Downhill, Deceleration Vibration

Ernie, If you have the stock motor mounts in the rear the rubber pads may be hard as rocks and not providing the cushioning that they were designed to do. Even with the Float-A-Motor mounts, the rubber pads should be replaced and the correct bolts and nuts installed with the spacers so that the pads do not get squashed. Plus, the original spring motor mounts in the front seem to work the best rather than any after market designs.

I am anxious to read your report after comparing the stock flywheel to the lightened one. There is nothing like test data to verify theory.
 
After enough people recommended the float-a-motor mounts, I decided to give them a try and ordered a steel set from Bratton's. The difference is AMAZING. I'd say the vibration is 95% improved. I really think that in my case at least, the vibration wasn't anything specifically wrong. I feel the higher compression of the burtz engine combined with the 4.11 rear end in my all steel slant window to cause the vibration. I even replaced the top wooden bows with steel. There was absolutely nothing there to absorb the vibration. So, I'm a happy camper. I expected that even with the FAM that I would still see a lot of vibration in the pedals and gear shift, but that's actually gone too. The vibration left occurs at 2000 rpm exactly (and still just at holdback). It actually starts going away again as the rpm's increase beyond 2000. So I think that just has to do with the harmonics of the engine. I am still quite curious how the phaeton with the same engine configuration and the original flywheel compares. It'll be a bit yet before it's done, but I will report back once I'm able to drive it and compare the two.

Thank you again to everyone who helped me think this through and offered suggestions!
 
Ernie, I have been thinking about the Burtz crankshaft and different weight flywheels as regards torsional vibration. The Burtz crankshaft is much heaver and stiffer than the original crankshaft. The original system had the node point, where there is no vibration, at the flywheel, so no torsional vibration was felt. With the stiffer and heaver Burtz crank, I think the node point will be forward of the flywheel, even with the heaver flywheel. So torsional vibration will probably be felt even with the original 65 pound flywheel.

Every engine has torsional vibration in the crankshaft. Vibration dampeners were introduced in the mid 1930's to keep the longer crankshafts from breaking. In the Burtz engine, I am not sure that there is enough room for a vibration dampener in the front that would be effective. I think there is an opportunity for someone to design a clutch disk that would dampen out the torsional vibration so that it would not be felt. Maybe the springs used on the current disks could be replaced by dampening components. The problem would be life of the dampening components as they are stressed and eventually wear out.
 
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