Setting up BP car, ? on Scrub Radius
 

Hey all i'm setting up another auto-X car and wonder if anyone has any experience on Scrub Radius or Steer Axis Inclination...

I've only experimented with this on 17x9" with a 22mm offset and it was not horrible just took a bit getting used to.

I will be doin custom Koni's and want to run a 16x12" wheel with a 6" backspace (0 offset) and wondered how much this would affect Scrub Radius or General steering response on the course.

If anyone has any foresight or possible downfalls/opinions to doing this please jump in....
Thanks
Jason


p.s. yes this is strictly a track car.

IIRC, a large scrub radius will make the car darty over bumps, and SAI creates corner jacking.
You can counteract the SAI effects with more caster, but then you end up with heavier steering.
I think Milliken and Milliken suggest you avoid as much SAI as possible. But with struts and very wide wheels it's hard to.
That's why I'm going to convert my FP car to double a-arms. It would be legal for you to do that as well for the BP car. I'm assuming that's autocross B-Prepared, not road racing B- Production.
Good luck

I am curious, will you "front half" the car, or something less drastic to do the A Arm conversion? Also, FP? I thought TII were in BP... Thanks, Carl

TII's are in BP... I have an NA.
At this point I don't think I'll have to tube frame the front end, since the boxed section of the subframe/chassis is right where the upper a-arm mounts will go.

Yea its B prepared..
I have talked with Tripoint a bit about this and when they did it on their car too. Seems like you wouldnt have to front half it to accomplish this but i've only started to scratch the surface of this.
I am still undecided if i wanna have the struts brought in farther so i can do the 16x12-6" in the front or just keep it to a 16x10 with a 5" backspace..
I guess thats the major thing at this point. I'm wondering if there is going to be a big difference between the 16x12-6" backspace and a 16x10-5" backspace... any thoughts?

Hey Carl would you mind PM'ing me some info on your wheel/tire choice?

I think for the power of a BP car you would want more tire. Besides the sidewall is really short on the 16x10 Hoosier most people run. Where as the 16x12 is a little more forgiving. I'm considering running a 15x10 for the extra tire selection.
If you modify the bottom of the strut for more clearance (by moving it in) I don't think that really changes anything. Doesn't the lower ball joint connect directly to the upright? So the line between the upper strut mount and the ball joint hasn't changed.

Yea i also thought about the 15x10 or 15x12 as there is plenty of stickier tires out for them i'll just have to do some more measuring for the brake caliper clearance since ill be using Diamond racing wheels...just to make sure theres not gunna be any clearance issues..

Yea i suppose that moving the strut in would have almost no affect.... I'm wondering if moving it in 1" at the mounting holes will be sufficient??

thanks for the help guys..

Actually, you can go 5.5" backspacing in front without moving anything. Since both Real, and Circle only offer full inch increments, I compromised in the front with a 16x11, 6" backspace with a 1/2" spacer. It is worth mentioning a 24.5' diameter rear maximizes the available clearance to the trailing arm. I am running a 13" slick in the rear. As far as my selection, I do not know if it is a good one yet, but I am currently set up with 16x12 rear with a 24.5x13x16 bias ply goodyear slick, and a 16x11 with a 23.5x11.5x16 front. I am going to try a little larger front tire shortly(24.5x12x16). I am hoping that Hoosier may come out with something trick for Grand Am next year, that I can "borrow". Currently my tires come from vintage Porsche 935 cars. My concern right now is the car may be too high with these tires. Carl