Dunlop Direzza Z1 Star Spec's tire pressure...

[ruffstik]

I'll be running these tires this spring at some autox events on my stock 18's - was just curious what a good starting tire pressure would be?

Thanks!

[JME]

Tire pressures are really something you are going to have to feel out. I would start with 37-39 psi and adjust as necessary. Honestly, the tire pressure is going to depend more on the surface of the track and the course you are following. Generally speaking, the slower the course (the more sharp corners it has) the lower the tire pressure you want to increase contact patch area and give you more lateral force at higher slip angles. Remember though, the lower the pressure, the more the tires will be worked by deformation and they will heat up faster as a result.

Are you in racing in stock class or STU? Do you know if you will be on concrete or asphalt?

[ruffstik]

stock class and it's run on asphalt around here.

[kwanzto]

I run the same setup and start each autoX day out with 39-40 lbs of pressure. There are many variables involved, and you will be playing with pressure constantly. I also like to run a little more pressure in the rear than in the front. Set a starting value somewhere around 40 lbs, then adjust from there.

You want to be using the entire tread width, while not riding on the sidewall. A good trick is to try a little chalk (they even sell special temporary white pens for this). Make a mark that starts maybe an inch in on your tread, and goes maybe 1/3 of the way down the sidewall, do this in 3 or 4 places around the tire. After your run you can see how much of the chalk you scrubbed off...it should come right to the edge of the tread. If you are not using all of the tread, use less pressure. If chalk is missing from the sidewall, use more pressure. You should be checking tire pressure before every run, so this is easy to work into your routine.

[larryn]

Definitely chalk the tires, and play with pressures. Both great ideas.

I have found that on hotter days with teh Star Spec on the 135i, slightly higher pressures work on them (41F, 39R or so). Otherwise, I'd run 37F, 35R, which dials out the understeer from the staggered tires.

If you happen to scrub a lot on a run, you can easily add 5lbs of pressure. So, be sure to bleed off air throughout your runs.

[Bimmer-Bob]

What are you guys running for tire pressure in daily driving? My manual says 32 F/ 39 R for 18" tires (128i) but that seems like a big difference, even for staggered tires. Getting ready to mount the new wheels (not even taking delivery of the car until next week) so can't report what the recommended pressures feel like...any opinions?

[Bimmer-Bob]

Originally Posted by Bimmer-Bob

What are you guys running for tire pressure in daily driving? My manual says 32 F/ 39 R for 18" tires (128i) but that seems like a big difference, even for staggered tires. Getting ready to mount the new wheels (not even taking delivery of the car until next week) so can't report what the recommended pressures feel like...any opinions?

BUMP! Anyone?

[larryn]

36 F and R

[Bimmer-Bob]

Originally Posted by larryn

36 F and R

Thanks!

[RaceTime]

Originally Posted by larryn

36 F and R

+1, somewhere right around there...depends on traction and surface grip.

[wkndracr]

I just did my first autox last weekend and was wondering the same thing. Using the "chalk method" I ended up putting 50lbs up front in my 245/40/17 Kumho Ecsta XS's, which everyone I spoke to said it was too high, to try 40lbs. I figured though that if I went lower, i would be on the sidewall. Any thoughts? The surface was asphalt and dry with temps in the 60's.

[jb2178]

I ran them for the first time a few weeks ago and will run in another AutoX on sunday. I've got them on my stock 17s but I also upsized to 225s up front and 235s in back. The difference in stock run-craps was huge, more grip and more controlled when you lose grip. I ran 35 psi all around last time and will try 30 on sunday... I'll report back.

GO AUTOCROSS!!!

[jsublime]

Originally Posted by larryn

Definitely chalk the tires, and play with pressures. Both great ideas.

If you get Roundel there is a great article about how to set tire pressure using chalk in the current issue, pg. 68.

[Evice]

Originally Posted by jsublime

If you get Roundel there is a great article about how to set tire pressure using chalk in the current issue, pg. 68.

latest Roundel?

[HP Autosport]

Originally Posted by Bimmer-Bob

What are you guys running for tire pressure in daily driving? My manual says 32 F/ 39 R for 18" tires (128i) but that seems like a big difference, even for staggered tires. Getting ready to mount the new wheels (not even taking delivery of the car until next week) so can't report what the recommended pressures feel like...any opinions?

My recommendation is 38-40 psi to protect your wheels. lower pressure and pothole means bent or crack wheels.

[axoid]

I was autocrossing yesterday. And got some OK results with 36 front and 41 rear. Stock suspension with 225/255 on 18" wheels. The car wont rotate the way I want, but I don't think I can fix that with air pressure. Sway bar and camber plates are in my future.

[Bimmer-Bob]

Originally Posted by HP Autowerks

My recommendation is 38-40 psi to protect your wheels. lower pressure and pothole means bent or crack wheels.

Same pressure all around? Or 38F, 40R?

[JME]

Originally Posted by axoid

I was autocrossing yesterday. And got some OK results with 36 front and 41 rear. Stock suspension with 225/255 on 18" wheels. The car wont rotate the way I want, but I don't think I can fix that with air pressure. Sway bar and camber plates are in my future.

Try just getting a decent alignment. As much neg camber in the front as possible (-0.6 deg) and 1/16" toe-out all around. Yeah, toe-out in the back isn't optimal, but it will help your car rotate.

[HP Autosport]

Originally Posted by Bimmer-Bob

Same pressure all around? Or 38F, 40R?

Same at all four.

[larryn]

Originally Posted by JME

Try just getting a decent alignment. As much neg camber in the front as possible (-0.6 deg) and 1/16" toe-out all around. Yeah, toe-out in the back isn't optimal, but it will help your car rotate.

The OP was asking about street pressures.. So, I would not go this route, unless your car is (at a minimum) dual-purpose of track or autox car/street car. You'll pee through tires with the factory rear camber, plus toe out (heck, the factory alignment already eats the inside edge of the rear tires quickly already).

[Bullitt]

Originally Posted by larryn

I have found that on hotter days with teh Star Spec on the 135i, slightly higher pressures work on them (41F, 39R or so). Otherwise, I'd run 37F, 35R, which dials out the understeer from the staggered tires.

By any chance, did you mistype and have it reversed? Wouldn't those tire pressures induce understeer? I've been running 33F and 38R for daily driving, and here I thought all along the understeer was reduced. There was even a thread here long ago recommending lower tire pressures in the front. I currently lose more traction in the rear on hard turns with this setup. I'm running these tires in 225/255.

[gotcone.com]

Sunday was my first event on the Star Specs and I ran 37f/35r and seemed to work well, no rolling onto the sidewalls, even when I over-drove a corner or 2 (or 8 ) and had great grip. Temps were a bit cooler around 60 and I was on concrete. The surface you are driving on asphalt vs. concrete can also play a part into what rates you want to run.

I am not on stock suspension though so not sure how much that helps.