Crankshaft Balancing: The Art of Mechanical Harmony
Crankshaft balancing is the precision process of matching the mass of the crankshaft's counterweights to the forces produced by the pistons and connecting rods. A perfectly balanced rotating assembly is the foundation of a smooth, reliable, high-revving engine, eliminating destructive vibrations that can rob power and destroy components.
The Forces at Play: Rotating vs. Reciprocating
To balance a crankshaft, we must account for two distinct types of mass.
Reciprocating Mass
This is the mass that moves up and down in the cylinder. It stops and starts violently, creating the primary source of vibration.
Rotating Mass
This is the mass that spins in a circle with the crankshaft journal. It includes the "big end" of the connecting rod.
The Bob Weight: Simulating the Forces
A "bob weight" is a precisely weighted fixture that is bolted to each crank journal during the balancing process. Its weight simulates the combined forces of the piston and rod assembly.
The bob weight is calculated as follows:
The Balancing Process
With bob weights attached, the crankshaft is spun on a balancing machine. The machine detects any imbalance, and the operator corrects it by drilling small holes in the counterweights to remove mass, or by adding heavy metal slugs to add mass.
The Balance Factor (%)
This percentage determines how much of the reciprocating force is counteracted. For V-type engines (like a Harley V-Twin), a factor around 50-60% is used to achieve the best compromise. For most inline and boxer engines, this factor is effectively 100%.