Re: Crank balancing
Posted: Mon Jul 14, 2014 3:38 am
I agree with Graeme and Nigel, that there is no fixed factor, it really depends on the frame and the way the engine is to be used.
To get a handle understanding why we balance you have to go back to basic functions. The piston force going up and down can be balanced completely. The bob weight that we add to the flywheels moves opposite to the force of the reciprocating piston to balance it in the vertical plane, but in doing so we produce another problem.
When this bob weight rotates to the front of the engine there is no reciprocating weight opposite to balance it, same happens when the bob weight is rotated to the rear of the engine, so we develop an out of balance in the horizontal plane.
There has to be a compromise made, if we don’t completely balance the reciprocating weight the horizontal force won’t be as bad, we could say 50% to go both ways, but as there is a difference in the effects of the two different forces we can favour the one that counts. The horizontal forces are felt greatest at low rpms and reducing as the rpm increases. The vertical forces increase as the rpm increases. So this is where we make the compromise to suit the frame and use.
The frame can dampen or amplify an out of balance; it may reduce the effects of the horizontal force, so more of the vertical force can be balanced. Or if the engine is to be used at lower rpms, less of the vertical force is balanced to keep the engine smooth at lower speeds. High speed engines a 70+% balance factor is used as they are running in the higher rpm, and don’t spend much time at low rpms.
The main point in doing a balance is to reduce the reciprocating mass as much as you can, to reduce the amount that you are going to balance the first place, as this reduces ALL the forces. The 88mm Wisco piston that I used was reduced to 470gms, a 70% factor made it very good at speed, 1000km rides were a possibility, but stopped at traffic lights the front brake had to be held on to prevent the front wheel from bouncing around from the horizontal balance.
To get a handle understanding why we balance you have to go back to basic functions. The piston force going up and down can be balanced completely. The bob weight that we add to the flywheels moves opposite to the force of the reciprocating piston to balance it in the vertical plane, but in doing so we produce another problem.
When this bob weight rotates to the front of the engine there is no reciprocating weight opposite to balance it, same happens when the bob weight is rotated to the rear of the engine, so we develop an out of balance in the horizontal plane.
There has to be a compromise made, if we don’t completely balance the reciprocating weight the horizontal force won’t be as bad, we could say 50% to go both ways, but as there is a difference in the effects of the two different forces we can favour the one that counts. The horizontal forces are felt greatest at low rpms and reducing as the rpm increases. The vertical forces increase as the rpm increases. So this is where we make the compromise to suit the frame and use.
The frame can dampen or amplify an out of balance; it may reduce the effects of the horizontal force, so more of the vertical force can be balanced. Or if the engine is to be used at lower rpms, less of the vertical force is balanced to keep the engine smooth at lower speeds. High speed engines a 70+% balance factor is used as they are running in the higher rpm, and don’t spend much time at low rpms.
The main point in doing a balance is to reduce the reciprocating mass as much as you can, to reduce the amount that you are going to balance the first place, as this reduces ALL the forces. The 88mm Wisco piston that I used was reduced to 470gms, a 70% factor made it very good at speed, 1000km rides were a possibility, but stopped at traffic lights the front brake had to be held on to prevent the front wheel from bouncing around from the horizontal balance.