Starter motor

The starter motor is a small DC electric motor that turns the crankshaft when you press the start button, until the engine fires. Because it draws a very high current, it is switched by a starter relay (solenoid) rather than directly by the button.

The starter drives the engine through gears, with a starter clutch (sprag clutch) that lets the engine run free of the starter once it has started.

Common faults: only a click when you press the button (flat battery, poor connection or the relay), slow cranking (weak battery or worn brushes), or the starter whirrs but the engine doesn't turn (starter clutch). Many bikes have safety switches that block the starter if a gear is engaged without the clutch pulled in, or with the sidestand down.