I have doing AutoX for about 6 months now. I have been running in a local Street Tire CStock class. I got my first upgrade, new tires, on my other wise stock '03 Track model. I put on 255/275 Dunlop Star Spec's on, the handling was a huge improvement over my all-seasons I have been learning with.

We ran a test and tune practice day this last weekend and I was running 38psi front and back to start. I quickly realized that as the tires warmed up, the pressure was rising and understeer got worse. So we went to resetting the tire pressure before each run to 38PSI, which helped a lot. But after lunch, we started running again, and I realized that my first run, when the tires were cool, the car had considerably more grip in the corners and much better turn-in. When we had a few back to back runs, the tires got so hot it felt like the fronts were on butter.

I thought about lowering the pressure more in the front, but we are already rolling over on the fronts to the edge of the tread. How low is too low for pressure on the tires? Do i need to be more pro-active about cooling the tires between runs like a water sprayer? Would a rear tire pressure adjustment help with the front tire temperatures and understeer?

Just a newbie, looking for a little advice and what to try next.

 

Yes get a sprayer, your tires overheat very fast autocrossing. Tire pressure, all depends on the tires. Some will like higher pressures , some like lower pressures. Play around till you find something that works. Also , get camber arms for the front, running -1.5 to -2 camber will help your tires last a lot longer.

 

Even more camber could be beneficial in the future as well. Although many people here prefer conservative camber settings for autox, I couldn't be happier with 3.0 degrees up front and 2.8 in the back. Tire wear isn't bad with minimal toe.

Use the tire pressures to fine tune the balance of the car. Throw a few more psi in the back and see if it rotates better. Water will help keep the temperatures in check.
Good luck

 

I usually start with my star specs at 31-32 psi cold. As they get hot I let them out to keep them around 32-34 psi. I am all stock but using 245's square setup. I usually have wear marks down to the lines at the end of the tread on the edge. When I look at other cars like S2000's running the tires they are similar wear patterns.

My first AutoX I started my stock tires at 42psi and had lots of understeer. I feel like I could go even lower with the star specs than the 32-34 psi.

 

Thanks for all the advice. Being C Stock, front camber is pretty much off limits. I will try lower pressures next time and invest in a water sprayer, I believe that may help a lot.

 

Star Specs are great tires, but they don't like being hot at all. Spray the hell out of them, until they are near ambient temperature between each run. This will require lots of water--like a full sprayer tank per set of runs with two drivers.

If you're already rolling onto the edge of the tread, you probably don't want to go any lower in the front. The rears generally don't roll as far as the front (and you can dial in more rear camber with the stock setup) so you can cut pressures a little bit to gain rear grip. When I was stuck with stock camber, I usually went with a 4-6 PSI difference front-to-rear, with the rears being lower. Race rubber may require more of a difference.

Also, if you go too low on the front pressures, you'll lose some steering feel, which can lead to more understeer even though it grips a bit better. At the very least it can be harder to attack slaloms, etc. You just have to find a balance that works for your setup. It sounds like you're close at 38 PSI, though you might try moving up or down 2 PSI to see if it makes a difference in feel while keeping tread usage the same.

As for adjusting the rears to balance the car away from understeer, I'm never a fan of taking away grip to improve balance. That makes you slower, even if the car is easier to drive. Better to drive around the understeer (i.e. make sure you don't enter corners too fast) and use the rear grip to get out of corners.

Anyway, sounds like you're off to a good start. Keep playing with it and be sure to keep a notepad with you so you can log temperatures/ambient conditions/times/etc. You'd be surprised how much you "think" you remember versus how much you actually forget or mis-remember. Writing it all down between runs or immediately after your set of runs will give you a good reference point to look back to from event to event and help keep you from spinning your wheels (metaphorically) in the pursuit of the right setup.

 

Simple solution? BSP

Whats worse, fighting off Rx8s or Evos with boost controllers?

Good luck. Its nice to read about other autoxers running Zs

 

try 35 front and 30 in the back !!! and you will stop your car!!

 

I ran 275 Dunlop Star Specs on my Z last year, and had tire pressures at 33 psi all around cold in 80+ degree weather. After 2 runs, I would bleed off to around 36-37 PSI hot, and water them down after each run.

For my set-up (18x10 w/ -2.0 camber all around), 36-37 PSI hot was the sweet spot in terms of grip and steering response.

 

Buy some sidewalk chalk and see where your tires are rolling down/over to. Rule of thumb is roll the tires down to the carrot see ^ on the edge of the tire. Most performance tires will have the carrot. Your pressures will then depend on the amount of camber and tire wear you have.

I'd need some hard emperical data to prove to myself that the water bottle thing does anything other than waste time. If your not getting faster by the end of your runs just from learning the corse something else is wrong. And tires like heat about 150-180F for a street tire, higher for a track slick, maybe lower for an autocross slick.

Edit: if you really want to get **** about tire pressures get yourself a tire pyrometer for example. Take temperatures at the inside, middle, and outside. Adjust your tire pressure so that the middle is the average of the inside and outside.

 

^This, also remember that tire pressures that work for one car may not work for your car. It is up to you to use theirs as a reference and tune from there.