tyre pressures: Changes due to temperature.
Discussion
Checked my pressures when before going for a drive last night:
21 rear and 23 front
Went for a 30 mile hack around fast and tight B roads (Dont you just love it when England are playing
).
When I got back measured them whilst warm:
25 rear and 27 front
I was surprised that there was such a significant change!
What pressures are advisible when cold and when warm?
Im running S03s all round and had thought it was R22F24 but cant remember if thats when wamr or cold.
Its not mentioned in the bible. But it has felt alright like this. Just wondered if it could feel better and I was missing out on something!
21 rear and 23 front
Went for a 30 mile hack around fast and tight B roads (Dont you just love it when England are playing
). When I got back measured them whilst warm:
25 rear and 27 front
I was surprised that there was such a significant change!
What pressures are advisible when cold and when warm?
Im running S03s all round and had thought it was R22F24 but cant remember if thats when wamr or cold.
Its not mentioned in the bible. But it has felt alright like this. Just wondered if it could feel better and I was missing out on something!
The advised pressures are 22 front, and 24 rear, for Chimaera and Griffith, and these seem to work with whatever wheel/tyre combination.
As regards track driving, it gets quite complex, but coincidentally, I was pondering this matter only yesterday, so I'll add my thoughts, and someone far more knowledgeable can then correct us both!
In general, you need to remember the two extremes of tyre pressure:
1. UNDER inflation - reduces the support to the centre of the tread and over time the outsides of the tread - supported by the sidewall - will wear quicker than the centre. You'll lose grip and have gerater tendancy to roll the tyre off the rim.
2. OVER inflation - reduced the contact patch by 'ballooning' the tyre and over time the insides of the tread will wear quicker than the outsides. The car will run 'harder' and will break traction much more suddenly. You can often get a very 'pointy' feeling through the steering wheel - as if there's very little resistance to turning.
TRACK USE: generally, it's best to increase pressures - by about 1 or 2 - in all tyres, to give greater support to the tryre for the higher forces generated. This stops the tyre flattening under braking, or rolling off the rim under cornering.
However, on sprints/hillclimbs this doesn;t seem to work and for a 1/4 miler (where you're tyres start below optimum temperature) lower pressures may tend to give better traction.
It all gets very complex with factors such as temperature, and also the interplay between tyre pressure (and stiffness) and suspension settings. Heat will cause the air to expand, so when you're doing a number of laps, the pressures can change drastically.
That's it now, I'm handing over the batton, someone else need to explain more...greenV8s, shpub et. al where are you?
>> Edited by Digga on Friday 25th June 10:01
As regards track driving, it gets quite complex, but coincidentally, I was pondering this matter only yesterday, so I'll add my thoughts, and someone far more knowledgeable can then correct us both!
In general, you need to remember the two extremes of tyre pressure:
1. UNDER inflation - reduces the support to the centre of the tread and over time the outsides of the tread - supported by the sidewall - will wear quicker than the centre. You'll lose grip and have gerater tendancy to roll the tyre off the rim.
2. OVER inflation - reduced the contact patch by 'ballooning' the tyre and over time the insides of the tread will wear quicker than the outsides. The car will run 'harder' and will break traction much more suddenly. You can often get a very 'pointy' feeling through the steering wheel - as if there's very little resistance to turning.
TRACK USE: generally, it's best to increase pressures - by about 1 or 2 - in all tyres, to give greater support to the tryre for the higher forces generated. This stops the tyre flattening under braking, or rolling off the rim under cornering.
However, on sprints/hillclimbs this doesn;t seem to work and for a 1/4 miler (where you're tyres start below optimum temperature) lower pressures may tend to give better traction.
It all gets very complex with factors such as temperature, and also the interplay between tyre pressure (and stiffness) and suspension settings. Heat will cause the air to expand, so when you're doing a number of laps, the pressures can change drastically.
That's it now, I'm handing over the batton, someone else need to explain more...greenV8s, shpub et. al where are you?
>> Edited by Digga on Friday 25th June 10:01
Measure them cold.
Recommended are 22/24
With SO3s running 1psi lower makes sense because of the stiffer sidewall. In my experience 22/24 on SO3s gives a wear pattern that looks as if they're over-inflated.
There's a lot of tyre to warm up and expand so the increase makes sense - and remember not to try any heroics on cold tyres!
As far as track pressure - over to teh experts! The arguments for up and down leave me confused! i do know that running on high pressures on the road (tyre fitters let me out with 32 psi once
) leaves you feeling like you're driving on marbles!!!
Recommended are 22/24
With SO3s running 1psi lower makes sense because of the stiffer sidewall. In my experience 22/24 on SO3s gives a wear pattern that looks as if they're over-inflated.
There's a lot of tyre to warm up and expand so the increase makes sense - and remember not to try any heroics on cold tyres!
As far as track pressure - over to teh experts! The arguments for up and down leave me confused! i do know that running on high pressures on the road (tyre fitters let me out with 32 psi once
) leaves you feeling like you're driving on marbles!!!beano500 said:Very good point - forgot that one.
