Weird vibration at 90-100 mph - doesn't exist with extra weight?!

[AndrewBird]

Have you checked wheel bearings?

[Simon S]

oh - and i'd also rotate tires..

double-check easy stuff first

.

[Simon S]

Maybe my diff is low on fluid?

easy to check - pull filler plug and stick your pinky finger in there

I'm pretty sure the driveshaft is good, but maybe my u-joint is goin or somethin.

CSB condition ?

[Sean]

Did anyone have anything to add on this matter?

[Sean]

UPDATE

So, I did some more high speed driving this weekend.

Out on the highway, and some other roads - some pretty loud/can definately feel in da butt, vibrations at 90mph and up to about 105. Pretty damn apparent.

On the way back from the hills, I realized that at certain RPM's, at any speed, there's a slight...rattle that comes from the rear. Almost like the diff is loose or something, and it's noticeable at that given load range.

I got udnerneath the car this morning, and everything is tight. I thought it might be the diff loose or a loose halfshaft - everything is tight.

So, here's another idea. Maybe my diff is low on fluid? Maybe it leaked some when I first filled it, and it's not full?

My other, and most likely idea, is that one of my bearings only makes noise at a given RPM. However, I changed the load ever so slightly, and the noise changed. This leaded me to believe it was driveshaft related or something around teh diff. I'm really at a loss.

I'm pretty sure the driveshaft is good, but maybe my u-joint is goin or somethin.

Just wanted to throw this up there. ;)

[Sean]

Thanks for the suggestion's guys - I'll be finding out if it's the bearing soon, hopefully. ;)

[Koony]

Just to add to that, bad wheel bearings sometimes create excessive amounts of heat. After one of those 90mph drives, feel the wheels around the hub. A hot one would indicate a problem. I had a front hub bearing go bad and it was glaringly obvious to the touch.

But....... the vibration was constant at any speed.

[AndrewBird]

Yea, the driveshaft won't be affected at all by ride-height, so I don't think it's that. It could be my differential, but I don't see that being affected all that much by ride height.

You are correct because the diff is stationary and does not move up and down with the suspension.

I would check wheel bearings. Here's how:

Jack the back of the car up so the wheels are off the ground. Take the parking break off and take the car out of gear. Spin the wheel as fast as you can by hand. It should spin smooth with near no noise. If there is any grinding noise or the wheel seems to "wobble" in any direction, it's a wheel bearing. You can also grab each side of the wheel and push and pull alternately on each side to see if there is any play. There shouldn't be any.

[gazellebeigem3]

The alignment is irrelevant because that rarely causes a vibration, so chuck that out. Personally i have had vibrations that come and go at certain speeds but not that vary under load.

[Sean]

Good luck getting me to go under 90 mph in the middle of Wyoming...yea, no. lol

Secondary imbalance? Tell me more. I've been meaning to switch the front and rears - although I coulda sworn I did that when I put them on after thier re-balance I had done about a month ago.

The trailing arms were not touched when I changed my subframe this summer. They are IE Urethane bushings - and I know they're not rubber. They did somehting to the rear. And, from the looks of it, the urethane bushings CAN move back and forth slightly, and they are nice and tight in their current position.

Yea, the driveshaft won't be affected at all by ride-height, so I don't think it's that. It could be my differential, but I don't see that being affected all that much by ride height.

[1991 318is]

Could be....

There is a secondary inbalance in wheels that occurs about 85-90 mph and again at 125 mph. Swap tires front to rear.
The rear camber may have been changed by installing BMW offset trailing arm bushings.
The vibration being sensitive to ride height sort of makes you think of half shafts. With a solid mounted differential there is minimal movent of the driveshaft. Also pinion and drive flange bearings (230,000 miles)
You could just not drive 90 mph on public highways, especially since you have a vibration. That is unless you have a death wish.

[PeaveyBassist]

U joints in driveshaft sounds like a good guess if the wheels arent bent and are balanced correctly.

Will

[Sean]

Wow, Sean...sounds like time for you to get a fat GF...or a mother in law.

I have a driveshaft sitting around that has aU-joint that sticks on a certain spot. Smooth and flexible all through its rotation, except at a certain spot. I would ecpect it is worn at that spot...you can feel an indent(sp?).

Seems to me that the angle would change when you add 300Lb to the trunk, thus getting past that indent...so it just might be a bad U joint.

Pure conjecture on my part, but you gotta look at something, right?

Luke
still wondering about that camber!

LOL thanks, Luke. :p

Yea, I'm really at a loss of what would cause it. I don't think the car being lower in the rear affects the driveshaft. Now...it COULD be the half-shafts. For all I know, they are original, and at 230,000 miles, that's pretty old.

Other than rear wheel bearings, rear alignment (although not ENTIRELY adjustable), and the half-shafts, I'm not sure what else to check in the rear.

[StereoInstaller1]

Wow, Sean...sounds like time for you to get a fat GF...or a mother in law.

I have a driveshaft sitting around that has aU-joint that sticks on a certain spot. Smooth and flexible all through its rotation, except at a certain spot. I would ecpect it is worn at that spot...you can feel an indent(sp?).

Seems to me that the angle would change when you add 300Lb to the trunk, thus getting past that indent...so it just might be a bad U joint.

Pure conjecture on my part, but you gotta look at something, right?

Luke
still wondering about that camber!

[Sean]

Could be an out of balance or bent wheel in the rear, or one/both of your rear wheel bearings are bad.

Wheels are not bent.

I also had all my wheels rebalanced, and they were perfect.

It was fine when I drove home - then when I finsihed my rebuild, it vibrated.

Only thing changed was the rear subframe BAR was changed out - everything else left the same.

I was thinking rear bearing too - any good way to test this? There's no noise from my car.