I'm wondering what's a good method to find the right tire pressures.

By the right pressure I mean the tire pressures which will result in the lowest time when the car is driven perfectly.

The theory I've subscribed to up till now is looking at tire temperatures. If you measure the temperatures across the tread (usually outside, middle, inside) a good pressure will result in a even temperatures across the tread, or at least a steady change in temperature. That worked pretty well for finding street and track pressures. The pressures were also validated by wear data. Another theory is that the right tire pressure will result in the most even wear across the tire.

Now I'm doing autocross that method doesn't seem to be working too well for me. I started off using the same pressures as I used on the track, these work pretty well, the temperatures look good, and the wear data looks reasonable. Though the shoulder was wearing a bit faster than I'd like.

Then we were at an autocross school and I was worried about wearing the shoulder completely off the tires, so I pumped up the fronts 5psi, and expected to mess up the handling. I was expecting understeer. What I did get was a better handling car, with a hint of oversteer. I Was surprised, but experimented at our last event. I started with the fronts at +5psi and let out the extra pressure after a few runs. I think it was faster with the higher pressures. Part of the problem here is the car is not being driven perfectly and its difficult to distinguish between the car's contribution and the driver's contribution.

If it makes any difference we're currently using Bridgestone RE-11 tires in 205/45-16, short for better gearing. I did notice that they're quite insensitive to pressure, the other day I got a puncture in one of the fronts, so it was down about 20 psi from the other one. Even with that difference in pressure I didn't notice any handling weirdness, and the car didn't pull towards the flat tire as I'd expect.

 

Testing. try to be consistent. get to a practice day. make 3 runs back to back (in a 12 min window) then cool tires with water to ambient and repeat as needed.

I like higher pressures than everyone I've met in a mini. I run pressures up till the tire isn't abused on the edge then stay there. This results in 46-52F pressures and the rear is just for changing rotation...so between 40-60 there.

In stock I was in the 50-60F range.

 

Start at the middle of the front bumper. Looking down, not up, go clockwise around the car.
The first two tires you come to will have the right tire pressures.
The next two will have the left tire pressures.
If you have a regular Cooper, ignore the spare in the above discussion.

 

My theory about tire temperatures may not be so far off after all. In order to get better data, I got a IR memory pyrometer. That allows me to quickly get tire temperature data, and then analyse it later. Just driving on the street, that seemed to tell me that the front pressures were way low, the tires were basically riding on their shoulder.

I pumped up the tires a bit and things looked a bit better, but still way off. I pumped them up a bit more, and a bit more. I started at 35psi, tried 40, 45 and now 48. I'm a little weary of going much higher, the rated max pressure for the tires are 50psi.

Now I'm wondering why the pressures were so way off, and how I managed to not notice. At the start of the season I changed from the old track tires in 205/55-16 to shortend 205/45-16. I didn't expect the change in profile to make any difference to the pressures, so I used the same pressures I'd used for the taller tire. I'm not sure if I measured the temperatures for the new tires, or just assumed they'd be the same. Obviously, I'm surprised by the difference, I'm wondering if its because the lower profile tires are XL load rated.

Now I've got an idea of what the right pressures might be, I'm going to experiment with them at the next event (which is Saturday). I'll probably start at 45psi and then drop down to 40 for the last run and see what happens.

After I get the front pressures right, I'll start on the rears. I've been concentrating on the fronts, a quick look at the rears suggest they're pretty good, maybe slightly high.

Here's a couple of profiles I took from driving on the street. The outside points are not interesting, they're from the very corner of the tire. I expect those to be more interesting with an autocross. I'll be measuring the temps after each run.

 

your data makes no sense to me since the only label is on the pressure colors...1-17 means what? the 100's #s means degrees I assume...


Max pressure is irrelevant in autocross. If you are making decisions purely off analytical stuff then you'll never get it right IMO...the stopwatch is what tells the truth.


What you need to decide is what pressure does what with a fast consistent driver. Lower pressures MIGHT be faster (upper 30s) or the higher ones may be (upper 40s) or one may wear better but be slightly slower. That's the data that matters. What's fast or slow.

Do you have camber plates? If not then the outside edge will always be much hotter. If it's not you're going way too slow.

