Post by 90GTVert on Feb 14, 2010 15:30:00 GMT -5
Scooter Glossary
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2T : Acronym for 2 takt. Takt is a German word, meaning cycle. See two-stroke.
4T : Acronym for 4 takt. Takt is a German word, meaning cycle. See four-stroke.
A
ABS : See anti-lock brake system.
aftermarket : Parts or accessories not from the original equipment manufacturer.
air/fuel ratio : A ratio expressing the mass of air versus the mass of fuel present in a mixture. For example, an air/fuel ratio of 12:1 means there would be 12 pounds of air for every 1 pound of fuel.
anti-lock brake system (ABS) : A brake system designed to prevent locking up the brakes and skidding the tires for safety and stability. ABS can be computer controlled, but scooters commonly use a pressure relief valve in the brake line(s) to help prevent lock-up.
ATGATT : An acronym for "All The Gear, All The Time." This refers to wearing the proper safety equipment, such as a helmet, gloves, an armored jacket, riding pants, and boots, on every ride.
auxilliary transfer port : The transfer ports located between the main and secondary transfer ports of some two-stroke cylinders.
B
baffle : The section of a tuned pipe just after the belly and just before the stinger. The baffle is a reverse-megaphone that reflects pressure waves and helps to push any fuel mixture that has made it's way into the exhaust port or header pipe back into the cylinder for combustion. Shorter baffles and steep angles help to produce a more instense but narrow powerband, while longer baffles or lesser angles produce a wider but less intense powerband. May also be part of a muffler/silencer assembly.
BBK : See big bore kit.
BDC : See bottom dead center.
belly : A section of pipe between the diffuser and the baffle in a tuned pipe. Altering the length of the belly can narrow or widen the RPM range in which the pipe makes it's power. Shorter bellies create a more narrow powerband and longer bellies broaden the powerband.
big bore kit : A package that usually contains a cylinder with a larger bore, a larger piston to accomodate the new bore, piston rings, and sometimes other items such as a wrist pin, cylinder head, gaskets, or a variety of other add-ons. Big bore kits create more displacement through the use of the large bore.
blow-by : When hot gases from compression or combustion escape past the piston rings and into the crankcase.
BMEP : See brake mean effective pressure.
boost : Positive manifold pressure. A pressure greater than atmospheric pressure, which is 14.7psi. If an engine makes 15psi of boost, this does not mean it is only 0.3psi more than atmospheric pressure. Boost is expressed only as the amount of pressure above 14.7psi, so 15psi of boost would equal a pressure of 29.7psi.
boost port : A port or ports located opposite of the exhaust port in a two-stroke cylinder. Boost ports aid in cylinder scavenging through the use of steep cylinder entry angles that help to push pockets of stale mixture out of the cylinder. Boost ports can also help deliver additional air/fuel mixture, which is especially beneficial at high engine speeds.
bore : The diameter of the cylinder, usually measured in millimeters(mm) when dealing with scooters.
bottom dead center (BDC) : The lowest point of a piston's travel.
brake mean effective pressure (BMEP) : A theoretical measurement of the average amount of pressure that would be necessary to produce force measured by a dynamometer. BMEP is useful for benchmarking any engine's ability and effectiveness at producing horsepower.
bridged port : A port with a center divider or support. Bridged ports are used in many aftermarket two-stroke cylinders for the exhaust port, because a single large port would not support the piston ring(s) properly and could cause rings to snag and break.
burnout : Spinning the drive tire(s). In racing this cleans and heats the tire compound for better traction. Burnouts are often done for fun, just to make some smoke.
C
cage : Slang used by scooterists and motorcyclists for a vehicle that confines the driver within the frame and/or body panels such as a car, truck, or SUV.
cager : A slang term used by riders of two-wheeled vehicles referring to opearators cars and trucks. Usually used in a derogatory manner.
capacitor discharge igntion (CDI) : CDI ignitions provide powerful, but short, sparks by charging a high voltage capacitor and then disscharging it quickly. There are two common types of CDI systems, AC-CDI and DC-CDI. AC(Alternating Current)-CDIs use power from the stator/alternator to charge the capacitor. DC(Direct Current)-CDIs use battery power to charge the capacitor.
