Help Me Understand Tire Pressures For Autocross - Vintage Mustang Forums
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post #1 of 37 (permalink) Old 05-05-2019, 09:18 PM Thread Starter
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Help Me Understand Tire Pressures For Autocross

So I set my tire pressures at 32psi last night for today's autocross in a 68º garage. The tires in question are Bridgstone Ponenza RE71's. It went from 60º on the morning runs to 75º on the afternoon runs. A lot of talking and mumbling back and forth regarding tire pressures in the afternoon. Since I'm not burning rice or driving a Porsche little of the mumbling applied. The 2005+ Mustang crowd, very well represented and kicking butt VS the Chebby guys didn't seem to care in the least about tire pressures that they were willing to share. Don't know were to start on this?

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post #2 of 37 (permalink) Old 05-05-2019, 09:34 PM
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Take some white shoe polish and paint a few stripes on the sidewalls of your tires. Your tire pressures will be optimal when the scrub off is equal to or a little less than the tire sidewall rollover marks. Most if not all track equitable tires will have rollover marks.
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post #3 of 37 (permalink) Old 05-05-2019, 09:36 PM
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Here’s an example...

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Patrick
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post #4 of 37 (permalink) Old 05-05-2019, 09:45 PM Thread Starter
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I saw the white strips today on other cars! Cool!

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post #5 of 37 (permalink) Old 05-05-2019, 09:49 PM
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...or use tape so it doesnt look like you were vandalized. Whatever else you might find reading about those tires and pressure add a couple PSIs to it since your car is likely lighter than most others.

Verbose Vocabularian
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post #6 of 37 (permalink) Old 05-05-2019, 10:00 PM
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Sidewalk chalk works well
I start at around 40 psi most days and go from there
Now if we could have a nice day for autox with no rain


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post #7 of 37 (permalink) Old 05-05-2019, 10:08 PM Thread Starter
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75 and sunny in Merica today! PSA. Don't leave your SPF 100 sunscreen in your work truck over the winter in sub zero conditions and actually expect it to work the next spring.
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post #8 of 37 (permalink) Old 05-05-2019, 10:10 PM
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Another tool that is probably more accurate is an actual tire temp probe, but you have to be diligent at checking the temps as soon as you get out of the car.

https://www.amazon.com/dp/B000FPVU5O...v_ov_lig_dp_it


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Patrick
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post #9 of 37 (permalink) Old 05-05-2019, 10:12 PM
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Quote:
Originally Posted by Nailbender View Post
75 and sunny in Merica today! PSA. Don't leave your SPF 100 sunscreen in your work truck over the winter in sub zero conditions and actually expect it to work the next spring.
55 and rainy in NY but I think we are scheduled for mid 70's and sunny tomorrow ,alas not an autox day just a back to work day. sigh


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post #10 of 37 (permalink) Old 05-05-2019, 10:23 PM
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Isn't a nitrogen fill a lot less temperature dependent? I don't autocross so just askin'.

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post #11 of 37 (permalink) Old 05-05-2019, 10:24 PM Thread Starter
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I had no visible scrubbing on the sidewalls but the last 2 runs were pathetic. I was not alone in my complaining about the track loosing it. I will definitely try the chalk on the sidewalls. This is a great place to learn stuff!

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post #12 of 37 (permalink) Old 05-05-2019, 10:39 PM
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Quote:
Originally Posted by Nailbender View Post
I had no visible scrubbing on the sidewalls but the last 2 runs were pathetic. I was not alone in my complaining about the track loosing it. I will definitely try the chalk on the sidewalls. This is a great place to learn stuff!


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post #13 of 37 (permalink) Old 05-06-2019, 06:23 AM
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I assume this applies to open track also?
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post #14 of 37 (permalink) Old 05-06-2019, 06:49 AM
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Isn't a nitrogen fill a lot less temperature dependent? I don't autocross so just askin'.
Not significantly, the important part is to have dry air. Moisture inside the tire will tend to cause a larger increase in pressure. I'm not sure it matters in Autocross, since you can adjust pressures before each run. I wouldn't think pressures build that much during a run ??
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post #15 of 37 (permalink) Old 05-11-2019, 09:38 PM
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Originally Posted by myfirstcar66 View Post
Isn't a nitrogen fill a lot less temperature dependent? I don't autocross so just askin'.
Air is 78% Nitrogen. But it was a good scam for tire shops to sell something you dont need.
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