Autocross Options

[ElMurcielago]

So Sunday I had my first experience at SCCA and it was a blast for me. My fastest time was 62 seconds which I find pretty good being my first time. The thing about it though was my car liked to oversteer some which didn't really make sense to me (or those I went there with). Suspension mods are tien lowering springs and KYB struts and front and rear strut bars. If anyone might know a thing or 2 about why I'd like to know.

Anyone else try autocross and like to share experience?

[Bobzilla]

There's a few of us auto-x'er's here. I started this crazy sport in 2007 with my 02 Elantra. Ran that for several years, bought and run a Swift GT, sold that and played with a C4 corvette, sold that and bought hte 2010 Forte SX I currently have. Been to Lincoln for solo nats the last 3 years and all that.

As for set up..... What spring rates are those coilovers? What tires? Tire pressures? Surface? Driving style etc. Lift throttle oversteer is common in FWD cars. When you brake or "lift" it transfers the weight to the front end, losing grip in the rear. Do this mid corner and the back will come around. Knowing more about the car would be helpful and a video of your driving style would as well. What tire pressures were you running? What tires etc.

[RPW00Mirage]

all day son. i posted up some runs yesterday.

[slowgls]

Me too man.

[bhorste]

I'm a 2-time regional autocross champ from way back (1996 and 1998), and that was in a '91 Ford Festiva. If the back end's coming around a little bit, that means you're going just about fast enough.

Chances are you need to bump up the rear tire pressures just a bit if it's coming around like that. An easy way to check the next time you run is to get some white shoe polish (in the bottle with the sponge applicator) and mark several spots on the sidewall right up where it transitions from the tread. If the polish is rubbed off after your run, you're rolling over onto the sidewall a bit and might want to up your tire pressure. If it's still there and you were comfortable with the way the car felt, you're good. If it felt like it was still understeering, you can drop your tire pressure a little bit.

[ElMurcielago]

There's a few of us auto-x'er's here. I started this crazy sport in 2007 with my 02 Elantra. Ran that for several years, bought and run a Swift GT, sold that and played with a C4 corvette, sold that and bought hte 2010 Forte SX I currently have. Been to Lincoln for solo nats the last 3 years and all that.

As for set up..... What spring rates are those coilovers? What tires? Tire pressures? Surface? Driving style etc. Lift throttle oversteer is common in FWD cars. When you brake or "lift" it transfers the weight to the front end, losing grip in the rear. Do this mid corner and the back will come around. Knowing more about the car would be helpful and a video of your driving style would as well. What tire pressures were you running? What tires etc.

Not 100% sure on spring rates, I'm running valera sports 205/45/15, after my first few runs I had to up the tire pressure to get good contact on the corners to 40 psi and on tarmac. I don't have any videos outside the car but I do have one inside, try to post that up when I get a chance.

I'm a 2-time regional autocross champ from way back (1996 and 1998), and that was in a '91 Ford Festiva. If the back end's coming around a little bit, that means you're going just about fast enough.

Chances are you need to bump up the rear tire pressures just a bit if it's coming around like that. An easy way to check the next time you run is to get some white shoe polish (in the bottle with the sponge applicator) and mark several spots on the sidewall right up where it transitions from the tread. If the polish is rubbed off after your run, you're rolling over onto the sidewall a bit and might want to up your tire pressure. If it's still there and you were comfortable with the way the car felt, you're good. If it felt like it was still understeering, you can drop your tire pressure a little bit.

Sounds good, I'll do that when I go to another event and check pressure more frequently.

[RPW00Mirage]

He's got the pressures backward.

To thwart oversteer you lower the rear pressure a bit.

[ElMurcielago]

He's got the pressures backward.

To thwart oversteer you lower the rear pressure a bit.

Will that increase understeer though?

[2000J25SP]

Will that increase understeer though?

should make it more neutral.

Want more oversteer? raise front psi
Want less oversteer? lower front psi
Want more oversteer? Raise rear psi
Want less oversteer? Lower rear psi

My J2 is on lowering springs + Tiburon rear bar. And it will only oversteer a bit if you let off midcorner.

[bhorste]

He's got the pressures backward.

To thwart oversteer you lower the rear pressure a bit.

Negative. By upping the rear pressure, you in essence add some stiffness to the sidewall, preventing rollover and keeping the contact patch on the tread.

[2000J25SP]

Negative. By upping the rear pressure, you in essence add some stiffness to the sidewall, preventing rollover and keeping the contact patch on the tread.

Raising psi adds stiffness but it does reduce the contact patch. I think It feels better, but it loses traction at a lower threshold. Same goes for understeer.

[Bobzilla]

Negative. By upping the rear pressure, you in essence add some stiffness to the sidewall, preventing rollover and keeping the contact patch on the tread.

Raising psi adds stiffness but it does reduce the contact patch. I think It feels better, but it loses traction at a lower threshold. Same goes for understeer.

You're both sort of right. It all depends on the tires, the starting pressures etc. I know National auto-x'ers that run low rear pressures, allowing the sidewalls to roll over to get the car to rotate. I know National Auot-x'ers that run high rear pressures to reduce the contact patch to make the car more nuetral. Again, it depends on the tires and where you start. If you're starting in the 40psi range, lowering the rear tire pressure will reduce oversteer and make it a little more pushy. If you're starting in the 20-25psi range, lowering it will do the same. Tire pressures are a bit of a bell curve. Lower pressures on the left, higher on the right with the "sweet spot in the middle.You can achieve the same effect at both ends of the spectrum.

The tires themselves make a heck of a difference in tire pressures needed/wanted. I ran the BFG Rivals when they came out. 225/45/17 on 7.5" wheels. That tire was happiest at 34psi front, 37 rear on the Forte with the spring rates and rear sway bar I am running. When I made the switch to RS-3 V2 Hankooks I've fought the tire pressure for a dozen events (including Nationals) because that particular tire, on the same wheel and same car likes 40/38 to get the same feel and keep it off the sidewalls (with -3.25* of front camber).

RFEgardless.... If you're getting a little oversteer, that is preferable for an auto-x car over push. I fought push with my 02 and with the Swift GT. NEver got either of them to be as good as the Forte is now.


Here's some videos of my last event in Bowling Green, KY.
https://www.facebook.com/bob.miller.1804109...91/?pnref=story