MIRROR Anti-Vibration Modification -- INT-116
|The MGA With An Attitude
On 31 May 2007 Jim Ferguson in North Carolina, USA, wrote:
".... had a persistent problem of the rear view mirror vibrating while driving. .... It as fine at some speeds but was almost unusable particularly at driving speed (a resonant point). Tightening or loosening the screws seemed not to help in a consistent manner. I tried replacing the fiber packing with rubber, foam, etc. That home 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 washers 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."
Nice work, Jim, for attacking a 50 year old problem. Thread lock adhesive may be used to prevent loosening of the "snugged but not tight" screws in long term usage.
There is hidden science here. The spring washers in combination with the slightly loose screws allow the mirror base to be "decoupled" 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. Fortunately it is easier to create a mismatch than an exact multiple match.
Now if I can just find an easy source for that double helical spring washer, ....
On 31 May 2007 Dominic Clancy in Switzerland wrote:
"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. My mirror constantly shifts in an arc from one side to another, so I am constantly adjusting it. I need to take it off and give it some attention with a BFH to tighten it up again."
On 31 May 2007 D C GRAHAME in South Yorkshire, UK, wrote:
"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."
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 fat 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
More recently a couple of people have mentioned application of Krazy Glue to the joint, allowing it to wick into the space between the loose parts. They say this makes the joint solid again, and no other action is necessary. At least it's worth a try before machining it for a screw.
Addendum August 1, 2009:
At 05:48 PM 3/18/2009 -0600, Bill Graham in Arizona wrote:
"The washers appear to be the same ones used to attach Weber carburetors to the manifold. It seems that these carbs have issues with harmonics also. Anybody selling these carbs will have these washers".
True. These bits are technically called Double Helical Spring Washers. Notice the word "Lock" is not part of the description, as these are definitely not lock washers. They are essentially a die spring with two active coils. They are used in a surprising range of applications. They are also often referred to as Railroad Washers, as they are commonly used with screw-type rail spikes to hold railroad rail retainer pates to the sleepers (wooden or concrete ties). You may be able to find the double helical spring washers at an Ace Hardware store (I haven't actually looked), or at a good auto parts store (like NAPA for instance).
Once you understand that these are not lockwashers, there is another possible substitute. Given enough space you can use Belleville (brand name) washers. These are conical shape spring disc washers. Go here: http://www.mcmaster.com/#disc-springs/=12alkt
Click on Disc Springs. Stepping through the specifications, McMaster catalog number 9712K58 may be a good candidate for this application, $3.65 for a pack of 12 (plus shipping).
If you stack up more than one in the same orientation it increase spring force with same deflection. If you stack them alternating orientation it increases the deflection range with the same spring force. If you stack them in alternating pairs it increases both the force and deflection range. If you stack them in an irregular pattern it may even damp out certain vibration frequencies. The dash mirror anti-vibration trick may be a good application for Belleville washers, as they fit into a small height space.
The trick with the mirror mount is that the mirror has a fixed natural harmonic vibration frequency (like a pendulum or a guitar string or a tuning fork). When vibration in the body cowl is close to the natural harmonic frequency of the mirror it will amplify the vibration (shake violently). Same applies when the excitation frequency is a integer multiplier of the natural harmonic frequency. For instance if the mirror likes to vibrate at 20-Hz (cycles per second), then cowl vibration close to 60, 40, 20, or 10-Hz will cause it to vibrate heavily.
One way to get rid of the mirror vibration requires making the natural harmonic vibration frequency substantially different than the excitation frequency. The problem here is that the excitation frequency might change with road speed and engine speed. Suppose you tune the mounting base so the mirror only vibrates at 15-Hz. Then speeding up or slowing down in the car might produce excitation frequency close to 60, 45, 30, 15, or 7.5-Hz. The mirror then vibrates at a different road speed, but maybe it still vibrates sometimes.
The idea of de-tuning the mirror so it doesn't vibrate may depend on the body cowl itself having a specific harmonic vibration frequency. For instance, the body cowl might naturally vibrate at 15-Hz while the mirror naturally vibrates at 30-Hz. The car chassis could be encountering several different excitation frequencies coming from road bumps, wheel imbalance, engine vibration, propshaft vibration, or the natural frequency of suspension springs. Enough energy input from multiple sources may cause the body cowl to vibrate at its single natural harmonic frequency. In this case it may be rather easy to tune (or de-tune) the mirror mount to vibrate at a different frequency that is not sympathetic to the input frequency.
Tuning the mirror mount implies that it has to be decoupled from the body cowling so it does not move as a single piece. This requires (among other things) that the thin elastomer pad must be in place between the body cowl and the mirror base to allow a little flexibility. Then you can put the springs under the mirror base and adjust screw tension, reducing force between the mirror base and the elastomer pad. This allows the elastomer pad to absorb some of the energy (turning energy into heat). That in turn reduces energy going into the mirror at a specific frequency, so it can squelch the build-up of vibration in the mirror. In other words, the elastomer pad is the energy damper (like a shock absorber), and the springs can be adjusted to make the pad damper most efficient at a selected frequency.
The other effect comes form the elastomer pad acting as a spring, and the double helical washer or disc washers also acting as springs but with a different natural harmonic frequency. When the pad and spring washers are in sync with sympathetic frequencies it will vibrate. When the pad and spring washers are out of sync (non-sympathetic frequencies) it will not vibrate. This may be sufficient to squelch the mirror vibration regardless of what combination of input vibrations you get from the body cowling.
I haven't tried to list sources for the double helical spring washers, because I never needed them and so have never been looking for the parts. I was a little surprised to find that McMaster -Carr doesn't have them listed. But McMaster-Carr does have Belleville washers that can serve the same purpose, and my be more versatile in mechanical tuning.