Tire PSI for AutoX

[Silver Ignition]

Let me start by saying I'm not new to AutoX or racing cars in general. I also know that tire pressure is a personal and experimental thing that will change with conditions, course layout, temperature, and how you personally want the car to react.


What I really want to know is what do you guys use as a BASELINE PSI; pull in the lot, register, tech inspection, back to the paddock, drop tire pressure. What do you set it to?




The reason I ask is because I recently did my first event with this car and found it struggling to touch the ground. At no point duing the course did I feel like the car was actually connected to the ground. There was understeer, there was oversteer, there was even a spinout. I also did not drop the pressure, I had all 4 tires set at 35psi. My car is stock other than a set of S-Techs and TRD exhaust.

[ayau]

You probably already know this, but you should be adjusting when the tires are hot. I like them at about 34F and 32R. When cold, I set them to about 30F and 28R. After a few laps, they should heat up to my ideal temps.

[TrqlessWonder]

Well, to answer the original question, I did my first event in the rain on stock everything save for an initial alignment, and ran at 37F/34R Car was lively to be sure, and I did spin it once, but I experienced no understeer with it. Just had to be a little patient/smooth on corner exit.

As for the erratic/unpredictable behavior, it's hard to tell how much of that is car versus you. However, looking at the specs for your springs, I think they are part of your problem. Especially if you're still on the factory dampers. For autocross, those springs drop the car too far. The fast guys are only dropping 20mmF/15mmR. Check to see how much you're hitting the bumpstops. Whacking bump stops will make most any car a handful, and kind of invalidates any tire pressure recommendations, or thoughts about spring rate suitability.

[Silver Ignition]

Originally Posted by TrqlessWonder

Well, to answer the original question, I did my first event in the rain on stock everything save for an initial alignment, and ran at 37F/34R Car was lively to be sure, and I did spin it once, but I experienced no understeer with it. Just had to be a little patient/smooth on corner exit.

As for the erratic/unpredictable behavior, it's hard to tell how much of that is car versus you. However, looking at the specs for your springs, I think they are part of your problem. Especially if you're still on the factory dampers. For autocross, those springs drop the car too far. The fast guys are only dropping 20mmF/15mmR. Check to see how much you're hitting the bumpstops. Whacking bump stops will make most any car a handful, and kind of invalidates any tire pressure recommendations, or thoughts about spring rate suitability.

Yeahh I was fully aware when I bought the springs that they wouldn't do me any favors when it came to AutoX but they make the car look gorgeous so the 345 days of the year that I'm NOT at AutoX it looks perfecttttt.
My main reason for asking is to see how low you guys run, and so far it seems like not very low. I was assuming guys would be running 22-25psi but it seems you're all only dropping down in the 30-32 range.

[f0rge]

I usually set them all at 32psi when I first get there. After the 1st run I'll check where the tire has been rolling to and adjust accordingly. Sometimes the front will need another psi or 2 but usually I don't have to change it.

If you're still on stock tires, that's why the car is a handful, especially if you drive it hard.

[ayau]

Originally Posted by Silver Ignition

Yeahh I was fully aware when I bought the springs that they wouldn't do me any favors when it came to AutoX but they make the car look gorgeous so the 345 days of the year that I'm NOT at AutoX it looks perfecttttt.
My main reason for asking is to see how low you guys run, and so far it seems like not very low. I was assuming guys would be running 22-25psi but it seems you're all only dropping down in the 30-32 range.

Autox generally won't heat up the tires a lot because of the short stints.

[travisb]

I'd like to tag along with OP's post if that is ok.

I've been autocrossing off and on for a few years. First in a Hyundai Accent and then in a turbo Miata.

I just got my BRZ last week and it is completely stock.

What PSI should I start at? It will be about 80 degrees and the pavement is a bit loose. Air pressure is something I never really got a hang of in the other cars.

[SubiePig]

Autox will heat tires more than you would think. Surface temp after 3 runs on a 60 second course will put mine at about 120. You get surface temp issues rather than on the track its the whole of the tire internally as well getting hot. This is why you see autox guys spraying the tread with water on a hot day as the surface is taking all the heat and they get really greasy.

As for pressures, on stock tires, I would probably run them as high as 40 because the sidewall is so soft. So first run I would run them about 38 and let them rise to 40 or whatever is low enough to get the sidewall to roll over the right amount.

[Silver Ignition]

Originally Posted by f0rge

If you're still on stock tires, that's why the car is a handful, especially if you drive it hard.

I am, it is, I do.

[Eric1855]

The recommended PSI can vary greatly by tire and how you want the car to perform.


When I had Rivals they liked lower pressures, but the RS3s like higher pressures.


Right now I have ZIIs and I go with 35F 32R warm which works well on concrete with the way I currently have the suspension set up.


The only way to get the real answer is to keep testing and trying different combinations. You want to watch the wear on the tire to make sure you aren't rolling over too much or too little. A Pyrometer can really help too by seeing if you are getting consistent temps across the tire or getting to much wear in the middle or on the shoulders and adjust pressures accordingly.

[TrqlessWonder]

I'll add in a bit more. Mostly because there's more than one way to skin a cat (or exorcise demons).

Bump stops means bump steer, which is a toss of the dice every time. If you've got your heart set on what those springs provide visually, you can still make them work better, I would think. Couple things to think about are maybe cutting/changing the bump stops if you're only engaging them a little, or, and I think this would be better long-term, get shocks that are valved for those spring rates. Sometimes you can use other things like tire pressures to compensate for some handling evils, and sometimes, you need to address it more directly.

To figure out your own pressures (because everyone's setup and driving style are different) maybe you could try the way I did it with my last car (still figuring out the FRS, myself, so grain of salt). Set the fronts so that at operating temp (like 3rd run if you're solo) you are using the full tread, but not rolling onto the shoulder. You can use chalk, or the arrows that are on the sidewalls. You want to get close, but not hit the arrows. Rear pressures are what you use to adjust the attitude of the car. Too loose? Drop a pound. Not rotating? Add a pound. And so on, and so forth.

Frankly, puzzles like this are one of the best parts of autocross. Not only is it the best adrenaline release this side of intercourse, but it's also a cerebral pursuit as well.