best way to have a "balanced" crank

[Deleted member 12676]

With the low power output of engines not ported to scream higher than 7000 rpm I think the following idea would work for a balanced crankshaft/flywheel.

Even if you imbalance the crank to offset the downward force of the piston,wrist-pin/upper-con-rod you have created a front/rear vibration due to the lessened weight around the quarter of the flywheel where the connecting rod connects to it. The only solution to this is lightening the piston/wrist-pin/rod so that the holes close to where the rod connects to the flywheels can be very minimal. Since a low rpm engine doesn't impose very high forces on the connecting rod or wrist pin or piston then these three can be lightened.

To somewhat test this idea without taking the crank out I lightened my piston and it did work somewhat. Before I could only rev up to 26mph (26" bike with std gearing) due to vibration but after the mod the engine vibration existed only between 24 and 28mph and I was able to rev out to go 31mph on flat streets with no vibration.
Next I need to take everything apart, lighten the rod and wrist pin, and then add weight to the flywheels where it was previously removed so that it matches the newly lightened parts.

If I can prove this method works better than the old method (leaving the piston/pin/rod the same weight) then maybe some company can sell balanced "sets" that are superior to anything previous.

 

[Dave C]

There are some threads dealing with balancing. It seems that the Chinese decided during the developement of the 66cc motor that it would be OK to just use the 49cc crank and bore it out and stroke it by moving the crank pin location. It's madly out-of-balance stock. You can spend the money to get a auto machine shop to do the job. I have a contact in So Cal that is a genius machinest. I'm just waiting for summer when I can get another motor to modify, maybe my BGF motor will last that long

 

[Deleted member 12676]

Yeah but what I'm proposing is a new approach for weak motors. Instead of worrying about balancing the weight of the parts moving up and down, just lighten those parts so less flywheel imbalance is necessary to offset the up/down weight/force. For existing flywheels more weight would probably have to be added to the flywheels instead of taken away. But I can't confirm that until I get mine apart.
Ideally the connecting rod should be replaced by one fabricated from a lighter material.

 

[Deleted member 12676]

To better understand this topic here is some info on it:

A single cylinder engine produces three main vibrations. In describing them, it will be assumed that the cylinder is vertical.
Firstly there would be an enormous vibration produced by the change in momentum of the piston, wrist pin, and connecting rod once every revolution. Nearly all single-cylinder crankshafts incorporate balancing weights to reduce this. [Removing metal from the flywheels close to where the connecting rod pivots has the same effect as putting additional weight on the flywheels opposite of the same area.]
While these weights can balance the crankshaft completely, they cannot completely balance the motion of the piston, for two reasons. The first reason is that the balancing weights have horizontal [front/back] motion as well as vertical motion, so balancing the purely vertical motion of the piston by a crankshaft weight adds a horizontal vibration.
Additionally, the piston travels faster in the top half of the cylinder than it does in the bottom half, while the vertical motion of the crankshaft weights is equally sinusoidal. The vertical motion of the piston is therefore not quite the same as that of the balancing weight, so they cannot be made to cancel out completely.
Secondly, there is a vibration produced by the change in speed of the piston. The crankshaft will tend to slow down as the piston speeds up and absorbs energy and to speed up again as the piston gives up energy in slowing down at the top and bottom of the stroke. This vibration has twice the frequency of the first vibration and absorbing it is one function of the flywheel.
Thirdly, there is a vibration produced by the fact that the engine is only producing power during the power stroke. In a two-stroke engine it will have the same frequency as the first vibration. This vibration is also absorbed by the flywheel.