Learn me: Setting tire pressures at the track.

I'd like to hear how you all go about setting tire pressures at the track for HPDE, not competition. I've been to 17 HPDE days. I have a tire pyrometer and a good pressure gauge. I've been running the Goodyear Eagle F1 Supercar 3 and Nankang CR-S V2 for their good heat tolerance and consistency. The door placard on my Cayman recommends 30/32PSI minimum. From my experience the optimum pressure for these tires is right around 33psi hot. That means I need to start out around 26psi cold to get them in the right range on a 70F day. 

My general procedure is to set the cold pressure before the first session (~27psi). Run a couple laps at 85% to get them up to temp (30psi on the dash) and then go 100% until the end of the session. I generally get passed a bunch on the first couple laps and then start reeling people in for the last 5-6 laps. I come straight in and measure the tire temps and then check pressures. Then I look at the temperature spread across the tires to see if I want to make any changes. I've been dropping my pressures a little each session as the air temperatures rise. Well, this last time out the air temp dropped 10+ degrees from the end of session 4 to the beginning of session 5. The tires wound up at 24psi cold. In session 5 I could feel the tires squirming around and after 3 laps they still weren't coming fully up to pressure. I finally gave up on lap and brought it in. 

So, I have two thoughts:

In hindsight, there's usually 80 minutes between sessions. The temperatures can change quite a bit in that time. I should probably set my pressures just before the session starts instead of just after it ends for better consistency.

Maybe I should shoot for a slightly higher starting PSI to consistently get the tires into their window at the cost of occasionally putting them out of their optimum range towards the end of a session.

I watched Andy's video here:

This can be as easy or as hard as you want to make it.  For HPDE usage there's usually a range of target temps that give similar performance, and it will matter less for an intermediate driver than for a pro who can consistently turn laps within a few 10ths of each other.

I should start by admitting my biases.  I think more expensive/accurate tire gauges are worth it; at a minimum you should be able to zero it for ambient temperature.  I think tire temps are only meaningful if taken with a needle pyrometer immediately after coming in from a hot lap, and only useful if you're willing to change setup parameters like camber.  For most people it's a waste of time, better to manage backwards from lap times.

I think it starts with understanding what the best target hot temperature really is.  Tire pressure is one of the most important variables for lap times, yet almost everyone I know has some arbitrary target that they heard on the internet or from friends with a similar car or tires.  You don't really know what YOU like best until you do a sweep. 

Have a friend with you on the day you want to do this, and make sure you can accurately record your lap times.  Start with cold pressure at the higher end of your range; turn 5-6 hard laps and come in for your friend to air the tires down to the high end of your target hot range in the pits; say 34psi at all corners.  Return to the track, do 5-6 more hot laps, come in and have your friend air down the hot temp another 0.5 psi.  Write some notes about how the car feels at each stop while your friend is airing down the tires.  Rinse and repeat until you're low in the target range, say 32psi.  At the end of your session, compare your lap times and notes from each pressure and determine which one was the fastest.  Don't be surprised if there's a few seconds difference between best and worst.  For my car and driving style, I was faster with a little less pressure in my rear tires than front tires, which I never would have figured out without the notes.

Once you have an accurate target hot pressure, it's a matter of managing your cold temps to arrive there during a session.  Easiest way is to inflate to a known cold pressure near the high end in the morning (say 27psi).  Take 5-6 laps to get the tires hot, come to the pits and air them down to the target hot pressure.  Remember how much air you let out to get there (say 2 psi).  If you subtract the amount of air released from the initial cold pressure, you have an approximation of new cold target (25 psi in this case).  Use this as your target before you start each session.  Get hot pressures after each session and refine your math accordingly.  If you really want to get fancy you can do it separately for each wheel.  This will yield different results at different tracks. Right tire temps will often be different from left tire temps depending on whether the track runs clockwise or counterclockwise.  When I'm racing (spec class on race slicks, needs to be very exact) it's not uncommon for my target cold temps to vary by 2psi or more between different wheels depending on what the track is asking of the car.

This is admittedly more work than most HPDE drivers want to put in, but it really does affect the lap times.  You can also go way further down the rabbit hole than this, but IMO it's unnecessary for HPDE.

For my 911, I shoot for 32 front/34 rear hot.  So I start them ~8psi lower than that.  I usually end up having to bleed some more out at the end of the session, but that's what works for me

I'm not sure if this qualifies as an unpopular opinion but I'm a firm believer that tire pyrometers are one of those things that do more harm than good for all but the most qualified of users.

HPDE with long gaps between sessions complicates things a bit but in general it's much easier to manage hot pressures than cold. Figure out a cold pressure that gets you in the ballpark to start, then manage hot pressures by checking and setting immediately off track while everything is still hot. Let it cool down and (barring major ambient temperature changes) they'll come back to the same temperature next session. If there is a major change than add or remove a psi or two to compensate.

I haven't driven the nankangs but I had a set of goodyears and one of the things I didn't like about them was that they seemed very sensitive to tire pressure changes. The lap times didn't change much but a couple psi difference either way had a pretty big impact on feel and feedback.

Thanks for the replies. 

I'm not using anyone else's target pressures or temperatures. My targets are based on my fastest times. I'm running all the camber I can get. The temperature gradient across the faces of the tires is pretty reasonable, somewhere around 10F across the faces. 

I guess it all boils down to I'm uncomfortable pushing my tires at 24psi cold so they get into their sweet spot when they are hot. It sounds like docwyte is comfortable at 24psi cold. What's your technique to warm the tires up?

Am I overthinking the possibility of damaging the tires or blowing a bead at 24psi?

24 is the minimum I personally go out on with the ND Miata.  And that's with lots of wheel width support and camber.  It's also a fairly light car.  Depending on ambients, it can take a bit to build pressure.

In reply to CAinCA :

Short answer?  Yes.  You're worried about nothing.  You're not going to damage the tires or blow a bead starting at 24 psi and they're not going to be there very long.  Within 1/2 of one hot lap they're most likely at 28psi+, then over 32psi in a hurry.  After my first session I usually come in and have to bleed more out of the tires, so honestly I should be starting them even lower than I am.

Thanks Andy and docwyte.