circumference of the tyre. Less resistant to tread
distortion than other tyre types.
RadiatorA liquid-to-air heat transfer device
designed to reduce the temperature of the
coolant In a liquid cooled engine.
RakeA feature of steering geometry - the angle
of the steering head in relation to the vertical (see
Steering geometry
Illustration).
ref.52 Technical Terms Explained
Rebore Providing a new working surface to the
cylinder bore by boring out the old surface.
Necessitates the use of oversize piston and
rings.
Rebound damping A means of controlling the
oscillation of a suspension unit spring after it has
been compressed. Resists the spring's natural
tendency to bounce back after being compressed.
Rectifier Device for converting the ac output of
an alternator into dc for battery charging.
Reed valve An induction system commonly
used on two-stroke engines.
Regulator Device for maintaining the charging
voltage from the generator or alternator within a
specified range.
Relay A electrical device used to switch heavy
current on and off by using a low current auxiliary
circuit.
Resistance Measured in ohms. An electrical
component's ability to pass electrical current.
RON (Research Octane Number) A measure of
a fuel's resistance to knock.
rpm revolutions per minute.
Runout The amount of wobble (in-and-out
movement) of a wheel or shaft as it's rotated.
The amount a shaft rotates out-of-true'. The out-
of-round condition of a rotating part.
s
SAE (Society of Automotive Engineers) A
standard for the viscosity of a fluid.
Sealant A liquid or paste used to prevent
leakage at a joint. Sometimes used in
conjunction with a gasket.
Service limit Term for the point where a
component is no longer useable and must be
renewed.
Shaft drive A method of transmitting drive from
the transmission to the rear wheel.
Shell bearings Plain bearings consisting of two
shell halves. Most often used as big-end and
main bearings in a four-stroke engine. Often
called bearing inserts.
Shim Thin spacer, commonly used to adjust the
clearance or relative positions between two
parts. For example, shims inserted into or under
tappets or followers to control valve clearances.
Clearance is adjusted by changing the thickness
of the shim.
Short-circuit An electrical condition where
current shorts to earth (ground) bypassing the
circuit components.
Skimming Process to correct warpage or repair
a damaged surface, eg on brake discs or drums.
Slide-hammer A special puller that screws into
or hooks onto a component such as a shaft or
bearing; a heavy sliding handle on the shaft
bottoms against the end of the shaft to knock the
component free.
Small-end bearing The bearing in the upper
end of the connecting rod at its joint with the
gudgeon pin.
Spalling Damage to camshaft lobes or bearing
journals shown as pitting of the working surface.
Specific gravity (SG) The state of charge of the
electrolyte in a lead-acid battery. A measure of
the electrolyte's density compared with water.
Straight-cut gears Common type gear used on
gearbox shafts and for oil pump and water pump
drives.
Stanchion The inner sliding part of the front
forks, held by the yokes. Often called a fork tube.
Stoichiometric ratio The optimum chemical
air/fuel ratio for a petrol engine, said to be 14.7
parts of air to 1 part of fuel.
Sulphuric acid The liquid (electrolyte) used in a
lead-acid battery. Poisonous and extremely
corrosive.
Surface grinding (lapping) Process to correct a
warped gasket face, commonly used on cylinder
heads.
T
Tapered-roller bearing Tapered inner race of caged needle rollers and separate tapered outer race. Examples of taper roller bearings can be found on steering heads.
Tappet A cylindrical component which transmits motion from the cam to the valve stem, either directly or via a pushrod and rocker arm. Also called a cam follower.
TCSTraction Control System. An electronically-controlled system which senses wheel spin and reduces engine speed accordingly. TDCTop Dead Centre denotes that the piston is at its highest point in the cylinder. Thread-locking compound Solution applied to fastener threads to prevent slackening. Select type to suit application.
Thrust washer A washer positioned between two moving components on a shaft. For example, between gear pinions on gearshaft. Timing chainSee Cam Chain. Timing light Stroboscopic lamp for carrying out ignition timing checks with the engine running. Top-end A description of an engine's cylinder block, head and valve gear components. Torque Turning or twisting force about a shaft. Torque setting A prescribed tightness specified by the motorcycle manufacturer to ensure that the bolt or nut is secured correctly. Undertightening can result in the bolt or nut coming loose or a surface not being sealed. Overtightening can result in stripped threads, distortion or damage to the component being retained. Torx key A six-point wrench. Tracer A stripe of a second colour applied to a wire insulator to distinguish that wire from another one with the same colour insulator. For example, BrAty is often used to denote a brown insulator with a white tracer. Trail A feature of steering geometry. Distance from the steering head axis to the tyre's central contact point.
Triple clamps The cast components which extend from the steering head and support the fork stanchions or tubes. Often called fork yokes. Turbocharger A centrifugal device, driven by exhaust gases, that pressurises the intake air. Normally used to increase the power output from a given engine displacement. TWIAbbreviation for Tyre Wear Indicator. Indicates the location of the tread depth indicator bars on tyres.
u
Universal joint or U-joint (UJ) A double-pivoted connection for transmitting power from a driving to a driven shaft through an angle. Typically found in shaft drive assemblies. Unsprung weight Anything not supported by the bike's suspension (ie the wheel, tyres, brakes, final drive and bottom (moving) part of the suspension).
V
Vacuum gauges Clock-type gauges for
measuring intake tract vacuum. Used for
carburettor synchronisation on multi-cylinder
engines.
Valve A device through which the flow of liquid,
gas or vacuum may be stopped, started or
regulated by a moveable part that opens, shuts
or partially obstructs one or more ports or
passageways. The intake and exhaust valves in
the cylinder head are of the poppet type.
Valve clearance The clearance between the
valve tip (the end of the valve stem) and the
rocker arm or tappet/follower. The valve
clearance is measured when the valve is closed.
The correct clearance is important - if too small
the valve won't close fully and will burn out,
whereas if too large noisy operation will result.
Valve lift The amount a valve is lifted off its seat
by the camshaft lobe.
Valve timing The exact setting for the opening
and closing of the valves in relation to piston
position.
Vernier caliper A precision measuring
instrument that measures inside and outside
dimensions. Not quite as accurate as a
micrometer, but more convenient.
VINVehicle Identification Number. Term for the
bike's engine and frame numbers.
Viscosity The thickness of a liquid or its
resistance to flow.
Volt A unit for expressing electrical "pressure" in
a circuit. Volts = current x ohms.
w
Water pump A mechanically-driven device for
moving coolant around the engine.
Watt A unit for expressing electrical power.
Watts = volts x current.
Wear limit see Service limit
Wet liner A liquid-cooled engine design where
the pistons run in liners which are directly
surrounded by coolant (see illustration).
Wet liner arrangement
Wheelbase Distance from the centre of the front
wheel to the centre of the rear wheel.
Wiring harness or loom Describes the electrical
wires running the length of the motorcycle and
enclosed in tape or plastic sheathing. Wiring
coming off the main harness is usually referred to