Mechanical Engineering Dictionary-Two Wheelers

Important Terms and their Meaning of Mechanical Engineering-Two Wheelers:



ANTI DIVE SYSTEM – A system installed with some brakes that uses front fork damping and the front brake assembly to prevent excessive fork compression and to improve handling when brakes are applied.

AUTOMATIC TRANSMISSION – A transmission not using a manually operated clutch.

AXLE – A shaft used to support a part or parts across the frame or forks. e.g., front and rear axles.

BACK BONE FRAME – Frame which uses the engine as a structural member for load carrying.

BEAD – The portion of the tyre which holds it onto the rim.

BEZEL – Piece of metal surrounding head lights, gauges or similar components, sometimes used to hold the glass face of a gauge in the dashboard.

BOOSTER PORT – In a two stroke engine, the port that allows an extra amount of air fuel mixture from the intake port into the combustion chamber.

BRAKE ACTUATOR CAM – Small cam that pivots in brake backing plate and forces brake shoes into brake drum.

BRAKE CALIPER – Part of a disc brake which holds friction pads and encloses disc. As the brake is applied, hydraulic fluid forces a piston in caliper towards disc, causing disc to be pinched between brake pads.

BRAKE DISC – A round, flat disc made of steel or cast iron. It is mounted on outside of wheel hub.

ANTI DIVE SYSTEM – A system installed with some brakes that uses front fork damping and the front brake assembly to prevent excessive fork compression and to improve handling when brakes are applied.

AUTOMATIC TRANSMISSION – A transmission not using a manually operated clutch.

AXLE – A shaft used to support a part or parts across the frame or forks. e.g., front and rear axles.

BACK BONE FRAME – Frame which uses the engine as a structural member for load carrying.

BEAD – The portion of the tyre which holds it onto the rim.

BEZEL – Piece of metal surrounding head lights, gauges or similar components, sometimes used to hold the glass face of a gauge in the dashboard.

BOOSTER PORT – In a two stroke engine, the port that allows an extra amount of air fuel mixture from the intake port into the combustion chamber.

BRAKE ACTUATOR CAM – Small cam that pivots in brake backing plate and forces brake shoes into brake drum.

BRAKE CALIPER – Part of a disc brake which holds friction pads and encloses disc. As the brake is applied, hydraulic fluid forces a piston in caliper towards disc, causing disc to be pinched between brake pads.

BRAKE DISC – A round, flat disc made of steel or cast iron. It is mounted on outside of wheel hub.

BRAKE DRUM – A circular ring of cast iron that is part of wheel hub. It provides a place for brake lining to be applied.

BRAKE FADE – Loss of braking power, usually caused by excessive heat after repeated brake applications.

BRAKE LINE – Special hydraulic tubing made of steel, plastic or reinforced rubber. Hydraulic lines must be capable of withstanding extreme pressure without deforming.

BRAKE LINING – A special high friction material made of asbestos and other materials bonded to brake shoes and brake pad plates. Brake lining produces friction and heat when it is forced against brake drum.

BRAKE PAD – The friction pad on a disc brake system.

BRAKE SHOE – The friction lining on a drum brake system.

BRAKE WEAR INDICATOR – Index grooves, tabs, or reference lines to indicate amount of brake lining or pad wear.

BRIDGED PORTS – A vertical port division in a two stroke cycle engine cylinder which allows use of a large port without the danger of ring or piston catching.

CAST ALLOY WHEEL – A one piece wheel made of cast aluminium or magnesium alloy. This design is more rigid than the wire spoked wheel.

CENTRIFUGAL CLUTCH – Clutch engaged by centrifugal force as engine speeds up.

CHAIN DRIVE – Use of a chain and sprockets to connect gear box output shaft to rear wheel.

CHAIN STRETCH – Wear of pins and bushings of a roller or hypo chain, causing the chain to lengthen.

CHAMFER – To bevel an edge of an object or to chamfer edges of port openings in a two stroke cycle cylinder to prevent piston ring breakage.

CLOSE RATIO GEAR BOX – A gearbox with gear ratios spaced close together.