 

They're temperatures across the tire from left to right. The points across the tire are somewhat arbitrary, but consistent. 1 is outside of the left tire, 8 is inside of the let tire, 2-7 are in between. Similarly for the right tire (10-17). The vertical axis is indeed degrees F.

How so? If the tire manufacturer says don't exceed that pressure, I'll listen to them.

I'm using the analytical stuff to give me ideas of which way to go. Then I've been doing back to back runs with different pressures to see if it makes a measurable difference. As I said, the car is not being driven perfectly, so its difficult to separate the difference the pressures make from the driver's random contribution.

What do I do if I don't have access to such a driver?

No camber plates, this a stock class car. Its not surprising I'm going slow, those temperatures are from driving on the street (as noted).

After autocross they look like these.

45, dropping to 40 for run 4. 2 drivers, strongs sun on the right. Felt much better at 45 psi:



48 dropping to 45 for run 5, only one driver, cold ambient temperature. Not much difference in feel:

 

Testing different pressures is definitely key. In a FWD the general rule is to start at 5psi over what the manufacturer recommends. After that I would I would lower until you start hitting the wear marks (the little triangles on the shoulder), usually just a couple psi, and then increase or decrease a couple psi to your liking. Check wear at every event, pressures may also need to be changed due to surface conditions and weather, so it can be an ever changing thing :/

 

5psi over the wear marks is going to be way too low on a stock camber mini. Start at 50psi front and 50 rear and drop the front till you get close to the triangles. Then raise the rear till it becomes balanced.

Most fast minis I've seen on Hoosiers run between 40 and 50 in the front depending on the driver with 16" wheels

 

Agreed, I forgot stock Minis don't allow for much camber change.

 

I try to get even temperature across the entire tire. If the center is warmer I drop air. I hear all about high pressure and maybe that works for autocross but I have not seen it at the track. Running stickies I start about 24Lbs cold and my conti's wet/street tires I start out at 28lbs and adjust as needed. The will heat up to the mid 30's. The MINI being light weight I find it hard to get the race tires to a good operating temperature in the cool PNW. On a hot day here Mid -80's they only get to the ideal temp even when being punished. Also 2 full laps at 70% to make sure they are warmed up before dropping the hammer.

Ran into a high pressure MINI fan at my last track day. After 2 sessions he was complaining his tires were bad etc. He was up in the 40's and his centers were hot. I think we dropped 8lbs and he said it was like a new car the next session. Hard and greasy is not fast.

 

As was mentioned above start alittle high on pressure and check your tires shoulder wear marks. Little triangles in 4 places around the tire. The point of the triangle is what the manufacture says is the starting point of the shoulder for said tire. Keep in mind different driving styles will mean different pressures to get to the shoulder point.( along with tire brands, suspension setups etc.. ) Hence the faster your driving the more force will be on the tire in the corners so more pressure will need to be added compared to a slower driver..

Island maser, as I'm sure you know at an Auto X event you don't get the chance to do a couple of runs at 70% to warm up the tires. You can ride around the lot maybe before getting inline for your run but that's about it.. The carts use tire warmers.. Tire pressures for the track vs auto-x just aren't the same..
Our runs are only 40-50 seconds each and 1 at a time with anywhere between 10-20 minutes in between..

 

Makes total sense. I never ran SOLO or ever really thought about it is all done on cold tires, lol. Its over before they are even warm. I can see how the shift from SOLO to track would be a totally different experience. I watched a 1200LB Lotus track car spin in front of me at what would be considered warm up speed. I was following behind and it was like he was on ice! Tires were cold as he had just pulled it out of the trailer 20 minutes before. We chatted afterwards and he was totally surprised too!
It was a warm day too and dry. Cold tires. Same reason guys get in trouble on the street with high HP cars.

I check the shoulders also and like to mark the tires with chaulk or a yellow lumber grease pencil(Best). Makes it easier to see exactly how far your rolled the tire over on that particular session. Remember to remark after each run. Where is the pit crew anyways?

 

I hear yeah. Pit crew would be nice. Just get in and drive!!!!!

 

Not necessarily, the tires warm up during the event, depending on a multitude of factors. If you see the tire temp graphs above the temperature can stay up from run to run. On the track the tires will heat up to about 160F. They seem to like 130-140 best.