Harley Balance Factors

[Bill Pedalino]

Thanks for the reply and information Jack.

If you read some of my ealier threads you will note that I have been balancing 74 and 61 motors for many years (since the '70's). My inquiry was whether the Factory had changed the balance factors for certain years. This question was promulgated by the fact that, in 1946/1947 the street-opinion in my area was that the 74" OHV's were 'vibrators' while the 61's were not.

I have always used a 51-to-55% balance factor (depending on the model and application) with great success. I was just wondering if this factor was factory-developed of just the street-developed factor that yielded the best 'sweet spot' , developed by common practice.

Back calculating is fine, as long as you have the original pistons, rods, etc. Over the years however, MC pistons, Taiwan rods, aluminum bearing cages, etc. make this harder to do. Although, if the motor ran smoothly in your favorite RPM range, then the weights would seem to be ok and a back-calculation would probably yield an accurate balance factor.

By the way, S&S used to make a balancing set-up for retail sale. I never liked it though, as it was kind of crude, although it probably worked well. Mine is custom-machined and consists of rolling knife edges and custom ground shafts.

Thanks again for your reply!!

[Jack_Hester]

Bill -

Actually, the reply was for 'gtsickle', and his question as to how to get started.

Jack

[Steve Slocombe]

The early Hints and Tips books imply a 50% balance factor for all bikes, as the calculation shows weights being halved. I'm having my VL flywheel assemblies balanced to a 46% balance factor, which my balancing guy says he found in a notebook for early Harleys. It works well with this fairly square engine (typically 3.5" bore x 4" stroke). A customer has tried 42% and says it is even smoother at high revs. My balacer says the modern Harleys use 58-60% balance factor as they are tuned more for torque at low revs rather than top end performance.

[Jack_Hester]

Steve -

60% is the number that I've heard for the late stuff, as they have skinny wheels, and this factor leaves some meat on them for torque. As to your balance factor, I've only seen it in print as to the 50% for the older stuff like yours. The lower factor would lighten the wheels, explaining the smoothness at higher revs. But, only experience from doing it proves it. I've never used a 42% factor. My WLA wheels reverse calculated to a 44% factor, though all the other typical H-D 45 wheels that I have done came out to be in the 50% range. I don't know the history on my wheels, but I'm leaving it at that, just to see how the engine feels. It's not that much trouble for me to change it later, if I don't like it.

Jack

[T. Cotten]

Please be aware that the chassis design has direct influence on the best factor for a motor.

The late model cradle frame is significantly different from single-downtube models in how it dampens vibrations.

Matt Elvenkemper gave a good explanation of this at http://virtualindian.org/Flywheeltheory.htm

This helps to understand the wide variation in factors between early flattys and later OHV's.