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leptronjohn
05-15-2010, 10:51 AM
Hi guys,
I have overhauled my motor and cleaned the lead out... good grief it was ugly.
Does anyone have the proper measurement on the tube spacer for the rubber motor mounts? I changed them a couple years ago after breaking a mount but believe it changed lengths after that..

john Oakley

DesertFox4
05-15-2010, 01:50 PM
Hope you're back in the air to stay John. Been quite a battle. Lots of flying to catch up on. Best of luck.

leptronjohn
05-15-2010, 03:54 PM
thansk Steve,
I too hope to catch up my flying this year...... the motor looks much better, should run it next week..

HighWing
05-17-2010, 07:02 AM
Hi John,

I wonder if there is a magic number here. I recall when I first built, the spacer length allowed a credit card to be inserted between the rubber and the large washer under the nut. It just didn't seem right to me, so I shortened them on my own authority so I got a bit of rubber compression and I got 900 hours over ten years with no engine mount issues. I do know that Rotax on their ring mount recommends a quarter inch of compression overall on their rubber donuts.

If no one comes up with a factory recommendation, maybe measuring at first rubber contact and then going in a bit shorter might work. I made my own mount for a Model IV and am currently in the thought process concerning this exact issue. I had a talk with an airospace engineer on this - Aerojet - and the gist of his opinion is that in shock mounting there is no magic bullet, but rather engineer the best you can then fly it and tweek. The interesting thing here is that he was not talking about structural integrity - that they thought they could engineer, but the part about protecting sensitive equpment, i.e. actual vibration damping. So my legs wouldn'g vibrate for the rest of the day after a long cross country flight. My approach will have to be the trial and error, because it is a one off design. One other thought he had is the shore hardness of the rubber. Mine is 70. That they also engineer, but sometimes they have to change hardness to get the damping they need. Think of all the shaking you see at the engines on a shuttle or other rocket launch. That is all planned.

Lowell

wannafly
05-19-2010, 06:52 AM
I believe if you go to the Kitfox web site and look at the service bulletins that you will find the right measurement. I bought an old firewall forward kit that did not have the right length spacer and had to cut mine down and that is where I found the measurement:D