Rubber Atlas

Derek Wilson

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I always wondered how one of these would work when the power train is no longer a stressed member in the frame... How does the frame handle the loss in rigidity?

And the swing arm still appears to be frame mounted... hmmmm...

Definitely some interesting ideas integrated into this bike. Don't know that I would want to own it, but it would be an interesting experience to take it for a ride...
 
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The isolastic mounts in this build cause for a rocking moment that is felt in the drive train making the rear chain go taught then slack.
 

mdt-son

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The rubber mounting must be very stiff on this one. No visible torque reactions at idle. Maybe this shouldn't surprise because there needs to be no less than 4 isolastic joints. I am curious as to how much the countershaft and sprocket displaces, and if the movement forms an elliptic circle, similar to the Commando. I doubt the movement is large enough to make the chain come off at the sprocket. There may be increased wear though.

- Knut
 

grandpaul

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The rubber mounting must be very stiff on this one. No visible torque reactions at idle. Maybe this shouldn't surprise because there needs to be no less than 4 isolastic joints. I am curious as to how much the countershaft and sprocket displaces, and if the movement forms an elliptic circle, similar to the Commando. I doubt the movement is large enough to make the chain come off at the sprocket. There may be increased wear though.

- Knut
Triangulation is sufficient for stability, four-point isolastic would be excessive. (I believe)
 

mdt-son

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GP
Agreed. I was referring to the Featherbed frame, for which one needs 3 main isolastic supports and one spring loaded head steady. The latter is kinematically overdefined and provides (elastic) restraint in one direction only, thereby limiting transverse rotation of the powerplant (provided the spring is stiff enough). Inertia forces from the top end heavy Norton engines are not counteracted by the main ISO supports.

On the Commando, the powerplant is actually dangling between two elastic supports, as the designers assumed a completely stiff powerplant. This precondition is not entirely fulfilled.
The head steady is needed to make the system statically determinate about the longitudinal axis. Now, the large distance between the line between the main supports and the head support ISO means that this bearing can be made much smaller than the other two. In addition, the head support ISO serves to limit transverse rotation of the powerplant due to inertia forces. The stock design doesn't transfer any of these loads well, and the only alternative head support providing restraint in two degrees of freedom is the Norvil racer head steady. The top end is the best location to counteract inertia forces arising from the powerplant.

Addendum
Here is an example of a Commando with the Norvil racer head steady (https://andover-norton.co.uk/en/shop-details/16979). That detail is commented at 04:30 into the video.

- Knut
 
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Fast Eddie

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Someone who built a feather-lastic, and kept the swinging arm pivot frame mounted, told me that the drive chain pulled the engine back badly under acceleration.
 

grandpaul

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Someone who built a feather-lastic, and kept the swinging arm pivot frame mounted, told me that the drive chain pulled the engine back badly under acceleration.
The "fix" is a loose chain, and a dirt bike type chain tensioner. Too easy.
 

Fast Eddie

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The "fix" is a loose chain, and a dirt bike type chain tensioner. Too easy.
No, my point wasn’t about chain tension, although that is of course a topic. My point was that the chain pulls the engine back a lot. With the swinging arm spindle mounted solid to the frame the isolastics are not working as intended and are subject to loads they were not designed for. The chain ‘leverages’ against the swing arm pivot and the rubber mounts yield, a lot.
 

baz

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Someone who built a feather-lastic, and kept the swinging arm pivot frame mounted, told me that the drive chain pulled the engine back badly under acceleration.
An fj1200 yam just has an adjustable metal cup the size of a car engine core plug and a rubber smartie to keep the 135 bhp motor from twisting in the frame too much
The xjr12 uses a simple parrell link
 

baz

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No, my point wasn’t about chain tension, although that is of course a topic. My point was that the chain pulls the engine back a lot. With the swinging arm spindle mounted solid to the frame the isolastics are not working as intended and are subject to loads they were not designed for. The chain ‘leverages’ against the swing arm pivot and the rubber mounts yield, a lot.
Part of how the isolastic system works is to shake the vibration out via the swing arm/tyre/ road I believe?
I have heard of rubber mounted commando motors in different frames pulling the chain off the sprocket
 
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