SCCA Autocross setup thread

Discussion in 'Fiesta ST Autocross' started by AlanBDahl, Oct 21, 2013.

  1. Des

    Des Active Member

    Congrats Alan! That's a tough crowd. And congrats to everyone who had a great opening weekend! Nice to see expectations confirmed.

    Luckily we can autox all year, so yesterday was Round 2 of the Fiestus Smackdown. This was concrete event set up primarily for CP and SSM cars, really. Nothing remotely technical, quite definitely a power course.

    In preparation I found a little more front camber in the Fiesta (now at -1.4) and took advice to go bold on rear pressures, so started the day at 42/56. I let the backs climb to 58. But neither Josh Salvage, who was co-driving, nor I could get anything more than a little lift-throttle looseness. At the same time, Mark Scroggs in his Focus on RS3s, didn't make it 300 feet before spinning off course, but he found his footing as the tires and day warmed up.

    My best: 40.000
    Josh's best: 39.996
    Mark's best: 38.6XX


    Afternoon, Mark and I switched, both cars set up as they were in the morning. Mark promptly cut a half-second off in the Fiesta, then burned the next three runs trying to force it to turn. For his last run, we dropped 33 pounds of air out of the back. THIRTY THREE.

    Running 25 PSI in back, he dropped another half second, down to 39.102. Which is pretty awesome, just a half second in back of his Focus time. But he said the difference was mainly from not fighting the car as much, though it did turn better.

    I was at 38.8 in the Focus, and it wasn't difficult to get there. I was dropping 2-3 tenths each run.

    While I'm disappointed it's not me making the Fiesta go faster, it's nice to see that it can. This was a worst-case scenario for comparison: Concrete makes any car pushier, we were on the rev limiter but the Focus wasn't, and there were three digs where power mattered a lot.

    But after four events in the Fiesta, I'm still not seeing anything resembling tail-happy.
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  3. AlanBDahl

    AlanBDahl Active Member

    Des, I think you have both the front and rear tire pressures way too high. I’m expecting to keep all my pressures in the 35-40 range unless I see extensive front tire wear and my taller sidewalls are sure to have more flex than the shorter sidewalls on your car. Just IMHO as I have no dry time in the car yet but I’ll probably start around 40 when I do run in the dry and go from there. WRT rotation that’s a tricky thing as there can be several stable plateaus as you go up/down with the rear pressure but one might be significantly faster than another. I encourage you to try a wide range of pressures before settling on one.
  4. Des

    Des Active Member

    You're welcome to come experiment with it in San Diego. :)
  5. k757

    k757 Active Member

  6. Des

    Des Active Member

    One add: Josh did Laguna Seca the day before in his car, on the 225/45/16s. He reported some rubbing at the bottom of the Corkscrew, thought it was the outside edge rubbing the fender liner but no damage done.
  7. MLKN

    MLKN Active Member

    I still cannot understand the logic of running low pressures in the rear. If the car tends to understeer, and you want more play in the back, one should have a smaller contact patch, hence higher pressure. Yes?
  8. Des

    Des Active Member

    In theory anything other than optimal pressures would result in a smaller contact patch.

    Usually you get a fairly obvious result with a dramatic increase or decrease. If 58 won't do it, there's not much headroom to go higher.

    But it may be that the bar I put on is skewing my result, or the higher pressures I am using in the front, or that the Dunlops are grippier at the margins than the RS3s are.
  9. jimclark

    jimclark Active Member

    MLKN wrote: "In theory anything other than optimal pressures would result in a smaller contact patch."

    Is this true? In cycling the argument is the other way 'round--lowered pressures do indeed result in a large contact patch, or so goes the reasoning; when I raced, we reduced pressures in wet for this very reason.
  10. MLKN

    MLKN Active Member

    Yes. In wet conditions you run lower pressures. However, too low and you are driving on the sidewalls, beyond the indicators, which is not good either. To summarize, optimal pressure on the front maximizes the contact patch (to the indicators) which will vary by tires. On the back you can use a range from optimal to overinflated. Overinflated (above optimal) pressure narrows the contact patch to the center of the tire, reducing grip, and increasing tendency to wag out, which in our car is desirable.
  11. MLKN

    MLKN Active Member

    That being said... I want to offer a sincere congratulations to everyone who has been having success with the Fiesta. In case you missed it I gave an in depth analysis in my post from Sonoma. The response from passengers in the car including people coming from STIs, Evos and BRZs was "Wow". This car punches weigh above its weight class in terms of handling and speed on autocross. It gives you the confidence to take on lesser drivers in superior (ie Porsche Turbo) cars.
  12. spangenb

    spangenb Active Member

    I believe that since car tires have a wide and flat area for the tread, unlike bike tires, running underinflated causes the center of the tread to "fold up" under load thereby reducing the contact patch. Like was said, going over or under the optimum pressure should reduce grip, and each will have other effects on handling or "manners". You just have to decide which way works best for your style and what the car wants.
    Last edited: Mar 17, 2014
  13. RodMoe

    RodMoe Well-Known Member

    Not sure if this is true with the short sideways but lower pressure would allow them to lean or roll over more letting the car rotate more or less with sideway flex .. Or no ??
  14. spangenb

    spangenb Active Member

    I still think with heavy side load rolling the sidewall over and not enough pressure to hold the shape of the tire you will buckle the tread area and cause a reduction in contact patch.
  15. Mightymango

    Mightymango Active Member

    Seeing all the posts about pressures and such it seems like there is a bit of difference as to what works for one may not work for another (which makes sense) Seems like the warmer areas and concrete events are benefiting from the higher pressures in the rear.

    6 more weeks until I can accurately post my results....come on spring!
  16. RodMoe

    RodMoe Well-Known Member

    Warmer temps and Diff tires too make a diff I had heard the BFG Rivals are not as picky about pressures as the DZll's are. But that is just internet chatter as I have not tested both only the DZ's last year and am planning on going BFG this year but have not had a need to aquire them yet waiting on Spring and my wheels to show first LOL
  17. Nik17

    Nik17 Member

    Step 1.) Acquire tires that may or may not fit
  18. limbo

    limbo Active Member

    So doing this in the rear would mean less grip or possibility of better rotation but only if you push it past the point of deforming and I'm not sure id be comfortable at that low of pressures. I like the idea of minimizing contact patch with more psi in the rear but haven't had results. The car tends to come around pretty well with a deliberate throttle lift for me. I've found reverse of oem specs 36f/39r to be my sweet spot.
  19. wash

    wash Active Member

    Radial tire construction is about as far from bicycle tire construction as you can get.

    A motorcycle tire might behave similarly, maybe a wrinkle wall slick or sand rail tires too but anything like a road car tire will behave quite differently than a bicycle tire.
  20. Slowclimb

    Slowclimb Active Member

    Really interested to see how these fit
    Nik17 likes this.
  21. jimclark

    jimclark Active Member

    I think I get what you're saying here. Such being the case, the amount of "fold up" will be very dependent on construction all around, including especially sidewall/tread curvature area as well as central tread area. This makes the whole thing very much a trial and re-trial sort of thing--very, very much tire dependent.

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