Tire Pressure Variation

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John_Heard
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Tire Pressure Variation

#1 Post by John_Heard » Mon Dec 09, 2013 8:39 am

Kinda slow here lately so thought I'd bring up a topic I'm a bit curious about to see what others think.

Just how accurate is setting tire pressure multiple tires during a race event? Let's look at this scenario, you get to the track let's say it's 75 degrees before your 1st pass of the day. Tires are "cold", rims cold, etc. You set your tire pressure at what you think works best on your car, keep the tire covered when it's in the sun, etc. etc. normal procedure.

You do the burnout, of course tire temps increase big time, along with air pressure. You make the run, get back to the pits turn the car around and head back to the lanes. Air temp is now 80+. Most likely you'll bleed some pressure off as the psi will go up with temp. Tires and rims are much warmer than the beginning of the day, rims are warm (brake heat), etc. You set the pressure back to the same setting. Do your burnout etc. and repeat each round with air temperatures and tire/wheel temperatures varying greatly depending on how much down time there is between runs.

Just how close do you really think the tire pressure is at staging with the different temperatures the tire / rim is at over the course of the race day?

Assuming the tire doesn't leak air, wouldn't it be more accurate to set the tire psi at the beginning of the day and call it good?

Any of you ever measure tire psi AFTER the burnout?

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wheels
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Re: Tire Pressure Variation

#2 Post by wheels » Tue Dec 10, 2013 9:54 am

I had the pressure checked after a burnout 1 time and there was no change. This was with a bias slick and tube. Even with the drag radials I have now the only thing that seems to drive up pressure is direct sunlight.

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Re: Tire Pressure Variation

#3 Post by John_Heard » Tue Dec 10, 2013 10:11 am

I'd bet money pressure goes up after the burnout if you're getting the tires hot. I know it's up when we get back to the pits after a pass and that's after cooling down for a while.

I guess what I'm getting at, are we doing more harm than good by resetting tire pressure multiple times per day at random times in the heat / cool cycles?

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wheels
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Re: Tire Pressure Variation

#4 Post by wheels » Tue Dec 10, 2013 10:26 am

I thought it was odd the pressure did not change...that was a long time ago and not anything to back it up. Not sure about your wonderment....I only check them in the lanes right before we start moving especially if the sun is beating on one side. My point was that the pressures in the radials stay the same all day as long as they are in the shade.

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Re: Tire Pressure Variation

#5 Post by bracketchev1221 » Tue Dec 10, 2013 1:46 pm

I reset mine to the same pressure every pass right before I get in the car. However, I will set mine a touch low before the first pass knowing they have a little more temperature to pick up. But I can see a pressure increase just driving up to the lanes on hot blacktop.

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Re: Tire Pressure Variation

#6 Post by sunsation540 » Tue Dec 10, 2013 10:29 pm

we start with the tires .5 low first pass and recheck about 10-15 min after the first pass and set to running pressure, and usually don't have to adjust unless it gets real late or a huge oil down, we start .5 low after long periods of time. one hour or more.. this has worked well for us, however always only use your tire gauge and adjust to what works best on your gauge! PS keep good records!
make a plan and stick to it !!

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Re: Tire Pressure Variation

#7 Post by John_Heard » Tue Dec 10, 2013 11:04 pm

Dang look what the cat drug in? How ya doing Mr. Cook?

1/2 lb sounds pretty close. I'd still like to test psi after the burnout to see what it's actually at.

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Re: Tire Pressure Variation

#8 Post by bracketchev1221 » Wed Dec 11, 2013 7:24 am

sunsation540 wrote: however always only use your tire gauge and adjust to what works best on your gauge! PS keep good records!
This is important. I use the same gauge all the time. To me gauges and dynos are reference numbers. If you use a different one, you may get different readings.

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Re: Tire Pressure Variation

#9 Post by clutchdude » Wed Dec 11, 2013 5:42 pm

I just ran a tnt a week ago, made 4 passes in a hr and tire pressure was up .5 lb after each pass

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Re: Tire Pressure Variation

#10 Post by Craig W. » Wed Jan 01, 2014 10:35 am

Setting the pressure once at the start of the day would set the amount of air molecules in the tires (assuming no leaks) but that doesn't really help us much. In my opinion, the best we can do is establish a pre-burnout pressure that yields a car that works for the current track conditions. Then do a consistent and adequate burnout followed by the same staging procedure on every pass. That will hopefully lead to a pretty consistent tire pressure and repeatable and predictable performance.
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