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Autocross Tire Pressures?

Discussion in 'Roadster' started by Pantera Dude, May 24, 2013.

  1. Pantera Dude

    Pantera Dude Member

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    I'm going to do my first autocross this weekend and I'd like to know what tire pressures to run. Anyone out there done this?
     
  2. Doug_G

    Doug_G Lead Moderator

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    Depends on what tires you are using.

    Simplest approach is to use the chalk method. Chalk the sidewall and see how far the tires roll over. Increase the pressure until they don't roll over too much.

    I found with the AD07's that if I increased the fronts more than a couple of PSI I'd get excessive understeer. I haven't run them for quite a while, but I think I was using 32 front and 44-45 rear.

    Later I switched to using A048's for autocross, and did a skid pad test with them. Go figure, the stock pressures seem to work just fine.

    I've also used the following technique: figure out the optimum hot tire pressures at a race track (best to use a tire pyrometer). Then for autocross set the cold pressure to what you were using as hot pressure at the track. Autocross is pretty brief so the tires don't really warm up very much during a run, so using "track hot" as "autocross cold" seems to work.

    Be sure to check pressures periodically through the day. You'll probably have to let some air out as things warm up.
     
  3. 7racer

    7racer Member

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    also, remember that "stiff slides first". So if you do get some understeer you can stiffen the rear sways or increase the rear pressure or decrease the rears.

    Typically at an autox I don't bring a tank of air or a compressor. I start a bit high on all the wheels then bleed them.

    But in all honesty, if this is your first autocross, put them at the stock pressure and just enjoy the day. You will be busy enough keeping track of the confusing cones!
     
  4. Doug_G

    Doug_G Lead Moderator

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    Good point. Sea-of-cones will be your biggest problem!

    If you do have the adjustable suspension, set the front to loose and the rear to middle. That will help control the understeer without making the car too twitchy.

    (Honestly it still understeers quite a bit at that setting, but you will be a bit more likely to spin it.)
     
  5. Kaivball

    Kaivball Member

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    I have not done auto cross but lots of track days in my various cars.

    On the track you will see increases in the 5-7 pound range.

    I usually back that out at the start of the day. So if my ideal pressure is 34 psi then I will reduce the pressure to 27-29 cold.

    I then check at the end of the session and expect to see around 34 psi on the hot tire.

    A temperature gauge will also tell you if the heat is evenly distributed.

    This assumes that you had your car four corner balanced and aligned perfectly to begin with... :)
     
  6. teslasguy

    teslasguy MSP P#1117

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    I've done lots of autocrossing and if this is your first then I agree with the previous recommendation. Just set them at the standard pressure and work on the fundamentals like looking where you want to go, not right in front of you. Go to some courses. This is really a case of less is often more. Focus on being smooth and properly timing scrubbing speed properly when hitting the apex. Have fun and ride with some experienced drivers.
     
  7. Doug_G

    Doug_G Lead Moderator

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    @Kaiball, that's good advice for the track, but autocross is different. You do one lap of a relatively short course, often around cones in a parking lot. Over the course of the day you might get 4 to 7 runs, depending on how the event is organized.

    Even the long course events don't heat your tires up much. Usually you have to let air out of them because the sun heats them up during the day.
     
  8. Kaivball

    Kaivball Member

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    Good point.

    Is auto cross much fun?

    On a typical track day I get close to 200 miles on the race track. Maybe 5 sessions of 25 minutes each.

    How many miles do you get in auto cross?

    Cost varies from $150-300 depending on the organizer (plus gas, tires, etc, etc :))
     
  9. brianman

    brianman Burrito Founder

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    YMMV. Last Saturday, I got 50 runs. This coming Monday I expect to get less than a dozen (different event, sponsors, number of cars, etc.) if I go.
     
  10. Doug_G

    Doug_G Lead Moderator

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    50 timed runs??? That's a LOT. The MCO events I go to have maybe 80 competitors, who get 7 runs each - 8 if things go really well. If we did 50 runs each we'd need to have no more than a dozen competitors!
     
  11. 7racer

    7racer Member

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    yep agree about the tires and the "YMMV".

    Usually here there are 3 "heats". You work one heat (shag cones), drive one heat, and sit one heat. In the heat you could get multiple runs but the autocross course might be about 2 mins at the longest. Some people who track their car don't like autocrossing because it's such a long day for such a short time on the track.

    I like it because you race against other people and can compete with other cars of different classes due to the PAX handicapping system. You might have the fastest time of the day but a Minicooper still beat you due to their PAX time. Plus autocrossing is more violent :) that is you are still "smooth", but not like in a road course as you are usually braking hard, accelerating hard and sometimes transitioning left to right so fast (slaloms for example) that it feels like you are in a cocktail shaker!
     
  12. brianman

    brianman Burrito Founder

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    Yah we had about a dozen at the beginning. And it wasn't formally timed -- it wasn't a competition. Mostly self-competition to improve, etc.

    Again, different kinds of events with different participation, costs, etc. But both under the general banner of autocross.


    In case you didn't notice it, I discussed the event and provided some pictures, video, and charts regarding last Saturday and Sunday:
    7th Matt Ammon Memorial Driving School at Bremerton Motors Park on Sat/Sun 5/18-19 - Page 2

    I was a bit verbose (multiple posts) but lots of interesting (I thought) stuff to share.
     
