[otay]

Could have also posted in the suspension sub-forum, but thought I start here to get more visibility.

2016 550i with Dynamic Handling Package (ZDH) that includes active anti-roll bars and rear-wheel steering. I've had highway speed vibrations since I purchased the car early last year.

Had it into multiple shops to have the wheels road force balanced, with limited success...always told it should be fine now. After the 2nd trip to the BMW dealer, they said both rear wheels were a bit out of round. Instead of paying them $750 each for the two 19" OEM wheels, I decided to replace all wheels with aftermarket 20" and new tires. This improved the overall vibrations at speed, but I'm still feeling the same intermittent "wobble" from the back end.

It seems to be a sort of oscillation from the back, and felt most often when gently accelerating up uphill. It's not like a vibration you'd feel at highway speeds (corrected with the new wheels)... it's lower frequency. Almost feels like something is loose or miscalibrated in the rear suspension...? Although this heavy beast corners well, the issue makes me wish it didn't have ZDH...but maybe it's not related.

I'm thinking of taking it to the other BMW dealer we have here in Raleigh to see if they can figure it out. Anyone else experienced similar? Any other thoughts?

[BrandBMW]

Does this "wobble" also happen in hard braking from high speeds?

[otay]

Originally Posted by BrandBMW

Does this "wobble" also happen in hard braking from high speeds?

No. Does not happen when braking. The brakes in this car feel great.

Another question for y'all: does anyone have the TIS info for the active roll stabilzation and rear steering? Would like to see how it works and possibly disconnect to troubleshoot...

[BrandBMW]

I ask about the hard braking not because of the brakes, but to understand the off throttle response.

When you say oscillation and wobble, I am assuming it is a rotational issue because you mentioned the wheels. The first place I would look is the rear guibo or flex disc, or even the rear diff bushing.

If it is rear sway or rear steering feel which may feel like the rear ARS sway bars are moving on their own, then that could be the rear balljoint (bearing at lower control arm) or upper control arms.

[otay]

Originally Posted by BrandBMW

I ask about the hard braking not because of the brakes, but to understand the off throttle response.

When you say oscillation and wobble, I am assuming it is a rotational issue because you mentioned the wheels. The first place I would look is the rear guibo or flex disc, or even the rear diff bushing.

If it is rear sway or rear steering feel which may feel like the rear ARS sway bars are moving on their own, then that could be the rear balljoint (bearing at lower control arm) or upper control arms.

Gotcha, thanks. The issue is more noticeable when gently accelerating going up an incline.

If flex disc, I would think the wobble would be higher frequency...more like a vibration.

Found this and read thru it, didn't realize the ARS was this complex. Guess I can't unplug it.
https://www.eeuroparts.com/blog/demy...l-bar-systems/

[BrandBMW]

Then another question since you are saying when gently accelerating. Have you changed and checked the fluid level for the ARS? A bad guibo flex disc can clunk/thud, doesn't have to vibrate, and may not be noticeable in all situations. If you take off harder, well then the rotational shift in the flex disc is less noticeable. Vibration in the driveline (felt in the seat) is more so the center bearing. If it is linked to rotational mass, it could also be the rear strut mount, bad strut, etc.

Again, is it a rotation issue or a sway issue? Your description and fix (wheels) talk about two different issues. What you are describing is confusing. Is it vibrations at speed or is it sway.

How many miles are on your car? What's the maintenance history? Have you taken any pics underneath your car to see if there are any strut oil leaks, ARS fluid leaks, if you can see the guibo/flex disc cracks, to diff bushing cracks. You say both rear wheels were out of round? Do you know what caused that, such as hitting a bad bump which could affect the rear suspension?

Remember, sway means you have to make adjustments with the steering wheel. Vibrations you generally do not, unless the tire loses contact of the road or toe is being affected, like with control arm bushings.

[otay]

It's confusing to me, too. More info:

Car has a bit less than 36k miles and I'm the 2nd owner. From what I see in the Carfax and Davis (warranty) report, the car has been well maintained. I do not see any leaks. Everything else about the car seems to be fine.

I did have steering wheel vibrations when I first got the car, but an indy shop fixed a bent front wheel and did a road force balance. Since I still felt rear-end issues (thru the seat), I took it to the dealer.

I've had it into the dealer twice, once to have the oil changed and "wobble" diagnosed, and then took it back for the wobble again. They found no issues the 1st time and said the rear wheels were out of round the 2nd time. That's when I purchased the new 20" wheel & tire setup, but the intermittent issue remains.

For the sake of discussion, let's say one of the various sensors in the system is introducing a small oscillation at a certain speed when on an incline. This is sent to the sway bar and causes it to stiffen and soften a bit. So...I'm not moving the steering wheel, but the sway bar changes stiffness. Crazy? Maybe. However, I don't feel anything from the front which also has an adjustable sway bar. Could be other things, too, like you've pointed out.

[BrandBMW]

So you had 1 Front bent wheel and then 2 out of round rear wheels. That right there is suspect.

Then you say there is a low speed oscillation on an incline and felt through the seat bottom.

First is to check and do your maintenance.
1. You say incline so check the pentosin fluid level
2. You say vibration from the seat. You cannot see the center bearing since it is covered by a heat shield. A dealership inspection might not see it from a lift. Ask someone to ride with you next time since this happens when you are moving.
3. Keep it simple. There could be other suspension issues with the history of 3 wheels appearing to be bent or out of round. The dealership tech might move through inspection fast due to the lower mileage. If there is a computer module issue, there should be a stored code.
4. Go find another dealership or have them ride with you. Be clear about what you know, like 3 bad wheels. Have them scan for any suspension codes, though I am not betting suspension electrical is the cause.
5. If it were a rear ARS sway bar, you would feel the lift/drop. When going straight, the ARS is designed to “disconnect” so each wheel can move independently, thus reducing transferring force/energy to the opposite side and the rest of the body as seen with traditional fixed sway bars. Is that 100% disconnect, probably not or it depends on suspension setting.
6. Even engine mounts and transmission mounts can shift thus on an incline, or when turning, there could be an oscillation vibration felt. In another BMW, I’ve had the transmission shift down due to collapsed mounts and the driveshaft rubbed against the heat shield. The balancing weight rubbed so it was an oscillating sound since it was once per revolution of the trans output. It was felt at lower speeds. On the same note, a bent heat shield or a heat shield that has a torn mounting hole (for a screw) could shift and rub on an incline.

The thing is it could be anything without knowing more. Do your diligence. Go video record it. Go have a dealership tech ride with you. Check your maintenance. And take pics of bushings and parts, perhaps when you zoom in you can see bad bushings or other bad parts.