D
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The crank of a 2 stroke has radial forces on it throughout the 360 degrees of crank rotation. These forces can be felt at the handlebars. A good crank balance equalizes the vertical forces with the horizontal so that the pattern of forces throughout the whole crank revolution is close to being a circle when graphed radially. If the counter balance of the crank is lacking then the circle becomes oblong in a vertical pattern. If the counter balance is too much then it becomes oblong in a horizontal pattern. Either too vertical or too horizontal have amplitudes greater than the circular pattern of a balanced engine which produces an annoying vibration at the handlebars. So I am theorizing that the resultant pattern can be seen by using a long thin metal connected to the crank cases while it is running. Here is my proposed method of testing:
Use a short/thin bicycle spoke to press onto the engines left side cases so the spoke is sticking out to the left. Start the engine and hold the clutch in. Press the spoke onto the cases with your left hand and position your body so your eye is inline with the spoke farther to the left and close to it while controlling the throttle with you right hand. Slowly increase RPM as you watch the spoke end. At top RPM it should remain centered. If not then the crank is out of balance. Top RPM is the most important because that is when the forces on the crankshaft are the strongest.
On my Suzuki 100 the spoke end only leaves the centered position around 3300 RPM. Then it vibrates from about 10:30 to 4:30 o’clock (making the upward direction of the cylinder at 12 o’clock). Below and above that RPM the spoke has only the slightest of vibration, mostly staying centered. This bike has very little vibration at the handlebars which is one thing I like about it.
If any of you have a bicycle wheel handy I’d like you to remove one spoke (I did it without removing the wheel from the bike.) and give this test a try and then report back here. Tell the engine size, whether or not it has a high compression head or aftermarket CDI, what year it was bought new, and where you bought it. I think the older engines have the most vibration.
If everyone has a similar experience that it only vibrates at one RPM then that means it is only showing the resonant vibratory frequency of the frame/engine and not the radial forces on the crankshaft.
Use a short/thin bicycle spoke to press onto the engines left side cases so the spoke is sticking out to the left. Start the engine and hold the clutch in. Press the spoke onto the cases with your left hand and position your body so your eye is inline with the spoke farther to the left and close to it while controlling the throttle with you right hand. Slowly increase RPM as you watch the spoke end. At top RPM it should remain centered. If not then the crank is out of balance. Top RPM is the most important because that is when the forces on the crankshaft are the strongest.
On my Suzuki 100 the spoke end only leaves the centered position around 3300 RPM. Then it vibrates from about 10:30 to 4:30 o’clock (making the upward direction of the cylinder at 12 o’clock). Below and above that RPM the spoke has only the slightest of vibration, mostly staying centered. This bike has very little vibration at the handlebars which is one thing I like about it.
If any of you have a bicycle wheel handy I’d like you to remove one spoke (I did it without removing the wheel from the bike.) and give this test a try and then report back here. Tell the engine size, whether or not it has a high compression head or aftermarket CDI, what year it was bought new, and where you bought it. I think the older engines have the most vibration.
If everyone has a similar experience that it only vibrates at one RPM then that means it is only showing the resonant vibratory frequency of the frame/engine and not the radial forces on the crankshaft.
