Technical Terms

A collection of technical terms to help you understand motorcycle terminology you may come across in day-to-day life.



Atmospheric/automatic inlet valve. System used on early engines where the inlet valve is held shut by a weak spring and opens by atmospheric pressure when the falling piston creates a vacuüm in the cylinder.


Electronic generator that makes alternating current by spinning a magnetic rotor inside a coil-wound stator.


bash plate

A protective plate fitted under the engines of off-road machines to prevent damage caused by grounding.


Bodywork resembling an upside-down bathtub used on rear of some Triumph motorcycles. It was introduced in 1957 and dropped in the early 1960s

bell mouth

Bell-shaped air intake fitted to some carburetors.

bevel gears

A pair of gears with faces cut at an angle of 45°, allowing drive to be turned through 90°.


Bottom dead centre. The point during crankshaft rotation at which the piston is in its lowest possible position.


Brake horse power. A unit of measurement for engine power output.

big end

The larger (crankshaft) end of the connecting rod.


The measurement of a cylinders diameter (see also stroke).

breakout (drag racing)

Used only in handicap racing, the term breakout refers to a contestant running quicker than he or she “dialed” his or her vehicle (predicted how quick it would run). Unless his or her opponent commits a more serious infringement (e.g., red-lights, crosses the centerline, or fails a post-race inspection), the driver who breaks out loses. If both drivers break out, the one who runs closest to his or her dial is the winner.

burned piston

When a cylinder runs lean (too much air in the air-to-fuel mixture) and excessive heat burns or melts the piston.

burnout (drag racing)

Spinning the rear tires in water to heat and clean them before a run for better traction. A burnout precedes every run.



An eccentrically shaped rotor that converts rotational movement into linear movement. Cams are used in the operation of valves, contact breaker points, and drum brakes.


A shaft with two or more cams used in the four-stroke engine to operate inlet and/or exhaust valves.


Device that mixes fuel and air into a combustible vapour.



christmas tree (drag racing)

The Tree, as it is often called, is the noticeable electronic starting device between lanes on the starting line. It displays a calibrated-light countdown for each driver.

clincher rims

Type of wheel rim used with early beaded-edge tyres.

clip-on engine

An engine that attaches to a conventional bicycle frame.


Low racing handlebars that clamp directly onto the fork legs.

clutch can

The bell-shaped housing, or bell housing, used to encase the clutch and flywheel.

clutch dust

Carbon dust created when the surface of the clutch discs wear as they slide together during the clutch-lockup process.

clutch lockup

The progression of clutch-disc engagement controlled by an air-timer management system.

compression ratio

A measurement of the difference between cylinder volume at t.d.c. and b.d.c.

contact breaker

The spring switch in the low-tension ignition circuit that controls the timing of the spark in the high-tension circuit. Operated by a cam, contact breakers are sometimes called points.

contracting-band brake

A brake in which a band is tightened around a rotation drum.

cradle frame

Frame with two tubes passing under the engine (see also open cradle frame).


The cranked shaft in an engine that changes the pistons linear motion into rotational motion.

cush drive

A transmission shock absorber, usually a rubber cushion in the rear hub.

cylinder head

A casting that caps the cylinder and contains the valves and combustion chamber.



Means of controlling speed of movement of the steering or suspension.

deep staged (drag racing)

A driver is deep staged when, after staging, he or she rolls a few inches farther, which causes the pre-stage light to go out. In that position, the driver is closer to the finish line but dangerously close to a foul start.


Valve gear operation in which the cam shaft actively closes as well as opens the valves.


Double overhead cam shaft. Two camshafts fitted into the cylinder head.

dial-under (drag racing)

Dialing under allows drivers in Super Stock and Stock, which are handicap categories, to select an elapsed time quicker than the national index. As with a dial-in, a driver selects a dial-under, or e.t., that he or she thinks the car will run based on qualifying performance. The breakout rule is in effect.

diamond frame

Tubular frame design common until World War II and derived from the bicycle layout. The engine cases often form part of the structure. In profile it resembles a diamond shape.

diaper (drag racing)

A blanket made from ballistic and absorbent, often Kevlar, that surrounds the oil pan and serves as a containment device during engine explosions. Required on Top Fuel dragsters, Funny Cars, Alcohol Dragsters, and Alcohol Funny Cars.


In an engine, displacement is the total volume of air-to-fuel mixture that an engine theoretically is capable of drawing into all cylinders during one operating cycle.


Device used in the ignition system of some multi-cylinder machines to send the high tension part of the correct cylinder.


