Motorcycle Glossary
Clear definitions of motorcycle parts and maintenance terms. Each entry explains what the part does, common symptoms when it fails and the related product category.
71 terms
A
- ABS (anti-lock braking system)
- ABS (anti-lock braking system) detects a wheel about to lock under hard braking and modulates brake pressure many times a second to keep grip.
- Aftermarket part
- An aftermarket part is made by another manufacturer to replace the original part with the same dimensions and function.
- Air filter
- An air filter stops dust and particles in the intake air, protecting the cylinder, piston and rings from wear.
B
- Battery
- A motorcycle battery is the 12-volt lead-acid or lithium battery that turns the starter motor and powers the electrics when the engine is off.
- Bearing
- A bearing is a machine element that reduces friction between a rotating shaft and a fixed housing using balls or rollers.
- Brake caliper
- A brake caliper uses hydraulic pressure to move its pistons and press the brake pads onto the disc.
- Brake disc
- The brake disc is the steel rotor fixed to the wheel that the brake pads clamp to slow the motorcycle.
- Brake fluid
- Brake fluid is the high-boiling-point fluid that carries pressure from the master cylinder to the caliper.
- Brake master cylinder
- The brake master cylinder converts force on the brake lever or pedal into hydraulic pressure and sends it to the caliper.
- Brake pad
- A brake pad is the friction-lined part that the caliper pistons press against the brake disc to slow the motorcycle.
C
- Cam chain
- The cam chain (timing chain) drives the camshaft from the crankshaft and keeps valve timing in sync.
- Camshaft
- The camshaft uses its cam lobes to open and close the valves at the right time.
- Carburettor
- A carburettor is the mechanical fuel system component that mixes petrol into the incoming air in the correct proportion.
- CDI (capacitor discharge ignition)
- A CDI (capacitor discharge ignition) unit stores energy in a capacitor and dumps it into the ignition coil to create the spark at the spark plug.
- Centrifugal clutch
- A centrifugal clutch is the automatic clutch in scooters whose shoes swing out under centrifugal force as revs rise and grip the clutch bell to drive the wheel.
- Chain and sprocket kit
- A chain and sprocket kit is the front sprocket, rear sprocket and chain that make up the final drive to the rear wheel.
- Chain slack
- Chain slack is the total up-and-down movement of the lower chain run measured midway between the sprockets.
- Choke
- The choke enriches the fuel-air mixture (adds more fuel) to help start a cold engine.
- Clutch
- The clutch connects and disconnects drive between the engine and the gearbox under the rider's control.
- Clutch cable
- A clutch cable is the sheathed steel cable that links the clutch lever to the clutch release mechanism on the engine.
- Clutch plates
- Clutch plates are the friction-lined discs in a wet multi-plate clutch that clamp against the steel plates to transmit engine power to the gearbox.
- Connecting rod
- The connecting rod links the piston to the crankshaft and transfers the piston's linear motion into crank rotation.
- Crankshaft
- The crankshaft converts the piston's up-and-down motion into rotation through the connecting rod.
- CVT (continuously variable transmission)
- A CVT is the automatic transmission used in scooters that varies the drive ratio continuously with variator pulleys and a belt, without fixed gears.
- CVT drive belt
- A CVT drive belt is the V-section, cogged rubber belt that carries power between the front and rear variator pulleys of a scooter.
- Cylinder
- The cylinder is the part of the engine in which the piston moves and combustion takes place.
- Cylinder head
- The cylinder head closes the top of the combustion chamber and carries the valves, camshaft and spark plug.
D
- Drum brake
- A drum brake works by brake shoes expanding against the inside of a drum in the wheel hub.
E
- Engine displacement (cc)
- Engine displacement is the total volume swept by the pistons from bottom to top dead centre, expressed in cubic centimetres (cc).
- Engine oil
- Engine oil lubricates, cools and cleans the engine's moving parts; on most motorcycles it also lubricates the clutch and gearbox.
- Engine speed (rpm)
- Engine speed is the number of crankshaft revolutions per minute (rpm), shown on the tachometer.
- Exhaust
- The exhaust system carries burnt gases away from the cylinder and consists of the header (manifold), catalytic converter and silencer.
F
- Fairing
- Fairings are the plastic or composite body panels that enclose a motorcycle's engine and frame for aerodynamics, protection and looks.
- Final drive ratio
- The final drive ratio is the rear sprocket tooth count divided by the front sprocket tooth count; it sets how engine power is geared to the wheel.
