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

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

autocycle  A 98cc moped

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

bathtub  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 carburettors.

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

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

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

big end  The larger (crankshaft) end of the connecting rod.

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

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c
cam  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.

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

carburettor   Device that mixes fuel and air into a combustible vapour.

chair  Sidecar.

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

clip-on engine   An engine that attaches to a conventional bicycle frame.

clip-ons  Low racing handlebars that clamp directly onto the fork legs.

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 brakers 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).

crankshaft   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.

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

desmodromic  Valve gear operation in which the camshaft actively closes as well as opens the valves.

d.o.h.c.  Double overhead camshaft. Two camshafts fitted into the cylinder head.

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

dope  Highly combustible alcohol/methanol-based fuel mixture.

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

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

dynamo  Electric generator that produces alternating current.

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e
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.

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

epicyclic gear  A gear that operates around the circumference of another.

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

fairing  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.

FIM  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.

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

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

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.

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

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.

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

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

i.o.e.  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.

ISDT  International Six Day Trial.

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

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j
JAP  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.

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

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l
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 component.

loop frame  Early frame design in which the downtube curves underneath the engine cased to become the seat-post

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m
MAG  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.

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

manifold  A branched collection of pipes for inlet or exhaust gases.

marque  Make or brand of motorcycle.

megaphone  An outwardly tapered high-performance exhaust.

monocoque  Fame in which 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.

moped  A pedal assisted motorcycle of less than 50cc.

motorcross   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.

MX  See motocross.

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n
nacelle  A housing, usually for mounting instruments

neander-pattern   Frame and fork design by Neander using pressed steel or Duralumin

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

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o
o.h.c.  Overhead camshaft

o.h.v.  Overhead valve. Valves situated 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 downtube and the swingarm 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

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p
pinion  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.

port  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

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

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

pivateers  Racers who do not have the backing of a manufacturer.

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

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

rectifier  A component 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.

rocker arm   Centrally pivoted arm acting as a lever to open valves.

rolling chassis  The assembled frame, wheels and suspension of the bike.

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

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

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

servo  Any system used to assist a mechanism to operate with greater force than that initially applied to it.

side valve   Valves positioned at the side of a cylinder.

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

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

snail-cam chain adjuster  Axle-mounted helical cams that allow accurate chain adjustment.

s.o.h.c. Single overhead camshaft

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

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

step-thru   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.

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

subframe  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.

supercharger  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.

s.v.  See side valve

swept volume  The volume displaced by a piston's travel.

swingarm   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.

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t
t.d.c.  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.

timing  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.

trials  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.

TT  Tourist Trophy races held on the Isle of Man.

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.

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

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u
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.

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v
V-twin  Two-cylinder engine layout in which the cylinders form a "V"

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w
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.

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