The crankshaft converts linear piston motion into rotational energy, but that conversion creates violent forces. Every time a piston changes direction at TDC or BDC, it generates an inertial force. Multiply that by thousands of RPM and you've got a vibration problem that can crack blocks, destroy bearings, and shake fillings loose. Counterweights are the cure.

Primary vs. secondary imbalance: Primary imbalance comes from pistons moving up and down once per revolution. Secondary imbalance is subtler — it comes from the fact that a piston moves faster in the top half of its stroke than the bottom half (because the connecting rod swings through an arc, not a straight line). Primary forces are at crank speed; secondary forces are at twice crank speed.

How counterweights work: Cast or forged into the crank's web (the thick section between journals), counterweights place mass opposite each crank pin. As the piston and rod move upward, the counterweight swings downward, canceling the force. But here's the critical detail — counterweights only balance the rotating mass (the big end of the rod plus the crank pin), not the full reciprocating mass (piston, rings, wrist pin, small end of the rod). A typical street engine uses 50% overbalance, meaning the counterweight is sized to offset all the rotating mass plus roughly half the reciprocating mass. This splits the remaining vibration equally between vertical and horizontal planes rather than letting it hammer in one direction.

Rule of thumb for bobweight: Bobweight = rotating mass + 50% of reciprocating mass. If your rotating assembly has 400g of rotating mass per journal and 600g of reciprocating mass, your target bobweight is 400 + 300 = 700g per journal. The crank is then spin-balanced on a machine to match that bobweight within about 1–2 grams.

Real-world example: GM's LS3 6.2L V8 uses an internally balanced crankshaft — all counterweight mass lives on the crank itself. Contrast that with older small-block Chevys (like the 400ci) that needed a heavy external balance weight on the harmonic balancer and flywheel because the stroke was too long to fit enough counterweight on the crank webs. Internal balancing is preferred because it's more stable across RPM ranges and simplifies swapping flywheels or dampers.

Why engine configuration matters: An inline-6 and a flat-6 are naturally balanced in both primary and secondary forces due to piston phasing — no counterweight trickery needed for secondaries. A 90° V8 with a cross-plane crank has inherent secondary balance but needs counterweights for primary. Single-cylinders and inline-4s are the worst offenders, which is why your Subaru WRX's flat-4 feels smoother than a comparable inline-4 — opposing pistons cancel each other's primary forces.