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
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
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