Tire Pressure Street vs AutoX - Z1 Star Specs

[SubieGuy4]

Ok the facts:
05 STi COBB Stage 2
Stock Suspension
Stock size Spec C BBS wheels
225/45R17 Dunlop Direzza Z1 Star Specs

I don't do Auto-X. All my hard cornering is done on the street so I will never be varying my tire pressure from when I'm cruising on the highway to when I'm at my grip limits in a corner.

Mechanical reliability is a big deal to me. I don't know if different pressures Front-to-Rear can mess with the driveline but if it can I won't want to do that. I'm looking for the optimal tire pressure for neutral handling (I hate understeer) and maximum grip, while maintaining mechanical durability and safety. Ride quality is of little concern and I replace my tires every 1-2 summers so tire wear isn't a huge deal.

Suggestions?

 

[IGOTASTi]

I run 35 psi in the front and 30 psi in the rear since the rear of the car is so light and it handles great.

 

[bugeye_fever]

I run the same size spec as you OP. With a stock suspension, I don't think you'll be able to even out the handling using tire presure, maybe if the DCCD can work some magic. Don't know I don't have any experience with it. For me to get the car to handle close to neutral I have to stiffen up the rear shocks, and run higher tire pressures in the rear. For AutoX I run 43 in the fron 45 in the back for what its worth, but I also have c/o's and a rear sway. How come you don't AutoX?

 

[SubieGuy4]

Auto-X is a bit more abuse than I want to put my car through regularly. I'm a reliability freak so the idea of Auto-X messes with me a bit. I'd rather get out on a road course once a year if there were a good local one.

With just a strut tower brace, my STi is pretty neutral. If I ease into a turn it will understeer slightly (not that I can usually find the limits of grip on my Z1s). If I dive into a turn the rear will step out for a second. I'm just looking for the optimal pressure for the best possible grip without killing the neutrality of the handling.

Here's a question. Why do people run higher pressures for Auto-X. It obviously must lead to better handling, so how come lower pressures are run on the street? (I'd assume it's either a matter of safety or ride comfort).

 

[bugeye_fever]

Well, here is how I read was the way to make you subie handle well. First off you have to maximize your front grip. Unlike most cars, to do that on a subie you have to tighten up the front which you've done a little of with the tower brace. Then you need to tailor the rear grip to suit your driving style, you can do that to an extent with tire pressures. If you want the rear looser, bump the pressures up, if its to loose, lower the pressures a little to increase contact patch.

To maximize your front grip using tire pressures, go out and take some hard turns. When finished look at the front tires. You want the wear on the tire to utilize the full tread block. If you see that the wear does not, lower tire pressure. If you can see the wear pattern is way down on the sidewall, you need to bump the tire pressures up. I see people doing this also by making portions of the sidewall with chalk if you have really worn tires. For changes I personally move tire pressure in 2 psi increments, then when its closer, change by 1 psi to fine tune.

Tuning this stuff in really gets harder the more things you have to adjust, so starting off with a stock suspension really isn't bad, probably the best way to learn.

Here's the thing about hard driving though. Before I started AutoX I thought I drove hard burning up the backroads. Now i'm not trying to be a dick or pull your punk card or anything, I don't know your background. But a couple seasons autoXing and my one track day I realized just how hard the car could be driven. Hell, I'm still learning.

Its really hard to give an optimal because its kind something everyone will probably tell you a little different on. Its just something you have to go out and experiment and find out on your own what works best for you(which is fun!) As far as you last question, I didn't say originally what I run on the street. Usually I run 50psi front and rear, rides a little worse, but theres less rolling resistance so I see a little better mileage. I dont drive hard on the street so having less grip isn't a concern.

I hope some of the other more experience autoXers will comment as well, as I have laid out a pretty basic way of reading tires, I'm sure they could go more indepth.

 

[adamkob32]

Do you have any kind of performance alignment on the car? 33psi all around always felt pretty well to be on the street with Star Specs.

Not to thread jack, bugeye what alignment are you running. 43psi in the front on star specs is super high. If your running any kind of camber that high is most certainly hurting you. Around -2.5 deg front camber and star specs 37-38 psi was about ideal. You can also run a lower rear offset then higher to get rotation. Say a 37 front 32 rear split. That will get you setup with a little slower and more predictable rotation then going higher in the rear. It all comes down to preference and car setup though.

--Adam

 

[bugeye_fever]

Right now, I'm running stock alignment. -1.1 in the front and -.8 in the rear. I know thats hurting me big time, but I still have stock tophats. Next year I'll be replacing them with some proper camber plates so I can adjust accordingly for events but be able to put something closer to stock specs for daily driving.

And thank you for posting up in here, good to have more experienced guys giving tips.

 

[adamkob32]

Haha yea no problem at all, definitely looking forward to growing with this site, I'm seeing a lot of great people on here.

 

[IGOTASTi]

I know the last autocross I went to I ran 33 in the front and 29 in the rear and it made a huge difference in my time and my STi handled great.

 

[John]

I don't auto-x. I do more mountain runs then anything. I've had good luck running 35psi for both DD and runs on both front and rear in my 235/45/18 Potenza RE01s.

 

[adamkob32]

I don't auto-x. I do more mountain runs then anything. I've had good luck running 35psi for both DD and runs on both front and rear in my 235/45/18 Potenza RE01s.

Wow that is a tall tire, what car are you running those on?

 

[John]

My 95 Legacy

 

[adamkob32]

That isn't an issue at all. Pretty much even will run lower then that at some point. We have already taken the newer ST tires below 20 psi especially in the rear in hopes to find rotation. Generally if we get that low and are still not succeeding we will raise the pressure in the rears higher then front to gain that back. I prefer higher in the rear on my car to start with, but our 91 ST civic is a different story.