Hams955

the castor pucks are fine. The adjustable thrust arms allow you to fine tune the castor reading.

these cars are quite sensitive to ride height and especially rake.

Since your ride height has been altered you need to lower the car back to factory spec at a minimum and be sure to set rake properly. If doing this I would consider corner balancing as it would have been altered as well.

What did did you end up with toe settings front and rear?

Chris.

Inrev

Front toe is slightly open
Rear toe is .30 degrees total inside
Height is around stock height they told me.

As tires wore out today I found the car very understeery with snap oversteer tendencies coming out of corners. I also felt the car didn't have as much front grip on high speed turns at turn in, feels like the aero didn't work as well. Could it be a rake issue?

Overall I shaved 1 sec off my best time in the morning but I don't think the suspension setup helped that much and it's more a matter of having a good coach with me in the car yesterday.

Bill Lehman

After reading what's going on with your car, I believe your "race shop", the dealer, is clueless. I'm running a similar camber set-up with no issues but do have proper offset in the wheels. I have no caster correction. Raising ride height to correct a camber issue seems wrong. Rake is important and can be affected by tire diameter. I dropped front ride height about 10 mm and is within factory specs (barely). I wanted to correct for tire diameter but also added a little extra because that helps understeer. Where are your sway bars set?

Inrev

Sway bars middle front and back. Running on the 19in OZ leggerra setup from tirerack.

Hams955

As I mentioned you have too much rear toe for the GT4. too much rear toe will result in exactly your symptoms. High speed understeer that moves to snap over.

I would get your ride height back in check. Set rake to spec.

Zero out toe rear and set front very slight front.

Also - what tires are you running?

let's discuss hot pressures too.

Chris.

Inrev

Running 245/40/19 and 285/35/19 RE-71R hot pressure targets 32-33 psi.

No toe in for rear, won't the wheels open up too much on accel? Using tpc toe links

Hams955

Question -

Did you set the rear toe angle to what TPC recommended?

.30 is around 20 minutes of toe in total or 10 minutes total. That is pretty safe setting for high speed turns.

If you want the car to free up a little reduce toe to around 5 minutes per side.

The car will not push as much on technical turns and be less bound-up. The bound-up feel will cause a push at low speed and can cause a snap-over if you're overdriving it.

Again, set your ride height and rake proper, drop toe just a little.

High speed tracks like RA and Watkins Glenn you need about 10 minutes total toe. Slower more technical tracks you could reduce it to get the car to rotate faster.

32-33 hot on RE-71's is good pressure by the way.

- Chris.

Inrev

I have no idea what tpc recommends to be honest, I went with the max toe in that would help stop that scary high speed braking rear end wiggle. That at least felt more planted. Calabogie is a technical track but I felt the car had a hard time rotating at times in slow corners. Thinking about playing with sways but worried about snap oversteer.

Bill Lehman

I initially ran the same wheel and tire combo. -2.8 F and -2.4 R camber, .03" F toe out. .16" R toe. Tarett R toe links. I don't agree with 0 R toe and have increased my R toe to .24" total to get stability under heavy braking. Stock ride height is 106 mm F and 133 mm R +/- 10 mm. I have switched to 19" CCW wheels and Hoosiers 295/30 R and 265/35 F. To compensate for the small rear tire diameter, I've dropped the F ride height to 92 mm. I'm running F bar Med and R bar Stiff. The car has some understeer with this setup and I'm changing to full soft F for my next event.

Hams955

If you reduce your rear toe the car will rotate faster and you can run less front bar. You're freeing up energy and the car will behave more consistently.

.24" rear toe isn't a bad place to be but reducing will help in high speed corners.

The instability you're describing is due to the bump steer in the rear suspension. Slightly increasing rake actually helps with that.

- Chris

Inrev

If I don't want fender rub, it makes sense that I would need to move the strut as inwards as possible. Is it possible to keep stock ride height with -3.0 camber up front without rubbing? Somehow I doubt it's only a caster issue?

Bill Lehman

My struts are as far inboard as they can go. I didn't realize that your rubbing issue was on the outside of the fender.

Hams955

I am running Tarett camber plates up top. That way you don't need to run so much track width to get the camber you want.

Chris.

Inrev

I'm starting to think I might need those, and since that's out might as well go for the Swift apring upgrade!

Inrev

Yeah, putting a bunch of tire marks on my paint. Doesn't make sense, maybe too much shimming? Could the caster pucks help get more camber with less shimming?