Tire pressure adjsutment due to ambient temperature
08-22-2001, 05:23 PM
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Tire pressure adjsutment due to ambient temperature
I've read on here and in Racing archives discussions on finding best tire pressures for your car/tire/suspension setup. Bottom line has largely been to arrive at a cold set of tire pressures to use as a reference (generally has been written as X/Y psi at 70 degrees ambient temperature.
Thought is also that these base temperatures should be adjusted due to changing ambient temperature conditions (i.e., temperatue changes from early morning to heat of the afternoon). Rough number I see is to adjust pressures by 1 psi for each 10 deg F change in ambient temperature. The problem is that I have read what I think is divergent opinions of which way to adjust them. I thought I read one opinion on this board to INCREASE pressure as outside temperature increases; one I read on TurnFast web site (www.turnfast.com) says to DECREASE pressure as outside temperature increases. Which is it?
My brain says to decrease it as outside temperatue increases. What you want is best hot (driving pressures). At higher ambient/track temperatures, tire heats up more and thereby increases pressure more. So, you should reduce pressure to compensate. Do the opposite as ambient temperatue increase since tires don't heat up as much and need more cold pressure to compensate.
Any of you willing to provide your expert help?
Thought is also that these base temperatures should be adjusted due to changing ambient temperature conditions (i.e., temperatue changes from early morning to heat of the afternoon). Rough number I see is to adjust pressures by 1 psi for each 10 deg F change in ambient temperature. The problem is that I have read what I think is divergent opinions of which way to adjust them. I thought I read one opinion on this board to INCREASE pressure as outside temperature increases; one I read on TurnFast web site (www.turnfast.com) says to DECREASE pressure as outside temperature increases. Which is it?
My brain says to decrease it as outside temperatue increases. What you want is best hot (driving pressures). At higher ambient/track temperatures, tire heats up more and thereby increases pressure more. So, you should reduce pressure to compensate. Do the opposite as ambient temperatue increase since tires don't heat up as much and need more cold pressure to compensate.
Any of you willing to provide your expert help?
08-23-2001, 08:51 AM
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Ray,
I run a modified 914-6 in the PCA DE format.You are correct is the statement that we are stiving for equal hot operating tire temperatures, so if any ambient adjustment is needed you would lower the the pressure as the ambient temperatures increase. My initial set up with the present suspension and tire combination was done at Watkins Glen. The next day I read the cold pressures and the ambient temperature was 45 degrees F. I have used these cold pressures at early morning temperatures of 70 deg. with no ill affect. You also must consider the humidity of the air in the tire if you are not using nitrogen for this can affect the hot tire pressure. I have found the method that works for me is to set the pressures in the morning and check the hot pressures after a good run and adjust if necessary, but 1 psi per 10 deg. F sounds about right.
Good luck.
Pete
I run a modified 914-6 in the PCA DE format.You are correct is the statement that we are stiving for equal hot operating tire temperatures, so if any ambient adjustment is needed you would lower the the pressure as the ambient temperatures increase. My initial set up with the present suspension and tire combination was done at Watkins Glen. The next day I read the cold pressures and the ambient temperature was 45 degrees F. I have used these cold pressures at early morning temperatures of 70 deg. with no ill affect. You also must consider the humidity of the air in the tire if you are not using nitrogen for this can affect the hot tire pressure. I have found the method that works for me is to set the pressures in the morning and check the hot pressures after a good run and adjust if necessary, but 1 psi per 10 deg. F sounds about right.
Good luck.
Pete
08-23-2001, 10:59 PM
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I agree with DERSIX. I run Dunlop slicks with 100% nitrogen, and I check hot pressures after every session, which has allowed me to develop a good database of tire temp data for my car and driving style. Although time consuming, I would recommend this to any-/everyone. What I've found is that my tire temps are not as sensitive to ambient temperature as one might expect. They seem to be a bit more sensitive to (1) prevalence of high speed corners, (2) actual speeds realized (re. amount of traffic), and (3) amount of direct sunlight on the track surface.
At this point, I can usually guess (given the track, the amount of direct sunlight and breeze, and my estimate of air temperature) where to start for the first run of the day -- usually anywhere from 5-8 psi below the target temp. Curiously, I seem to have to bleed off a pound or a pound and a half after each run on a sunny day to keep to my optimal temperature ranges. Also, a run where I can't run clean laps near the end of the session will definitely show lower pressures. YMMV
- Nick
ncamos@bellatlantic.net
At this point, I can usually guess (given the track, the amount of direct sunlight and breeze, and my estimate of air temperature) where to start for the first run of the day -- usually anywhere from 5-8 psi below the target temp. Curiously, I seem to have to bleed off a pound or a pound and a half after each run on a sunny day to keep to my optimal temperature ranges. Also, a run where I can't run clean laps near the end of the session will definitely show lower pressures. YMMV
- Nick
ncamos@bellatlantic.net
08-23-2001, 11:45 PM
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After 10 years of working the tire pressure issue from DE to Club Racing the best advice I can provide is adjust for HOT tire pressures.
This means for Dunlop slicks 32 psi HOT and for Hoosiers 40 psi HOT. This really takes all of the guess work out and responds to all of the variables of track, temperature and setup issues.
Generally I start with about 24 psi for Dunlop slicks and about 34 psi for Hoosiers on 70 degree days. Start about 1 psi less for each additional 10 degrees of ambient air temp more than 70 and 1 psi more for every 10 degrees less than 70. Of course this is only a starting point. The key is HOT temps so what you begin with cold is only academic.
Be sure and check HOT temps frequently and immediately after hot laps. Generally pressures will vary based on changing conditions including tire wear or usage. Dry gas, such as nitrogen, is highly recommended because it lacks the moisture in normal air pump air which can have a measurable affect on tire pressure changes during a race.
This means for Dunlop slicks 32 psi HOT and for Hoosiers 40 psi HOT. This really takes all of the guess work out and responds to all of the variables of track, temperature and setup issues.
Generally I start with about 24 psi for Dunlop slicks and about 34 psi for Hoosiers on 70 degree days. Start about 1 psi less for each additional 10 degrees of ambient air temp more than 70 and 1 psi more for every 10 degrees less than 70. Of course this is only a starting point. The key is HOT temps so what you begin with cold is only academic.
Be sure and check HOT temps frequently and immediately after hot laps. Generally pressures will vary based on changing conditions including tire wear or usage. Dry gas, such as nitrogen, is highly recommended because it lacks the moisture in normal air pump air which can have a measurable affect on tire pressure changes during a race.
09-07-2001, 12:16 PM
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C,
I too start with 34 front and 35 rear and end up with 40 f and 41-42 rear. The ambient temp is quite hot at Willow Springs getting up to 100 degrees in the afternoon. You could have an alignment problem causing the excessive tire temp.
I too start with 34 front and 35 rear and end up with 40 f and 41-42 rear. The ambient temp is quite hot at Willow Springs getting up to 100 degrees in the afternoon. You could have an alignment problem causing the excessive tire temp.
09-10-2001, 03:12 PM
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This is based on the gas law, Pv=nRT, Pressure changes proportionately with Temperature. So you would be correct, to maintain same pressures (as when cold) you will need to decrease pressure as ambient temp increases. You will probably also need to account for increased tire temps due to frictional changes.