- Flywheel magneto
- The flywheel is the heavy rotating disc on the crankshaft that carries magnets; with the stator it generates electricity and smooths engine rotation.
- Front fork
- The front fork is the twin-tube telescopic assembly that holds the front wheel and provides front suspension with springs and oil.
- Fuel injector
- A fuel injector is the solenoid valve in fuel-injected engines that sprays pressurised fuel into the intake on command from the engine control unit (ECU).
- Fuel pump
- The fuel pump is the electric pump in fuel-injected motorcycles that delivers fuel from the tank to the injector at constant pressure.
G
I
- Idle speed
- Idle speed is the lowest engine speed at which the engine keeps running with the throttle closed.
- Ignition coil
- The ignition coil is a transformer that steps up the low voltage from the CDI or ECU to the many thousands of volts needed to fire the spark plug.
J
- JASO (MA, MA2, MB)
- JASO T903 is the Japanese standard that classifies four-stroke motorcycle oils by their compatibility with wet clutches: MA, MA1, MA2 and MB.
O
- O-ring chain
- An O-ring chain has rubber rings between the link plates that seal grease inside the pin and bushing for longer life.
- OEM part
- An OEM part is made by the motorcycle manufacturer or on its behalf and matches the part fitted at the factory.
- Oil filter
- An oil filter traps metal particles and dirt in the engine oil so that clean oil circulates through the engine.
- Oil seal
- An oil seal is a lipped seal on a rotating or sliding shaft that keeps oil in and dirt out.
P
- Piston
- The piston moves up and down in the cylinder and transfers combustion pressure to the crankshaft through the connecting rod.
- Piston ring
- Piston rings sit in grooves on the piston; they seal combustion pressure, scrape oil off the cylinder wall and carry heat to the cylinder.
R
- Radiator
- A radiator is the heat exchanger in liquid-cooled engines that transfers heat from the hot coolant to the air.
- Regulator rectifier
- A regulator rectifier converts the stator's AC output to DC and limits the charging voltage to protect the battery and electrics.
S
- Shock absorber
- A shock absorber combines a spring and a damper to soak up bumps and keep the wheel in contact with the road.
- Spark plug
- A spark plug ignites the fuel-air mixture in the cylinder with a spark across its electrodes.
- Speedometer cable
- A speedometer cable is the flexible sheathed cable that carries rotation from the speedometer drive gear at the wheel hub to a mechanical speedometer.
- Starter clutch
- The starter clutch is a one-way clutch that passes the starter motor's drive to the crankshaft and freewheels once the engine fires.
- Starter motor
- The starter motor is a small DC electric motor that turns the crankshaft using battery power to start the engine.
- Stator
- The stator is the set of fixed coil windings inside the flywheel that generate alternating current together with the flywheel magnets.
- Steering head bearing
- Steering head bearings are the upper and lower bearings that let the front fork turn smoothly in the frame's steering head.
- Swingarm
- The swingarm is the rear suspension arm that connects the rear wheel to the frame and pivots up and down with the shock.
T
- Throttle body
- The throttle body controls how much air enters a fuel-injected engine using a butterfly valve linked to the twist grip.
- Throttle cable
- A throttle cable transmits the twist grip's movement to the carburettor slide or throttle butterfly.
- Torque
- Torque is the turning force the engine produces at the crankshaft, measured in newton-metres (Nm).
- Tyre pressure
- Tyre pressure is the air pressure inside the tyre, measured in bar or psi; it directly affects grip, braking distance, tyre life and fuel use.
- Tyre size
- A tyre size is the code giving tread width (mm), aspect ratio (%) and rim diameter (inches), e.g. 120/70-17.
V
- Valve
- A valve is the mushroom-shaped part in the cylinder head that opens and closes the intake and exhaust ports.
- Valve clearance
- Valve clearance adjustment sets the gap between the valve stem and the part that opens it (rocker or bucket) to the value in the manual.
- Variator
- A variator is the pulley system in scooters and other CVT motorcycles that changes the drive ratio steplessly according to engine speed.
- Variator rollers
- Variator rollers are cylindrical weights in the front variator that move outwards under centrifugal force and increase the belt's running diameter.
- VIN (vehicle identification number)
- The VIN (vehicle identification number) uniquely identifies a vehicle; on modern vehicles it is a standardised 17-character code.
W
- Water pump
- The water pump circulates coolant between the engine and the radiator in liquid-cooled engines.
- Wheel rim
- A wheel rim is the metal hoop the tyre sits on; it is joined to the hub by spokes or cast spokes to form the wheel.
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