Establishing Tire Pressure

[e129745]

Haven't seen anything on tire pressures. I suspect tires are part of the suspension package and I have historically trial and error-ed my way to a PSI for a given brand. After some time, wear certainly is something I look at, by then I'm committed to the loss of tire life, and probably performance as well over that time. Tread temps would likely be helpful, esp. on the track, but this is a daily driver to and from work, etc. Would temp. be the best method to determine PSI, even if I'm not pushing the limits? I.e. elevating temperatures? What does one use to meaningfully measure tread temp. with enough resolution to be helpful?
Car is 1968 Olds Cutlass with 15x7 all around. Tire size are P235-60s. I ran BF Radial TAs at 32PSI, but both fnt/rear centers typically wear thinner than both inner/outer edges suggesting too much PSI, after 30 or more years with the same car, I resist going down in pressure cause the sidewalls feel less responsive. Of course, now I'm trying Cooper Cobra Radial GTs for the fist time and sidewall may be stiffer, if that even matters with radials. Fire Hawk SS's were way to soft in the sidewall in my opinion and would not venture back into a set again, although that was at least 16-18 years ago and construction may be different.
Thanks in advance.

[e129745]

No comments? What is used to measure tread temps at the track? I'm going to bet a needle type is used to get below the surface. Would've liked to know. Surprised I didn't get something about using my hand as a last resort....lol

[marolf101x]

You won't get much response on 15" tires as everyone competing is on 17's minimum.

[Ron Sutton]

Don't be bothered by the lack of response. Most of us will chime in where we have knowledge, but are hesitant to post something if we don't know.

I'm not sure many of us have set our daily driver tire pressures using a tire pyrometer & probes like race teams use. It will work. Obviously if the center is cooler, you need to add air & if it is hotter, you need to reduce air pressure.

Best wishes.

[e129745]

Thanks guys, I understand, I might lack some confidence, but not bothered or insulted. I thought the hand thing was funny...lol

I think I'd do well with 17s, if I could find late 1960s/early 70s SSII format/style in 17x8. I understand wheels can be made to customer desired sizes, just not sure of the styles available. That would be in the future$.

May I ask, is the larger wheel diameter predominantly to allow clearance for larger disc systems? or are there dynamics that come into play that play a noticeable role in cornering?
Thanks

[Bryce]

May I ask, is the larger wheel diameter predominantly to allow clearance for larger disc systems? or are there dynamics that come into play that play a noticeable role in cornering?
Thanks

THe shorter sidewall increases the lateral stiffness of the tire; therefore, less sidewall flex. This promotes a better cornering car and allows the suspension to work as designed.

[High Plains Mopars]

Since wear is a by product of friction, you can indeed use a pyrometer to set tire psi on street car. Another less technical method is to use chalk, roll the car back and forth, see where the chalk is worn off and reduce/increase psi until you have full contact across the tread face.

However, you've also pointed out a shortcoming of reduced psi on a tall side wall tire and that is flex and reduced response. In your case, you have to decide where the compromise is at with wear vs feel. If you're wearing the centers out, you certainly are running at too high a psi. Also consider as you run the car, the tire generate heats and it adds psi to the combo so if you are slightly above recommendations cold, you are going to be even higher after the evening commute on 105* asphalt at highway speeds, hence, center wear.

Back when you could get 15" tires in 4 or 6 ply sidewall configurations, it was a different issue on comfort and handling than things now. Now most 15" tires these days are used on show poodles and comfort and longevity have lead to 2 ply sidewalls and rock hard rubber combos.

[Ron Sutton]

show poodles

Hahahahaha :rotfl:

I have a new term to add to my repertoire.

[Henesian]

It's all about tire temperature and your effective patch. I know that this little tidbit doesn't apply to you, but theoretically it does. On a drag radial, you lower the tire pressure until you get as much patch as possible after a burnout in the water box at the strip. It's very visible. On a street car, you have to use a pyrometer, because 400 treadwear tires are not going to show a patch like a drag radial will. Bottom line, if you're serious about really trying to dial in the tire pressure on the street tires, looking at wear and using a pyrometer is the best way. Typically, you measure the temperature on the inside, middle, and outside of a tire and adjust it until all 3 temperatures are as close as possible. I had 245 and 255/60/15 Radial TAs on my TA, and I know for a fact that the car handled the best if I had 30-32 psi cold in each tire. There was a significant increase in cornering speed if I ran 30-32 psi cold over 20-30 psi cold.

[e129745]

Thanks guys, that`s what I was looking for. I never heard or thought of the chalk idea before but that strikes me as very clever. I'll try it and compare what I find with temperature. I tried using a simple (cheap) non-contact or Infrared Laser thermometer that seems to differentiate well enough, I guess.

It's a tough thing, but true is the limited choices for the 15s that are likely offered more for looks than performance. 15s just don't perform against the more common sizes of today.
Thanks for the feed back guys.

[High Plains Mopars]

There are still a couple of fair 15" tires for street performance. They are the Mickey Thompson S/R radial and the Maxxis Marauder MA-S1. Both are modern carcass designs with H speed ratings. The MT does not carry a UTQG rating as MT says it is a specialty tire. They are claiming 15k worth of mileage out of them, which would mean they are somewhat soft. The MA-S1 carries a UTQC rating of 360, which is a bit softer than the more popular BFG and Cooper ratings of 400+, but still fairly hard for a performance tire.

For race tire performance out of a 15" d.o.t. rated tire, you have Avon CR5ZZ, which is a V rated tire with UTQG rating of 80. Available in a wide variety of wide sizes with fairly stiff sidewalls, it is very effective but not cheap. Expect to spend around $300 a tire for these.