Help Me Understand Tire Pressures For Autocross - Page 2 - Vintage Mustang Forums
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post #16 of 38 (permalink) Old 05-13-2019, 10:09 AM
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When I went to pick up my SHO, they had an up charge on the sticker for nitrogen inflation. I told them I didn't want it, and I wasn't going to pay for it, and they could keep the car. They took the up charge off.

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post #17 of 38 (permalink) Old 05-24-2019, 10:37 PM
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I notice big differences in how my car handles during an autocross day as I get them dialed to the scrub arrows. I used to bring an infared gun and have tried a borrowed temp probe after getting the scrub lines right, the tires were evenly heated. I over inflate slightly at home and bring an air tank. Before my first run, I drop the tires to around 38 up front and 30 out back. Then run and dial from there. I use sidewalk chalk. Usually I'm dialed by the third run but check and re-chalk between every run.
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post #18 of 38 (permalink) Old 05-24-2019, 10:44 PM
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Originally Posted by dobrostang View Post
I notice big differences in how my car handles during an autocross day as I get them dialed to the scrub arrows. I used to bring an infared gun and have tried a borrowed temp probe after getting the scrub lines right, the tires were evenly heated. I over inflate slightly at home and bring an air tank. Before my first run, I drop the tires to around 38 up front and 30 out back. Then run and dial from there. I use sidewalk chalk. Usually I'm dialed by the third run but check and re-chalk between every run.
I think I have your chalk BTW


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post #19 of 38 (permalink) Old 05-28-2019, 07:04 AM
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SO to finish this discussion, once you get your tires scrubbling correctly at the arrows then start playing:


TO add Oversteer; Increase front pressure and or decrease rear pressure - vice versa, unless its dramatic, I use 2 lb increments, my personal preference is to get scrubbing all 4 scrubbing right then adjust the rears to dial it in for the conditions of the day and set up for the fastest part of the course e.g. I want to be fast in the fasted part of the course...

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post #20 of 38 (permalink) Old 05-28-2019, 07:17 PM Thread Starter
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Thanks for the advice everyone. I did notice once I got the car on the lift after the last event that the front tires were at one point in the day scrubbing to the arrows, but almost nothing at the rear. I have a potential autocross this Sunday at a county cop shop/ government center. The same one with the horrible pavement where I snapped my axle. I'll be starting in second gear this time if I go.
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post #21 of 38 (permalink) Old 05-28-2019, 09:05 PM
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Thanks for the advice everyone. I did notice once I got the car on the lift after the last event that the front tires were at one point in the day scrubbing to the arrows, but almost nothing at the rear. I have a potential autocross this Sunday at a county cop shop/ government center. The same one with the horrible pavement where I snapped my axle. I'll be starting in second gear this time if I go.
to much rear air


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post #22 of 38 (permalink) Old 05-30-2019, 07:09 PM
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Tires have a pressure range based on weight; first thing to do is find out what the manufacturer suggests is that safe range for your cars weight. Often TireRack has that and if not it exists elsewhere on the net. Start to the low side of what is the recommended pressure in order to allow for pressure increases due to heat and moisture. Make a run and immediately after square all four tires to the middle of what is recommended as the range by the manufacturer. Now you have a starting place. Note that it’s important to check pressure very quickly at the end of the run, heat and thus pressure will dissipate very quickly.

The sidewall marker, the triangle, is a wear indicator pointer, nothing else. Trying to get your tread to roll over to or near the triangle is not how you’ll see the professional teams do it. Lots of people like to try and use wear on the sidewall, that started way back before radial tires and the super strong sidewalls of today. It’s out of date.

Remember, when you start with low pressure, that lower pressure will cause squirm under hard breaking which can be unsettling, so consider that as you start your run, as the pressure comes up that will decrease. After you know to expect it, then it’s no big deal.

I’d suggest after you’ve made some runs and you know what cold pressure will place you in the middle of the tires pressure range, then start to adjust front to back to help balance the cars handling. Harder pressure is akin to changing to higher spring pressure. The front tires will most certainly start from different cold temps to arrive at the same hot temp; if your car has too much understeer or your not pushing hard enough, then the rears may start and end at complementing (same/same) pressures, meaning both sides starting the same and growing to the same pressure. If thats the case you want to change the balance to induce some oversteer, or drive the car with more throttle.

This approach of using the manufacturers pressure range and shooting for the middle to start, then adjusting for balance I’d suggest is far better than looking at a sidewall tread wear pointer and trying to move the wear up and down on the side of a radial tire. Old school is fine when it applies, but rolling over sidewalls is not applicable to a radial high performance tire. How you use the brakes too can have a big influence on how the cars handles, it’s an ever evolving game of data, learning, and skill. Start by requiring the tires to do only one thing at a time, brake, release, turn in, exit and open the wheel, then accelerate hard. Don’t overlap the actions and the tires will perform best, with more skill there are other techniques to learn but start here.

