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MG MGA - Dash mirror vibration - solved

I got my '59 1500 back on the road a year ago but had a persistent problem of the rear view mirror vibrating while driving. There was no other evidence of vibration or anything out of balance. It as fine as some speeds but was almost unusable particularly at driving speed (a resonant point).

Tightening or loosening the screws seemed not to help at in a consistent manner. I tried replacing the fiber packing with rubber, foam, etc. Some helped somewhat but didn't solve the problem at all speeds. Either too loose or too tight.

My solution turned out to be a two coil spring washer under the mirror on top of a rubber packing pad. The spring (image attached) was part of one of Todd Clarke's F-83 kit. The mounting screws were snugged but not really tight. It worked like a charm, NO vibration and a steady rear view at all speeds.

Apparently the spring has damped out the engine and road vibration and any resonance that was resulting.

Jim


Jim Ferguson

Nice solution. The other possibility is a cork gasket material like under the window stantion.
T McCarthy

I had a very similar problem recently, except that it was a rattle at low speeds. I found that the pedestal was moving relative to the base. On the reproduction mirrors this is held together with a set screw, so easy enough to fix (tighten). The original mirror appears to be rivetted (so shouldn't come loose anyway).

The only visible differences I can see between the original and the reproduction is WINGARD MADE IN ENGLAND stamped on the back of the mirror and a slightly different shade of gold paint (perhaps a more copper shade).

Neil
Neil McGurk

Nice work, Jim, for attacking a 50 year old problem. How about a picture of the final installation? I would like to post this on MGAguru.com

There is hidden science here. The resilient pad underneath and spring washer on top hold the mirror base between two flexible surfaces, effectively decoupling the base from the body cowling. The mirror itself still has the original resonance frequency from the base up. Combination of the new soft mount with original mirror mass creates another lower resonance frequency. If the higher frequency is an even multiple of the lower frequency, it will still vibrate at the higher frequency. When the two frequencies are not closely matched (by multiple) they damp each other out and it will not vibrate. Success of the setup depends on mismatched harmonics, but it is easier to create a mismatch than an exact multiple match.

A flat washer and rubber washer on top might work as well as the spring washer. A threaded stud with acorn nut and rubber washer might clean up the appearance a bit. Thread lock adhesive or a self-locking nut may be used to prevent loosening in long term usage.
Barney Gaylord

On my car the vibration is not a problem for visibility, but the poor manufacturing of the mirror base is.

The stem passes through the mounting plate, and then is distorted to hold it in place. The trouble is that vibration over the years has caused the fitting to become loose, in that the hole has worn. My mirror constantly shifts in an arc from one side to another- so I am constantly adjusting it. (but at least I can always see what is hapenning behind me when I do)

I need to take it off and give it some attention with a BFH to tighten it up again, but why are they so shoddily made (this one is about five years old)
dominic clancy

Dominic,I had a similar problem although maybe not so severe and solved it by running a few drops of instant glue round the joint between stem and base.
Might be worth a try ?
David
D C GRAHAME

Dominic - strangely mine has also recently started to do what yours does - slowly moves out of alignment - Might try your fix David - cheers Cam
Cam Cunningham

I had the same problem with multiple copies of replacement mirrors in the late 1980's. Some of them were so loose as to vibrate dramatically. At one time I was using a rubber band around the mirror stem and two nearby LTD studs to stabilize the mirror. After enough frustration, it was apparent that buying another one would not solve the problem of poor manufacture. I finally drilled and tapped the bottom of the stem to installed a flat washer and machine screw, and banished that problem entirely.
Barney Gaylord

Okay, I have Jim Ferguson's pictures on line. See here: http://mgaguru.com/mgtech/interior/int116.htm

It surprised me a bit that the spring washer was underneath the mirror base rather than on top. That caused me to slightly revise my explanation of the science involved. Apparently the exact "snugness" of the mounting screws will be important, and one should probably use some thread locking adhesive to prevent them from rattling loose.

From one of the pictures it appears that Jim did the same thing I did, installing a screw to secure the mirror stem to the base.
Barney Gaylord

This thread was discussed between 31/05/2007 and 03/06/2007

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