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- Nov 26, 2009
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Spent all day messing with balance factor ideas. Made a jig with a tiny brass tube superglued & taped to a heavy block, set it on a jack stand next to the Norton 750 motor. Inserted a 1/32 music wire sharpened to a needle point. The tubing & wire are available at hobby shops. Taped a piece of sheetmetal to the timing cover. My motor is balanced at 72% in solid featherbed frame (vertical engine). Sat on the bike with brakes on & Revved it to 6000+ while my wife Beth pushed the sharp wire against the sheet metal. Put the sheet metal under a microscope.
At 4000 to 5000 there is a round inscribed circle indicating perfect balance.
At 6000+ the circle elongates to the front and rear indicating that the balance factor is too high for higher RPM and the flywheel counterweight is shaking the motor to the front and rear more than up and down. But you way want it that way so it doesn’t shake you up & down as much as forward & backwards – you’ll feel it less..
Repeated this several tries and we found that by tapping the wire several times against the sheet metal at the desired RPM gave more and better readings. The inscribed circle at 4000 RPM is only about .007" in diameter and 5000 its about .010" in diameter and at 6000+ it turns into an ellipse about .010" tall and .020 wide. Photos of the witness marks came out blurry so I didn't post them. Its hard to believe that such as small shake amplifies into a buzz by the time it gets through the frame to the rider.
This is not the last word on balance factors but it sheds light on what's going on. Others are welcome to repeat it with other frames and other balance factors to get an idea what balance factors really should be used instead of by guess and by golly. I would like to see the same test done with 52% and 85% factors. Its not an easy test to make and its no fun revving your motor above 6000 over and over again in neutral. For instance the wire must be short on the pointed end or it will flex, Watching the tool and the tach at the same time is almost impossible. There are still a lot of unknowns until other balance factors are tried, But I hope I've given some people some ideas. If anyone else repeats this test successfully - please get in touch with me (jim@jsmotorsport.com).
At 4000 to 5000 there is a round inscribed circle indicating perfect balance.
At 6000+ the circle elongates to the front and rear indicating that the balance factor is too high for higher RPM and the flywheel counterweight is shaking the motor to the front and rear more than up and down. But you way want it that way so it doesn’t shake you up & down as much as forward & backwards – you’ll feel it less..
Repeated this several tries and we found that by tapping the wire several times against the sheet metal at the desired RPM gave more and better readings. The inscribed circle at 4000 RPM is only about .007" in diameter and 5000 its about .010" in diameter and at 6000+ it turns into an ellipse about .010" tall and .020 wide. Photos of the witness marks came out blurry so I didn't post them. Its hard to believe that such as small shake amplifies into a buzz by the time it gets through the frame to the rider.
This is not the last word on balance factors but it sheds light on what's going on. Others are welcome to repeat it with other frames and other balance factors to get an idea what balance factors really should be used instead of by guess and by golly. I would like to see the same test done with 52% and 85% factors. Its not an easy test to make and its no fun revving your motor above 6000 over and over again in neutral. For instance the wire must be short on the pointed end or it will flex, Watching the tool and the tach at the same time is almost impossible. There are still a lot of unknowns until other balance factors are tried, But I hope I've given some people some ideas. If anyone else repeats this test successfully - please get in touch with me (jim@jsmotorsport.com).