VelocityXvolumE

Recently did a track day at COTA and had some interesting behavior at the rear of the car. The setup is as follows...

Square 255 tire setup, Kumho V730's
Ohlins FPSpec 13k front, 11k rear
Dampers set to 7 clicks from full stiff Front, 9 clicks from full stiff Rear.
Alignment, Front: –3.0 deg. Camber, 0 Toe. Rear: -2.5 deg. Camber, 1/16” toe out.
SBG Wilwood track day kit with larger pistons, SBG RX8 rear brake upgrade
No aero
Stock drivetrain

The car was generally loose and rear limited. Lots of oversteer when trail braking at initial steering input, rear recovers pretty quickly after the brakes are released and the car leans on the tires into the corner. The most interesting behavior was through the esses braking into turn 4, here the rear feels very unstable almost floaty(like a water bed type feeling) under braking with minimal steering input. The car must still have some lateral movement from the exit of turn 3. The rear seems to stay unstable for a good period of time, maybe a second and then I can add steering input and the car once again leans on its tires through the corner and is stable.

I'm not sure what direction to take to correct this. No doubt my driving can be better, lets get that out of the way first. I'm thinking the feeling through the esses could be damper related but the overall loose oversteering type balance is related to wheel rates one way or the other(spring or sway bar rates). I'm thinking reducing the sway bar size in the rear could help overall but I'm not sure it will fix the floaty feeling through the esses. I'm curious if anyone else has had this experience and if there is a good fix or compromise to be made here.

I'm also thinking a wing would help a lot through the esses but I'd like to work on the balance mechanically. My overall goal is to get the car well balanced mechanically and then work on aero.

Any input would be appreciated.

Thanks,
Julian

engifineer

sway bar setup?

Sounds like you are mostly feeling a lot of rotation under turn in that goes away when back on power? That is actually pretty much what you want (And is part of trail braking) but it sounds like it is more extreme than you like.

First and foremost of course is making sure your inputs are smooth. When you brake, you put the weight on the front and lighten the rear, when you start adding throttle the opposite occurs so this is how you kinda want it to work to a degree. But if you over do it of course, it will get sketchy. So pay close attention to when you are performing each part of that

On setup, if the car is really rotating too much under braking and you want a quick bandaid, you can stiffen the front shocks. I cannot recall how the adjustments affect rebound and compression on the front though but basically your dampers determine how fast weight transfers (not how much). So if you add front compression stiffness it can help slow down weight transfer a bit. You could also soften the rear shock setting and perform about the same function. I would make large changes (small ones are too hard to judge). So try going 3-5 clicks softer in the rear and see if that feels better to you or if it just feels like trash. Do the same by stiffening the fronts.

you can also go stiffer on the front bar or softer on the rear depending on which bars you are running. However, this will affect how it behaves on steady state cornering as well, not just on entry/under braking. So if the car feels right in the steady state corners but is just behaving this way under trail braking, then maybe try shock settings.

Nothing about your setup screams bad to me. I run the same tires, I run a bit more camber all around but you should be good there really with what you have in terms of balance. Spring rates are sane.

michaelnyden

Is that a typo? Are you really running toe out in the rear? If so, there's your issue, even if you meant toe in, 1/16" toe in especially on an AP1 rear subframe is very aggressive and loose. You are running more front camber than rear so you are going to be more oversteer biased. Also please include details on your sway bar situation as well.

engifineer

Ah good catch, I totally missed that in the post! You want a small amount of toe in on this car, not toe out. Toe out will 100% amplify the oversteer upon lift/braking!

noodels

What have you modified to induce this behaviour,what year ?

VelocityXvolumE

Good point on the toe out, I was surprised to see that when i pulled it from my notes. I guess I should start there. Should I start at 0 or start with a smidge of toe in?

The car is a 2004, sway bars are stock.

Thanks for the advice, I regret not making some damper adjustments while there and I agree I do have to work on my inputs, particularly with the brake pedal.

I’m starting to suspect that there are 2 problems here a balance issue(the car is rear limited in turn 16, 17, 18) and maybe a damper issue through the esses.

VelocityXvolumE

engifeer, you mention running the same tires, what tire pressure do you shoot for hot?

Jub

Well, toe out will definitely do that. These cars generally rotate super *well* under braking. It is one of their strengths but can also catch you out. Every other car I drive, I feel like plows on brakes but I've finally realized it's just that the S drives very on the nose. There are many threads here on toe and people have strong feelings about that debate. Personally, I like on the higher end of things after having tried both. I run .4 total degrees toe in rear on my AP2. That works out to .17 inches total toe in which is slightly less than 3/16".

I ran V730s last year. Those things are stiff AF and were pretty happy at low pressures. I didn't really test much at higher pressures because I didn't have rollover and a local fast guy in a RX-8 ran them dumb low, like 23-24 psi kinda low. I ran mine 26-28 PSI. I did have half a degree more camber than what you're running.

yamahaSHO

Yeah, get the toe fixed in the rear and try again. This has worked well for me:

.Boston.

This is pretty much what I run, no complaints. Add a little more rear toe for it being an Ap1.