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      12-31-2019, 02:07 PM   #1
romero1990
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Tracking down vibration at certain speed band

This thread will likely become a slow work-in-progress as I am also learning what actually is going on with the car. I bought the car last year with about 120K on the clock, spent a year ~5k miles driving around to make sure there is no major engine/drive train problem before taking it to tracks. The vibration is the most annoying bit that I would definitely want to get rid off.

The setup
  • 18x9.5F/18x10.5R with A/S3+ 265/35/18 F; PSS 295/35/18 R (this set is inherited from the previous owner. I already plan to change them to PS4S 265/285 35 18. And maybe dedicated square set down the road).
  • KW V3, also inherited from previous owner, it is set to fairly stiff, I can barely get any movement pushing on front axle. One of the valve nut is stripped when I bought it. Going to swap it out for B12 pro kit for now.
  • RD Sport front sway bar + bimmerworld end link (also inherited)
  • Front suspension arm/kit were refreshed with new ones about 30-40K miles ago by previous owner.

The symptom:
1. Sort of rolling noise as the car starts to move at low speed. It hard to describe the sound, but certainly related to vehicle speed.
2. Other than the noise, the car runs fine, minimum feeling of vibration all the way up to 75mph.
3. Somewhere between X-Y mph track speed on fairly perfect pavement, there is a persistent periodic vibration. It is very pronounced that the interior rear view mirror will vibrate along. The vibration can be felt throughout the car
4. >Y mph, I think it goes away.
** side note, I got the wheel balanced and alignment done about 1yr ago.(because I felt a bit vibration back then) It went away for a while then came back. So I figured there has to be something fundamentally out of shape causing the problem

The annoying part is, the magnitude of the vibration is inconsistent when it comes up. Sometime it is very very bad that give me chills, while other times not as severe.

Steps I planned to take to figure it out. (feel free to chime in)
1. Swapping the entire suspension set + new mounts. I read from other thread a broken strut mount can cause it. I bought an entirely new set for suspension refresh.
2. Get the wheels checked for roundness. Might just swap the wheels if they are the cause.
3. I know the tires are not matched and has some cupping. (sort of confirmed need to get suspension+ alignment + wheels figured out). Then putting on the new set of PS4S. I don't want to mess up the perfectly new tires.
4. Wheel hub/bearings. I don't hear grinding sound, but the low speed rotational noise could also be from the wheel bearing. The hub is slightly rusted. I am debating if I should just replace it while I am at it or refresh it. (see pic)
5. I am going to replace all wheel studs and lug nuts. Just in case they are bent/streched/bent that caused the vibration.


I will update once I finished something. I really hope it is wheel/tire that is bad. But I don't have a second pair to swap and try.
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      12-31-2019, 06:15 PM   #2
Redd
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Wheel bearing noise will increase with speed. Where is the vibration felt? Is it thru the steering wheel, the chassis, the seat?
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      12-31-2019, 06:20 PM   #3
romero1990
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Quote:
Originally Posted by Redd View Post
Wheel bearing noise will increase with speed. Where is the vibration felt? Is it thru the steering wheel, the chassis, the seat?
I can feel it at steering wheel. But I can also visually see shakes on the interior rear view mirror. I would say it can be felt from the front half chassis.

It sounds more like flapping sound at low speed (~30 mph). Then the engine/exhaust sounds get loud enough to cover it.

not sure if related, I got squeaking sound from wheel well when turning all the way left/right at very low speed, say 5mph at parking lot
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      01-14-2020, 01:31 AM   #4
romero1990
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A minor update as I only have weekends to work on my car. First thing done was actually swapped the wheel+tires. Now that flapping sound at around 40 MPH is mostly from the left side instead of right. I also checked the tires, with -2 degrees camber, the inner is worn down to almost minimum. Running the car at higher speed now came out less shock/vibration at the same high speed.

I tried to wiggle the wheel on the hub, it does not move at all. Nothing loose there.

However, as I rotated the brake rotor, there is a very faint click/tick sound. very hard to tell if it was from wheel bearing or just rotor rubbing against pads. (rotor not 100% clear of pads.)
I'll need rotor change soon (about minimum thickness now). I'll remove the caliper then and see if it was actually the bearing ticking.

The main suspect is the tire now. I will try to find/borrow a spear set just to check if different sets of tire solves it.
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