Help with tire pressure

[robsontuning]

Hey guys, we will be running this Sunday in the Solo2 SCCA autocross event. I will be running my new tires and wheels. I need a suggestion of tire pressure to at least start with, and then dial down from there.

 

My setup is OZ Ultraleggera 17x8 with Kumho MX 245/40/17

 

Any help would be great.

 

It kind of sucks, it took 3 autocrosses to finally get the re92s dialed in and now I am starting over again.

[Nutter]

Hey guys, we will be running this Sunday in the Solo2 SCCA autocross event. I will be running my new tires and wheels. I need a suggestion of tire pressure to at least start with, and then dial down from there.

 

My setup is OZ Ultraleggera 17x8 with Kumho MX 245/40/17

 

Any help would be great.

 

It kind of sucks, it took 3 autocrosses to finally get the re92s dialed in and now I am starting over again.

 

I used the exact same tire all last season on my old WRX. I got them new shaved from TireRack to 6/32. Fronts I used to run 40-44psi some times up to 46psi depending on the surface. Rears 48-52psi and up to 54psi depending on how well the car rotated. The high rear pressure made the car tail happy but once used to it, it was the faster way to go.

 

I would recomend useing chalk on the fronts, marking the outside edge of the tread and down the side wall about an inch or 2. This way you can see how much the tire rolls over by how much chalk gets rubbed off. This is a great help in setting tire pressures, you want the chalk rubbed off just to the edge of the shoulder tread and no further.

 

John

[robsontuning]

thanks for the help

[EL PAALO]

my setup is certainly different, but with the 17x7.5 sti wheels and 225/45 17 azenis 615, I'm dialed in at 38 front and 37 rear for just the correct amount of rotation ........ going with the same pressures front and rear gave too much rear rotation and higher pressures seemed to lessen all around grip.

I remove my spare, tools, etc when I run, but am likely still a bit heavier in back than the sedan I'd guess.

[Xenonk]

how's your shoulders look? You can try some tire chalk next time around and just make a mark at the shoulders to see if you are rolling over. Do you have a pyrometer (probe type)?

[Xenonk]

my setup is certainly different, but with the 17x7.5 sti wheels and 225/45 17 azenis 615, I'm dialed in at 38 front and 37 rear for just the correct amount of rotation ........ going with the same pressures front and rear gave too much rear rotation and higher pressures seemed to lessen all around grip.

I remove my spare, tools, etc when I run, but am likely still a bit heavier in back than the sedan I'd guess.

 

I use less for my car with those same tires.. 35 front/33 rear.. and depending on the course, I may drop the rear another 1 or 2 psi, while topping off the fronts to no more than 37. Wet conditions is a whole other story and I run much MUCH lower.

 

You just want to put in enough pressures to prevent rollover.. there's really no sense of inflating to the point where you start to lose contact patch.

[NewScooby]

I ran 16 runs in a Autocross school several weeks ago with 225/40/18 Kumho MX's on 18X8 OZ Superleggera's. At the end of two days I was happy with 42 PSI in the front and 39 in the rear. My car is stock except for wheels and ION springs.

 

I was getting rotation in the rear when desired, even with 3 people in the car.

 

There was a 300WHP EVO with Toyo all season tires with the same times as me all day!!!! You'll be amazed with the improvement over all season tires

[EL PAALO]

I use less for my car with those same tires.. 35 front/33 rear.. and depending on the course, I may drop the rear another 1 or 2 psi, while topping off the fronts to no more than 37. Wet conditions is a whole other story and I run much MUCH lower.

 

You just want to put in enough pressures to prevent rollover.. there's really no sense of inflating to the point where you start to lose contact patch.

 

what kind of temps are you seeing outside? and do you have any weight reduction?

my times were slower when I dropped.

[Xenonk]

my temps were around the 120F to 140F across the tire with a friends IR pyrometer (not effective since the brakes are generating heat as well, but it's a good indication worth something). The ambient temps have been around the 60F to 75F.

 

My car weighs in a 3425ish with a quarter tank and me in the car.. so I havent done too much of a weight reduction at all.. most is probably dropping out 100 lbs total (all the stuff in the trunk such as the spare, swapping to coilovers, swapping out the rotors up front, and swapping out the rims and tires).

 

I usually have to bleed down to get better times, especially when the tires start to balloon up after the first 2 runs.. one event, I gained over 11 psi in the front.. I always check my pressures right after each run to see where they are at and how harsh the course layout is. I want to be as consistent as possible.

[EL PAALO]

yes, I have to bleed down after every run. that's partially because I typically double enter and often have a codriver that's double entered as well so 16 runs in a single heat isn't unheard of.

 

I haven't gotten a good weight without junk in the car ....... about 3425 with the spare and misc tools and random stuff ..... like a carseat and without me in it.

[Xenonk]

did you get the chance to corner weigh your car just to see where the numbers are? Best thing to do is to move that battery to the trunk or at least replace it with a smaller one that weigh like 9 lbs instead of the 35 lbs anvil that's in there now.

 

My front left corner is now just reading under 1000 lbs for that corner, which I am happy about (it actually comes down to having less tire wear overall).

[BRP800]

Opinion.: Take it or leave it. Far too much effort is put into getting tires that are a 1/2" wider, or rank higher on TireRack, or running different tire pressures, or bigger sway bars or coil-overs or a million other modifications. Luckily the SCCA Stock classes are bound by street stock class modification limits.

 

You will eventally learn that the driver is the biggest, by far, factor. This means the fastest driver in one class can borrow someone's elses car (or double enter), and also take that classes fastest. Pure

technique and experience. The same driving principles apply to all classes.

 

For 20 years I always had to have the absolute best tires, fastest car in the class, and read everything I could get my hands on. It was virtually a full time hobby if there is such a thing.

 

Your course times will increase with each successive run. Typically the last run is your fastest. This will true whether you fool around with tire pressures or not. Just getting more familiar with the course is the biggest factor.

 

The National run-offs have two different courses, or at least used to. Always had problems because the courses were long and time was short and I never walked the course the night before..usually because I couldn't leave work soon enough and it was a 500 mile drive to Kansas.

 

When you take a turn and see a field of pylons ahead and you're not sure which way to go, you're dead. All the earliest running cars had no tire scuffs around the corners as the the later runs do... just a penneys worth of stuff learned the long way.