Balance Masters®
Hot Rod Bikes

Occasionally, in the process of putting together this fine magazine, we quite by accident come across a product or process that truly amazes us. While we were shooting the Keck motor buildup that ran in our April '97 issue, Quality Assemblies President Pete Baker insisted that we test-ride a custom bike his company had recently completed. His purpose was to enlighten us about a new modification they had used on this custom bike, the Keck motor, and several of their stroker motor assemblies.

HFB Editor Frank Kaisler and I took turns riding the bike, and to say we were impressed is an understatement. We both agreed that it was among the smoothest-running solid-mounted Softail-style bikes we had ever ridden. That its motor was 98 ci in size made this even more impressive. Pete explained that the bike had been modified by Sun-Tech Innovations, the same company that makes Balance Masters®, using its active balancing technology.

Well, being the naturally inquisitive types we are, we contacted Chris Gamble at Sun-Tech and asked him to share his secrets with our readers. After huge amounts of cash changed hands, he allowed us into the Sun-Tech manufacturing plant-spy camera in hand-and gave us the lowdown on this revolutionary modification and the science behind it. Basically, the process involves machining a groove into one flywheel half and epoxying in a mercury-filled bladder, much like those used in Sun-Tech's Balance Masters® products.

First, a simple understanding of flywheel balancing is necessary. Because of variations in material density and in manufacturing and machining tolerances, no two flywheel halves or complete assemblies are exactly identical. Add to this the fact that the centrifugal force of a reciprocating mass (a mass that rotates or spins) increases in relation to its speed, and you'll understand that an imbalance of just a few grams can cause tremendous vibration at high rpm. In order to offset the centrifugal force of the crankpin, connecting rods, and piston assemblies as they move outward-away from the flywheel's centerline-additional weight (referred to as counterweight) is cast into the flywheels directly opposite those parts at full extension.

The idea is that since the piston and rod assemblies create inertia in one direction as the flywheels rotate, the counterweight creates a near-equal inertia in the opposite direction, resulting in opposing centrifugal forces that move away from the flywheels' axes and thereby cancel out each other. This helps reduce the forces that contribute to vibration and the inertial load on the bearings. Remember, Newton's Law says that for every action there is an equal and opposite reaction. With us so far?

The factory-using a complex equation to estimate average crankpin, pistons, and rod assemblies' weights-calculates the amount and location of counterweight material removal on each flywheel half. Holes are drilled in the counterweight until it meets the calculated weight; then the flywheels are sent for final assembly. This is all the factory does to balance its flywheel assemblies. It explains why one Harley can vary from another in the amount of transmitted vibration it exhibits. This simple balancing method doesn't account for variations in manufacturing and machining tolerances.

Traditional Harley V-twin balancing is accomplished using one or both of two additional methods. The first is called static balancing. This is done by supporting the flywheels' pinion and sprocket shaft centers, attaching a bob-weight to the connecting rods to simulate the weight of the piston assemblies, and allowing gravity to indicate the heavy spots. This method is only a ball-park indicator of major out-of-balance areas. Then there's dynamic balancing. Using a bob-weight as described earlier, the flywheel assembly is spun at one or two different speeds and a computer determines what additional material must be removed from the counterweight. The machine that does this works much like the modern tire balancer you'd find in an automotive shop.

The biggest drawback to both of these methods is accuracy. Because the centrifugal force of a fixed mass increases with rotational speed, a flywheel assembly that is relatively smooth at 1,500 rpm can shake like a paint mixer above 4,000 rpm.

Here's where Sun-Tech's active balancing technology comes in. It uses a circumferential mercury-filled bladder attached directly to the rotating mass. As the object spins, centrifugal forces cause the mercury to migrate within the bladder to an area that is directly opposite any imbalance. The mercury migrates not in a blob but in a thin bead, so it doesn't add any imbalance of its own. Since the mercury is not constrained, it can move around as needed in response to changes in rotational speed-hence the designation "active balancing."

Now, you might be curious why Sun-Tech uses mercury in its products. So were we. The reason is surface tension. Mercury has the highest surface tension of any liquid. Why does this matter?

Well, because the mercury is constantly moving within the bladder. This creates friction, and friction causes wear. A liquid with a lower surface tension will eventually wear through the bladder and leak out. Mercury's surface tension is so high that it is the only liquid known to man that cannot adhere to glass.

Now, the Balance Masters® products have been on the market for several years. We highlighted their installation back in our September '94 issue, and they're in use on a couple of our own staff's motorcycles as well. But only recently did Sun-Tech develop a process for installing this technology directly into a Harley flywheel assembly, which they claim reduces vibration by 80 percent. This is an even better alternative for eliminating vibration at its source.

At the Sun-Tech plant in Northridge, California, the company can machine your flywheels and install the bladder without the need for complete flywheel disassembly. And once modified, your flywheels never again need balancing, even if you change pistons or rods. This technology has been proven by years of fleet use on over-the-road trucks, and carries a U.S. patent. Companies such as UPS use Balance Masters® on their truck wheels to reduce maintenance costs and tire wear. Hallcraft's Industries is even using this technology on its new "never-need-balancing" laced wheels.

If you're contemplating an engine teardown or building a big-bore or stroker monster, the benefits of Sun-Tech's active balancing could be the difference your bike needs to be more Jekyll than Hyde. So, if your bike is giving you the shakes, check into Sun-Tech's Balance Masters® or active balancing flywheel modification. If you're still shaking after that, we recommend you cut down on caffeine. HRB