Tire Pressures

[Pantera1889]

Can someone recommend a starting point for me on tire pressures?

I am hitting the track for the first time with the new car and am interested in hearing opinions from people who have a great deal of experience with similar cars.

The car is a SN95 Mustang and the tires are Toyo RA1s and R888s.

 

Thanks in advance!

 

Ron

[stevepoe]

RA1s and R888s

 

which one ? ra1's shoot for 40 hot. since you have no data on the track , car or temp then I would start at 32 cold and try and find 40 hot.

[Pantera1889]

Thanks. I will be trying both sets of tires .

 

Ron

[Holster Maker]

On my SN95 w/RA1's (2950 lbs/impound) I have determined that cold starts to be 28 front and 26 rear. This always shows all 3 cross temps the same and has proved to be quick enough (2:02-4's at Thunderhill).

I know everybody else probably goes higher, but it didn't work for my car.

1.5 cents

[Walther]

There are several factors that come into play when determining the correct tire pressure for a race car. Wheel rate, spring rate, alignment settings, track surface, turn banking, driving style, etc. A car with a soft spring/big bar set up will generally need more tire pressure than a car with stiffer springs. While I have never used Toyo tires, the principle remains the same. My personal suggestion would be to start at the lower pressures of 28-29 pounds. (Tire pressures affect the cars wheel rate, and too much pressure can therefore make the car a hand full once the tires come up to temp, IF you haven't optimized your spring rates) Bring the tires up to temperature for a few laps and then "hot pit" and have somebody standing by with a pyrometer. Do not wait until you get back to the paddock. Probe style pyrometers are preferred but more expensive. You can get a infra red Pyrometer fairly cheap. In my opinion, tire temperatures are far more important than tire pressures. You are looking for uniform temps across tread**, and those temps should be within the manufacturers specs. Generally, the temp in the middle of the tread is the best indicator of proper tire pressure, and the inner and outer temps are indicators of alignment setting. Also, record and retain EVERY time you take temps, along with what track your at and what the weather conditions are. It'll pay off down the line. I raced over twenty years on ovals and have been road racing for the past three years. I cannot stress enough; tire temps are critical indicators of the cars performance in the turns. I don't want to sound like a know-it-all jerk, but if it is your first time out in this kind of racing, be cautious on the first few hot laps. Cold tires (with lower pressures) do not inspire a bunch of confidence. New tires with low pressures can be outright dangerous at first, until they are properly heat-cycled. The difference you'll feel in the cars handling and attitude once the tires come up to temperature is remarkable. As Harry Hoge said in Days of Thunder; "Tires is what wins races."

 

** On your SN95, if you are still running the strut style suspension, you will most likely have to run a substantial amount of negative camber with (about) 1000# springs to get the temps even. On my (former) A-Sedan Fox Mustang, I had to run anywhere from 3.5 to 4.25 degrees of neg. camber- depending on the track. If your outside temps are disproportionately high, you are not running enough neg. camber. I had to modify the strut mounts to get that much neg. camber.

 

Hope this helps,

 

Mark

[swhiteh3]

A lot of what Mark says is absolutely true, but I'll add one thing. IR Pyrometers are useless for tire temps. The carcass equalizes too quickly. Tire temps should be probe only. IR pyrometers are good for brake temps, header temps, etc.

 

And his comment about "not waiting to get back to the paddock", well, let me stress that several times over. IT'S CRITICAL.

[Walther]

IR Pyrometers are useless for tire temps. The carcass equalizes too quickly.

 

 

I have often wondered about that. I knew when I finally got a good probe style pyrometer that my archive of tire temps all seemed too low as compared to the new readings. I had a cheap IR from Harbor Freight tools, but it did help to get closer to the starting point for tire pressures. I ended up with the Intercomp Digital Pyrometer with memory.

[Red Tornado]

Way to many factors here as bottom line is take temps with alignment for right starting point.

 

General rule for me was for qualifying RA1's was 26-28 cold weighing about 3,350lbs.

For race would start at 28-30lbs weighing about 3,450 (full load gas)

 

Found higher lbs meant better feel and tires lasted longer although about 3-4tenths slower usually.

 

Again I kept very specific track and temp, etc.. records so wasn't shooting in the dark