Crank Balancing

Let Thumper racing get the smoothest power you can get by having us balance your cranks for better performance, smoother power and less stress on the
motor.  If you have ever done a long ride and found your hands buzzing afterwards you may need to balance your crank.  We've seen factory cranks be off as far as 100g.

We offer two levels of balance for your single cylinder motor. First and foremost is primary crank balancing, this should be done any time a crank is stroked, lightened, or a heavier/lighter piston rod combination is used. 

Simply put, the main reason is to eliminate damaging and parasitic forces that cause unwelcome mechanical activity and robs the engine of its ability to freely transmit all of its power directly to the wheels.

Most people overlook the fact that all engines, from the first stage of manufacturing, go through some process of balancing. When the crankshaft and all related components are manufactured they have to be shaped and weighed to meet a target weight specification.

The crankshaft is then designed with counter weights that offset the thrusting and rotating forces generated via the combustion activity that causes the engine to rotate. Ok this is generally known and understood.

Forces In Action

To better understand the mechanics of balancing, let's look at the theory behind it. As everybody knows, a rotating object generates "centripetal force." Centripetal force is an actual force or load generated perpendicular
to the direction of rotation. Tie a rope to a brick and twirl it around and you'll feel the pull of centripetal force generated by the "unbalanced" weight of the brick. The faster you spin it, the harder it pulls. In fact, the magnitude of the force increases exponentially with speed. Double the speed and you quadruple the force.

The centripetal force created by a crankshaft imbalance will depend upon the amount of imbalance and distance from the axis of rotation (which is expressed in units of grams, ounces or ounce-inches). A crankshaft with only
two ounce-inches of imbalance at 2,000 rpm will be subjected to a force of 14.2 lbs. At 4,000 rpm, the force grows to 56.8 lbs! Double the speed again to 8,000 rpm and the force becomes 227.2 lbs.

This may not sound like much when you consider the torque loads placed upon the crankshaft by the forces of combustion. But centripetal imbalance is not torque twisting the crank. It is a sideways deflection force that tries to
bend the crank with every revolution. Depending on the magnitude of the force, the back and forth flexing can eventually pound out the main bearings or induce stress cracks that can cause the crank to snap.

Centripetal force should not be confused with "centrifugal" force, which is the tendency of an object to continue in a straight trajectory when released while rotating. Let go of the rope while you're twirling the brick and the
brick will fly off in a straight line (we don't recommend trying this because it's difficult to control the trajectory of the brick).

Back to centripetal force. As long as the amount of centripetal force is offset by an equal force in the opposite direction, an object will rotate with no vibration. Tie a brick on each end of a yardstick and you can twirl it like a baton because the weight of one brick balances the other. If we're talking about a flywheel, the flywheel will spin without wobbling as long as the weight is evenly distributed about the circumference. A heavy spot at any one point, however, will create a vibration because there's no offsetting weight to balance out the centripetal force.

This brings us to another law of physics. Every object wants to rotate about its own center of gravity. Toss a chunk of irregular shaped metal into the air while giving it a spin and it will automatically rotate about its exact center of gravity. If the chunk of metal happens to be a flywheel, the center of gravity should be the flywheel's axis. As long as the center of gravity for the flywheel and the center of rotation on the crankshaft coincide, the flywheel will spin without vibrating.

But if there's a heavy spot on the flywheel, or if the flywheel isn't mounted dead center on the crank, the center of gravity and axis of rotation will be misaligned and the resulting imbalance will create a vibration.

Price doesn't include HEAVY METAL