Balance shaft
In piston engine engineering, a balance shaft is an eccentric weighted shaft which offsets the vibrations of unstable engines. Balance shafts are usually mounted low in the cylinder block, driven by and rotating at twice the speed of the crankshaft. Balance shafts are most common in straight-4 engines, since these have fewer cylinders to balance the force of reciprocating masses. Large (greater than 2.0 L) straight-4s in particular are prone to rocking due to long-stroke rods or large-bore pistons.
General Motors' Quad 4 was notorious for vibration until balance shafts were installed. Other engines with one or two balance shafts include:
- Chrysler's 2.4 L Neon engine
- Ford's Modular V8
- Ford Taunus V4 engine.