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      10-24-2010, 06:34 PM   #1
MarkR171
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AutoX Tire Pressure?

Just got back from my first even with the Star Specs. My first 3 runs were sloppy and felt like the grip was worse than the run flats. I let out 4 psi all around and took a full second off my time and the tires felt a lot better. What PSI do you guys recommend to run? I'm assuming lower than run flats, as I started with the door jam recommended pressures.

Managed to take 1st in D-Stock on the last run but it was close! Video shortly.
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      10-24-2010, 10:31 PM   #2
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Congrats on the win! I'm interested to hear the responses, but I think it has to be dependent on the tire as well. I found that with my Hankook tires I have to increase pressure over the door jam. I found 40psi (cold) all around seems to be the best. Sometimes I will lower the rear slightly to 38ish. The sidewalls on the tires are fairly soft, so I need the higher pressure to stop them from rolling significantly. With my older tires (Michelin Pilot Sport A/S Plus), I found lower pressures worked better and they rarely rolled onto the sidewalls.

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      10-25-2010, 12:52 AM   #3
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Quote:
Originally Posted by MarkR171 View Post
Just got back from my first even with the Star Specs. My first 3 runs were sloppy and felt like the grip was worse than the run flats. I let out 4 psi all around and took a full second off my time and the tires felt a lot better. What PSI do you guys recommend to run? I'm assuming lower than run flats, as I started with the door jam recommended pressures.

Managed to take 1st in D-Stock on the last run but it was close! Video shortly.
What were your actual tire pressures? 4 psi lower than the tire jam seems waaay too low to me. I start with 39 front 41/42 rear COLD and work from there. Direzzas have fairly stiff sidewalls, but it's still not enough to keep the tires from rolling over without higher pressures. (On a lighter car it might be.) Chalk your tires and you'll see - I ran with lower pressures (34/36ish) and they rolled over like crazy and my times showed it. A large bump up made an immediate and large difference. I like some extra pressure in the rear (vs the front) to try and get some corner entry rotation. Some prefer slightly lower pressures in the rear for corner exit traction, but I've been able to modulate well enough to work around this, and I find the corner entry rotation counters some of the innate understeer largely experienced in autocrosses. (If you're plowing massively into a turn with heavy understeer, corner exit traction isn't gonna be what you're worried about, and you've already blown your time.)


So to answer your question, no, I run way higher than the runflats, not lower.
Continue to use large changes for testing, like you did (small changes can be difficult to discern positive/negative results from normal variation), but next time try going 4psi HIGHER instead of lower and see what happens.
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      10-25-2010, 01:04 AM   #4
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Will try that, thanks. Next event in 2 weeks! I honestly haven't done much reading on effects of tire pressures yet, but I knew something was up when I realized the run flats felt better and was easier to drive faster. I decided to drop pressures as I always see people dropping their pressure after their first run. I'm still a novice here! (Less than 10 events in me.) On my first 3 runs, I actually noticed a lot of "looseness" and didn't feel like I had the grip. The last run wasn't a huge improvement in grip, but I did manage a better time, but I think that was more driving than the difference in tire pressure.

Thanks again Xaeryan!


My baseline was the door jam recommended pressure, which iirc is 36 front 39 rear.
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      10-25-2010, 06:51 AM   #5
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I used the white stuff sold at auto part stores to mark windshields with prices to mark my tires last autocross a couple weeks ago. There are little triangles to mark the edge of the tread on the upper sidewall of your tires. I put the white stuff in this area extending onto the tread and the sidewall. Then I made some practice runs and inspected. I started 4 over the door jamb which for me in my 128i convertible meant going from 32/36 to 36/40. That was plenty in the rear (white stuff still on all the sidewall and a little of the tread (but very little), but not enough in the front (rubbed off the white stuff well onto the sidewall). But my tires would not get back down to where they started between runs. So I made the fronts the same as the backs. I also had about 2psi difference between the sunny side of the car and the shaded side. So I made the front on each side back the rear on each side. When I got home and measured them cold, the fronts were about 38.5 - so not totally up to 40 psi cold. I was still on the sidewalls a little bit so I will go with 40 psi front and 38 or 40 psi rear next time.

