Originally Posted by KCvale
Ahhh, OK, I think get it now. So a vibration at a give rpm may not be the motor or drive train at all, but just the handlebars (or frame) resonating?
Right. Or more specifically, it's the handlebars' natural tendency to buzz at a certain frequency being stoked when the motor vibrates at that same frequency. (I don't have any reason to believe one way or the other whether that's the nature of the OP's complaint.)
Handlebars are basically prongs that can twang if you pluck them hard enough. Compliant parts like grips and tires will quickly damp out that vibration as long as a motor isn't feeding it back in at the same time.