Autocross tire pressures

Discussion in 'Fiesta ST Autocross' started by Smokin, Aug 5, 2014.

  1. Smokin

    Smokin Active Member

    Ok, I'm seeing huge differences in tire pressures guys use to autocross. Just like me I think we use what the "locals" recommend and never sway far from it. So for those of you who compete regularly and seriously, please post the tire and size and cold and hot tire pressures you use. Track guys and tuner dudes post on another thread....

    I started the season with 225/45x16 Direzza ZIIs. 38/50 cold. bleed off to 40F when hot and leave R alone (+/-53).
    New Falken 615ks 215/45x16 last weekend, same 38/50 cold. 40/50 hot.

    This weekend I'm in Lincoln for a regional where we might 6+ runs so I am going to start with absurdly low pressures and go from there. I'll report.
     
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  3. CrookedRacer

    CrookedRacer Active Member

    I currently run 225/45-16 Dunlop Direzza ZII's on 16x7" rims.

    Cold temps, I run 37psi in front, 48psi in back.
    Hot temps, I bleed off pressure between runs (as the tires warm up) to maintain about 37-38psi in front, and I just leave the rears alone.

    I find that at 36 psi or lower, and the sidewalls just buckle under extremely hard braking. It gets uglier at turn-in. It sounds horrible (FWAP FWAP FWAP). Above 38 psi it still can happen - just not every damn time.

    At 38 psi and above, there's noticeably more wheelspin on acceleration. The contact patch is just not as large, and so the wheels just spin.

    So I've found 37psi to be the sweet spot for me. It's an unhappy compromise: At that pressure, I'm still not quite happy with the contact patch, yet still not quite happy with the sidewall stiffness. And even when I'm diligent in keeping that pressure, the tires get greasier and floppier at higher temperatures anyway. My first runs are usually my best.

    Incidentally, I just bought a sprayer, so that might help this weekend.

    Can't wait to hear how your Falken Azenis work out for you!
     
    Last edited: Aug 6, 2014
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  4. eRic

    eRic Active Member

    I have been autocrossing with 42 psi front and 39 psi rear cold in the OE tires with the OE suspension.
     
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  5. Nik17

    Nik17 Member

    At the beginning of the season I was still experimenting with my 215/45/R17 Rivals and was running them at about 45 psi front and about 38 psi rear hot.

    The last two events I've lowered them to 37/38psi front and the same 38 psi rear hot. I've definitely seen a noticeable difference in front end grip without any real negative consequences.
     
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  6. CrookedRacer

    CrookedRacer Active Member

    So I edited my post above for the pressures I run... but here's my philosophy for FWD cars. Not meaning to offend anyone who knows this already, just trying to help.

    Fronts: You want to run your pressures as low as you can without rolling onto your sidewalls excessively. That's it.
    Which means you have to find out for yourself what that pressure is going to be.

    Rears: Play with this to get your car to "rotate" if need be. I'll run very high pressures (48-52) on small, tight courses where speeds are low and I want to really allow the car to come around. I'll run more normal pressures (40-45) on large, sweeping courses where I want the rear end to get more grip and "stick" through the sweepers without getting unnecessarily squirrelly.

    People can suggest good starting points for you to try, but it will come down to you, your car, and the conditions you're in.​
     
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  7. Smokin

    Smokin Active Member

    I've not tried to go much below 38 on the fronts as I do get roll over on the sidewalls. Especially on the Dunlops. The Falkens are working good and at 39 they are wearing right to the pointer arrows. I might go to 37 like CrookedRacer this weekend. BTW we had 4 runs last weekend at Lincoln (I had 5 because run 1 had a timing error) with no water to cool them and short time between runs. The last run was my fastest by a little under 0.4 sec and they were hot! That concrete up there has giant grip and the turns were medium speed. I did not hit the rev limiter like at Topeka the week before and were doing 61 second runs.

    I didn't have a chance to compare runs with the 17" Federals even though I took them along. I just think now, comparing times between Ft. Riley and Lincoln, they are just too heavy with the OEM tires. They can work on the FoST because of all that torque.
     
  8. Cligedy

    Cligedy Active Member

    Was that with the Falkens? On the old 615 if you didn't spray, 2 runs would overheat them.
     
  9. Smokin

    Smokin Active Member

    Yes the new Falkens, heat cycled by Discount Tire Direct. Same thing again this weekend at the Midiv 2-day event: 4 runs, 80 second courses (last year's nationals courses). They did not get as hot as the week before but I measured them with infrared Sunday at 140 deg. They stuck great. I am really happy with that tire. If only they would do a 225/45 in either 16 or 17. Was 0.9 sec off Umholz's FoST Sat with the 80 sec course. Was in a position to catch him Sunday in run 4, had fantastic grip in a slalomfest course and then had a brain fart and missed a cone for a DNF. I watered them on Sat and not on Sunday and there was no diff. BTW I started with 37 psi but could not resist the urge to bleed off less than 38 frt after run 2. I think they would do just fine at 36-37 psi but just couldn't make myself do it. The best grip was 38/50. When the rears got to 54-55 they got too loose.

    I'm happy to get brain-fade out of the way for Nationals. I have no aspirations other than being the fastest Fiesta ST there.....
     
    Last edited: Aug 18, 2014
  10. CrookedRacer

    CrookedRacer Active Member

    My thoughts exactly. :)
     

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