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      05-16-2014, 10:50 AM   #1
intoflatlines
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Recommended PSI for autocross?

I know each course/driver/tire/etc. is different but I'm trying to figure out where to start for tire pressure. I'm not planning on doing any fine tuning in between runs, I just want to know if there's a good cold pressure to start with. I did 37F/38R cold on 245/40 and 275/35 BFG Rivals. I'm not sure what to look for in terms of adjustments/feel.
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      05-16-2014, 11:21 AM   #2
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I'm running 29psi cold on semi slick Ns-2r nankangs, i like to get to around 35ish HOT really, and this seems to work well for me, i think your running those pressure high alittle but not really to sure what the autocross scene do in this respect.
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      05-16-2014, 11:43 AM   #3
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You do track days though, right? That would make sense that you start with much lower temps since your tires get hot and stay hot throughout your session... autocross runs are like 45-60 seconds, then 5 minutes of waiting. So tires are only warm at the end of each run.
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      05-16-2014, 01:53 PM   #4
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Quote:
Originally Posted by intoflatlines View Post
You do track days though, right? That would make sense that you start with much lower temps since your tires get hot and stay hot throughout your session... autocross runs are like 45-60 seconds, then 5 minutes of waiting. So tires are only warm at the end of each run.

i do sprint sessions on a full course over a timed lap... id still lower to 35lbs cold and try again, how you feel the car behaves with your current pressures?
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      05-16-2014, 02:06 PM   #5
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I've been running my rivals at around 37 hot. But this psi was really just a guess. Feels ok, and using the right amount of shoulder. Balance pretty good.
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      05-16-2014, 03:45 PM   #6
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Quote:
Originally Posted by Beedub View Post
i do sprint sessions on a full course over a timed lap... id still lower to 35lbs cold and try again, how you feel the car behaves with your current pressures?
Well I'm very new to autocross (only 4 events last year) and plus the new tires, it's really hard to say. I really am not experienced enough to put to words how it feels.

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Originally Posted by funkle View Post
I've been running my rivals at around 37 hot. But this psi was really just a guess. Feels ok, and using the right amount of shoulder. Balance pretty good.
37 hot for autocross? or for track?

Here's a picture of a front tire after 8 runs. I was freaked out about the rub mark close to the wheel, but was told that it was probably just a cone mark. Excuse the self portrait stamp thing, I pressed the wrong button on my phone

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      05-16-2014, 08:29 PM   #7
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Quote:
Originally Posted by intoflatlines View Post
37 hot for autocross? or for track?
Autocross.
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      05-17-2014, 02:17 AM   #8
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Quote:
Originally Posted by intoflatlines View Post
Well I'm very new to autocross (only 4 events last year) and plus the new tires, it's really hard to say. I really am not experienced enough to put to words how it feels.

37 hot for autocross? or for track?

Here's a picture of a front tire after 8 runs. I was freaked out about the rub mark close to the wheel, but was told that it was probably just a cone mark. Excuse the self portrait stamp thing, I pressed the wrong button on my phone


you could do with dialling in abit of camber their, you can see the feathering on the outer shoulder , you don't want this.
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      05-19-2014, 10:58 AM   #9
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Quote:
Originally Posted by funkle View Post
Autocross.
Interesting... seems pretty low! Do you have pics of treadwear?

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Originally Posted by Beedub View Post
you could do with dialling in abit of camber their, you can see the feathering on the outer shoulder , you don't want this.
How much do you think? I'm on stock suspension/alignment. I've no plans for suspension mods in the near future.
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      05-19-2014, 01:00 PM   #10
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BFG Rivals typically do very well on camber challenged cars.

I'm surprised you're running such a high pressure and seeing the wear in that picture...any chance you could get more pictures of each tire?
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      05-19-2014, 04:50 PM   #11
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Quote:
Originally Posted by Kgolf31 View Post
BFG Rivals typically do very well on camber challenged cars.

