Re: Passive Mass Oscillation Damping...
After reading Tony Foale's Motorcycle Handling and Chassis Design, it's easier to understand this phenomena.
In section 6, under Suspension Principles, there are complete descriptions on how the suspension plays a large role in resonance... essentially that the tire, wheel bearings, fork stanchion material's hardness, fork tube material's hardness, oil volume, damping method and material, spring material, triple clamp; material, shape, and hardness, handle bar mount material, and bar material, are all directly responsible for the total resonance in the front end. These variables, combined with weight bias induced by raising the rear, or lowering the front, are responsible for noticeable oscillation.
The effect of weight connected to the resonating materials, i.e. bar end-weights, is directly related to the type of material they are connected to. The more weight at the end of a material that is likely to resonate at the same frequency as other components, the more drastic the effects on the human attached to the machine.
Conversely, the less weight attached to a material, the more likely it is to resonate with the harder components it is attached to.... sort of a damned if you don't do your research, and buy what they sell you, and expect it to always be a wonderful tame little motorcycle scenario.