Highly combustible alcohol/methanol-based fuel mixture.

dropped cylinder

When a cylinder becomes too rich (too much fuel in the air-to-fuel mixture) and prevents the spark plug or (plugs) from firing.

druid forks

Side-sprung girder forks. Druid were the original makers.


Double, having two parts. Applies to frames with two down tubes, and chains with double rows of rollers.


Electric generator that produces alternating current.


earles forks

Long leading-link forks, ie front suspension by pivoting fork controlled by twin shock absorbers. Designed by Earnie Earles, they were used by many manufacturers in the 1950s.

elapsed time (drag racing)

An elapsed time, or e.t., is the time it takes a vehicle to travel from the starting line to the finish line.

eliminations (drag racing)

After qualifying, vehicles race two at a time, resulting in one winner and one loser. Winners continue to race in tournament-style competition until one remains.


Off-road competition against the clock and usually over long distances.

epicyclic gear

A gear that operates around the circumference of another.


face cam

A cam system in which the eccentrics are situated on the face of a rotating disc.


An enclosure fitted to improve the aerodynamic performance of the machine and/or rider comfort.

featherbed frame

Famous Norton frame design by the McCandless brothers. It was introduced in 1950 and was given its name by factory rider Harold Daniell.


Fédération Internationale Motorcyclistes, the governing body of the international motorcycle sport.

final drive

Means of transmitting power to the driven wheel, usually by chain, shaft, or belt.

flat tank

Fuel tank shape used on early motorcycles.

flat twin

See horizontally opposed

float bowl

The fuel reservoir on a carburettor into which fuel flow is controlled by a valve operated by a float.

fore-and-aft flat twin

A flat-twin engine mounted with cylinders positioned in line with the frame.


Early three-wheeled vehicle with two-front wheels fitted to a motorcycle based fame. Passenger accommodation was sited above the front axle.

foul start (drag racing)

A foul start is indicated by a red-light on the Christmas Tree when a car has left the starting line before receiving the green light, or starting signal.

fuel injection

A fuel-delivery system that replaces conventional carburetion. Fuel injection delivers fuel under pressure directly into the combustion chamber or indirectly through the airflow chamber.

full tree (drag racing)

Used in Competition, Super Stock, and Stock, for which a handicap starting system is used to equalize competition. The three amber bulbs on the Christmas Tree flash consecutively five-tenths of a second apart, followed five-tenths later by the green starting light. A perfect reaction time on a full Tree is .500.


“garden gate”

Nickname for the plunger-sprung frames used on Norton machines from the late 1930s.

guard beam (drag racing)

A light beam-to-photocell connection located 16 inches past the staged beam that is used to prevent a competitor from gaining an unfair starting-line advantage by blocking the stage beam with a low-installed object such as an oil pan or header collector pipe. If the guard beam is activated while the staged beam is still blocked, the red foul light is triggered on the Christmas Tree and the offender is automatically disqualified.

gear cluster

A set of gears.

gear ratio

The ratio of the turning speeds of a driving and driven gear or the cumulative ratios for a series of gears.

girder forks

Type of forks common on early machines, comprising rigid beams attached to the steering head by parallel links that allowed movement.



Fine-tuned exhaust system that routes exhaust from the engine. Replaces conventional exhaust manifolds.

helical gear

A gear with a spiral or semi-spiral meshing face.


A hemi engine has a hemispherical shaped cylinder-head combustion chamber, like a ball cut in half.

hole-shot (drag racing)

Reacting quicker to the Christmas Tree starting lights to win a race against a quicker opponent.

horizontally opposed

Type of engine layout in which the cylinders are placed at 180° to one another. It is also described as a flat twin/four etc. or a boxer engine.

hub-centre steering

Steering system in which the wheel pivots about its centre point; the axle is normally fixed.


When a cylinder fills with too much fuel, thus prohibiting compression by the cylinder and causing a mechanical malfunction, usually an explosive one.


ignition timing

Point at which, relative to crankshaft rotation or piston position, the ignition spark occurs.


Engine layout in which the cylinders are arranged in a row, and in-line with the wheels of the machine.

index (drag racing)

The expected performance for vehicles in a given class as assigned by NHRA. It allows various classes of cars in the same category to race against each other competitively.

interval timers (drag racing)

Part of a secondary timing system that records elapsed times, primarily for the racers’ benefit, at 60, 330, 660, and 1,000 feet.


Inlet over exhaust. Valve layout used on some early machines in which the exhaust valve was mounted to the side of the engine and the mechanically operated inlet valve was positioned above the exhaust.


