Tire pressure rise - FNG question

[LUCKY DAVE]

How much rise do you experienced AXers plan on seeing if your starting tire pressure is correct?
If I knew how much rise was desired, or what running temp I was looking for,(I have a non contact IR gage) I could at least have a target cold pressure.
Last time I started at 32/30, and the fronts got too hot and went off. The rears were marginal.
For sustained lapping on big tracks, I have a pretty good idea what I'm looking for, with a one lap format I'm clueless.
Street tires on a 944t, KSS class.

[4est]

Dave,
If members seem evasive on this question, it's not because they're being "that way". This is a tough thing to determine. Maybe you'll get lucky and find someone who runs the exact same tire (brand/size) and they will hook you up.
I suggest you start wherever you think is right, and have a buddy wait by the cue line with a pressure gauge so he can check your pressure every lap. Then, once you get it where you want, you can just re-measure it after it gets cold and there you are.
A ton of things go into determining this value. How hot is the track surface(sunny/cloudy)? How hot is it outside (ambient air)? How many cars are running the event (interval time)? Did you get to the Q early enough to get parking in the shade? Etc., etc.
My suggestion for getting help is a good one; you get to have current readings, and may make a friend or two...

[LUCKY DAVE]

I'm not looking for an exact number (the hot setup for free) I'm just trying to get a feel for the general value. 6psi? 26psi? 26,000psi?
I figure it's somewhere between 3 and 10, but.........maybe my motorcycle Gp tire experience means nothing here and I'm out in space.
Should the tires be too hot to lay your hand on between runs, with ~5 minutes dwell time in line?
Is that way too hot?
It seemed like it to me, but what do I know?

[ttweed]

I'm not looking for an exact number (the hot setup for free) I'm just trying to get a feel for the general value.

It will depend a lot on how much moisture is in the air in your tires, and how hard you drive them, but I usually see about a 4-5 psi gain in an autox session, running 4 laps over about a 20-minute period. You could start 1-2 lbs. under your ideal pressure and either get out of the car each lap and bleed off the gain, or have someone do it for you, but I just start 4 lbs. low and let them heat up. If you fill them with nitrogen instead of air, the tires will gain much less pressure, but few people go to that trouble.

Should the tires be too hot to lay your hand on between runs, with ~5 minutes dwell time in line?

Only on the hottest days will you be able to get that much heat into the tire running one lap at a time, unless you are sliding the car constantly. There are people who actually cool their tires with water out of a spray bottle between runs on the hottest days, but when the weather is cooler, you are lucky to get them up to ideal operating temp by the 3rd or 4th lap, unless you are running a very soft compound race tire, and/or you are a very agressive driver.

Ideal "pit-lane" temps for a soft, autox compound tire would be in the range of 140-160F, for a road race tire, about 180-200F. For your street tires, who knows? What brand and model are they? Ask the manufacturer. You aren't going to get accurate temps with the infrared, non-contact pyrometers, though, as the surface cools much faster than the core of the tread. You need a probe type to be accurate about it.

HTH, and welcome to the club,
TT

[gulf911]

Tom,
And just when I was considering an IR type... Ok, would the IR type be ok if you were to measure right away? Aren't you mainly looking for temp diffs from inside to outside? Would these variances change as the tire cools? What contact type of pyrometer do you like?

Thanks,
Dan

[Kim Crosser]

GMG Racing was using an Intercomp memory contact pyrometer up at SOW on Monday - it was very fast to use as it recorded three temps per tire automatically, without having to stop and write them down (or try to remember 12 temps). (http://store.eastcoastspeed.com/productdisplay.php?sku=971975&hdwt=31101&loc=101&dealer=no&company_id=10021486&PHPSESSID=0763de81190c008a704bffacd1a69bdf)

Longacre has similar units (http://store.eastcoastspeed.com/productdisplay.php?sku=972425&hdwt=31101&loc=101&dealer=no&company_id=10021486&PHPSESSID=0763de81190c008a704bffacd1a69bdf).

They both automatically compute average tire temps, and the Intercomp unit can also record brake temps.

[ttweed]

Ok, would the IR type be ok if you were to measure right away? Aren't you mainly looking for temp diffs from inside to outside? Would these variances change as the tire cools?

Look at the physics of radiant heat loss, and you will see what the problem is. The surface loses heat much faster than the rest of the tire. The hottest portions radiate the heat and cool much quicker. By the time you get to the pits, a lot has changed from the condition the tires were in on the track.

You can use the IR tool to get some idea of the difference in temps inside to out, but don't depend on the absolute values to be accurate. If you keep in mind that the hot parts have cooled more quickly than the cooler parts, and that the actual interior core temps are higher than your surface readings, it may be helpful, just not very accurate.

What contact type of pyrometer do you like?

What Kim said.

If you could take simultaneous readings with a probe type and the IR type a few times, you might get an idea of the magnitude of the errors and be able to adjust your readings mentally in the future. There is indeed a big cost difference between the two types, but having a recording pyrometer certainly makes things a lot easier and quicker.

TT

[LUCKY DAVE]

Thanks for the input about contact vs non contact temperature measurement. I hadn't really thought about it in those terms, I'd only considered the black body error issues involved with IR type temperature measurement. Tires are black after all.
Back when I worked on GP bike tires, we used probes, but hand held IR tools weren't for sale then.

[LUCKY DAVE]

I think I found the answer to my tire pressure question by myself.
My old dirt tracker driving style and the current chassis setup (the rears are for steering, the fronts just keep the body from scraping on the ground) quickly incinerates the back tires regardless of starting presure.
Note to self: adjust the chassis setup so the car will turn with the brakes on......