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      10-26-2010, 12:01 PM   #6
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Z1 Star Specs
fronts were 32 cold
rears were 29 cold (could probably go 27-28)
I aim for 36 to 37 hot and typically have exceptional front end bite. Could have used another pound or two based on that sidewall pic. Tire pyrometers are your friend
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      10-26-2010, 12:44 PM   #7
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I found this article online that explains in depth how to adjust tire pressures. It's long, but a great read! This site also has info on everything else about handling (springs, anti-roll bars, etc.)

http://www.turnfast.com/tech_handling/handling_pressure

I think I'm going to try 35/35 cold next time. With 39 cold on Sunday, my rear end was coming loose on the tighter corners and lowering it by ~4psi seemed to solve the issue. I checked my pressures when the tires cooled down and I had them at about 33psi F/R after adjustments at the event.
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      10-26-2010, 02:16 PM   #8
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I have yokohama AD08s. 31 psi all around gave me the best autocross times, and I verified I was using the entire shoulder but not the sidewall by using paint dots.

Yes, it feels more mushy, but it hangs on MUCH better than at 40 psi.
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      10-27-2010, 08:38 AM   #9
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Maybe non run flat tires need lower tire pressure but I am pretty sure my RFTs need around 40 psi in the front to keep tread on the track, not sidewall. The concept of determining pressure by tire temperature is somewhat interesting, and probably has a sound technical basis, but I doubt many of us will buy the necessary equipment. A little white stuff on the sidewall is a lot cheaper.

I also struggle with the use of "feel" or even time results to determine what is the right pressure. Time is a lot less subjective but there are obviously a number of factors other than tire pressure that affect it. Regardless of how it "feels", I think you want to keep tread on the pavement, not sidewall. I wish I had thought to ask the instructors at the M-school. It would have made for an interesting lunch time discussion.

Jim
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      10-30-2010, 05:51 AM   #10
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It's very dependent on a lot of things.

Before camber plates
RE50 RFT 38F/38R hot
Conti DW's I run 44F/40R hot, (rolled them over at 38)
V710's 40F/38R

After camber plates
Conti DW's, 40F/38R
V710's will run 38F/38R

Like said above, you put enough tire pressure in the tire so that you stay on the treats and don't wear below the triangle indicators. Camber plates will give your tires much higher resistance to rolling over as they change the angle of the tire.

Star Specs have stiff Sidewalls. I'd start 40/38 without camber plates.

Check your tire pressures immediately after each run and bleed off the excess pressure you get from heat. After 4-5 runs, your pressures will level out after each run then you no longer have to check them. You're set.
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      10-30-2010, 11:36 AM   #11
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Quote:

Star Specs have stiff Sidewalls. I'd start 40/38 without camber plates.
40F/38R? And that's hot pressures right? That's what I was going to consider running. When I let air out on my run last week, I dropped the pressures to about ~41 hot F/R and they improved. A bit lower might be good.
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      10-30-2010, 05:51 PM   #12
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Yeah, the pressures I listed are hot pressures that I've been running.

I'm not an expert so use your own good judgment. Those pressures have just worked for me.
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      11-03-2010, 08:59 PM   #13
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Played around at this weekend's auto-x and basically verified my previous result. The ambient was quite cold, and tires were not gripping for anyone. I found with 40-41 (cold) in the front I had pretty good traction and did not roll onto the sidewalls excessively. I started with 43 in the front, and found that a little too high. I vent down to about 40-41 and found that best. In the rear I started with 42 for experimentation, and I noticed a lot of oversteer (as expected). I had good times with this, but spent most of the track sideways. I went down to 38-39 (cold) and it did indeed improve the grip in the rear end. This seemed like enough to keep traction in the rear, without having so much that my fronts start to break loose first (i.e. understeering).

