[ZDan]

Originally Posted by Icecreamtruk

I dont want to argue or anything, it seems like you want some honest answers to some of the questions you raised after my post, so here they are.

Excellent, I don't consider this an argument as much as a discussion I have an open mind and I know I don't have all the answers.

For cold pressure, depending on day or track I go out at 22 to 24psi. I dont drive like I stole it on my first session, Im aware of my tires being underflated, and even if somehow I wasnt, I can feel the tire distorting under braking and turning. I rarely get the pressure I want in one session, it takes 2 or 3 sessions doing small adjustments to get it right. I also dont set equal presures on all corners, more on that in a second.

FWIW:
I go out with ~27psi first session (sometimes as low as 24 on the corner that gets worked hardest), and I drive it hard from lap 1. Even when I didn't plan to! Bleed pressures down to 36 all around after 1st session, repeat as required throughout the day.

Lower pressure = more heat. I've heard this a number of times, but I wonder if its actually true, because the pyro says completely the opposite. Higher pressures = more heat, lower pressures end up with less heat on the tire. I imagine it has to do with people thinking the rubber moving more with lower pressures or the bigger contact patch makes it heat up more, but thats not what the pyro says to me.

There is a difference between tread heating and carcass heating. Lower pressures will work the carcass of the tire more and generate heat from hysteresis. Smaller contact patch from high pressures at high slip angles may give more short-term transient localized heat at the tread, some of which heats the carcass but some (most?) of which is quickly convected and radiated away.

Im not running 2-3 laps at a time, I run 20 minutes sessions. But at the same time, I run time trials, so only the fastest lap count, the rest I use it as practice if I know I have tires to spare and that Im not destroying them by overheating them. On a street tire, the fastest lap is almost always the first lap out of the pits (first full lap after the outlap). When I say it takes 2-3 laps to get the ideal pressure, what I mean is that 2-3 laps in will be my fastest lap on that tire, meaning usually that pressures were too low and I was having to deal with the tire rolling over, or not heated up enough yet (im in canada, track days can be cold sometimes).

Are you able to monitor pressures while on track? I'd always assumed that tires got up to pressure relatively quickly, within a couple or 3 hot laps. But in the Cayman where real-time tire pressures can be displayed on the dash I found that I would only gain ~1.5 ish psi per lap at Watkins Glen a couple of weeks ago. Pretty much crept up the whole session.

I think that what happens is that the tread does get up to temp very quickly, but that pressure, which is related to *carcass* temperature and wheel temperature, increases at a much slower rate. It takes some time for the mass of air in the tire to heat up.

Worth noting that pressures can continue to rise on the cooldown lap and even in the pits for short time as heat from the hubs and brakes continues to go into the wheels and the air in the tire, unlike tread temperature which falls off quickly.

Regarding pressure changes, 1-2 PSI dont change my car into a mustang, but might be enough to gain or drop half a second.

Half a second is huge. Obviously you don't want to be there in your critical 1st laps. So again, better to err on the high side. If pressures go "too high" after 2-3 laps, for your purposes, so what?

But given that track and weather conditions are changing throughout the day, and repeatability variations, how do you *know* that half second is due to pressure and nothing else?

Half a second can be the difference between first and sixth place in a time trial, so to me, it is drastic. If you go under the point where the tire starts rolling over, it will handle quite different, taking more time to settle up during turns, and getting easily upset by bumps and curbs (and times suffer much more).

If 1st lap is the opportunity for your absolute fastest lap, you'd like to be right at optimal pressure for *that* lap, which again implies highish starting pressure given that you aren't likely getting to max pressure for several laps.

Thats all I got, more or less, if you have more questions, shoot away. I know this all sounds very subjective and non scientific way but the stopwatch doesnt lie, even if a driver isnt Senna, as long as he can drive to his limits every time out, the stopwatch will tell you if the change was good, or bad. Its then up to the driver/team to either record information on what changed and the effect it had, or chuck it all up to external factors and decide it did nothing or little.

For most of us who track regularly, there's just too much going on to be able to say that 1psi changed lap time by a few tenths to a half a second. Not to mention that for most of us, all else exactly the same, there's going to be at least 2-3 tenths variablity due to driver anyway. Obviously it can be a lot more than that depending on driver!

Edit: forgot to add, this all works very well for a car that is track ready, at least suspension and alignment wise. Trying to optimise pressures too much when you have too little camber is just going to end up killing the tires.

I kinda think optimizing pressures will somewhat automatically preserve tires a bit. Optimal pressure for a street car not 100% set up for the track will likely be a bit higher vs. dedicated track car.

I imagine that somebody looking to extract the absolute maximum of their tires will have taken the time to get an alignment dialed up to maximise tire performance, otherwise, yeah, "36 hot and your good" is as good as anything else.

I daresay it's better than 28-30psi hot! Speaking for my situation, I have -3/-2.5 camber and slightly stiffer springs, but mods are minimal for classification purposes for time trials. I would say that for my car and for a lot of street/track cars, "34-36psi hot" is a good approach.
But based on my observations at the Glen I may revise my approach to cold pressures for the 3-lap time-trial...