Z4M "rocks" when it changes from accelerating to decelerating (or vice versa).

[wyshyvanuk]

Tough to describe the symptoms, but it feels like the suspension is taking a fraction of a second to "catch up" to inputs from the accelerator or brake pedal. The symptoms pre-dated the installation of TC Kline suspension components. New motor mounts have been recently installed. I'm thinking that the rubber suspension components might be at fault (48k miles), but I'd like to be pointed in the most likely direction. Thanks for any insight!! Ted

[Phatty]

sounds like your suspension is too soft, either dampening or spring rates, if they are adjustable, crank up the rear stiffness and see if it helps.

[mmm-five]

I found the standard mounts (even when new) allowed to much movement during hard acceleration/braking. I fitted Vibratechnics road mounts (360M were c.£130 each, which is cheaper than the price for OE parts from BMW; US site shows $325/pair) and it removed the excessive squatting & diving completely.

This was about a year before I went further and fitted Bilstein B16 PSS10 suspension for better/less choppy road manners.

OEM


Vibra-Technics


Stiffness comparison


They're rebuildable too, but I've had mine on for over 60,000 road & track miles and they still look great.

https://www.vibra-technics.com/bmw/z...t_engine_mount

[wyshyvanuk]

I actually replaced a set of Vibra-Technics with 034MotorSport mounts. This was part of a project to get the oil pan off the anti-sway bar. Yes, the bolts securing the engine supporting bracket on the passenger side were either missing or backing out. I know this is difficult to believe, but the visible rubber portion of the Vibra-Technics were looking a bit "squished", after not too many miles. The 034 mounts are solid rubber, and, supposedly, provide a stiffer mount than OEM.

[wyshyvanuk]

Phatty, The shock absorber/strut settings are very soft, in compliance with TC's very detailed instructions. But that issue with the rocking existed prior to the TC Kline installation, and the TC Klines replaced Bilstein B6s. There's another Z4M in the club here and I may beg for a test drive to compare mine to his (I've been known to occasionally suffer from delusions).
Although I was about to start randomly replacing suspension parts that have rubber components, I'll get the car in the air first and hopefully see something that obviously needs attention. The fun continues.

[StealYourFace2]

Crazy question but I like to rule out the easy stuff -not meant as an insult but have you checked the rear tire pressure? If on of the rears are low it will change direction with throttle (don’t have to be hard on it)

Then there’s every rear bushing from trailing arms to the diff -

If it’s forward/rear movement like drivetrain play I agree with checking the easy stuff - motor and trans mounts, subframe bushings, diff bolt and bushing, guibo …

Does a clunk happen at the same time you experience this?

[wyshyvanuk]

Well, I check tire pressures frequently enough to rule that out as a cause. "...forward/rear movement like drivetrain play...." that's a perfect description. I was hoping for a solution like "squirt some WD-40 on bolt #17", but it's starting to look more like me under the car again for an extended period. I started watching a series on dropping the rear subframe until it got too scary. No clunks that I've noticed, but I'll pay closer attention. I have a QuickJack....the sad part is that it gives you just enough courage to make you think you can handle a project like this ;~). If you have any diagnosis tricks, I'm eager to hear them!! Thanks very much, Ted.

[StealYourFace2]

Are you saying the car rocks on and off power as in the front raises and dives? Is this on track?

Without changing any parts can you get away from that uncomfortable section in the next highest gear?

Worked wonders for me at the glen at the suggestion of a driver more experienced with a more powerful car

Edit - I was experiencing some unwanted compression up the esses at the glen in 4th - the car was right at the top of the power band.

Running up it in 5th calmed the car down and increased my exit speed.

[wyshyvanuk]

More a forward/backward movement than an up/down movement.