You can balance perfect all you want but when it seats a bit differently on the crank then on the balance mandrel things will change. To know best (applied dynamics aside) the rotor needs to be true'd for the balance device and then DTI'd on the crank. Static rotor balancing will be fine for these bikes in the right hands if reducing rotor mass (it's not rocket science either) and no need for high speed dual plane balance say like a turbine or supercharger might require. Because every rotor and crank has tiny production variances we cannot make things perfect for mass production. If a rotor can be positioned at various radial locations on the crank, which it can, we have introduced a dynamic variable with the finished product, however small that might be. These engines as good as they are, are still low rpm clunkers, so expect some vibration from various dynamic interactions. There are so many harmonic dances going on that a perfectly applied and balanced rotor could actually increase vibrations over that of a slightly imperfect one located at a corrective position, when splitting hairs. You might (situation dependent) be able to measure this change with a dynamic balance device with the engine running and you might not, so basically most understand that a static rotor balance method is good enough for SOHC government work in these cases.
Couldn’t agree more.
Thanks for the insight, guys! While I have another 500 engine here [for the book], I plan on selling it after I'm done, so it's not a 'donor' that could borrow the [end of the?] crankshaft, or the like, for the purposes of this little project. In the end, if I could get the typical rotor to actually produce the 150+ watts these were rated, that would be enough for the bikes - they need about 95-105 watts with regular incandescent bulbs aboard and the lights always on (thanks a lot, ex-President Carter...). The trouble is that they often make 80-90 watts, in my experience, and the things that cause the 750 alternator output to drop are not the same on the 500/550 version: they start out low when new if their rotors are especially bad, and never get better. That is the #1 reason this particular bike gets parked: the owner gets tired of struggling with low battery charge and/or constant battery tending issues. On those bikes that have the better alternators I've seen their mileage roll the odometer over, like the 750, from touring America's many interstates and commuting daily to work. Even here, I think Crazy PJ has rolled his over before?
I only discovered this issue (many years ago) when a damaged 550 crankshaft had to be replaced and the donor engine for it happened to have a better rotor (and all but 1 of the same crank journal sizes!). The alternator rotor was on it and I left it there, assuming it would work: after running the bike at idle for a while the voltage was up over 13.6 volts, which I'd only ever seen on one other 550 before. The 'new' rotor was from the last of the 550F series bikes, and had a very 'automated' look to the welds, like they were done by machine and not a newbie welder. The 750 bikes after the F1 have similar-looking rotors.