Measure them cold.
beano500 said:
i do know that running on high pressures on the road (tyre fitters let me out with 32 psi once
Why do they insist on doing that? Always right up there in the high 30's for some reason, and with all the wheel nuts air-gunned up to bu66ery, so there's no chance whatsoever of removing them with a wrench. During test sessions I've seen race teams measuring the whole width of te tread with those needle-probe type thermometers as soon as the car arrives back in the pits from a session, and then dropping the pressures....
My tyre bay fitted S03s for me and put 36psi in all round. I didnt know this till I left their premises sideways! 
So recommended for S03's is:
rear 21
front 23
Correct?
For the track the bible recommends a couple of PSI extra all round.
BTW: Rear = 245/45/16, Front = 205/55/15
(May have the profiles wrong)
yep, non-PAS and proud of it!
So recommended for S03's is:
rear 21
front 23
Correct?
For the track the bible recommends a couple of PSI extra all round.
BTW: Rear = 245/45/16, Front = 205/55/15
(May have the profiles wrong)
yep, non-PAS and proud of it!
I'm no expert here, but it seems to make sense for track applications to find a stable set of conditions for your tyres.
Hence, when you come in from a session check the temperature across the width of the tyre - you are trying to have an even temperature profile indicating that the maximum width of the tyre is remaining in contact with the track. Also measure the pressure and if necessary reduce it; you will find that at some point you reach a steady state where the pressure after (and hopefully during) the session doesn't change - this is what you are aiming for.
At the start of the day you might want to increase pressures by 10% or so, but monitor them regularly and adjust after each session.
As with all setups, personal preferences vary as do the responses of different cars and models so its more a question of finding something that works for you.
Hence, when you come in from a session check the temperature across the width of the tyre - you are trying to have an even temperature profile indicating that the maximum width of the tyre is remaining in contact with the track. Also measure the pressure and if necessary reduce it; you will find that at some point you reach a steady state where the pressure after (and hopefully during) the session doesn't change - this is what you are aiming for.
At the start of the day you might want to increase pressures by 10% or so, but monitor them regularly and adjust after each session.
As with all setups, personal preferences vary as do the responses of different cars and models so its more a question of finding something that works for you.
Digga said:
Think it's to make sure the tyre is properly seated on the rim. and of course air leaks out of tyres not into them, so for Mr & Mrs Numpty it's 'safer'.
does altitude make a difference?
I had an odd situation abroad. When I was in Andorra (an dthe boot was almost empty as all the stuff was in the hotel room) I checked my pressures and they seemed high. I left them at around 28 back 24 front. Got down to toulousse and checked them again - 24 back and 22 front
and that is after driving in hot weather and with a boot full of stuff
I had an odd situation abroad. When I was in Andorra (an dthe boot was almost empty as all the stuff was in the hotel room) I checked my pressures and they seemed high. I left them at around 28 back 24 front. Got down to toulousse and checked them again - 24 back and 22 front
and that is after driving in hot weather and with a boot full of stuffjust use a CO2 bottle with a regulator and pressure guage, CO2 gets less thermal expansion than dirty compressor air with oil and 5hite in it !!!
Used to use it on a racing gokart (0-100 in 9 secs)
round most of the brit circuits and it was v. good
beano, you still sellin your motor? that green griff might have not been used too much so am a bit wary?
Used to use it on a racing gokart (0-100 in 9 secs)
round most of the brit circuits and it was v. good
beano, you still sellin your motor? that green griff might have not been used too much so am a bit wary?
shnozz said:
does altitude make a difference?
I had an odd situation abroad. When I was in Andorra (an dthe boot was almost empty as all the stuff was in the hotel room) I checked my pressures and they seemed high. I left them at around 28 back 24 front. Got down to toulousse and checked them again - 24 back and 22 front
Altitude and atmospheric conditions will both make a difference as the pressure measurements are not absolute.
If you check your tyres when its low pressure atmosphere (ie when its raining/cold or at high altitude) the tyre pressure will show higher.
When its a scorcher of a day (high atmospheric pressures) then you'll notice they are lower.
These could be such small differences that you barely notice though.
PIERSY said:
Did not shpub suggest 20 front and rear 'cos of the hard side walls of the SO3's ? (or am I miss-quoting)
I think you are correct.
My Griff has SO3's and recently had the rears replaced and those kind tyre fitters sent me out the door with 36psi. Like Dusty, it scared me to death trying to keep it in a straight line!!! Frightening
I read the same in the bible or a forum on PH and set them at 20PSI front and back and feels spot on for general road use!
shnozz said:
does altitude make a difference?
Yes. As does barometric pressure.
As altitude increases, ambient air pressure drops (this is the main reason why humans can't survive at very high altitudes), so the pressure difference between the indisde of the tyre, and the outside air will increase.
Similarly, you can set your tyre pressures one day, not move the car, and check them again and they've moved 1 (or in extreme cases 2) depending on the barometric pressure - all those funny isobart lines on the weather forecast, for those of us not busy oggling the weather girl (or Michael Fish, if that's what floats your boat).
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