Why do 2 rotors need counter weights, shouldnt both chambers be perfectly out of phase with one another, and thus not need it?

 

You'd like to think that, but have you ever held a counterweight in your hand? Or weighed one? They weigh less than the rotor itself. That's not really an answer to your question, but it should at least get you thinking.

 

the 2 offsets do have perfect static balance, but not dynamic balance. When at speed, if offset is up at the front rotor and dn at the rear, then there is a bending/moment load on the shaft, due to centrifical force, that pushes up on front brg and down on rear bearing.

to negate this moment, 2 more counter weights are added at the shaft ends, out of phase with the adjacent eccentrics, this maintains both static and dynamic balance.

 

Ahhh, thank you very much.