Vague rear end on higher speed corners at trackdays

[bloke79]

Hi All,

Wanted to ask a question for those of you who regularly track your car.

I've got a 2017 m4 comp with a bm3 GTS style tune (stage 1 low torque), a050 semi comps, BBK, and MSS sports adjustable springs. I've also got a front splitter (with brake ducts) and a rear boot lip fwiw. Car has CS coding on the suspension + GTS diff coding. MDM and stabililty off.

On higher speed corners (think 80++ mph-ish), the car feels very rear-end limited. The front end of the car feels planted, however the back end of the car feels a lot more nervous. I find myself having to constantly make small steering adjustments to keep the back following the front. I've had the back step out a few times doing 100mph++ which was a bit sketchy.

I'd like to think I'm a reasonable steerer, and I wonder if this behaviour might be as I'm approaching the limits of the car as it is. It's only really started to present itself as I've gotten faster.

TL;DR - is there anything (other than aero) I can do to the car to remove some of this "vague-ness" of the rear end at higher speeds? Rear end sway bar links, changes to kinematics, bushing changes? Playing with height?

It's still largely a street car, so don't particularly want to go lower, but happy to experiment with adjusting the front/rear of the car...

Anyway - would appreciate any thoughts of those who regularly track their cars and may have encountered this?

Cheers!

[jfritz27]

Are you running some toe-in for the rear?

[bloke79]

Thanks for the quick reply jfritz27. Good question - from what I remember when the alignment was done - I don't believe there is any rear toe.

Suspension and alignment is a bit of a dark art to me - toe-in in theory should make it less likely to step out yeah?

[jfritz27]

Yes, I think most track junkies on this platform around here run some rear toe-in (I do). This should assist with keeping the rear in check and following the front. I would make an alignment tweak your next step and see what that does. For reference, on the GTS the OEM spec rear toe (in) is 9' per side (18' total). That is same as 0.15 deg per side.

I presume the issue you're describing is NOT due to excessive trailbraking where you're shifting too much weight off the rear too quickly.

[bloke79]

Excellent - well that is probably the first thing to try then. Appreciate the suggestion.

Yep - I run 18's too (should have mentioned that), 255/40 front, 295/35 rear.

re: trail braking and weight transfer - While I can't say with 100% certainty, I don't believe so, as it does it under braking, acceleration and on an even throttle and seems fine at lower speeds.

Originally Posted by jfritz27

Yes, I think most track junkies on this platform around here run some rear toe-in (I do). This should assist with keeping the rear in check and following the front. I would make an alignment tweak your next step and see what that does. For reference, on the GTS the OEM spec rear toe (in) is 9' per side (18' total). That is same as 0.15 deg per side.

I presume the issue you're describing is NOT due to excessive trailbraking where you're shifting too much weight off the rear too quickly.

[bloke79]

To close out an old thread - the rear toe adjustment looks to have fixed it!