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How to determine tire pressure

Old 04-06-2005, 11:18 AM
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How to determine tire pressure

This past weekend at our local event there was a lot of discussion about how to set tire pressures. I thought I'd share the process I use and hopefully spark some useful discussion on other ways this could be done and/or ideas.

So what do you do when you don't really know what pressure to use? The easiest is to find someone that runs something similar and use that as your baseline. If you can't find someone using something similar try asking the manufacture. They will generally give offer some advice on pressure. And if you're using tires not made for your car you'll get an earful of advice about why you shouldn't be running what you have.

You'll need a good air pressure guage (buy your own and use only it), a tire pyrometer, and keep accurate notes.

1. Run the car and when you come in immediately check air pressures and write them down (try to follow the same pattern for measuring each tire). While doing this measure the tire temps.

2. The optimimum pressure will be when the average of the inners and outers is about the same as the middle. Figure a 3 to 5 degree window on this one.

3. Check car balance by noting the tire pressure rise. A balanced car will have equal rise in the tires front to rear per side (not necessarily the same per side). For autox tire pressure is going to be a better indicator than tire temp for how you are working the tire. We don't run long enough to build core temp.

4. Make initial changes at about ten percent the tire pressure. For 20 pounds you want to move about 2 pounds at a time. As you get closer move to 1 pound changes and try to get down to a half pound. The idea is to make a large change and then follow up with smaller changes while honing in on the correct number.

5. If the car oversteers on corner entry and understeers on exit, lower all pressures. If you had the opposite problem try raising all the pressures.

6. If you need to add air do it early, not right before the run. When you fill the tire the air coming in will lower in temperature and take time to stabilize.

That's pretty much it. You know have a hot pressure to use with your car and tires. Remember that for every ten degrees in track temperature change will change by about 1 pound. This is really key. As long as you don't have a lot of water vapor in your tires you can use this rule of thumb to set a cold pressure that will give you the hot pressure you need for best handling.

Hope this helps,

Cary
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Old 04-06-2005, 12:50 PM
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Things seems to work best if I go with as low as I can with pressures and still get good responsive turn-in without the tires feeling too squishy. I've run as low as 18 to try and get some grip when there wasn't any (really squishy, no response), as high as 27 psi (no squish, good response) when the car won't turn and there seems to be good grip. The "normal" seems to be 23-25 psi. I've tried using pyrometers for autox and for me they weren't helpful in determining the right tire pressure. It's just how they feel when I drive. The rubber compound, age of the tire, car weight, and ambient temperature all have a large influcence on what pressure feels right. Oh, and feel free to dismiss whatever I have to say about anything as I was unable to put a decent run together last weekend in Houston.
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Old 04-06-2005, 01:11 PM
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Originally Posted by jburge01
Oh, and feel free to dismiss whatever I have to say about anything as I was unable to put a decent run together last weekend in Houston.
Nah, we're just waiting for you to get those spacers done to see how much better the car works.

Cary
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Old 04-06-2005, 01:12 PM
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I wish I could, but I can't blame Houston on the car.
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Old 04-15-2005, 08:10 AM
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Ok, I finally have the time to post what I have. It's from a booklet that was givin to me back in '97, at a classroom racing school. This is definetly a beginners guide, and Cary is more on the right track (no pun intended), but here's what it says:

Originally Posted by Midwest Council

This is for an ITC car on 185/60/13's, but maybe a guidline:

deg. 60F 70F 80F 90F 100F
Front 24# 28# 30# 30# 32#
Rear 28# 32# 34# 34# 36#

After each session, check tire pressure immediatley and match fronts to the highest front pressure, match rears to lowest rear pressure. At the end of the day, pressures should match.

Once you get a starting pressure, you can chalk (often called the poor man's pyrometer) your sidewalls to check roll over. This will indicate if more air is required and allow you to adjust for maximum tire life.

Sometimes mild handling problems can be corrected by adjusting tire pressures. If your car is pushing, or understeering slightly, adding air to your front tires will help as long as you are not exceeding the maximun limits of adhesion. If you are at the limit in the front, you can also reduce air pressures in the rear tires to accomplish a similar correction. Should your car be oversteering, or loose in the rear, adding air to the rear tires will help as long as you are not exceeding the maximun limits of adhesion. Reducing air in the front will provide a similar correction if you are at the limits in the rear.

Last edited by preith; 04-15-2005 at 08:18 AM.
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Old 04-18-2005, 02:25 AM
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A LF tyre after some clockwise circuit laps with mostly right corners, outside edge of the tyre at the top of the pic. We reckon that the tyre was over inflated and the front could use some toe out (Ackermann), note the heavy wear about 1/3 of the way in and that the outer edge is relatively OK.

Intend to drop pressure by about 5 psi and then be guided by what happens to the tyre at the outer edge.
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Old 04-18-2005, 05:10 PM
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That Kumho tire needs more negative camber IMHO. Those tires are the best in lack of adjustment applications. Try getting as much camber as you can and then take a look.

Alan
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