Best of luck.

Darrell George
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Last edited by 66stangFb; 05-30-2019 at 07:38 PM. Reason: Edit to indicate proper name of triangle as per manufacturer
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post #23 of 38 (permalink) Old 05-31-2019, 09:30 AM
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If sidewall rollover marks are not useful why do manufacturers still put them on performance radials? Curious.
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post #24 of 38 (permalink) Old 06-01-2019, 11:30 AM
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If sidewall rollover marks are not useful why do manufacturers still put them on performance radials? Curious.
It’s there not as a roll over indicator, but as a: Pointer to the - Tread Wear Indicator. Performance radials don’t have many places to place a wear bar, at least not typically and at least generally not as obvious as most inexpensive street tires where the wear bar goes the width of the tire.

If you have a High Performance tire like maybe a Toyo (common in NASA and Vintage these days) you can go straight up and over from this pointer to find the tires wear indicator. Seems kind of stupid that such an item has been mandated, but that’s what it is. Search for “triangle tread wear indicator” and you’ll find very little but you will find this:

Triangle marks ( △ ) on the sidewall show the tread groove position of the “tread wear indicators”, the “tread wear indicators” represent 1.6mm of remaining tread depth at which time the tyre should be replaced.

So I guess the idea is, one Wear Indicator gets pointed out and then those with limited background will know what the indicator looks like and where to find it. Someone misunderstood the use of the triangle and it spread, I guess. But: Making Radial tires roll to this pointer is silly, and could be unsafe since someone could have too low a pressure, suffer a serious tire failure and get killed or kill someone else. Use the manufacturers safe pressure range and play within that range, the tire will last longer and perform better.

This really is an example where bad advice could get people hurt.

Darrell George
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post #25 of 38 (permalink) Old 06-01-2019, 02:10 PM
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Modern (since at least the 80s) tires are designed with a very stiff and flat tread section. A few PSI doesn't make a huge difference. I don't do autocross, but I don't think there would be any benefits to using more than 40 or under 30 PSI on modern road tires.
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post #26 of 38 (permalink) Old 06-01-2019, 07:51 PM
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Thumbs Up Size matters, as do PSI

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Originally Posted by cougar70 View Post
Modern (since at least the 80s) tires are designed with a very stiff and flat tread section. A few PSI doesn't make a huge difference. I don't do autocross, but I don't think there would be any benefits to using more than 40 or under 30 PSI on modern road tires.
You are correct that 30 to 40 is generally useful guideline/range, but best to know for sure what the manufacturer suggest for the specified car weight. For example, Toyo R888R is 26 cold to 40 hot for my car, I’ve never started that low and have never been able to compile back to back test data, so I just shoot for the 35-36 area front and play with the rear. I use to run the BFG R1 and tended to like it more towards 40 but I don’t know if I was cheating myself out of better lap times or not, it just seemed right, not much data.

A couple of pounds will add to or take away from lap times, especially as driver skill increases. This article while not real technical does have some useful information. https://www.turnology.com/tech-stori...petition-tire/

And the NASA tech article has some additional info.
https://nasaspeed.news/tech/wheels-t...-spring-rates/

Suspension tuning.
https://speed.academy/how-to-fix-und...40954589843750

There is just tons of reading for those who want more professional opinions, plus the Carol Smith books and the like.

Darrell George
'66 Fastback
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post #27 of 38 (permalink) Old 06-01-2019, 09:21 PM Thread Starter
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Originally Posted by 66stangFb View Post
Tires have a pressure range based on weight; first thing to do is find out what the manufacturer suggests is that safe range for your cars weight. Often TireRack has that and if not it exists elsewhere on the net. Start to the low side of what is the recommended pressure in order to allow for pressure increases due to heat and moisture. Make a run and immediately after square all four tires to the middle of what is recommended as the range by the manufacturer. Now you have a starting place. Note that it’s important to check pressure very quickly at the end of the run, heat and thus pressure will dissipate very quickly.

The sidewall marker, the triangle, is a wear indicator pointer, nothing else. Trying to get your tread to roll over to or near the triangle is not how you’ll see the professional teams do it. Lots of people like to try and use wear on the sidewall, that started way back before radial tires and the super strong sidewalls of today. It’s out of date.

Remember, when you start with low pressure, that lower pressure will cause squirm under hard breaking which can be unsettling, so consider that as you start your run, as the pressure comes up that will decrease. After you know to expect it, then it’s no big deal.

