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Good Vibrations

by

David Tocher

I had severe problems with the central mirror vibrating so much that it was impossible to see anything. I sorted the problem by mounting the mirror on a plate which used the right and left wing nuts on the left and right Brooklands fly screens. The increased rigidity seemed to sort the problem. I read Model Engineers Workshop and in the #266 edition Mark Noel describes his solution to a very similar problem with the mirrors on his Royal Enfield Bullet motorcycle.

The cure for vibration is either to minimise any imbalance or use a damper to reduce the vibration. One method is to use tuned mass dampers and a good example was the inertia dampers fitted originally to front and rear of 2CVs then only on the front and finally removed from the front. The principle behind a tuned mass damper is for the natural frequency of the damper to be the same as the frequency of vibration of the affected part.There's a nice demonstration of tuned mas damper written by Hugh Hunt. The device Mark describes Stockbridge damper , the article again by Hugh Hunt. Stockbridge dampers are fitted to cables such as power lines and suspension bridges (e.g. Humber and Severn suspension bridges) to stop them vibrating in the wind. The vibrations are caused by the wind eddies shed from the wire. The dampers can look like dogbones and can be designed to damp more than one frequency.

For copyright reasons I'm reluctant to just copy the article. His first task was to determine the natural frequency of the rear view mirror. He didn't have a rev counter so he used a small magnet, pick-up coil and an oscilloscope to measure the frequency. His damper was a flexible rod, fixed to the mirror stem near the top, with a brass weight on the free end and the clamp at the other end. The natural frequency was adjusted to match the mirror's natural frequency. A plastic tube slipped over the rod and mass absorbs the energy. Mark reported that it worked giving a clear rearward view at all speeds. One minor problem - the rod broke from fatigue after about 10 miles! He replaced the steel rod with a pultruded carbon fibre rod which has survived hundreds of miles of road use.

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