This weekend my student will be driving an Evo X with 18x10.5 wheels with 265 width street tires (Kumhos I believe). What tire pressure should I have him start with?

 

28-30 when hot itll go up so play : depends on tires tho

 

I usually do about 34-36psi HOT on my HPDE days.

 

I usually go 32psi hot, then adjust based on wear indicators and chalking. I've never had to lower, but sometimes I end up adding a couple pounds. Works well for R1R's and R6's.

 

It depends on the tires. For R compound, you should find info in the manufacturer's site. For street tires a bit more difficult. Hot temps are usually around 38 psi, but depending on driving, colds can be up to 10 psi less.

 

When making tire pressure adjustments, you really need to know the temps to make educated adjustments. Granted you don't want the tires to roll over onto the sidewalls too far but that is only one part of the equasion.

Get a laser tire temp gauge for like $10-$20 and have some fun keeping track of what works best!

It's plenty accurate to get an inner, middle, and outer tread tire temp +-1 *F quite consistantly. You want more or less even temps across the board although depending on how you drive the car and it's suspension settings that will vary widely.

Low temps in center relative to outside edges means you can add some tire pressure, high temps means drop it down. Mainly as long as they are in between the inside and outside temps you are in the right ballpark. Low temps on inside edge, higher on outside means you could use more negative camber (or less body roll). If Inside temps are tons higher, probably less negative camber is needed, or dial in less toe out if you aren't at zero.

All tires are going to react differently from cold to fully heated track temps, some get greasy when tracked for 20 min while others seem to love being heated assuming you aren't totally overdriving them.

I shoot for 30-34 psi as a good starting point keeping in mind that most street tires want to be around 36-40 psi when hot. Usually higher pressures counter the tires from rolling over but may feel overly responsive and greasy when hot actually reducing traction as the outside edges of the tire pull away from the pavement. Go too low and they might start to feel sloppy as they roll over onto the sidewalls more and you will start to lose some traction again as the center of the tire makes less contact with the pavement. If I drove on the street on a warm day they might be at 34 psi "cold", where as a cold day and not being driven on they might be at 30 psi. On hot days you can see a good 8-10 psi increase from the "cold" temps where as colder days the temps in the tires don't increase as much.

It's really all about getting your tires to read within 10*F on the inside, middle and outside assuming you have your suspension dialed in to match so that the tire is contacting the pavement as evenly as possible when under load. I would say 38 psi is a good target to reach on all four tires when they are considered "hot". Since some tracks are clockwise which puts more heat into driver side tires while others are counterclockwise increasing passenger side tire temps, you have to make your pressure adjustments right after you get off the track when they are still relatively "hot".

Hope you followed all that!

 

Here's what I was told by instructor @ Englishtown NJ HPDE event.

This is from Motortrend:

...get a bottle of white shoe polish and dab about 3 or 4 dots around your tire. there are small arrows on most performance tires right where the tread meets the sidewall. the ideal setting to get the most traction out of your tire is when you get the sidewalls rolling (and wearing off the shoe polish) to that point. more air than that and you aren't using all of your treads surface area, and less air than that and your sidewalls are rolling over too much.

Get a few laps in 1st then check wear the shoe polish is worn off. Adjust tire pressure accordingly.

 

The shoe polish trick is a good one, but the problem tends to be that a few wild corner entries where you over drove the car can screw up the results. It works as a pretty good indicator of how much rollover onto the sidewalls you have but still won't tell you temps at each point across your contact patch if you want to get more exacting or diagnose what your suspension might need to allow you to run optimal pressures all the time.

 

http://www.turnfast.com/tech_handlin...ling_pressure2