CLUTCH – Device used to connect and disconnect engine power to gearbox input shaft.

CLUTCH BASKET – Part of clutch assembly containing drive plates. Primary drive gear engages teeth on the outside of the clutch basket.

CLUTCH HUB – Part of the clutch that engages with plain driven clutch plates. Clutch hub is mounted on the gearbox input shaft.

CLUTCH PRESSURE PLATE – Part of a clutch assembly providing pressure against the clutch disc or clutch plates.

CLUTCH RELEASE MECHANISM – Mechanism that moves the clutch pressure plate away from the clutch pack allowing the clutch to slip.

CONICAL HUB – A wheel hub (wire wheel) with spoke holes on the brake side of a wheel at a greater distance from the center of the hub than spoke holes on the opposite side of the hub.

CONVENTIONAL REAR SUSPENSION – Suspension used on dual purpose and road bikes which provide less than 152 mm of suspension travel.

COUNTERSHAFT SPROCKET – Output sprocket from gearbox. Mounted on output shaft in indirect drive gearbox and on high gear pinion in direct drive gear box.

CRADLE FRAME – Frame built of tubing which supports and surrounds the engine.

CRANKSHAFT AXLES – Extension at each end of the crankshaft to provide a mounting place for the main bearings, primary drive gear or sprocket, and alternator rotor or magneto flywheel.

CURB WEIGHT – The weight of a vehicle without passengers or payload, but including all fluids (oil, gas, coolant etc.) and other equipment specified as standard.

DAMPER – Device which uses oil metered through orifices to control abrupt suspension movement during expansion and compression.

DAMPER ROD – Tube secured to the bottom of each fork slider to hold the slider onto the fork leg. Damper rod controls movement of front suspension by metering hydraulic fluid through orifices in the rod.

DIAPHRAGM SPRING – A slightly cone shaped metal disc which acts as a clutch pressure plate spring when flattened.

DIRECT DRIVE GEARBOX – Power is transmitted from clutch to input shaft (main shaft), to layshaft, to high gear pinion, which has output sprocket mounted on it.

DISC BRAKE – A brake consisting of a flat circular disc attached to the wheel. A hydraulic or mechanical caliper applies pressure to two brake pads and press them on the disc to slow or stop disc rotation.

DOUBLE PISTON CALIPER – A hydraulic brake caliper with two pistons and provisions for applying hydraulic pressure equally to both pistons. The caliper body is fixed solidly.

DOUBLE ROW CHAIN – A chain having two rows of rollers. Duplex (double row) chains are used for primary drives.

DRIVE PLATE – A clutch plate which is indexed into the clutch basket (outer hub) by tabs. Drive plate has friction material bonded to its surface. When clutch is engaged, drive plate transfers power to driven plate.

DRIVEN PLATE – A clutch plate which is indexed onto the clutch inner hub by tabs or splines around its inside diameter. Driven plate is usually a plain plate (no friction material) and drives gearbox input shaft through clutch inner hub.

DRIVE TRAIN – The drive system that transfers the power produced by the engine to the rear wheel, includes primary drive, clutch, transmission and fluid drive systems.

DRUM BRAKE – A brake consisting of two brake shoes mounted on a backing plate. One or two cams cause the shoes to expand against inside of the brake drum. Brake drum is part of hub or is bolted to hub.

DRY CLUTCH – A clutch assembly that does not run in an oil bath.

ELLIPTICAL PORT SHAPE – Rounded port shape designed to prevent ring catching in large ports of two stroke cycle engines.

ENDLESS CHAIN – A roller chain without a master link for connection of ends. All pin links are permanently riveted.

EXHAUST PORT – In a two stroke engine, the passage way located in the upper portion of the cylinder opposite from the intake port, that channels spent combustion gases to the exhaust system.

EXHAUST PORT TIMING – Amount of time, two stroke cycle exhaust port is open, expressed in crankshaft degrees or piston travel.

EXTENSION – The return or stretching outward of suspension components (after compression) caused by spring pressure.

FINAL DRIVE RATIO – The number of times the transmission output shaft turns to produce one revolution of the rear wheel.