  13. Doug_G

    Doug_G Lead Moderator

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    Okay, you were doing autocross-style driving, but not really an autocross competition. In that case, yes, you could get many runs in a short time. 50 is still a lot, though!
     
  14. brianman

    brianman Burrito Founder

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    Yah, it was a long - FUN - day. I discovered tired muscles all over the place.
     
  15. 7racer

    7racer Member

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    please report back on how your day went!
     
  16. Pantera Dude

    Pantera Dude Member

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    Thank you guys for all the help. I had a blast! Saturday was practice and Sunday was the competition. I was exhausted after practice day. I made 12 timed runs, (all the runs were timed), and started with five or six ride a longs. I found the rides to be much more helpful than the course walk. I suppose the walk will now make a little more sense since I will have some idea as to what to look for. I was lost on the course for most of the weekend, it is not like being on a track where you usually don't get lost. I'm sure that being lost was costing me a fair amount of time. There was one guy out there that went the wrong way a couple of times, yikes! (though I can see how it could happen). Even some of the old pros were having some trouble following the course, I guess it was more difficult than most. I was slooooooooooooow! People were running from the high 70's into the 90 second range. When I got my first time slip, I couldn't understand the number. I didn't feel all that slow but I did a 113! I hadn't seen anyone out of the 90's. So I'm looking at my time and thinking "did I do a minute and 13"? Obviously not!:biggrin: I knocked nine seconds off on my second run so that was fun. It took me eight runs before I got into the 90's but over the course of the day I cut my time by 16 seconds so I was happy about that. The second day I had an instructor take me around the course before I drove it. (Sunday we drove the reverse direction). She then rode with and coached me for three laps and that helped a lot. I had another instructor ride with me later just because he wanted a ride in the car! (a LOT of people wanted a ride).:biggrin: The second instructor was also telling me to look ahead but he had me looking so far ahead that I had no idea where I was going! I had to modify that looking ahead a bit.:biggrin: In fact the run with him was a 99, the next run by myself was a 93! I made four competition runs on race day and took four extra runs for a total of 20 for the weekend. On the first day I probably could have made a couple more runs had I not gone for the rides. I'll have to go do a better inspection of my new Dunlop tires but I did see a few very small chunks out of the fronts. We had the event on an old military air strip where the grip was very high and apparently hard on tires. I did all the runs in standard mode and had more than enough acceleration. I can see that horsepower is not a huge factor in autocross. The course is about 25 miles from my home so I did a range charge both days and I'm a little concerned that I got an ideal range of 169 this morning where I had been getting 174-175. I hope some of those miles will come back? The hour and a half stint of course working seemed like a LONG time but I guess that is one of the reasons that autocross is less expensive than track events? The instructor that drove my car commented on how weak the brakes were but I really don't get it? The brakes slow the car as much as the tires and abs allow. I felt no brake fade. I'm used to the feel of the brakes. I don't see how different brakes could slow the car any faster? Anyway I had a great time I think I'll get out and do it again soon! I did get some significant rubbing between the tail light surrounds and the decklid. I put some blue masking tape on the surrounds to prevent any more damage. The damage can't be seen with the lid closed. Has anyone else experienced this?

    - - - Updated - - -

    I did run the standard 30/40 psi.
     
  17. Doug_G

    Doug_G Lead Moderator

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    The stock brake pads have very poor initial bite. You have to hammer the pedal. I switched my Roadster to Carbotech AX6 and it's like it's a different car.

    Sounds like you had a good time.
     
  18. Tesla 940

    Tesla 940 Member

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    Pantera Dude - I would like to give this a try - can you provide more information so I can research and attend next time? I suspect I will make your 100+ times look fast.
     
  19. 7racer

    7racer Member

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    Agree with Doug_g...the brakes have poor initial bite and you really have to stomp on it to slow the car down. Regen helps but when you are going flat out, you need the brakes. I really hope they come out with the rumored brake upgrade for the roadster.

    Another thing about looking ahead. As you get more comfortable, you will look further and further ahead. There are times I am spotting the exit of the turn before I am even in it. Unlike a HPDE, in autox I am looking so far out each side window it looks like I'm staring directly to the side. It will really help you drive the course. Also, you probably heard from the walk thru that the closer you are to the cone the less time around the track. Each foot away from the cone cost a second.

    And yes, shagging cones suck...that's why most of my friends hate autox and would rather do track days. If you do autox enough sometimes you can do more of the other jobs that don't require the course work. Also, if you look the guys that are the instructors etc..usually have those jobs and the newbies get the cones.
     
  20. Pantera Dude

    Pantera Dude Member

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    After taking a closer look at my tires, it appears that I was wrong about the chunks out of the front tires. It is just the tire design.

    - - - Updated - - -

    You can click on solo2.com for all the info. It would be great to have another roadster out there! Let me know if you have any questions.

    - - - Updated - - -

    Thanks for the brake info and yes I had a blast!

    - - - Updated - - -

    [. Also, you probably heard from the walk thru that the closer you are to the cone the less time around the track. Each foot away from the cone cost a second.


    That I didn't hear but it sounds like info worth remembering, thanks!
     

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