Motorcycle handle Bar vibration & Bar End Weights 
Pic 1 is a normal motorcycle bar end weights.
My question, if I replace the bar end weights with a long threaded rod (M8x150) and the thread rod is inserted inside the handle bar(pic 2), will it reduce vibration or it will increase vibration? And what will happen if the threaded rod is longer?
 A: I suppose that it's to hard to say, giving a general rule.
As a mathematical model, let's assume that it's possible to find a linearized equation describing the motorbike dynamical system around different equilibrium conditions (riding on a straight line, bending, ...).
The vibrational behaviour of your system really depends on how this modification affect the eigen-values and the eigen-function of the system in each particular riding condition, considering the spectrum of the perturbation introduced by the rolling of the wheel on the street, or by other sources of vibrations, like the engine.
And you can't say anything for sure if you don't know the behaviour of your system.
Assuming that nothing terrible happens, if you made this kind of modification, you could try by yourself and directly feel if something change in the riding comfort (I don't know if you have any instrument to measure the level of vibration on the handle bar). Anyway, I wouldn't do anything, relying on the constructor design.
A: Bar-end vibration is blocked by mass. The most mass comes from a solid cylinder of metal inserted into the bar end where the diameter of the cylinder is slightly less than the inside diameter of the bar. The cylinder is long enough to just reach the first bend in the bar. You hold the weight in place by gooping its surface up with RTV silicone caulk before you slip it into the bar. the viscoelasticity of the goop will help dissipate the vibrations.
