Squirelly rear, what could be the cause

[BishamontenZC6]

I've been reading thousands of posts on 3 forums but still cannot get a good answer to my problem. So here it is :

I installed ST STX coilovers on my 2014 BRZ. It's now at -20mm, which is the minimum lowering allowed by those coilovers. Front are mounted with STI Group N top mounts.

At the same time, I installed 17x8 ET35 wheels, on which I'm finishing my current set of tires, 215/45-17 Bridgestone Potenza Sport. 215 being the minimum size for 8' so there's a very little stretch.

In terms of chassis, I already had STI Flexible Tower Bar, STI Flexible Draw Stiffener, Cusco steering rack bushes and STI steering rack long bolts in the front. For the lowering, I added SPC LCAs in the back.

With all that, I had the car aligned last week, with the current specs (both sides equal) :

Front :
Camber : -1.8°
Toe : 0
Caster : 4.9

Rear :
Camber : -1.4°
Toe in : 1.5mm (1/16')

Front axle tracks dead straight and is very stable and responsive during turns. Steering is pretty direct and precise. There's only a little bit of tramlining. My concern is more about the rear, which I find quite "squirelly", especially under the rain. In essence, if the pavement is not in a perfect shape, it's constantly making the car bounce in a lateral way, as if it was looking for lateral grip, even going straight. Like it's slipping and getting grip again, but with high frequency and with a lateral movement.

I'm trying to narrow down what can be the cause, as I will have to change the tires soon. Could it be :

- Setup related (coilovers or wheels) ?
- Alignement related ?
- Tire size related (should I switch to 225 with 8' wheels) ?
- Tire wear related (about 1.3mm thread left) ?

[Muskoka800]

Perhaps you can define ‘BOUNCE in a lateral way’.
Do you mean it’s sliding (horizontally) or literally bouncing (vertically) which is hard for me to imagine from new dampers.

[BishamontenZC6]

It's bouncing horizontally, from left to right, following the shape of the pavement. There's no noticeable vertical bouncing other than the coilovers being a little bit more firm than stock setup. It's like every road imperfection upsets the car in a lateral movement and makes it feel not planted. On dry road, it's less noticeable except if the pavement is really bad.

I drove the car with stock wheels/tire size for 500km right after installing the coils (and before alignement) and there was no such thing, so I don't really think it's a tire wear problem. I could drive fast as usual without anything special, bouncing or grip problem that would make the car feel unstable. I'm used to the car as I've had it for more than 3 years now.

It all started after installing the wheels and alignment (done at the same time). But rim size and specs are nothing special (I think ?), and alignment specs are regular recommandation for street driving. I keep on thinking it might be the extra sidewall stiffness of 215 on 8' stretch, but I'm not sure. That's weird, never had that before, especially with such a conservative setup...

[Ohio Enthusiast]

Originally Posted by BishamontenZC6

It all started after installing the wheels and alignment (done at the same time). But rim size and specs are nothing special (I think ?)

I would agree that wheel size isn't different enough to explain the issue (unless they are extremely heavy wheels or something). Do you have the full alignment printout, especially the "before"?

Might be worth it to get another alignment (or at least measure current angles), it might be that the toe in the rear slipped (which is reported fairly often).

[BishamontenZC6]

Sure, here's the printout. It's in French though. Top is front and bottom is rear.

[BishamontenZC6]

Oh I think I know where the problem is... I asked for 1.5mm toe in in the rear, and they put 1.50 degrees... With 17' wheels, 1.50° is 11.3mm (7/16') !!! It has to be that, correct me if I'm wrong.

[whataboutbob]

I'd be looking at changes in rear toe. Check the rear LCAs and toe arms to make sure their adjustments are fixed (nothing loose). I'd also second verifying your current alignment. Toe out can cause the issues you are describing. If the LCA and/or toe arms aren't fixed (nothing loose), you can get dynamic changes to toe, which will make the rear end move around.


I experienced this once while adjusting my rear toe arms, and the car was kinda terrifying to drive.


Hope this helps.

[BishamontenZC6]

Many thanks for your help guys. I don't think there has been any slippage as I only drove 50km on the highway since the alignment and I didn't hit anything.

But what's sure is that with 7/16' instead of 1/16' toe in at the rear, the car isn't going to behave properly and the symptoms I'm having make sense under that light... I will call the garage who made the alignment and ask them to correct that under warranty.

[autoracer86]

In the past I have asked for X amount of TOTAL rear toe and they would put the total amount on each wheel lol. Sounds like you just have too much toe as you found