FINAL DRIVE SYSTEM – The part of the drive train that uses a chain, belt or shaft to direct the power flow from the transmission to the rear wheel.

FORK SLIDERS – Lower portion of fork which slides over the fork leg.

FORK TUBES – Long sturdy tubes attached to triple clamps and fitted inside fork sliders.

FRAME – The skeleton of the two wheeler made of tubes, steel plates or pressed steel that supports the rider and engine and provides attachment points for the frame components.

FRONT FORK – The spring and damping device that holds the front wheel in place.

FUEL PET COCK – An on-off valve located at the bottom of fuel tank. It may provide for reserve fuel supply and may have a filter screen and sediment bowl. Fuel line to carburettor is attached to fuel petcock.

GAS CHARGED SHOCK ABSORBER – A shock absorber using a pressurized gas such as nitrogen to help prevent changes in damping as shock absorber heats up.

GUSSET – A reinforcing plate or boxed section used to prevent flexing of frame or swing arm.

HUB – The center part of a wheel.

HYDRAULIC DAMPING – A conversion of oil resistance to heat energy to create a force that opposes input motion.

HYDRAULIC DISC BRAKE – A brake system that exerts pressure through brake fluid to press brake pads against a disc to slow or stop the vehicle.

HYDRO-PLANING – A phenomenon of driving when water builds up under the tyre tread, causing it to lose contact with the road. Slowing down will usually restore normal tyre contact with the road.

HY-VO-CHAIN – A very strong chain made of toothed plates positioned side by side and held together by pins. Advantage of this chain is great strength and quiet operation.

INDIRECT DRIVE GEARBOX – A gearbox where power is transferred from clutch to input shaft and from that to output shaft. Output shaft has a sprocket mounted on one end.

INPUT SHAFT – Shaft of a gearbox which carries power into gearbox.
Clutch assembly is mounted on gearbox input shaft.

INTAKE PORT – In a two stroke engine, the passage way that allows the air fuel oil mixture to travel from the carburettor into the crankcase, located on the lower half of the cylinder opposite from the exhaust port on all but rotary valve engine.

INTAKE TIMING – Amount of time, two stroke cycle engine intake port is open, expressed in crankshaft degrees or piston position.

INTERNAL COOLING – Engine cooling provided by oil, fuel mixtures, and valve overlap.

LACING – The positioning of spokes in the hub and rim of a wheel.

LATERAL RUNOUT – Side to side movement (wobble) of a wheel rim.

LAY SHAFT – Second shaft in a direct drive gearbox. It transmits power from input shaft to high gear pinion.

LOAD RANGE – Indicates the number of plies at which a tyre is rated, load range B equals 4 ply rating, C equals 6 ply rating, and D equals 8 ply rating.

LONG TRAVEL SUSPENSION – Suspension used on current competition dirt bikes to provide more than 150 mm of travel.

MASTER LINK – A pin link which has one removable side plate located by a clip. This allows a convenient way of separating the chain.

MECHANICAL BRAKE – A brake system which uses a mechanical advantage by way of levers and cables or rods to apply brakes. A brake system not using hydraulic fluids or hydraulics.

MILD PORT TIMING – Two stroke cycle engine ports open for a relatively short time, providing for a broad power band.

MOTOR CYCLE – A two wheeled vehicle powered by an internal combustion engine.

MULTI PLATE CLUTCH – A clutch assembly using more than one driving plate and more than one driven plate.

MULTIPLE PORTS – Use of many small transfer ports rather than two large ports in the two stroke cycle engine cylinder.

NON-PRIMARY KICKSTART – A kick-start system which uses gearbox input shaft and clutch hub to connect kick-start lever to crank shaft. For starting, gearbox must be in neutral and clutch engaged.

NON-UNIT CONSTRUCTION – Engine design with separate engine crankcase and gearbox case.

O-RING CHAIN – Roller chain which uses ‘O’ rings to permanently seal lubricant into area between pins and bushings.

OUTPUT SHAFT – Gearbox shaft that transmits power to final drive at a selected ratio.

OVERALL GEAR RATIO – Ratio of crankshaft revolutions to rear wheel revolutions.