I’d suggest after you’ve made some runs and you know what cold pressure will place you in the middle of the tires pressure range, then start to adjust front to back to help balance the cars handling. Harder pressure is akin to changing to higher spring pressure. The front tires will most certainly start from different cold temps to arrive at the same hot temp; if your car has too much understeer or your not pushing hard enough, then the rears may start and end at complementing (same/same) pressures, meaning both sides starting the same and growing to the same pressure. If thats the case you want to change the balance to induce some oversteer, or drive the car with more throttle.

This approach of using the manufacturers pressure range and shooting for the middle to start, then adjusting for balance I’d suggest is far better than looking at a sidewall tread wear pointer and trying to move the wear up and down on the side of a radial tire. Old school is fine when it applies, but rolling over sidewalls is not applicable to a radial high performance tire. How you use the brakes too can have a big influence on how the cars handles, it’s an ever evolving game of data, learning, and skill. Start by requiring the tires to do only one thing at a time, brake, release, turn in, exit and open the wheel, then accelerate hard. Don’t overlap the actions and the tires will perform best, with more skill there are other techniques to learn but start here.

Best of luck.
So, it's not that I didn't believe you it's just that I didn't believe you on the triangle thing. I went out put the Mustang on the lift to have a closer look at my Bridgestone Potenza re71r tires. Those triangles definitely line up with the tread wear indicators! I also decided to read the sidewall while I was at it and found out the max inflation pressure is 50 psi. I still remember when the max pressure on a car tire was 32 psi. I don't actually know what each corner of my 1965 hardtop weighs and don't have an easy way to find out.

I guess I'll bump the fronts up from 32 to 36 to start at the next event and see what happens.

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post #28 of 38 (permalink) Old 06-02-2019, 11:02 AM
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Originally Posted by Nailbender View Post
So, it's not that I didn't believe you it's just that I didn't believe you on the triangle thing. I went out put the Mustang on the lift to have a closer look at my Bridgestone Potenza re71r tires. Those triangles definitely line up with the tread wear indicators! I also decided to read the sidewall while I was at it and found out the max inflation pressure is 50 psi. I still remember when the max pressure on a car tire was 32 psi. I don't actually know what each corner of my 1965 hardtop weighs and don't have an easy way to find out.

I guess I'll bump the fronts up from 32 to 36 to start at the next event and see what happens.
That 50 is mostly a number to use if your going to drive slowly with a ton (or several) loaded on the tire. I poked around a little but didn’t find anything too meaningful on that tire however there was one site that said 36 was the appropriate max. It appears to be a very capable tire and probably hard’ish given its wear score of 200 and maybe why 36 was suggested. Your car and mine probably come in near each other at say 3200 full wet with driver and cool suit. I’d just suggest that a hot target of 36 is probably in the area, maybe even a little under but I don’t think I’d start at 36 since you’d likely end in the 42-43 area. High pressures cause heat which cause higher pressures. But who knows, only the data will say for sure.

Go get ‘em,

Darrell George
'66 Fastback

Last edited by 66stangFb; 06-02-2019 at 11:07 AM.
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post #29 of 38 (permalink) Old 06-15-2019, 09:09 AM
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Well folks, you heard it here first, on the 15th of June in the year of our Lord 2019, I hereby admit (gulp) I WAS WRONG, and have been for some 30 years. Those triangles are just to point out where the wear bars between the treads are. I just researched this and went out and checked tires in the parking lot and sure enough, the wear bars are remarkably coincidentally aligned with the arrows. I was told 30 years ago, to get my scrub aligned with those arrows and frankly, when I get them right, the car is very well balanced AND the few times I used a tire probe, the temps were consistent across the tire. That said, could have been a particular tire I had happened to scrub correctly at the arrows.



That said, consistent scrub lines do set the car up for a neutral balance and has been a consistent way to balance my car which has more weight up front. Man those tire probes are expensive...
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post #30 of 38 (permalink) Old 06-27-2019, 03:18 PM
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I'm glad 66stangFB cleared up the triangles. This seems to get debated all the time and some folks remain convinced that it points to a scrub line.

That said, I still find that the best way to find a good target pressure is based on tire rollover. As mentioned a modern radial shouldn't be using any sidewall and if you are rolling onto a sidewall then your pressure is too low. I like to find the point where the car is just starting to roll past the outside edge of the tread to set my lower boundary. And then start tuning based on the feel of the car.

I've also found that the current crop of 200 tw tires are less pressure sensitive than Hoosiers and tires of the past. the BFG Rival 1.0 was perhaps the most pressure sensitive of the current crop because it had the softest sidewall. But with the stiffer 1.5 and in the case of RE71's or RT615K+'s and their stiff sidewalls I've found they can tolerate a wide range of pressure (5+ lbs differences) and still provide excellent grip.

I'm not a fan of using temp probes for targeting ideal tire pressures for autocross. I have a nice memory pyrometer but it only comes out at the road course.
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