ZX-Tex

iTrader: (15)

So today I tried out the new (to me) 15x8 6ULs with the brand new NT01s in 225/45/15. Very sticky, love them.

But, I have a small suspension setup issue. The car was aligned with 205/50/15 tires on stock 15x6 wheels. With these tires/wheels, there are no issues on the track or street, pretty balanced with a slight bit of throttle-on-exit oversteer. Car tracks straight. I would have to go find the alignment sheet since I forget all of the numbers. But, off the top of my head:
front camber - about 2.0-2.5 (all I could get)
rear camber - about 1.5-2 (all I could get)
front caster - about 4
I forget the front and rear toe in, but it is not much.

Under heavy braking the car is very twitchy with the new wheel/tire combo. It wanders back and forth slowly, maybe at about 1-2 Hz, and is sensitive to road imperfections, especially under braking. Once or twice it was quite instable, and I had to fight the wheel to keep it pointed in the right direction while braking over an uneven surface. I think what I need is a bit more front toe in. I am thinking I could just turn the tie rods equally to bring it in a bit.

Thoughts? Hoping the 6UL big tire crowd has seen this problem and has an answer.

y8s

iTrader: (8)

it could be the front and rear towing out from the strain on the bushings

it could be exacerbated by the increased weight transfer from the stickier tires taking the load off the rear of the car and causing it to lighten up.

I haven't done a lot of hard braking yet with the 15x8s but i dont have the sticky tires either.

ZX-Tex

iTrader: (15)

Funny you mention those as I was wondering the same things. On the former, I could see that happening. On the latter, the back-and-forth feels similar to what happens when I brake really hard on the race bike, especially when the rear is sliding a bit.

crashnscar

iTrader: (9)

You have bent control arms or subframe. -2.5 front is pretty typical to be limited to (a lot can get to -3.0 or so) but the rear being limited to -1.5 or -2 means you've got something going on back there. I'd figure that out before trying to fix this wandering issue, as it could be the same thing causing the problems. Rear should be able to get in the range of -3 to -3.5.

Not sure why changing wheels/tires would change it though. You said new to you, maybe a wheel is bent?

And you don't want toe in up front. 0 or a tiny bit of toe out (1/16 or so). Tiny bit of toe in on the rear (between 0 and 1/8th).

Savington

iTrader: (31)

What's your ride height?

ZX-Tex

iTrader: (15)

11.75" front, 12.5" rear. I may raise the front 1/4".

ScottFW

iTrader: (4)

The behavior you describe under braking is "typically" a result of a toe-out condition in the rear. And remember that suspension bushings compress when the car is being driven so your static measurement in the garage is not what it actually is on track.

An instructor I know spent most of a day at VIR tinkering with only the rear toe settings on his car. And the SM driver who corner balanced and aligned my car paid more attention to getting the rear toe set accurately than he did on any other measurement. A small adjustment of rear toe can make a big difference in handling, and even a little toe-out in the rear will lead to what you describe. With 225 RS2s (not shitty, but not that grippy either) and 1/16" total rear toe-in my car brakes straight and doesn't wander, but with NT01s you could probably use a little more. I think most SM guys run around 1/8".