Instructor Question

[c4s4pcs]

As a new AX instructor, I have a question about tire pressures. My Porsche competition experience has all been in a 997 with dedicated competition tires. Does anyone have suggestions on starting AX tire pressures for students for different Porsche models, stock cars with street tires?

THanks in advance,

Phil

[pdy]

Phil:

I recommend using the factory pressure settings, then adjust upward depending on tire profile.
* Lower profile (like 30 aspect ratio) go just a couple pounds over (or leave at stock).
* Higher profile (like 50 aspect ratio) maybe 5 psi above, or more if even higher profile.
You can adjust front and/or rear to make minor changes to understeer/oversteer as needed.
This is based on my experience - both as an instructor, and driving quite a number of "stock" cars and tires at AX.

A better way is to mark the tire edges (like with white shoe polish) to see how far they roll over onto the sidewall.
Ideally, you want to run at the lowest pressure that allows the tire to roll over no more than halfway (45° roughly).
Problem is that it takes more times, and students are sometimes not consistent enough to make the this process work.

HTH

[Jad]

Phil,

Opinions vary greatly. My answer would be very different than Pauls and we both have a lot of experience on this. I would want HOT pressures slightly below factory with modern tires. Especially the newer cars, I don't like pressure above 40 psi for max grip. The new car with 44 psi cold recommended will be 50-60 psi hot if driven really hard.

Basically, you are safe going with stock, plus no corrections are required before or after the event. (If they run 40 psi hot, the pressures are too low to protect the rim on the street the next morning. ) New students don't have the skill or feel to appreciate differences in air pressure and the fewer the variables the better to help develop a feel for the car in my opinion.

If you can keep a new student looking ahead, in the general area of the line, with smooth inputs, you did a great job. Worry more about oil than adjusting tire pressures as long as they are safe

Welcome and hold on!

[Kim Crosser]

Concur with the starting pressures. I always recommend running the pressures as stated on the driver's door panel or in the manual as a start point, then adjusting.

One key point is that different tires and different weather conditions often require significantly different tire pressures. At an AX, the tires are going to be cold for the first run of each session, and will cool down between runs. If the weather is cold, you might need more pressure to get and keep more heat in the tires, while if it is 100 degrees in August, you might want lower pressures to avoid overheating.

If you are serious about getting the last hundredths (or maybe tenths) of seconds, you should check your pressures with a contact pyrometer after every lap and ensure you have relatively even heating across the full width of the contact surface (i.e., three points across every tire in the grooves). If the center temp is higher than the outsides, you have too much pressure. If the outer temps are higher than the center, then you have not enough pressure. If you have a temperature "slope" (e.g., high temp on outside, middle temp in center, low temp on inside, or vice-versa), then you may need to correct the wheel alignment.

This all assumes you are already driving smoothly and consistently. Until you get to that point, tire temps/pressures aren't going to make much difference.

One obvious thing to look at. If you find your times get worse lap after lap, and it isn't due to driving mistakes, you may have too much pressure and the tires are slipping more as they heat up and the contact patch is shrinking. You can take out a couple of pounds of pressure and may find your times get more consistent.

For a DE/TT event with continuous lapping, it is especially important not to have too much pressure at the start, as there is no cool-down between laps, and your tires can wind up with a LOT more pressure than at an AX.

[Sawfish]

I drive a pretty light weight car (1840 lbs) and I start about 4-6lbs under desired pressure for an AX. After they heat up I'm at about 30-31psi. Good luck.
Chris

[pdy]

Phil:

As others have diplomaticly pointed out, my info is suspect!

What I said works fine for older cars, like with 50 series or taller tires, and factory settings of 32 psi (I was thinking older cars, as I am an older guy...).
But newer Porsches have much higher stock psi settings, like 44psi (duh - I should know - I've had several!)

As Jad says, one does not want to end up with 50psi (or more), so ideally check after a run, and adjust to stay well under that
.

And it's best to adjust back to factory pressures for the drive home, if possible.

[c4s4pcs]

Thank you very much for all of your input- it was extremely helpful. Paul, I really was looking for information on the older cars, which I am totally unknowledgeable about, and I feel that your recommendations are spot on. Jad, thanks for chiming in - leaving things stock is a really good idea, in general. I like to check with my student about tire pressure, but much more to make sure that it has been checked rather than trying to strive for optimum performance. Having suggested checking pressure, I do feel that I should have some idea as what to suggest for a starting point. At this point, I feel comfortable in suggesting stock pressures for late model cars running low profile tires, with older cars adding up to 5 psi as tires get taller. I did try to use my white shoe polish today, but my 5 year old bottle was not very effective - one more racing expense.

Again, thanks for the feedback - I hope to see you all soon at the track.

Phil

[ttweed]

I did try to use my white shoe polish today, but my 5 year old bottle was not very effective - one more racing expense.

Phil-

In a pinch, you can grab a piece of the drywall we use for chalking cone boxes and mark the tire shoulder. Not the best solution, but it is always available at an AX and it works.

TT