| A single cylinder engine will always vibrate as there is nothing to cancel out the secondary balance of the crankshaft. The only way this can be achieved is through a balance shaft turning at engine speed in the opposite direction to the crank as is used in the TZ125 gp bike and large 4-stroke singles. Multi-cylinder engines use the other cylinders to provide this function.
I am not saying that this is a futile excercise, espesially with chinese bikes. I think the external balancing is a good idea to minimize excessive vibration through trial and error. What i am saying is that single-cylinder crank balancing is always a compromise and proves to be more of an art form than an exercise in science.
The other thing to consider with the external balancing is the idea of a heavy or light crank. Moto GP bikes have a flywheel which is divided into sections like a cake. In wet conditions, or for a higher top speed they will put more dense material into the slots. Or if they want quicker acceleration they will remove slices or use lighter materials. 125 gp bikes also run heavy cranks to prevent the rear wheel from breaking loose and for higher top speeds (also called inertia cranks).
The pocket bike flywheel could be drilled so that extra weight could be bolted to it. This has three applications:
1. To change the crank balance as discussed
2. To add weight to the crankshaft by using extra bolts close to the edge of the flywheel
3. To reduce the rotating mass by removing all bolts and weights, leaving holes where aluminium used to be.
Just another tuning tool
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Trying to get more people to pocket bike events held around Tasmania
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