I have been auto crossing both my Fiesta ST and Boss 302 but keep having issues with the Boss. The rear end keeps wanting to come around and I have spun a few times. I have been trying to be gentle on the gas and wait until it is straight to accelerate. I have tried lowering the rear tire pressure but it didn't help much. The S550 Mustangs seem to stick extremely well. Is it because of the live axel? Would changing something help? I would enjoy auto crossing the Boss but it has been frustrating.
Thanks,
Jon

 

 

Lowering rear pressure usually increases oversteer. Raising the rear pressure and/or lowering the front usually helps. So does stiffening the front suspension or softening the rear. This can be done with adjustable shocks or sway bars.

 

I guess I had it backwards. I was running about 40 in the front and 35 in the rear. The shocks were set on 3 on the front and 5 on the rear.

 

Have you considered the Watts Link?

 

hmmm . . . I am not so sure you have it backwards, as I understand it "softer" will increase traction on that end. When you reduce pressure, you make the tire softer and increase the contact patch which should increase grip. A softer sway bar also makes that end act softer and increases grip on that end. I am not an expert on this by any means, but I don't understand why the tire pressure and sway bars would act opposite to each other.

that said -- I think your tire pressure is way high. I assume you are talking about cold pressures, so hot pressure is higher. On the road track I usually shoot for about 35 psi hot, which is around 30 cold. If you are at 35 cold in the rear, that is probably close to 40 when the tires heat up. Since the runs are short in autocross the tires might not get a chance to heat up much, so I'd probably try 32 cold and see how that works.

You can check tire pressure using the "roll over" test. If you look closely at the sidewall, you should see little marks molded in; I think they are triangles on most brands. The tire "should" roll over to the middle of that mark on the hardest cornering, right before it loses grip. You can often see the scuff marks, or you can mark the tire with chalk before a run to make it easier to see. If the tire is not rolling over to that mark, the pressure is too high; if it is rolling over past that mark, then the pressure is too low.

Also, how are your tires? Are they the original Perelli's? If so they are probably shot, and they weren't that great to begin with. A good sticky tire probably would help.

You could try softening up the shocks also; stiffer in the rear is not what you want if you are having oversteer issues.

Last thing, I'm wondering if you have a real stiff sway bar on the back. "Bigger is better" is not always true with sway bars; if you or the prior owner "upgraded" to a bigger (stiffer) rear bar, that could be a contributor also.

 

Bar, pressures and a watts link all help with rear grip off corner.

Tire pressures, search around for what may work. Cheapest

I went to a 18MM bar, off a stock mustang. It was a takeoff piece. Cheap. Helps keep the inside tire planted.

Watt Link helps plant the rear. Expect to spend a Grand.