I'm surprised you're running such a high pressure and seeing the wear in that picture...any chance you could get more pictures of each tire?
I can take more pictures tonight. I drove home with the pressure up still so the "fresh" dirt/pavement might indicate high pressures. You want shots of the tread or of the shoulders?
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      05-19-2014, 05:30 PM   #12
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Quote:
Originally Posted by intoflatlines View Post
I can take more pictures tonight. I drove home with the pressure up still so the "fresh" dirt/pavement might indicate high pressures. You want shots of the tread or of the shoulders?
Same profile as before. I want to see where the tire is rolling over at.
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      05-19-2014, 09:44 PM   #13
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Wow, the harsh lighting with the flash really exposes just how dirty my stuff is...

Front Left:





Front Right:







Rear Left:





Rear Right:



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      05-20-2014, 11:06 AM   #14
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In my experience autoxing on Rivals in various cars, they do not need nor like a lot of pressure. 37 psi hot is about what I've used on Rivals - a good place to start. And then maybe lower, more like 34-35 hot in the rears if you are struggling with traction to accelerate off the apex.
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      05-20-2014, 03:15 PM   #15
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Quote:
Originally Posted by Beedub View Post
I'm running 29psi cold on semi slick Ns-2r nankangs, i like to get to around 35ish HOT really, and this seems to work well for me, i think your running those pressure high alittle but not really to sure what the autocross scene do in this respect.
i repeat, 35lb HOT is a very good starting point imo.
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      05-20-2014, 07:00 PM   #16
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Quote:
Originally Posted by aklucsarits View Post
In my experience autoxing on Rivals in various cars, they do not need nor like a lot of pressure. 37 psi hot is about what I've used on Rivals - a good place to start. And then maybe lower, more like 34-35 hot in the rears if you are struggling with traction to accelerate off the apex.
Yea, this is basically what I would go with.

OP - Get chalk and mark up your tires, mark the whole sidewall and then on tread. There should be an arrow indicating where rollover should be. I apologize but I'm not too familiar with the Rival layout.

This will help you identify exactly where your tires are rolling over and adjust from there. Ideally, your rollover should be at the arrow tip, but there is obviously some room on this if you adjust up/down for better grip of feel.

Rivals have a very hard sidewall. Sam Strano on his FRS liked to run low 20 PSI on Rivals. Granted, this is a 2800 lb car, but still...I think the pressures are too high.


I use to run 30 PSI front and 26 PSI rear on my RS3s for the Z4M when I autocrossed it, for reference...and they have VERY soft sidewalls and typically do not like low pressures.
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      05-23-2014, 11:07 AM   #17
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Last night I dropped my PSI down to 34 cold from 38ish. Of course I didn't have an autocross course to run but I did mess around a little and it felt pretty grippy. A lot softer feeling especially when turning in, but it felt like once you were cornering, the tires were gripping pretty nicely. Not really sure how to explain it...

Last edited by intoflatlines; 05-25-2014 at 11:56 PM.
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      05-26-2014, 12:08 AM   #18
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So I ran 34 front 35 rear cold on yesterday's autocross. I found the arrows on the tires. On the fronts they had mostly worn away but were still somewhat visible. The rears were still visible. The car felt MUCH better with the pressures lowered by the ~3psi. I even won my class! I might drop them down a pound more for the next event. I don't think I'd want to do any more especially since the fronts looked like they had already rubbed past the arrows. Anyway, here are pics from afterwards (7 runs @ 60ish seconds each)

Front Right:




Front Left:



Rear Right:



Rear Left:

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      05-26-2014, 07:11 PM   #19
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35 - 37 is a good starting point. I adjust pressures between every run if I'm not getting the grip or release I want in the rear tires, or too much under steer in the fronts.
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      05-26-2014, 07:22 PM   #20
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Rears are over inflated in the last pics...
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      05-26-2014, 08:12 PM   #21
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Get some chalk and it will better tell how your rollover is doing with pressure changes.

With that being said, the arrows are basically just a starting point. As long as you're not rolling past and onto the sidewall, you can run pressures higher/lower than that arrow.
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      05-27-2014, 07:37 PM   #22
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Don't adjust your tire pressure to decrease roll over. Adjust tire pressure for to increase contact patch. You should have 15 degrees hotter on the inside than the outside. Check them when they are immediately hot.

You need more camber if you are rolling over the tire. You will need to adjust when changing to different tires, sway bars, spring rates, car weight, or the track.
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