BALANCE FACTOR SCRATCH TEST TOOL
First of all - when people suggest a BF how do they qualify it? By the sensitivity of their butt and how well they remember how a Nort vibrated one day and they how it vibrated a few days later after they changed the balance factor?
The first clue is that Nortons are balanced anywhere from 52% to 80% and that 67% is the midpoint average.
Then you hear that different frames, isolastics and tilt of the motor effects everything.
And the science is confusing. If you switch a heavy rod cap to a lighter rod cap on the same rod you get difference weight measurements on the small end of the rod even though in fact the small end of the rod did not change weight (because of the way that each end of a rod is weighed horizontally). This means that OEM alum rods with heavy steel caps will give different BFs than all steel or all alum aftermarket rods - so things are screwy, complicated and messy.
There is a way to actually verify correct BF. This is the only way I know of to figure out which BFs are correct and which are just regurgitated BS. NOTE THAT THE TEST BELOW CHALLENGES THE NOTION THAT HIGHER BALANCE FACTORS WORK BETTER AT HIGHER RPMS (I USED TO BELIEVE THIS BUT NO MORE).
Here is a crude example below:
The scratch tool is a piece of 1/32 ID brass tubing with a 1/32 piano wire sharpened to a needle point (hobby shop stuff) . The brass tubing is taped to a heavy steel block and the block mounted on something solid like a car jack or a stack of bricks. The needle should not extend more than 1/8" from the brass tube or the steel block or it will flex and not give a accurate reading. The opposite end of the wire should extend out of the brass tube so you can push it at the right moment. A piece of polished steel sheet metal is taped to the timing cover NEAR THE AXIS CENTER-LINE OF THE CRANKSHAFT (not on the frame).
Start the bike, hold the front brake.
Rev it the the RPM you want to test (4000 or 6000 etc) and have a helper tap the end of the wire as quickly as possible against the sheet metal. Move the bike forward 1/2 inch and repeat the process.
This test is very difficult to get right and it needs refining. I could only get a good witness mark about 2 times out of 10 tries. And you have to have a microscope to see the elliptical scratch clearly. It was not possible to get a good photograph so I drew an image (below) of the scratch test of my featherbed with a 72% wet balance factor with JS lightweight pistons & longer rods. The ellipse measured about .020" or so in dimension. It will probably measure a lot more in an isolastic frame or in any bike with heavy stock pistons. As you can see the 72% is too high because the motor shakes horizontally more than vertically at 6000RPM. A lower balance factor would shake less horizontally but more vertically. More tests need to be made and a good time to do it is when the bike is on the Dyno with various frames - isolastics etc. I'm guessing that tests will show the best BFs will be in the 60 to 70% range. Note that 65% wet BF(accounting for oil in the crank) = about 72% dry BF
