I am looking for input from experienced MINI autocrossers (or simply front wheel drive autocrossers). I watched an excellent video that was likely made in the 80s. I am assuming this all applies to the Mini as well?


I understand that its good to run a higher pressure in the front for a front drive car but no really sure how much with a lower profile tire like 205/45/17. I also realize that this question can't really be answered without physically trying out a pressure and measuring tread temps with a pyrometer but I am looking for a good starting point. Autocross events offer such a small amount of run time that starting in the dark and using them for testing is tuff.

I am running Dunlop ZII tires.

 

As in all cases of performance driving, you want the tire pressure that will allow you to use the most tire footprint along the course you drive.

As a result how you drive plays a major role in how you heat up your tires, but so does road surface, environment at the time, your suspension tuning etc.

Therefore there is not one tire pressure that works for all drivers, you can start with one pressure then adjust after the first or second runs to allow you to fine tune the wear and use of your tire.

Here is one approach-

Cold tire pressures- Start with pressures higher than you use on the street. 40 psi front and 36 psi in the rear or higher for both is common. Some of this will depend on which tire you are using (OEM runflat vs non runflat street tire vs R compound tire). Usually front pressure is higher to start. I have seen some OEM runflats at 42-44 psi.

Chalking your tires- before the first run get some sidewalk chalk and mark the edges of your tire tread in about four places on all tires. After your runs the chalk will rub off where you are using the edges, you want to wear out the chalk up to the end of the shoulder of the treads. If the shoulder still shows chalk then let out some air 2-3 psi. If all of the chalk is gone including the part on the sidewall then add some air.

Tire pyrometer- Use a probe style not infrared pyrometer as you want to measure the tire temperature on the inside of the tread under the hot surface. You will rarely see tire temperatures uniform from the inner to middle and outer parts of the tread from autocrossing. Usually one side is hotter than the other or very hot in front and cool in the rear. If you are has stock alignment then usually the front outer treads will be the hottest. If you have more front negative camber you can reduce overusing the outer treads.

Reading the pyrometer- if the temperature is hottest in the middle tread then let some air out 2 psi, if the temperature is hottest on both inner and outer edges then add some air. If the temperature of the middle and one of the edges is the same then leave it alone, depending on what the other temperature reading is.

It is possible you will run different tire pressures on each tire by the third run. Outer edges and front tires get hot the fastest. The higher speed the turn the more the outer edges on the far side tires will heat up.

Cooling of tires- You may see many autocrossers using water spray pumps to cool down tires between runs, usually for R compound tires. They claim that it helps allow for better grip. Most drivers with street tires will be fine. If the tire as it gets hot seems to be less grippy then you can try the water spray.

 

The usual thorough and informative approach from the distinguished Minihune!

I normally see pressure increases of 2-4 psi after the first run, and sometimes will still see further increase of 1-2 psi on the 2nd run.

I'd use the chalk approach to get the front end into the right ballpark, because until you've enough air to keep the tire in a reasonable shape during competition further adjustment is pointless.

Across a variety of tires (Z1, R1R, Hoosier, RS3) I've run 38-42 front, and 42-48 rear. The high pressures out back have nothing to do with maintaining the tire shapre or avoiding roll-over however, and everything to do with deliberately making the car slightly tail-happy by over inflating the rears to reduce the contact patch out back.

As always, your mileage may vary as Minihune wisely observes - there are many variables that matter including rim width to section width, tire brand, and even driving style.

Cheers,

Charlie

 

We had to run 45 psi in the front on the stock suspension to keep the tires from rolling over. They were the original Z1 star specs. Definitely less than ideal but you do what you gotta do to keep the tire from rolling over.

Now we're on the Z2s and coilovers. With -2.5 degrees of camber we run 38psi front and pyrometer measurements show even temps across the tire. We normally run 41psi in the rear although that varies depending on the course design.

 

49 front, 44-45 rear. Sport suspension, 16" Star Specs. Very sensitive to heat and need double cooling on a hot day.