froggy47

Let's assume 5 or so 20 min sessions.

Ambients will increase thru the day from as low as 40f to as high as 95f.

Your target "perfect" pressures from previous experience are 38f 34r (these are just for example purposes). Tires/alignment/etc. are not in the equation at this time.

Do you:

A) Take your tire pressures at the end (or half way) of the first session and lower them to ideal? Then don't touch a thing thru the second session and predict that they will end up at "perfect" each time.

And so on thru the day.

B) Calculate the pressure gain that happened, say xxxx psi, and use that number to set the cold pressure at the beginning of each session predicting that the gain will be a constant each subsequent session.

C) Explain another way.

Thanks.

crimlwC6

A. Assume 7 psi gain for first session.

UrbanKnight

Set it after 1st session and generally good to go after that. I'll randomly check them, but unless I'm driving like a granny first session, I don't have to touch them after the first session..

froggy47

Thanks guys,

any more?

Bill32

Buy a tire pyrometer (NOT an inferred) and do it the correct way.

Buy the tire temperature sheets from LongAcre or make your own and keep them in a logbook.

Everything else is just guessing.

The only real way to get to your "perfect pressure" is to use nitrogen but using tire temps with regular air will still work for that day. BTW, there is no such thing as "perfect pressure, it varies on track conditions.
You won't have an accurate history to go back to because you don't know the moisture content of the air in your tires.

The best way to evaluate what's going on is to take temps and pressure readings mid-session (preferably in hot pit and done by someone other than the driver) after some good hot laps, adjust the pressure. Then at the end of the session for comparison.

redtopz

What Bill32 said is how you find your ideal pressures and alignment. But assuming you know your desired hot pressures, then you want to basically do option A above. Start the morning at a cold pressure 5-8 psi below your optimal hot pressures. Then come in and bleed them down to the desired hot pressure as quickly as possible. You should never have to add air to the tires after the first session unless you started too low. Sometimes you will have to bleed the tires down in the 2nd session if you didn't get enough heat into them the first session (ie. short session, cold track, etc).

Bill32

5 to 8 is a 3 psi spread Bill.

3 psi could make as much as a 20 degree difference in the center tire temp.

Tire pressure is only a means to adjust tire temps.
A desired hot pressure may not give you the optimal contact patch for changing track conditions.

Again, there is no such thing as an ideal pressure

froggy47

Thanks guys, I use a pyrometer to set alignment mostly, seems like using it to set pressures, well I guess you can, but I like to "read" the tire, how it feels, how it's wearing, etc.

Good stuff.

Thanks!

redtopz

Good stuff Bill! Unfortunately I don't have a crew to help me at the track and it's all I can do to keep my car full of gas and running all weekend. I'm not one of those porsche drivers with an entourage of people holding cold drinks and umbrellas . No time to check tire temps, only pressures. It's not ideal, but it's all some of can do when we are the driver, engineer, crew, all in one.

Bill32

Doing tire temps may be a bit much for HPDE but how it feels and how it wears will ALWAYS be better if the temperatures are correct.

One thing for HPDE, running even temps across the tire (accounting for the camber increase on the inside) will give you better tire wear.
Since tires are your highest cost consumable, it may be a good idea to do it.

Bill32

Yea, I know. Been there, done that. I had to hold my own umbrella to keep the hot sun of me on pre-grid.

What we do when I'm paid to be a crewchief is always pull the car in during qualifying at 5 laps, run the temps and make a pressure adjustment. The driver usually sets a faster lap after doing it.

I can take 12 temp readings in under one minute. My pyrometer displays all 12 readings on one screen.

With how fast you're driving, it's not as important as it is with other drivers.

redtopz

Thanks Bill, you are making me want to dust off my old pyrometer and check tire temps at my next test day! I think many of us get used to running a certain pressure and alignment and forget to check things once in a while. You are doing it the right way.

Bill32

Thanks,

I live by the temps, I really don't care what the hot pressures are.
But I do record all the temps and pressures so when we go back to that track we have a good starting point.
I also record the ambient & track temp.

froggy47

How do you get the actual track temp? Pit surface temp?

Bill32

I used to my tire pyrometer (gently).

Now I use my inferred gun. I found that the gun is within 2 degrees of the probe.

froggy47