Rearview mirror vibration

[John F. Russo]

Rearview mirror vibration
I had new rear tires (same Pirelli's as before) installed and I immediately developed a shimmy (a vibration which then pauses for about three times the length of the time of the vibration;this cycle repeats) at about 80 mph. It doesn't happen all of the time on the highway. If the highway is very smooth, it is barely perceptible. The vibration is high frequency because you really don't notice it in the steering wheel or hear it.

I recently installed METCO control arms and the shimmy is barely perceptible all of the time. But now my rear view mirror vibrates at 75 mph and gets worse as I go faster.

(I use to have a Volkswagen Rabbit that had the same condition at any speed above 50 mph.)

MM History
I have 30000 miles and never had a shimmy with the road force balancing equipment my dealer uses. They have serviced my car since the beginning.

[SergntMac]

John...You have an uncanny knack for producing weekend riddles just for us. Thanks!

Starting where the tires meet the road and working backwards, you're sure all four tires have been correctly balanced. Is this happening now? After your tranny rebuild? If so, I would suspect the tranny mount may need a bit of adjustment. There's some play in this mount and throwing the left to right centering off a tad may produce a driveline "off center" vibration. Get up to crusing speed where the vibration is visible, slip into neutral and coast, and tell us what happens?

[David Morton]

Oh, man, the rearview mirror! I thought the woofer was just really badass, shaking the ground, but now I realize it's been the mirror all along. :D

I recently installed METCO control arms and the shimmy is barely perceptible all of the time. But now my rear view mirror vibrates at 75 mph and gets worse as I go faster.Seriously now, when you installed the Metco control arms it's possible you can have the bolts (I'm sure they're all tight) all pushed to one side of the bolt holes, stacking up to tilt the rear end just enough to make for a propshaft phasing vibration. The important thing to know about phasing is that the working angles of the u-joints (for example: the difference between the yoke and driveshaft angles, hence the movement of the joint) have to match as closely as possible. 0 degrees is best, for instance yoke to driveshaft moves 2D and driveshaft to pinion flange 2D. {transmission yoke-->/\/<--pinion flange} (\ is the propshaft)

Less than 1D is usually not noticeable except at very high speeds, but if yours was near this and the bolts went in stacking up in the wrong direction when the Metcos were installed, your angles might have gotten messed up.

The tool used to measure these angles is called an inclinometer and is like a bubble level with a scale for degrees and a magnetic mounting to put on the u-joint caps.

Just try loosening the mounting bolts on the control arms (wheels on the ground, front wheels chocked and car in neutral) and see if they move inside the mounts any and retighten. See if it's better. If not try to move them one way (push up on the nose of the differential with a jack) and re-tighten them. Re-test. Worse? Move them the other way. If the working angles are the cause, and high frequency vibrations at higher speeds are a symptom of this condition, then it's likely the Metco control arm installation is the cause.

And if you haven't deduced this from my post so far, if the control arms aren't the correct length exactly they themselves could be the cause. A small amount, 1/4" too long or short, can change the angle of the pinion flange enough to make the angles go south and make for a bad vibration.

[SergntMac]

Thank you for your descriptive reply, David, very informative. Have you made any observations of the MM specifically? If so, I'de like to hear more.

It's my observation that the drive line is canted every so slightly towards the passenger side of the car. The tip of the tail shaft housing is close to being in the center of the car at 6 o'clock, but as the imaginary centerline travels towards the front of the car from there (towards 12 o'clock), it shucks out towards the battery side. Have you noticed this too, or is it just my MM?

[David Morton]

Thank you for your descriptive reply, David, very informative. Have you made any observations of the MM specifically? If so, I'de like to hear more.

It's my observation that the drive line is canted every so slightly towards the passenger side of the car. The tip of the tail shaft housing is close to being in the center of the car at 6 o'clock, but as the imaginary centerline travels towards the front of the car from there (towards 12 o'clock), it shucks out towards the battery side. Have you noticed this too, or is it just my MM?I've seen her up in the air but I didn't look for that specifically.

About that centerline issue though, although an offset like you describe may exist as long as the transmission output and rear end pinion are pointed along parallel lines, this unmeasureable angle will cancel itself out.

This is one of those things that can get missed in a car that has been wrecked and not repaired properly, like improper frame straightening and such. Stabbing around with the up-and-down angles will eventually correct it, but most of the time the car gets wholesaled and the poor guy that buys it winds up with something that's commonly called a lemon.