I hear people talking about putting different engines into quad frames and others saying it won't work because the engine doesn't have a a counter balancer. What difference does it make whether you have the engine in a quad frame or a bike frame? I realize that vibration is the reason for the counter balancer but I can't see the difference.

 

Some frames aren't made to with stand bigger engines in some frames and If it isn't counter-balanced it will tear the frame apart from vibration. Counter-balancing will kepp the vibration to a minium.