Being the ambient was cold (low 40's F), the tires never did heat up much, and would usually only go up 1-2 psi during/after runs. I'm surprised those with the Direzza's and similar tires are able to run so low. I like the Hankook's but I'm excited to ditch them and get a tire with a stiffer sidewall.

Tim
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      11-04-2010, 01:48 AM   #14
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When I started at 38/39 cold at my last event, my pressures went up to the mid 40s after three 40 second runs. That's why I think I'll start around 35-36 cold and shoot for 40 hot.
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      11-04-2010, 08:16 AM   #15
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I always start with 36F / 36 R cold. Bleed down the front accordingly to stay at 36 Hot and rears I adjust depending on the course. There have been times where I ran 34 hot in rear on very tight courses, but on faster / open courses I like a bit more rotation from rear and have been running about 39 hot in those cases
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      11-04-2010, 10:34 AM   #16
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Quote:
Originally Posted by timhatimay View Post
Played around at this weekend's auto-x and basically verified my previous result. The ambient was quite cold, and tires were not gripping for anyone. I found with 40-41 (cold) in the front I had pretty good traction and did not roll onto the sidewalls excessively. I started with 43 in the front, and found that a little too high. I vent down to about 40-41 and found that best. In the rear I started with 42 for experimentation, and I noticed a lot of oversteer (as expected). I had good times with this, but spent most of the track sideways. I went down to 38-39 (cold) and it did indeed improve the grip in the rear end. This seemed like enough to keep traction in the rear, without having so much that my fronts start to break loose first (i.e. understeering).

Being the ambient was cold (low 40's F), the tires never did heat up much, and would usually only go up 1-2 psi during/after runs. I'm surprised those with the Direzza's and similar tires are able to run so low. I like the Hankook's but I'm excited to ditch them and get a tire with a stiffer sidewall.

Tim
You're results are consistent with what I observe (on Direzzas). If you can get the rear pressures just right, you can get it to induce some oversteer on hard braking turn-ins, but still not have to "hero" it throughout the course sideways. Works well to point the front and combat some of the understeer.
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      11-04-2010, 10:56 AM   #17
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since temps tires and surfaces differ, I would suggest using the chalk method, and start at 39F/36R, and see if you are rolling onto the tire shoulder. If so, you need more air.

It's more likely that you'll wind up around 36F/33R, but again, that will differ greatly on differing concrete versus tarmac surfaces.

Use chalk. It is your friend.. and keep checking pressures throughout the day and keep a log for what works best (including tire and ambient air temps).
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      11-04-2010, 04:21 PM   #18
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I was up to 42 front and 41 rear with my PS2s last weekend and I was still rolling the sidewalls. Its really dependent on a lot of things but one things for sure...I need star specs

lost C-stock by a bit over a half second to an E36 M3. I just had too much power and it was too cold, about 40-45 degrees, for me to put it down, so I spun the rears out of almost every turn if I wasnt careful and it was really easy to goose the rear end out in a sweeper.

If I was any lower than 41 in the rear I rolled the sidewalls like crazy...so it was cold and I had high pressure...of course I had no traction...oi vey


I miss summer.
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      11-04-2010, 04:46 PM   #19
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C-Stock? The 1 series is in D-Stock...
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      11-04-2010, 04:55 PM   #20
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C-Stock? The 1 series is in D-Stock...
verts are at the top of C because of our extra weight
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      11-04-2010, 04:57 PM   #21
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Really? Shows all 128s and 135s in D-Stock in the 2010 rule book... No mention of 1er verts in C-Stock.

C-Stock is a faster class too, .840 PAX modifier vs .825 in D-Stock.
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      11-04-2010, 04:58 PM   #22
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Really? Shows all 128s and 135s in D-Stock in the 2010 rule book... No mention of 1er verts in C-Stock.
sorry youre talking SCCA im talking BMW CCA. Im in D stock SCCA. but the SCCA guys here are a bit dickish so i dont really run with them.
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