Rear Instability

[Gneiss]

I made a few changes to my 135i recently and now I am having much more trouble controlling the car at the track. I am hoping someone might have some suggestions.

I got a new set of RT615K+s, StopTech BBK front and an alignment.

Normally at the track I drive with just DTC off and DSC does not noticeably intervene. The last two times as the track DSC was cutting power constantly, making proper driving virtually impossible, so I deactivated both.

Under hard braking the car now squirms and slides, I took out a braking point marker pole my last lap out. It feels like the rear end is what is moving and unbalancing the car.

I know, especially tuned, 135i have been high on power in relation to grip, but I have spun the car twice now on corner exit applying slightly too much throttle too early. This can be corrected with my driving, but it has not felt this unstable before. At my track High Plains Raceway, there is a high speed (~80 mph) down-hill chicane (9a/b) where the car simply feels much looser and scarier than before. Braking instability is however maybe the most unnerving and not something I can correct with my driving. And I don't understand the change in DSC behavior.

Alignment is
[Front] -2.4° camber 0.1° toe-in
[Rear] -1.7° camber 0.3° toe-in

This is however extremely similar to my previous alignment, only notable change is front toe was previously 0.25° toe-in. I thought maybe the new tires were low on grip as they had almost no miles on them before the first track day, but the second day at the track did not seem improved and I figure they must be scrubbed in at this point. A brake-bias issue with the new kit, it is still 6 pot front 2 rear, but my front pads are very fresh and my rears are years old?

Any alignment advice or potential other suggestions would be appreciated.

[spidertri]

What's the condition of your rear subframe bushings, toe arm bushings, etc? The alignment you have looks like the car should be very stable but if it's loose feeling then it seems like maybe something is moving back there.

What tires were you running previously?

[Gneiss]

Rear subframe bushing are M3 as of ~3k miles.

I have no idea the state of other rear bushings, the car currently has 81k miles on it.

I was previously also running the RT615K+ which I thought had performed fairly well.

[amg6975]

Front tension strut bushings make a big difference on braking stability and wear pretty quickly with track use. I was starting to get a similar feeling after two seasons on new TRW arms so I just ordered the Bimmerworld monoball conversion. Results TBD.

[tsk94]

Is that 0.3* toe-in for the rear per side or total? I'd assume total, in which case you could certainly add some more toe in to add traction at corner exit.
Lot's of other things you can do as well. Soften the rear shocks if they are adjustable. Softer rear springs. Lower rear ride height. Lower tire pressure.

Braking instability sounds bushing related as others have mentioned. With the alignment you have, the car should track nice and straight and be stable on the brakes. ABS is still functioning properly?

[Gneiss]

Front control arms were replaced with M3 parts maybe ~7 track days ago. Hopefully those bushings haven't been destroyed yet, but I will have to see.

Rear toe-in is 0.3° total. Unfortunately my budget felt constrained at the time so I am just on Bilstein B14, non-adjustable, I wish I had waited a bit longer and bought something a bit better.

There are certainly some things I can do to try to improve the rear end, but I was wondering if anyone had experienced something that would result in this sudden change. Although to some extent a feeling of "looseness" is subjective, the DSC intervention is a more objective, and confusing, indicator.

I will try to inspect the suspension components and look for failing bushings.

The ABS question is interesting and a component I had not been thinking of. At the end of the longest straight I am hauling the car down from ~140 to ~60 mph, stomping the pedal pretty hard, and I have not felt ABS activate.

[beattiecj]

I ran this alignment if this helps, make you you code out all of the electronic Nannie’s those may be making the car unstable, resolved the issues I had in hard braking zones.