CrashDummy
CrashDummy None
4/23/12 4:39 p.m.

I have an 08 Mazdaspeed3 that I autocross. It is stock with Star Specs (new tires for this season). 225/45/R18 I chalked the front tires for every run at a recent event. I started with 40psi, then 42, and then 45 but the tires were still rolling over too much based on the chalk. My question is: Is there a point at which adding pressure becomes counter productive and can actually make things worse? My car could really use more camber (but it can't be adjusted) so it's going to abuse the outside edge of the tire...maybe it is rolling over too much because of the (lack of) camber and not a lack of pressure? Any tips would be appreciated.

On the rear tires chalk doesn't tell me much because the tires don't roll over very much. I have been told that both high rear pressures or low rear pressures will help the car to rotate. Is this true? Any advice on rear pressures? Any tips for a good starting point or some upper/lower bounds that I shouldn't exceed? Again any advice is appreciated.

For the record I think the stock (aka door jam) pressures are 34 front and 32 rear.

N Sperlo
N Sperlo UberDork
4/23/12 4:51 p.m.

I can't answer, but you may want to include the type and size of tire.

mazdeuce
mazdeuce Reader
4/23/12 4:51 p.m.

Can you not run camber bolts for some reason?
A girl I autocross with runs a VW Rabbit on star specs with no camber to speak of and she has been happy at 36 psi if that is useful at all. She drives quick, but very smooth and very rarely overdrives a corner and I think thats a big part of it. I've only just done my first event on my star specs and I settled at 34 psi for now but I'm in a lighter car with less power and 2.5 degrees of camber so I'm not sure if that really applies. The star specs do react quite a lot differently to over cooking a corner though. Things I could get away with and massage back into a useful corner before I can't now. They certainly trade grip for forgiveness compared to all seasons.

CrashDummy
CrashDummy New Reader
4/23/12 4:56 p.m.
N Sperlo wrote: I can't answer, but you may want to include the type and size of tire.

Edit made...225/45/R18. Thanks.

mazdeuce wrote: Can you not run camber bolts for some reason?

I am not aware of camber bolts for the Speed3. I would be very interested if they are avaliable. To be Stock-Class legal I believe they need to be specified in the factory service manual? If anyone knows of Speed3 camber bolts please tell me!

vwgene
vwgene
4/23/12 4:58 p.m.

I've always heard up the tire pressure by 10psi from the factory spec. Up by ten in the front and rear.

Could also try reversing the pressures, higher in the rear than front. The idea is extra pressure in the rear will mean slightly less grip and hence more rotation. That's what I've heard, I've always done +10 front and rear

mazdeuce
mazdeuce Reader
4/23/12 5:01 p.m.

I'd just run the damn things and not worry about it. As long as you're not in the trophies nobody should care and they'll pay for themselves in increased tire life. Why run stock instead of a ST class? They're certainly legal there.
Edit for bad advice. Don't cheat. If you're going to stay in stock then don't run the bolts. If you don't have a compelling reason to stay in stock though, look into ST (the speed is in STU?) where you'll almost certainly be outgunned, but you have a bit more freedom. Some people are really driven to pick a class and win at it. Others just want to learn to drive faster and let the finishes fall where they may.

jrw1621
jrw1621 PowerDork
4/23/12 5:28 p.m.

Video Instruction:
http://grassrootsmotorsports.com/forum/grm/video-training-autocrossing-with-dick-turner/33988/page1/
Not unheard of to have 50psi in the fronts of a fwd car.

Since you are chalking, keep this in mind... 3lbs of air for every 1/4" you want to move the wear mark.

Try setting up a skidpad and use a themometer on the treads http://www.harborfreight.com/catalogsearch/result?q=thermometer

wbjones
wbjones UltraDork
4/23/12 7:30 p.m.

this will answer many questions anyone might have when it comes to ST tires

http://www.sccaforums.com/forums/forumid/23/threadid/425216/scope/posts

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