PISTON (brake system) – A movable part of a master cylinder or wheel cylinder. Master cylinder piston produces pressure and caliper (wheel cylinder) piston applies that pressure to brake disc, slowing or stopping the wheel rotation.

PISTON PORT SYSTEM – The two stroke intake system that uses the piston to open and close the intake and exhaust ports.

PLY RATING – A rating given to a tyre which indicates strength (but not necessarily actual number of plies).

PORTS – The passage ways cut into a two stroke engine cylinder, that channel gases into and out of the engine. Also called PORT WINDOWS.

POWER TO WEIGHT RATIO – Ratio of horse power to weight of a vehicle.

POWER TRANSMISSION – A system of gears, chains, sprockets and shafts that transfer power from the crankshaft to rear wheel in varying ratios.

PREMIX – Two stroke cycle engine lubrication provided by oil mixed with fuel in fuel tank.

PRIMARY CHAIN DRIVE – A primary drive design that uses a chain and sprockets to transmit engine power from crankshaft to clutch.

PRIMARY DRIVE SYSTEM – A system of gears, chains or belts that transmits power from the crankshaft to the transmission, includes the clutch.

PRIMARY KICKSTART – A kick-start system which connects kick-start lever to crankshaft through clutch basket. Gearbox can be in gear for starting if clutch is disengaged.

PRIMARY REDUCTION – A gear reduction usually about three to one, accomplished by primary drive.

RADICAL PORT TIMING – Ports open for a relatively long period of time.

REED CAGE – An aluminium frame providing a mounting place for reed petals in a two stroke cycle engine.

REED PETALS – Movable part of a reed valve assembly.

REED VALVE SYSTEM – A two stroke intake system that uses thin flapper valves that are opened by vacuum and closed by pressure.

REAR SUSPENSION – The suspension system that consists of the swing arm, rear shock absorbers, springs and linkage.

RIM BAND – A strip of soft rubber that protects the inner tube from the spoke nipples.

RIM OFFSET – The deliberate lacing of spokes off center to the hub to achieve rear chain or front brake clearance.

ROLLER CHAIN – A chain made up of pins, side plates, bushings and rollers. Roller links are connected by pin links to achieve desired length. The ends are usually connected by a pin link which has a removable side plate, called master link.

ROTARY VALVE – A two stroke engine intake system that uses a partially cut away rotating disc to open and close the intake passage into the crankcase.

SCAVENGING – Inertia caused movement of gases through cylinder and exhaust system of a two stroke cycle or four stroke cycle engine by the incoming fresh charge.

SEALED BEAM – A modern automotive headlight. The lens, reflector and filament form a single unit.

SEDIMENT BOWL – A cup located at the bottom of the fuel petcock, designed to prevent flow of dirt and water into the fuel line.

SHAFT DRIVE – Use of a drive shaft, universal joints, and gears to connect gearbox output shaft to rear wheel.

SHOCK ABSORBER – Rear suspension unit made up of a coil spring and a hydraulic damper. It prevents spring oscillations.

SINGLE ROW CHAIN – A conventional roller chain with one row of rollers.

SOLID ROLLER CHAIN – Roller chain using a one piece roller and bushing assembly.

SPINDLE (steering fork) – A shaft which is secured to upper and lower triple clamps. It pivots on bearings mounted in the steering head.

SPOKE – A metal part, often a wire, that runs between the rim and hub of a spoked wheel to transfer force to the hub.

SPOKE CROSSING PATTERN – Number of spokes crossed on the same side of the hub by anyone spoke.

SPOKED WHEEL – A wheel consisting of a rim, spokes, nipples and hub. Spokes are laced between hub and rim and are attached to the rim by nipples.

SPOKE TORQUE WRENCH – A small torque wrench used to adjust accurately the spoke tension.

SPONGY LEVER or PEDAL – A soft or spongy feeling when the brake lever or pedal is depressed. It is usually due to air in the brake lines.

SPROCKET – A circular plate with teeth machined around outside to engage links of a chain.

SPRUNG WEIGHT – The weight of the two wheeler components that are supported on springs, includes the fork tube, engine, triple clamp and frame.

