Stability issues at high speed E92 M3

[blackjackT7]

So I am an instructor with PCA and have a good buddy who requests me as an instructor at each event. I am BMW obsessed and have several M cars that I have tracked but currently run an E46M race car. I am however unfamiliar with the E92M chassis and am struggling to give advise on his car. Car is an 08 M3 with 90K on the clock, TC kline coil overs, single adjustable up front, double adjustable rear. We ran at Road Atlanta as his first event with his new car which was our base line. Car was setup to TC klines recommended rebound and dampening. I cannot remember the alignment specs we ran at RA. Car was great but on the high speed back strait going through the kink before the brake zone the car was super unsettled. To the point we would let off the throttle make the kink and get back on it. If you have ever raced in high winds where on the straits you catch a high cross wind that upsets the car, that's what it felt like. We firmed up the dampening and rebound but it made little difference. After that event we decided to replace the front upper control arm and install a monoball bearing in it, along with front and rear adjustable sway bars by hotchkis. We took it back to the alignment shop and had them align it to TC Klines recommended settings. This last weekend we were at Mid-O, the car did great everywhere except the back strait high speed kink. same damn thing, to the point the traction control light was kicking on at about 125mph. During the heavy brake zone the rear would dance around more than expected as well. I have never had this type of issue before on any of my track cars, E30, E36, E46 none were unstable at high speed. One thing to note is that the full coil over kit was new from TC with the exception of the rear double adjustable shocks, they were rebuilt. The next thing i can think to recommend it send them out to be rebuilt/tested again and or put a wing on the damn thing to see if that tames it. The wing is a long shot. What do you guys think?

[blackjackT7]

Alignment settings are:
Front -3 degrees
Rear -1.5
Im not sure on front toe
rear is 1/8" in

[olemiss540]

So the instability is felt during acceleration events? Are you on the brakes at all? Where is the weight transfer while this is happening? Has the car started doing this recently after a change or is this normal since the owner has had the car in their possession at these tracks?

[blackjackT7]

The instability is only felt at high speed, on throttle it is exaggerated, off throttle high speed the issue is still present. Not on the brakes when it happens. He bough the car and immediately put all the stuff on it to track it. So it has done it from the beginning.

[NeilM]

I'm wondering how the stability control can be invoked in Mid Ohio's back straight kink, which only even counts as a turn either in the wet or at pro race speeds. That's downright scary. I've instructed there in stock E9x M3's a couple of times and they've always been rock solid. Might want to call and talk to TC Kline himself — he's not exactly unfamiliar with Mid Ohio.

Assuming a careful suspension check doesn't show anything bad, I start to wonder about tires. And about that unknown front toe?

[blackjackT7]

It is scary, road Atlanta was the worst. NeilM im glad this is not a common issue with this car. The tires are new (two track weekends) Dunlop star spec's. The car feels great in the technical sections, front strait its decent. Thats a good idea, I will reach out to TC.

[TheJuggernaut]

Not a lot to go on from the description but unless there's a suspension/body motion event that goes under or over damped, like a direction change or a bump, no sense worrying about the dampers. Again hard to say without specifics but say if the front end is darting, then I'd check front toe and caster. Or could just be something loose.

[blackjackT7]

Front end seems planted, rear is where the issue is. Its hard to describe which makes it hard to diagnose without driving or seeing the car. Again im not familiar with this chassis so one of the goal of posting this up was to make sure there wasn't a known flaw or bushing in the rear that causes that type of issue. Which i think one of you would have said something if that were the case. The car has passed the pre-track inspection that i do, which includes having it on jack stands, checking for bearing slop, or anything loose. nothing is glaring.

[TheJuggernaut]

Just sounds like something very fundamental is wrong. A bus can feel stable at 125mph on a smooth straight road, things like damping and wings start to matter when you're not just sailing along. Did you drive the car and experience this? Does anything in particular seem to trigger it, or settle it (hitting a bump, steering/brake input etc)? It could even be driver induced (I've had guys who start sawing at the wheel for no reason when things get intense).

I'd start by checking everything over in the rear, and up on a 4 post hoist because with the wheels unloaded, the loose component can get bound up and seem tight. Every adjustable arm, bushing etc (incl top damper mounts)

[aeronaut]

So at high speed, letting off the throttle doesn't invoke the problem, only steering input causes it to happen?
I know you've had a re-alignment....this almost sounds like rear toe-out induced by something?

[blackjackT7]

aeronaut you are correct, letting off doesn't invoke or exaggerate the problem just steering input. Now after talking about it, it has to be a bushing failing.

[aeronaut]

Time for a crowbar party!

[blackjackT7]

Yup exactly, I will report back once I find it in hopes it might shorten someone else's learning curve.

[AJLM34A]

Car was great but on the high speed back strait going through the kink before the brake zone the car was super unsettled.

This most likely is not to the scale you were experiencing but this is from my dash cam going through 9 at Road Atlanta wide open.

[97m3john]

Can you post what you found as solution?
I'm having same problem with high speed stability. Exactly what you explained

[circuit.heart]

Can you post what you found as solution?
I'm having same problem with high speed stability. Exactly what you explained

Instability exaggerated by wheel torque is likely bushing related, as the guys mentioned above. If you just have a toe or aero problem it'll be consistently not stable.

[lmtfi]

Bushings are the first place I'd look.