home and have to say they felt a bit less grippy. Just generally wonder what people's thoughts are.
gannmaster said:
Just got a new set of Toyos on the back of my Chim. Initial pressure set at 26. Yesterday checked 'em cold before I set out and found one to be at 26 the other at 22 so headed to the Petrol station to top up. When I got there (went the long way ) the one which had been at 22 was at 25 and the one at 26 was at 29. 3 psi increase seems a lot to me. Would be interested to hear whether this is experienced by others here. After setting both at 29 hot drove home and have to say they felt a bit less grippy.
Just generally wonder what people's thoughts are.
) the one which had been at 22 was at 25 and the one at 26 was at 29. 3 psi increase seems a lot to me. Would be interested to hear whether this is experienced by others here. After setting both at 29 hot drove home and have to say they felt a bit less grippy. Just generally wonder what people's thoughts are.
Tyre pressures increase with tyre heat though I don't know by how much,thats why its always reccommended that you check pressures when cold.
A similiar thing happened to me and it turned out to be a faulty valve.
Get yourself a mobile compressor, this makes sure you are always using the same gauge. Alternatively fill them with nitrogen, which is inert and not affected by temperature. www.pistonheads.com/gassing/topic.asp?t=291600&f=23&h=0
Tyre pressures as stated in the book and those like me who need to increase pressures to counter the softer walls on Toyos, are 'cold' pressures. The optimum pressure that the car should be operated at is higher. The cold set pressure is the pressure that is required in order for the tyres to generate the optimum pressure when the tyres are at road running temperatures. Warm pressures should therefore be higher. I have found 4 to 6 pis higher when hot is normal.
jwt63 said:
Get yourself a mobile compressor, this makes sure you are always using the same gauge. Alternatively fill them with nitrogen, which is inert and not affected by temperature. www.pistonheads.com/gassing/topic.asp?t=291600&f=23&h=0
Hi John - I haven't read the thread you've referred to , but nitrogen is affected by temperature, like air. It's advantage is that it doesn't have as much water vapour in it as air but then again as soon as you top your tyres up with air (and water vapour), the advantage has gone. On track, my tyres typically rise by 8 - 10 psi so 4 - 6 psi on road sounds about right.
I have a pencil type pressure gauge which is surprisingly accurate and keep it in the car to check pressures. I would never trust a garage forecourt gauge; my local one a few years ago read about 10psi under!
tvrbob said:
Tyre pressures as stated in the book and those like me who need to increase pressures to counter the softer walls on Toyos, are 'cold' pressures. The optimum pressure that the car should be operated at is higher. The cold set pressure is the pressure that is required in order for the tyres to generate the optimum pressure when the tyres are at road running temperatures. Warm pressures should therefore be higher. I have found 4 to 6 pis higher when hot is normal.
Wow.. I am surprised that 4 - 6 psi is normal. I have to admit I am also surprised by the whole change in handling caused by the new tyres... so much better grip at the back that instead of vaguely drifting out on turning, the rears actually grip and force the front to turn in. I have a bit more experimentation to do to find the right cold pressure obviously. Given how sloppy they felt at 29 hot they're down to 25 cold tomorrow.
GarryM said:
jwt63 said:
Get yourself a mobile compressor, this makes sure you are always using the same gauge. Alternatively fill them with nitrogen, which is inert and not affected by temperature. www.pistonheads.com/gassing/topic.asp?t=291600&f=23&h=0
Hi John - I haven't read the thread you've referred to , but nitrogen is affected by temperature, like air. It's advantage is that it doesn't have as much water vapour in it as air but then again as soon as you top your tyres up with air (and water vapour), the advantage has gone. On track, my tyres typically rise by 8 - 10 psi so 4 - 6 psi on road sounds about right.
I have a pencil type pressure gauge which is surprisingly accurate and keep it in the car to check pressures. I would never trust a garage forecourt gauge; my local one a few years ago read about 10psi under!
Hi Gary,
Maybe you should read the article and refer to tyre services that will fill your tyres with pure Nitrogen which is inert! Aircraft / F1 have been using this for decades!
www.merityre.co.uk/
Edited by jwt63 on Sunday 13th August 12:41
jwt63 said:
Hi Gary,
Maybe you should read the article and refer to tyre services that will fill your tyres with pure Nitrogen which is inert! Aircraft / F1 have been using this for decades!
www.merityre.co.uk/
I think GarryM's point is that the Nitrogen doesn't stay pure unless you take your car back to your specialist supplier every time you want to increase the tyre pressure. For many people that simply isn't practical.
jwt63 said:
Maybe you should read the article and refer to tyre services that will fill your tyres with pure Nitrogen which is inert! Aircraft / F1 have been using this for decades!
Er... Nitrogen is quite definitely NOT inert. IF it was we wouldn't have such enjoyable compounds as Nitrous Oxide...
However, when used to fill tyres it contains far less water vapour than normal atmospheric air and therefore tends to result in significantly less long-term corrosion of thw wheel. It also tends to permeate the tyre casing less, and thus tyre pressures remain more consistent over a longer period.
These are the reasons why Nitrogen is used in the aircraft industry.
The same advantages will obviously apply to road cars, but only if you always top up the tyres with nitrogen -- which is largely impractical for most people.
Hugerr
hugerr said:
jwt63 said:
Maybe you should read the article and refer to tyre services that will fill your tyres with pure Nitrogen which is inert! Aircraft / F1 have been using this for decades!
Er... Nitrogen is quite definitely NOT inert. IF it was we wouldn't have such enjoyable compounds as Nitrous Oxide...
However, when used to fill tyres it contains far less water vapour than normal atmospheric air and therefore tends to result in significantly less long-term corrosion of thw wheel. It also tends to permeate the tyre casing less, and thus tyre pressures remain more consistent over a longer period.
These are the reasons why Nitrogen is used in the aircraft industry.
The same advantages will obviously apply to road cars, but only if you always top up the tyres with nitrogen -- which is largely impractical for most people.
Hugerr
It is inert under normal conditions ... http://en.wikipedia.org/wiki/Nitrogen
It is the oxygen in the nitrous oxide that supports combustion
H'mmm... Nitrogen will respond to variations in temperature more predictably than air owing to the latter's variable water content, as has already been mentioned. It may also convey other small advantages (corrosion less etc) that have already been covered. However, nitrogen's pressure change with temperature is governed by the ideal gas law just as all gases'. That means it's pressure is directly proportional to its temperature (in Kelvin). Though using air will make the relationship non-linear, the actual difference in pressure change in pure nitrogen vs. air over the sort of temperatures tyres experience will be insignificant for road driving. Of course that's not to say the change itself is insignificant - conceivably on a cold day with spirited driving you might see a 50 degree change in tyre temperature (or more depending on the car/tyre etc etc). That would cause a pressure change in the region of 15-20%.
And have a look here: www.racq.com.au/cps/rde/xchg/racq_cms_production/hs.xsl/Motoring_Maint_Repairs_Foun_factsheet_nitrogen_tyres_ENA_HTML.htm
Andrew
And have a look here: www.racq.com.au/cps/rde/xchg/racq_cms_production/hs.xsl/Motoring_Maint_Repairs_Foun_factsheet_nitrogen_tyres_ENA_HTML.htm
Andrew
Also, the larger the tyre the more the pressure diff will be as there is more air to expand. Always set cold as this is your ambient temp and more relative to the conditions at the time, setting at hot presures can be useful, but only when race testing and can be regulated in pit lane, and track temp monitored etc.
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