I've just switched from 17-inch rims down to 15-inch rims with 195-55 Dunlop Sport Z1 Star Spec rubber and would like suggestions on tire pressure for a Track Day.

Around town I've been running 34 psi but would like suggestions from others running 15 inch rims on the track. My experience has been to start with a slightly lower cold temperature pressure for track days as the tires will really heat up providing the higher pressure. I'm just not sure how low to go with 15 inch rims.

Suggestions are greatly appreciated.

I'd start with 39 F 42 R cold, and adjust from there!

You are probably better off starting with your street pressures, since that is what you are most used to....your pressure is only going to rise with increasing temps...39 and 42 sound a bit high as a cold setting.

I run 17" rims so it may be a bit different than 15". I generally start with 32 psi front and back cold. I am looking for about 42-45 psi hot.

If you want to rotate the car, you can have differential pressures between the front and back.

Forgot to mention, use chalk to mark the sidewalls to see how far the are rolling over. Also use a pyrometer to read the temperature differences over the width of the tires.

Its going to depend on the ambinet air temp, track temp, but this time of year, I go out at 26F/30R and come in 40-41 front 36-39 rear. More pressure than that and the car feels greasy.

So I am letting air out from what I drive around normally, and don't push until after a warm up lap.

I started with street pressures cold and adjusted down to 40f/40r hot. Seemed to work ok.

Thanks for the replys. I think I'll drop the front pressures 2 pounds or so, leave the rears where they are and adjust as needed during the day.

I'm in Houston and the expected air temp is upper 80's for this coming Saturday.

It's interesting that Alex @ Tire Rack recommended a much higher cold temp than anyone seems to be comfortable with, even slinger688, who I defer to on almost every track-tech question.

At a HPDE I asked an instructor the very same question and his answer was to get as close as you can get to the tire's maximum pressure (when hot). At another HPDE I got a very similar answer.

I'm not sure what the "correct" answer is, or if there is one, but I've been trying to up the pressure a little at every HPDE and have been very satisfied with the performance. Last week at Lime Rock I started at about 36 with my 16" Dunlop Direzzas. They heated up to about 46. The max pressure on the sidewall is listed at 51.

I hope others weigh in with their expertise and experience.

.mmm

I'd have expected a pressure rise of 3-4 lbs from cold to hot, with slightly more increase at the front axle.

I start at 38/44, and bleed after each run for autocross. For HPDE, given that one cannot adjust after a lap or two, I'd start lower (35/42) to reach the target pressures.

I do run 15" rims with Star Specs (the 205).

So I can see where Alex is coming from.

I assume you are running stock camber? If so then chalking would be a good idea.

Every year when I get a new set (RS3, R1R, Star Spec for the last 3) I have found a safe area and tested for max lateral acceleration (GTech) by varying the front pressures while holding the rear at a constant 40 psi. Once I've found the best front pressure I then vary the rear to optimize the balance of the car, going higher to gain rotation or lower to avoid it.

Trying for optimum balance using this approach at relatively low speeds (e.g. 40 mph or so) can however lead to a solution that is a handful at track speed so I run less rear pressure (44 not 48) at the track.

FWIW,

Charlie

Interesting. I may have to rethink my initial thoughts on lowering the pressures from street to track use. I had always been told that autocross guys raise the pressures as they aren’t out long enough to gain heat and track guys lower pressures since they are out long enough to build up heat and raise pressures.

Can someone either provide a link or explanation as to how the chalk marking works. I understand I’m trying to see how much the tire is rolling over, but where do I want it to stop? Also, do I mark the entire sidewall or just a few simple vertical marks?

My Mini should be a totally new car compared to the last time I was at the track. I’ve gone from bone stock with all season tires to : 15% pulley, CAI, Exhaust, Rear Sway Bar, Slotted Rotors, Hawk HPS pads, S.S. Brake Lines, Team Dynamics Rims and Star Spec rubber.

Using a probe style pyrometer, and adjusting to get even tire temps across the face of the tire would be the most accurate and effective way. You need a buddy to take the temps, the time you'd take to get out and probe them yourself will loose valueable temp data.

Once your dialed in after a gang of runs, park the car. The next morning take your pressure cold, and you will have the sweet spot exactrly where your cold pressure should be to achive the handeling you dialed in the day before.


Alex

I'll probably have to do that next time, I doubt I'll find a pyrometer in time for this saturday - but that sounds like a great plan.

Thanks!

You can still do the chalk markings to see how far the tire is rolling over.

The pyrometer readings are not so easy to interpret.

At my first HPDE (high in the mid 80's) I ran really close to the max pressure on the stock 175/65R15s on my MC. 49psi front and 46psi rear HOT. This was due to trying to keep the shoulders from rolling too much.
I lost my notes on the cold temp from the following morning, but IIRC they were around 39-40psi