VTwin Flywheel Balancing

I build stock engines. Mostly for others restorations. So, the only reason I try to learn as much as I can, is to try and figure out what the Harley engineers were trying to accomplish. Also, as a result of this digging, I have decided to incorporate it in all my rebuilds. Not to change the balance. Just determine what it is for the customers' bike. It may not matter at all. But, just part of record-keeping for future reference, when someone does figure it out. Now, for some basic info that has come out of this discussion on many websites:

1 - Lower balance factors mean that more metal is removed from the flyweight end (opposite the crankpin) of the flywheel. Makes for a quicker revving engine. But, some loss (maybe a lot) of ability to sustain torque throughout the rpm range of the engine. You would probably use this in a bike that has lots of close ratio gears, and running at higher than normal speeds (racing?).

2 - Higher balance factors mean that less metal is removed from the flyweight end of the flywheel. Makes for a machine that will pull good throughout the rpm range. This will be good for heavier machines, machines with passengers, machines towing trailers. The smoothness of the engine will be unique to that engine, rpms, loads, etc..

Just my opinion now. The lower factor probably will make for smoother overall, with less harmonics. The higher factor will have a 'sweet spot' of no vibration, and many harmonic ranges that are smooth. But, some mighty 'un-smooth' spots in between that will rattle the mirrors, and loosen the bolts.

For my personal road machine, if I had to choose, I would build in the 55% to 60% range. I always built heavy wheeled Big Twin engines at 55%, because that was a Harley standard, as confirmed to me by the shops that even knew 'back in the day'. 60%+ is what the factory uses now, I'm told. A couple of things to keep in mind: The older machines had massive flywheels (55% used). The newer machines have skinny flywheels (60%+ used). I'm thinking that the loss of mass on the newer engines require keeping more metal on the wheels for sustaining torque.

If the engine that you are building is not a race engine (meaning very high RPMs), a few grams difference in piston weight will not make any noticible difference in the overall assembly balance, as they are both connected to the same crankpin. Of course, there's nothing wrong with matching them. I tend to do it. They will be the only thing matched in the reciprocating components, though. The rods are not matched, just because the female rod is heavier than the male rod. T&O balances their wheels to a 55% factor, and S&S usually balance theirs to a 60% to 65% factor, unless otherwise specified by the customer. When balancing an assembly, the total reciprocating component weights are summed (your pistons fall into this catagory), and multiplied by the balance factor. Then, added to the rotating weight. This new total is divided by 2, and becomes the bobweight value to balance the wheel halves.

My point is, V-Twins are balanced as if they were single-cylinder machines. If your rods occupied a separate and different location on the crank, then matching would be critical for proper balance. As is, they become basically one weight, as far as the crankpin is concerned.

Male rod is lighter. Logic would say put heavier piston on the lighter rod. But, remember what I said. They are both attached to the same crankpin. So, the balance comes out the same either way. Pistons are reciprocating weight. Summed, they become one weight in the calculation to balance the whole assembly. If you really wanted to split hairs, there may be some way to calculate the affect of separating the pistons by 45 degrees. But, no one does.

One thing to keep in mind is that splitting hairs doesn't work on a V-Twin street assembly. It is very forgiving with any balance that falls within the factors in use. Just the vibration range changes, along with it's harmonics. And, that can be affected by load on the engine. The heavier the load, the less likely that vibrations will be an issue. But, it really shows up on a lighter machine (choppers, bobbers, race).

The way builders really found out what worked and what didn't usually came from racing. Engines are built to vibrate the least in the RPM range that they are most ridden. Race dictates high, sustained RPMs. Street dictates lower, sustained RPMs. That's where we get our numbers. Low numbers approaching 50% were used for race applications. Higher numbers approaching 65% were used for street, with 55% becoming the standard for heavy wheels and 60% becoming standard for skinny, light wheels. Higher factors leave more meat on the flywheels.

Matching weights is a good practice. Within reason. I'm not in the habit of grinding stock rods to get the light ends to match. But, some aftermarket makers (usually automotive) put a tab on the wristpin end just for this reason. Not enough meat on a stock rod to do so. I have a set of fresh Carrillo rods that I had made for my 45 racer project. Beautiful, and heavy duty. And, almost the exact weight of OEM rods for this machine. No tabs for grinding. Just a smooth, satin (from shot peening) finish. As to pistons, cast pistons can be ground a bit (just a bit) to compensate for weight differences. Forged provide more meat and are easier to find material to grind. I don't like forged. Or at least, the the old technology behind them. They came from old race technology that compensated for the weakness of cast pistons on the race track, and really are not the best for long life street engines. Meaning, open clearances necessary for expansion. Noisy. Eventually, very sloppy. Modern technology has brought us hypereutectic pistons that are beefier (read that as lots of meat), stronger, lighter, and can be run with closer tolerances than cast. Did I mention that I really like this type of piston? However, I build primarily old stock engines for others. So, quality cast is what I use when available. When it's not, hypereutectic is the number one choice. All my builds get a fresh balance as part of the build. No extra cost. Just what I practice. Reverse calculate to see where they fall, to begin with. Then, recalculate if necessary, to correct to the customer's application.

Back in the late 60's, the first time I really began to wonder about balance factors, I was given an explanation of the missing percentages. I was told that it was necessary to remove this weight from the total in order to compensate for the force the exploding fuel created, which the engine sees briefly as weight on the spinning assembly. Whether it was just the opinion of the wrench, who told me. Or, first hand knowledge. I don't know. Never had anyone to tell me different. Not a very scientific explanation. But, I'm happy with it.

There's a really good discussion of balancing (aside from my ramblings), from several knowledgeble perspectives at the following website: