There are other things I've done to calm the bikes down...lined the fenders and chaingard with thick weatherstrip, wrapped the seat springs with solder, and used a small amount of gaffers tape where the engine mounts to the frame...not a rubber mount, which can make things worse sometimes, just a double layer of gaffers or "friction" tape.

I filled one bikes tubing with expanding foam and it helped, but not as much as I thought it would.

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If it ain't broke, and you mess with it long enough, it will be.