International Six Day Trial.


Proprietary name for an engine/swingarm rubber mounting system used by Norton on its early Commando models.



English engine manufacturer. Founded in 1903 by John A Prestwich, the company was bought by Villiers in 1957.

jockey wheel

A wheel used to maintain tension in a chain or belt.


keystone frame

An American term to describe a diamond-type frame in which the engine serves as part of the structure.


leading link

Front suspension design in which the axle is mounted at the front end of two short links that pivot at the bottom of solid forks. The links are sprung to control movement A long leading-link system has a complete fork that pivots behind the wheel.

leading shoe

A brake shoe whose operating mechanism is adjacent to its leading edge. Twin leading shoe brakes have two operating cams.

leaf spring

A spring that comprises strips of spring steel clamped together One end is fixed and the other is attached to the spring part.

loop frame

Early frame design in which the down tube curves underneath the engine cased to become the seat-post



Swiss engine makers, the initials stand for Motosacoche Accacias Geneva. The company also produced complete Motosacoche bikes.

mag alloy

Magnesium alloy, a strong lightweight metal used for many components, particularly wheels.


A high-tension spark generator for the ignition system that does not need an external power source.


A branched collection of pipes for inlet or exhaust gases.


Make or brand of motorcycle.


An outwardly tapered high-performance exhaust.


Pure methyl alcohol produced by synthesis for use in Alcohol Dragsters and Alcohol Funny Cars.


Frame where the structure is made as one unit from a sheet material (ie alloy or steel). It may also include bodywork or fuel container in the structure. The Vespa scooter and the F750 Norton of c.1973 are among the rare examples in motorcycles.


A pedal assisted motorcycle of less than 50cc.


Off-road racing over a rough circuit. Formerly known as scrambling.

multi-plate clutch

A clutch with several friction and drive plates. Its compact size makes it ideal for motorcycles.


See motocross.



A housing, usually for mounting instruments


Frame and fork design by Neander using pressed steel or Duralumin


Werner frame Frame design first used on the 1901 Werner, the engine was fitted between the front downtube and the bottom bracket.

nitro methane (drag racing)

Produced specifically as a fuel for drag racing. It is the result of a chemical reaction between nitric acid and propane.



Overhead camshaft


Overhead valve. Valves in the cylinder head above the combustion chamber. They are operated by pushrods controlled by a camshaft situated below the cylinder head

open frame

See step-thru

open cradle frame

Frame without tubes running under the engine. The engine unit bolts into place between the front down tube and the swing arm pivot area as a semi-stressed or stressed member.

over square

An engine with a greater cylinder bore than piston stroke.

parallel twin

A two-cylinder engine layout in which both cylinders are side by side and mounted across the frame



A small gear

planetary gear

A gear driven by a central sun gear or crownwheel.

plunger suspension

A suspension system in which the vertical movement of the axle is controlled by springs mounted above and below the axle.


Opening into a cylinder.

port timing

In two-stroke engines the critical moment when ports are covered or uncovered by the piston.

pressed steel

Sheet steel made into curved sections by press forming. It is often used in the construction of the frame and forks


An engine and gearbox that are not built together in the same casing Ore-unit construction was common on earlier motorcycles.

pre-staged (drag racing)

When a driver is about seven inches behind the starting line and the small yellow light atop his of her side of the Christmas Tree is glowing.

pro tree (drag racing)

Used in Top Fuel, Funny Car, Pro Stock, Pro Stock Bike, Alcohol Dragster, Alcohol Funny Car, Super Comp, Super Gas, and Super Street, which feature heads-up competition. All three large amber lights on the Christmas Tree flash simultaneously, followed four-tenths of a second later by the green starting light. A perfect reaction time on a Pro Tree is .400.

primary drive

The system of transferring power from the engine to the gearbox. Usually achieved by chain or gear.


Racers who do not have the backing of a manufacturer.


Metal rod used to transmit linear motion, most often from camshaft to rocker arm on o.h.v. engines.


reaction time (drag racing)

The time it takes a driver to react to the green starting light on the Christmas Tree, measured in thousandths of a second. The reaction-time counter begins when the last amber light flashes on the Tree and stops when the vehicle clears the stage beam.

rear sets

Racing-style footrests fitted towards the rear of the machine to allow the rider to adopt a racing crouch.


A part that converts alternating current into direct current.

rising rate

A suspension system that becomes harder to compress the further it is compressed. This is usually achieved by a mechanical lineage with variable leverage ratio.


