Tracy and Chuck now use small rubber bushings mounted

in close to the engine. Only about 14 to 16 inches apart. IMHO it

will help

to move them outboard as the compression will be less under the

torque

load from the prop.

Al Gietzen wrote:

Offsetting this is that, generally, in isolating vibration you like to keep
the vibrating mass to a minumum, which would suggest getting the mount
bushings close to the engine.  As Paul has already suggested; the
flexibility in the mount leaves the possibility for some sort of resonance.

Al

Yes but a properly triangulated steel tubing space frame is not all that 
flexible compared to rubber bushings. True, individual tubes can vibrate 
if too long or too small in diameter. I really don't think this is much 
of a factor in our motor mounts. I have extensive experience with space
frame race car chassis made from both steel and aluminum tubing and compared
to the suspension flexibility the frame stiffness is at least
and order of magnitude higher. If you have any doubts about your motor mount
design  triangulation build a quarter scale model out of balsa wood 
or welding rod. It will immediately become apparent if your design  
is not properly triangulated when you attempt to twist or bend it by 
hand.

I like Tracy's vibration test. I.E. stand beside the engine
at wide open throttle with the cowl off and feel things for vibration. 
Best done with a short test club prop so the engine can be run all the 
way up to takeoff RPM.

Paul Lamar
 
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