What do you guys run for pressures in Dunlop ZII tires front and rear? Off to my first Autocross Sunday.

 

What size ZIIs?

I havn't run them yet (and mine isn't an STX car), but my 225/45-16 seem to like very high pressures up front. I'm up to 50/40.5 and the fronts still aren't showing even temperatures.

 

I ran 40 front and 38 rear last summer.

 

On my 2010 Hardtop my Dunlop Direzza Star Specs didn't stop rolling over until I had 49 in the front and 44 or 45 in the rear. I haven't had the Roadster out yet but I will chalk the tires and start at 49/44 when I do.

 

Our STX car we usually start at 39 front and around 41 rear then tweak from there depending on course/surface

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Wow I'm way off you giuys. I went with 35 front and 32 rear. That kept things ok. What's the thought behind running high rear pressures? I was fastest STX car with couple of BRZ etc. Fastest ST class car overall. Beat plenty of dot r clad cars too. I'm thniking with LSD and wider tires and wheels this car may be pretty quick at regional level.

 

Surface temps, grip levels, tires, alignment numbers all contribute to tire pressures. So it's not surprising that you're a few psi off other people.

We run a higher rear pressure to encourage the car to rotate more.

 

Interesting I have heard of FWD guys doing this but shouldnt rotation be managed by proper suspension setup? To me it seems suboptimal to overinflate rear tires (reducing contact patch) to get rotation? Im a FWD rookie so excuse my ignorance. I have driven mostly FP and XP rear wheel drive cars so this is all new to me but I know a bit about suspension dynamics and setup.

My car seemed to rotate fine. If anything it tended to over rotate but I can live with that!

I have to look at whether I have a little preload in my rear bar as car snap oversteered in couple places.

 

Well in STX you can fix some of the shortcomings of a strut/multilink suspension but you don't have nearly the allowances you have in the prepared classes like you are used to. Tire pressures are a tuning tool just like everything else and they're used to dial in the car to surface conditions just like shock settings. Some people will change springs/alignment/shock settings/tire pressures just to switch from asphalt to concrete. We just use shock settings/tire pressures.

Do you have an aftermarket rear swaybar? If so I'm not surprised to hear your car has snap oversteer. Mismatched roll rates from a large rear swaybar will do that. I know everyone does suspension setup differently but I'm a "more spring less bar" person which means our car rotates just fine with stock swaybars.

But hey if you're happy with what you have than stick with it. Seems like you're doing pretty good at a regional level so unless your local competition is getting faster or you're planning on going to national level events there probably isn't any reason to mess with it. If the car is fun to drive that's all that really matters.

 

A friend who races pointed out you shouldn't ignore the tire spring rates. Increasing the tire pressures will increase the overall spring rates at that end, which would shift the grip to the other end.

My tires seemed to like hot pressures of 49.5/46.

 

Also keep in mind you can't get to the top of the shocks in the rear of the mini without removing the suspension so if you need to make changes on site sometimes tire pressures are the only thing you have. Cutting holes to get to the top of the shock is not STX legal.

 

THanks!