STAMPED FRAME – A frame stamped from pieces of sheet metal which are welded together to provide support for engine and suspension.

STAMPED WHEEL – A wheel assembly using stamped sheet metal spokes in place of small wire type spokes. A stamped wheel resembles a cast alloy wheel in appearance.

STEERING DAMPER – A device which uses friction or a hydraulic damper to reduce steering oscillation.

STEERING HEAD – The tube at the top and front of the frame that supports the steering stem and is welded at a specific angle to provide the proper fork angle.

STEERING OFFSET – Distance between steering axis and axis on which the axle pivots. Steering offset is accomplished by offsetting axle or triple clamps.

STEERING RAKE – Angle of steering axis from vertical, given in degrees.

STEERING STEM – A shaft positioned through the steering head that enables the front end to turn.

SUSPENSION SYSTEM – A system that consists of front forks, shock absorbers, springs and the swing arm, used to support the two wheeler on its axles and wheels.

SWING ARM – The assembly that links the rear wheel to the frame and allows the rear wheel to move up and down, and prevents the wheel from moving laterally and flexing at the pivot point.

SWING ARM PIVOT – Forward part of swing arm, where it is attached to the frame and pivots on bushings or bearings.

SYMMETRICAL HUB – A wheel hub (wire wheel) which has spoke holes on each side of the hub at the same distance from hub center.

TELESCOPIC FORK – Front suspension unit made up of two fork tubes and two sliders that telescope up and down the tubes against spring pressure.

THROTTLE CABLE – A cable consisting of an outer housing and an inner cable which connects carburettor to throttle twist grip.

THROTTLE STOP SCREW – An idle speed adjustment screw used in slide type carburettor. This screw contacts base of throttle slide. As the screw is turned in, throttle slide is raised, increasing idle speed.

THROTTLE TWIST GRIP – A device mounted on the end of handle bar which locates one end of outer throttle cable and pulls inner cable as the twist grip is rotated.

TRAIL – The distance between the center of the tyre contact patch and the imaginary point where the steering head axis strikes the ground.

TRANSFER PORT – Opening in the cylinder wall of a two stroke engine which connects cylinder to crankcase.

TRANSFER PORT TIMING – Amount of time transfer port is open, expressed in crankshaft degrees or piston position.

TRANSMISSION – The part of the drive train that uses a series of gears and shafts to increase the torque and provides gear shifting to allow increases in speed without overworking the engine.

TREAD WEAR PATTERN – The pattern of wear on the tyre which can be read to diagnose problems in the front suspension.

TRIPPLE CLAMPS – An assembly that consists of the steering stem and two brackets that position the fork tubes.

TRIPPLE ROW CHAIN – A chain having three rows of rollers. Triple row chains are used for primary drives.

TWO STROKE CYCLE – A cycle of overlapping events (intake, transfer, compression, power and, exhaust) which all occur during one revolution of crankshaft.

TYRE PRESSURE GAUGE – A pressure gauge used to check tyre air pressure.

UNIFIED BRAKE SYSTEM – A system that automatically coordinates the front and rear brake systems when the rider applies pressure to the brake pedal.

UNIT CONSTRUCTION – Engine design which has one crankcase to house gearbox, clutch, primary drive and engine.

UNIVERSAL JOINT – A flexible joint which allows changes in angle of drive shaft.

UNSPRUNG WEIGHT – The weight of the two wheeler components that are not supported on springs, includes the wheel assembly, fork sliders, rear shocks, a portion of the swing arm, the chain or shaft drive, one half of the spring and on some models, the front fender.

WET CLUTCH – A multiple plate clutch which runs in an oil bath (primary drive).

WHEEL BALANCING – An equal distribution of weight achieved by determining where the wheel is heavier and then placing weights opposite the heavy portion.

WHEEL BASE – Distance between the center of front wheel and the center of rear wheel.

WHEEL WEIGHT – Small weights attached to the wheel to balance the wheel and the tyre assembly.

WIDE RATIO GEARBOX – A gearbox having wide ratio spacing between gears. A wide ratio gear box is used in off road and trials motor cycles.



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