Flywheel

Manual cars have a flywheel with a friction face for the clutch; automatics have a lighter flexplate that carries the torque converter. Both carry the toothed ring the starter motor engages. Its mass stores energy between the power strokes of each cylinder. Lightweight flywheels let an engine rev more quickly but can make it less smooth at low speed. Production cars balance smoothness, weight and cost.

Because it is heavy, the flywheel resists sudden changes of speed, so the engine runs smoother at low revs. The clutch disc is clamped against it to transfer torque to the gearbox.

A worn or damaged flywheel surface can cause clutch judder. Damaged starter ring teeth cause grinding noises when starting. The flywheel is usually inspected when the clutch is replaced; scoring, heat spots or cracks mean it should be resurfaced or replaced if permitted.