Revolutions per minute, or rpm, is a measure of engine speed as determined by crankshaft spin.

rocker arm

Centrally pivoted arm acting as a lever to open valves.

rolling chassis

The assembled frame, wheels and suspension of the bike.


saddle tank

A fuel tank that fits over the top tube of the frame.


Small-wheeled utilitarian machine with a step-thru frame.


A sensor unit conveying information about an engine to a gauge warning light, or other component.


Any system used to help a mechanism to work with greater force than that initially applied to it.

side valve

Valves positioned at the side of a cylinder.


Single, on of. Usually applied to frames with a single downtube.

sixty-foot time (drag racing)

The time it takes a vehicle to cover the first 60 feet of the racetrack. It is the most exact measure of the launch from the starting line, which usually determines how quick the rest of the run will be.


The part of a part that hangs down, particularly the area of a piston.

slider clutch

A multi-disc clutch designed to slip until a predetermined rpm is reached. Decreases shock load to the drive wheels.

snail-cam chain adjuster

Axle-mounted helical cams that allow accurate chain adjustment.


Single overhead camshaft


Specialised sport run on short, oval dirt tracks with four riders from two teams in each race.

speed trap (drag racing)

The last 66 feet to the finish line, known as the speed trap, where speed is recorded.

spine frame

A frame with a single main structural member from which the engine is suspended.

split single

A two-stroke engine with two pistons that share a single combustion chamber.

staged (drag racing)

A driver is staged when the front wheels of the car are right on the starting line and the small yellow light below the pre-staged light on his or her side of the Christmas Tree is glowing. Once a driver is staged, the calibrated countdown (see Christmas Tree) may begin at any time.


A frame layout with a low structure between the seat and the steering head. On early machines it was commonly called an open frame

stressed member

A component that is an integral part of the whole structure.


Measurement of length of piston travel in the bore, usually expressed in millimetres (see also bore).


Any auxiliary framework attached to the main body of a vehicle. On a motorcycle this often refers to the rear section of the frame, which supports the seat etc.


Mechanically powered device that compresses the combustible charge into the cylinder, artificially increasing the compression ratio. When the “blower” is driven by exhaust gas, the device is called a turbocharger.


See side valve

swept volume

The volume displaced by a piston’s travel.


Suspension member pivoted at one end and supporting the wheel at the other. In most cases the swingarm is more accurately described as a pivoted fork, as the wheel is supported at both sides.



Top dead centre. The point at which the crankshaft and piston are in their highest possible position.

telescopic forks

Front suspension system with two fork legs, each with sliding and fixed tubular members that telescope together to allow suspension movement.


Measurement of the moment at which valves open or close, or when the spark occurs. It is normally expressed in degrees or millimetres before t.d.c. (see ignition timing and port timing).

total loss

Ignition or lubrication system in which electricity or oil is used without being generated or recirculated. The ignition system uses power from a battery, eventually running it flat. The lubrication system uses oil without returning it to a tank. Both systems were common on early motorcycles. Two-stroke engines use a total-loss lubrication system.

trail bike

Dual purpose machine for use on or off-road.

trailing link

Front suspension design similar to leading link except the layout is reversed: the links pivot forward of the axle.


Off-road competition in which the rider has to surmount obstacles. Points are deducted if the rider puts his feet on the ground, goes outside the marked course, or fails to clear an obstacle.

twin-spar frame

Frame in which the steering head and the swingarm pivot are connected by two metal, usually alloy members that wrap around the engine.


A cylinder head design in which there are two exhaust ports.


unit construction

Integrated unit consisting of the engine and gearbox within the same casings.

unsprung weight

The weight of that part of the machine that is not sprung, ie wheels, brakes, tyres, and half of the suspension.

upside-down forks

Telescopic forks in which the lower section, on which the steel is mounted, telescopes into the fixed upper tube. They are sometimes called inverted telescopic forks on earlier bikes.



Two-cylinder engine layout in which the cylinders form a “V”



An engine with a wedge combustion chamber, a combustion chamber resembling a wedge in shape. Need not have parallel intake and exhaust valve stems.

weight transfer

Weight transfer is critical to traction. Vehicles are set up to provide a desired weight transfer to rear wheels. When the vehicle accelerates, the front wheels lift and the weight shifts to the rear wheels, which makes them less likely to spin.

wheelie bars (drag racing)

Used to prevent excessive front-wheel lift

works racers

Racing machines built and operated by the factory

worm and pinion gear

System for turning rotational movement through 90°, in which a pinion is turned by a spirally cut gear.