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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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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 It’s out of date.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.
 

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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.
 

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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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Size matters, as do PSI

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-stor...sted-toyos-proxes-r888r-dot-competition-tire/

And the NASA tech article has some additional info.
https://nasaspeed.news/tech/wheels-...ly-understanding-tire-sizes-and-spring-rates/

Suspension tuning.
https://speed.academy/how-to-fix-un...ing_wp_cron=1559432108.9348731040954589843750

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

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Discussion Starter · #27 ·
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.:wink: 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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So, it's not that I didn't believe you it's just that I didn't believe you on the triangle thing.:wink: 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,
 

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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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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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Discussion Starter · #31 · (Edited)
The local SSCA chapter had a autocross school today limited to 20 students. I had 2 very good instructors was able to do 16 runs. My car seems to be happy with 40psi front and 35psi rear. That seems really high at both ends but I was happy with the way it rotated with the higher pressures in the back. They are having a test and tune session tomorrow! Had a blast! One of the first time students was a 75 year old man with a Focus ST!
 
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Sorry, late to the party, I also run re71's on my 66 (235/40x17) and it may be an ambient air temp issue. But I never run above 32psi. I typically start at 30 in the front and 28 in the rear, check temps after the first run and settle at 32 and 28 for the rest of the day. I found when I went up to 34 or 36 the tires would get real slick and loose in the heat. Typical day is 85+ outside, course is 90+ on the pavement which may be why I run so low. We have a couple of drivers who own tire shops and they said the re71's don't like heat, if the get hot, drop your pressure.
 

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Discussion Starter · #33 ·
Sorry, late to the party, I also run re71's on my 66 (235/40x17) and it may be an ambient air temp issue. But I never run above 32psi. I typically start at 30 in the front and 28 in the rear, check temps after the first run and settle at 32 and 28 for the rest of the day. I found when I went up to 34 or 36 the tires would get real slick and loose in the heat. Typical day is 85+ outside, course is 90+ on the pavement which may be why I run so low. We have a couple of drivers who own tire shops and they said the re71's don't like heat, if the get hot, drop your pressure.
Thanks for bringing your first hand experience to the thread. 32 and 28 are exactly were I started and thought the car should be but it seemed like the tires were rolling over. Not just my opinion but drivers with much more experience than me. Obviously every suspension setup is different as are driving styles. I'm running the SoT "sport" front coilover system with CSRP 1.4" drop spindles and I did the alignment myself so I have no real idea were my car really is setup wise.
 

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My suggestion. If it under 90+ start dropping your pressure, not much. Just a pound or two. Looks like your weather is cooler then out here, I'd run 33 up front, and 30 in the rear and see how it feels. I too run similar, but SOT heim up front, and 5 leaf w/pan hard in the rear. My camber is 2.5 using a camber kit and caster is 4.5+ using the adjustable struts. That camber really helps the tires grip without pushing in the turns yet keeps the tires from overheating cuz they're not pushing and scrubbing.
 

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Discussion Starter · #35 ·
My suggestion. If it under 90+ start dropping your pressure, not much. Just a pound or two. Looks like your weather is cooler then out here, I'd run 33 up front, and 30 in the rear and see how it feels. I too run similar, but SOT heim up front, and 5 leaf w/pan hard in the rear. My camber is 2.5 using a camber kit and caster is 4.5+ using the adjustable struts. That camber really helps the tires grip without pushing in the turns yet keeps the tires from overheating cuz they're not pushing and scrubbing.
I had my first experience on concrete yesterday at the Hoosier Challenge at Grissom AFB in Peru Indiana. 179 entrants so I was limited to 4 runs in quick succession. My first run was at 38/32 and it was miserable with no front grip. I had my friend Nick riding along and he thought something was wrong with the car as he has driven it many times. Checking the tires they were at 43/36 so we went for a big swing down to 33/30 and the second run. Despite the NDF it was a much better run. Pressures went up to 36/32 so we went down to 32/28 for the last 2 runs with pressures going up 2psi after the third run so we backed them down again. Concrete is a totally different animal than the seal coated asphalt parking lots we run locally. An amazing amount of rear traction on concrete VS none on the seal coated asphalt. I'm kinda used to drifting?
Next event on the 25th is a non points charity event for INCOPS, a fallen officer charity, at a county cop shop parking lot. I should get a lot of runs in and am going to do a lot of experimentation with tire pressures along with scaring the @#$% out of some constabulary ride alongs!

groho, thanks for the input!

The second pic was right side up when I uploaded it.:wink:
 

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Discussion Starter · #38 ·
Sorry, late to the party, I also run re71's on my 66 (235/40x17) and it may be an ambient air temp issue. But I never run above 32psi. I typically start at 30 in the front and 28 in the rear, check temps after the first run and settle at 32 and 28 for the rest of the day. I found when I went up to 34 or 36 the tires would get real slick and loose in the heat. Typical day is 85+ outside, course is 90+ on the pavement which may be why I run so low. We have a couple of drivers who own tire shops and they said the re71's don't like heat, if the get hot, drop your pressure.
I've got to say groho you are spot on on the pressures for RE71's. In the warmer temps this summer 30/28 was great. As temps dropped and the tires got hurt I found myself running 28/24 last weekend @ 60º before I finally got a respectable run in with zero roll over.

I killed my tires 2 weeks ago at Tire Racks test track in South Bend. That is the most aggressive asphalt by far that I have ever seen! Take the track test info on their website as being done on 24 grit sandpaper! Being not the sharpest tool in the shed I will be back there Sunday to really finish them off in what was supposed to be South Bend SCCA's first event of the year that was rescheduled because it got snowed out!

Again, thanks groho, I'd still be messing with 42/36 if I had listened to the wisdom of the late model crowd!
 

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Discussion Starter · #39 ·
A year and a half and 3 sets of RE71's later I've got to say the treadwear triangle was spot on for that tire even though it might not be the official way to judge proper pressures. RIP RE71:confused: The new Falken's, if I can find someone to mount them and if "life" lets me autocross again this year, looks like the bottom of the arrow will be the sweet spot. 31 / 29 was good for the Re71's in the cold and 30 / 28 in the heat.

798026


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I had my first experience on concrete yesterday at the Hoosier Challenge at Grissom AFB in Peru Indiana. 179 entrants so I was limited to 4 runs in quick succession. My first run was at 38/32 and it was miserable with no front grip. I had my friend Nick riding along and he thought something was wrong with the car as he has driven it many times. Checking the tires they were at 43/36 so we went for a big swing down to 33/30 and the second run. Despite the NDF it was a much better run. Pressures went up to 36/32 so we went down to 32/28 for the last 2 runs with pressures going up 2psi after the third run so we backed them down again. Concrete is a totally different animal than the seal coated asphalt parking lots we run locally. An amazing amount of rear traction on concrete VS none on the seal coated asphalt. I'm kinda used to drifting?
Next event on the 25th is a non points charity event for INCOPS, a fallen officer charity, at a county cop shop parking lot. I should get a lot of runs in and am going to do a lot of experimentation with tire pressures along with scaring the @#$% out of some constabulary ride alongs!

groho, thanks for the input!

The second pic was right side up when I uploaded it.😉
This reminded me of some additional advice for anyone else reading this thread and trying to learn pressures for autocross. Check your tire pressures after every run! Many do not do this which means every run is a little different. Across your runs, you are either going to start too low or end too high or maybe both. On asphalt my tires usually grow 1-1.5 lbs on the first and second run. On a longer faster course they can grow 2 lbs the first run. On concrete I've seen them grow as much as 3 lbs in a run. In a 4-5 run session that can really add up.
 
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