Originally Posted by base2

Only from added mass. I don't think that a rubber plug in isolation would do much as it is not an intermediary between 2 different structures of differing rates..

As long as we are fantasizing: Between the drop outs & the axle would have an effect.

But in reality land: So would between the rim & the road, between the headset & the stem or the stem and the handlebars.

Or as added mass on the bar ends. Effectivly minimizing the distance x force moves the lever.

Originally Posted by duanedr

In practice (sample=1), I remember my Trek Madone from like 2006 or so came with these little rubber bar plugs that suspended a small (maybe 5/16 dia x 3/4" length?) rod in rubber webbing. I was using 44cm alum bars and 23mm tubulars on that bike and I couldn't tell a difference with them in or out. They were Bontrager branded and looked pretty cool so, I guess that's something.