Just to throw out another balance factor, I believe every flywheel set sold by S&S was, and still is, set at 60%. You also have the option of getting certain wheels "raw" and balance them as you choose.
This subject is far more complex than has been discussed. IMHO S&S uses 60% in part because a high percentage of their products have a smaller rod/stroke ratio than stock (stock 45, K, KR (except lowboy) XL: 1.95:1; stock FL big twin: 1.87/1.88:1). This results in higher secondary out-of-balance force and higher side-thrust vector - which automatically changes the ideal ratio, and based on how many of these are done to numbers above 50% it appears that the best factor goes up when the rod ratio goes down. Exactly what factor to use would, again IMHO, depend in some part on the new ratio, but there are other contributing factors beyond my comprehension so this is best done empirically (which is what S&S did). What is to be avoided at all cost is the natural impulse to interpolate known data and produce a compromise by "taking the best of both". This is the exact same fallacy as that which suggests that parents with 130 and 120 IQs will produce a child with 125 IQ. They won't - the chances are better that the child will be below 120 (of course, the reason is different, in this case it's due to regression towards the mean). You don't get the best of both by averaging etc. 50% and 60%, or averaging the RPM or stroke length. There is no direct relationship (linear or otherwise) between any dimensions of the subject engine and the model engine and their best balance factors. The forces at work are incredibly complex, and have never (as far as I know) been quantified, so any attempt at prediction of what factor between 50 and 60 etc. to use for an intermediate stroke length, rod ratio etc. is absolutely futile.
On the plus side: not only will the factor change with rod angle (~ as ratio), but there is also a range of tolerance where both the exact factor, and the accuracy of its application affect vibration in terms of amplitude (severity of the shaking force) and frequency (where does it occur). Engines like the KR with: 1. high RPM power peak 2. high rod ratio 3. low compression 4. low reciprocating weight are all relatively insensitive to small errors in balance.
As any of 1. through 4. reverse (lower RPM range, sharp rod angle, high compression, or high reciprocating weight) the effects of a bad balance are exaggerated.
The factory race 50% factor is for both s.v KR and ohv XLR in the 60's... Don't know what the street ones were at that time... The factory balance sheet lists all the rod lenghts as well (they calculate the reciprocating and rotary weights of the rod by balancing it on a knife edge and apply some calcs), so could be applied to big twins as well.
The guy I send my Sportster flywheels out to rebalance is always double checking with me, so he must know the street ones are balanced with a higher factor!!! I found this factor suits the way I ride, using all the available rpms, I get none of the mid-range vibration that all Sportys owners talk about, seems to "free"up the engine as well somehow, looses a lot of "lumpiness"...
No need for math or guesswork. You can achieve perfect balance empirically using the method Phil Irving suggests in TUNING FOR SPEED - just drill holes in the flywheel, thread them to accept weight plugs, then drill matching holes in the crankcase to allow access, thus permitting altering the balance factor in minutes.
Jim (XLR) always did the balance on our KR. According to my notes he used 53% factor,( Motor is very smooth at race RPM's. He also told me he did each flywheel and shaft separately not as an assembly. Static not dynamic. Mike