What tire pressure should I run for autocross with the stock 18" Track Model wheels? I have only raced once and I kept the tire pressure at the normal 40psi and I had way too much understeer. Can changing the psi help that? Thanks.

 

I tried 35/30 at my last autoX event. Supposedly the lower rear pressure help reduce understeer. I can't tell yet if it helps or not.

 

Kinda contrary to theory as I understand it. Every 1 lb increase/decrease in tire pressure equals about 10 lbs of spring rate, either up or down. To increase oversteer, you would increase rear spring rate or decrease front spring rate. That would suggest that lowering tire pressure in the rear would actually increase oversteer.

 

Correct, I want to decrease understeer by increasing oversteer. There are a number of sites that have charts that list the effects of various changes to your setup. Check out

http://www.susquehanna.com/susq/other/stuning.htm

http://track-days.org/page8.html

 

Thanks. What tire pressure do you guys run?

 

mbonanni1856,

Is 40 PSI what you run on the street (i.e. "normal") ?

 

Yeah I don't have an owners manual so I don't know the recomended tire pressure so I just ran the same as I do on my 1970 Chevelle that has about the same size wheels and tires. Is that too much? I know the PSI warning light comes on at around 33psi so I upped it to 40 and it hasen't come on.

 

The default for your Z should be 35 PSI. It should be marked that way on the plate attached to your drivers door frame.

My TPS warning has not come on until the pressure got below 25 PSI. 33 sounds too high for it to kick in.

 

Huh maybe I should get it checked then. Thanks. I didn't even think of looking in the door jam.

 

fyi, you mistyped, your logic says lowering rear tire pressure decreases oversteer.

but i also wanted to comment that's it's not just spring rate. tire pressures will effect tire contact patch and tire temperatures. the lower the presures the higher the temps will be, and thus more grip.

i was told to increase front tire pressure to increase understeer. but i haven't tried it yet to see if can tell.

iris and i just ran a track day on sunday. i had 35psi cold all around on the miata, this is on Toyo T1-S 195/55 R15. it felt more stable and more grippy than before when i had 29psi cold all round. she spun the car. but i thought it was pretty stable as long as you didn't lift throttle when you're at 9/10th's.

oh yeah, 40psi cold sound a little high. but if you're doing autocross your tires can't heat up that much anyway, so that may be ok. but 40 during normal driving is too high.

 

I ran last weekend on 18" wheels with Potenza S03s. I was running 42front 37rear Hot. So figure 40/35 cold.

 

On Kumho MXs 245/40/18 all round I run 35 psi on the street. For Solo2 I run 45 (f) 44 (r) psi. The MXs have fairly soft sidewalls.