two things really got of rid of most of the unpleasant vibration I had when I first built up a new Micargi cruiser:
1. Narrow street tires- the straight tread of more narrow 1.75 tires (only $10 each and used the same tubes) gave a much smoother and easier roll- knobby tires must be even worse on the road,
2. A bigger gear- going from the stock 44 down to a 36 on the back gave me the same cruising speed with a lot less throttle and motor revs. Made all the dfference really.
I also found that putting some BMX bars that are perpendicular to the frame gives amuch better grip than cruiser moon bars.
I've since gone even more narrow on the frontwith an alloy 27 x 1 1/4 wheel