The ignition box is commonly referred to as a CDI.
cam chain : A chain that allows the crankshaft to rotate the camshaft via sprockets or gears attached to each of them.
cam sprocket : A sprocket attached to the camshaft that allows the cam chain, driven by a gear or sprocket on the crankshaft, to turn the camshaft.
camshaft : Sometimes simply referred to as a "cam." A shaft inside the engine with lobes (cams) machined into it to allow it to control the intake and/or exhaust valve(s).
cc : An acronym for "cubic centimeters". The most common unit of measurement for engine displacement in scooters.
CDI : See capacitor discharge ignition.
chamfer : A beveled edge. Exhaust ports in two-stroke engines are often chamferred and smoothed to reduce piston ring wear and promote flow.
chassis : The frame or structure of the scooter.
choke : A device that blocks airflow into an engine, which creates a very rich condition. Primarily used for easier cold starting.
CHT : An acronym for "cylinder head temperature."
circlip : A circular, or semi-circular, clip used to hold something in place. Circlips usually fit into a receiver groove of some sort.
combustion chamber : The area inside a cylinder, between the top of the piston and a cavity in the cylinder head, where combustion occurs. The cavity in the cylinder head is also called a combustion chamber, though the true combustion chamber is the area defined in the previous sentence.
compression ratio : A ratio expressing the difference between the volume of the cylinder and combustion chamber when the piston is at BDC and the volume of the combustion chamber when the piston is at TDC. For example, an engine with a 10:1 compression ratio has a total cylinder volume 10 times the volume of it's combustion chamber.
compression relief : A valve that opens to release cylinder pressure and aid in starting high compression engines.
compression ring : A piston ring used to prevent compression and combustion forces from escaping past the piston and into the crankcase.
compression test : A method of checking cylinder pressure created during compression. Low cylinder pressures can indicate many problems such as piston ring sealing issues, stuck valves, valve seating or sealing issues, and more.
connecting rod : A rod that connects a piston to a crankshaft. The end that the piston attaches to is often called the small end. The end that the crankshaft connects to is often referred to as the big end.
conrod : An abbreviation for connecting rod.
contact patch : The area of a tire that actually touches the surface of the road.
continuously variable transmission (CVT) : A type of automatic transmission that changes steplessly through gear ratios. The minimum and maximum effective gearing are determined by variables such as pulley diameter and belt travel on most scooters. Effective gears within the minimum and maximum limits are infinite.
contra spring : A large spring installed between the rear pulley assembly and the clutch in a CVT transmission. The contra spring puts pressure on the rear pulley to help keep the transmission in a low gear longer and return the transmission to a lower gear ratio when necessary. May also be known as main spring or torque spring.
convergent cone : See baffle.
countersteer : Turning the handlebars in the opposite direction that you wish to travel. For example, pushing the handlebars left to make a right hand turn. By turning the handlebars, and subsequently the wheel/tire, to the left the bike will actually lean right. The easiest way to watch this in action is to sit still and turn the handlebars to the left and right. You should notice that you will lean in the opposite direction. Countersteering should only be used at speeds above 5-10MPH. Steer normally below those speeds.
crankcase : The section of an engine that encases the crankshaft.
crankshaft : Often shortened to "crank." A shaft with one or more offset journals that rotates as the piston(s) travel in the cylinder(s). Journals on greater or lesser offsets create more or less stroke respectively. A crankshaft converts the reciprocating linear motion of the piston(s) to a rotational motion, often used to turn the variator and flywheel.
crown : The top of a piston.
CVT : See continuously variable transmission.
D
deck : The top surface of the cylinder.
deck height : The distance between the top of the cylinder and the top of the piston. If the piston is below the deck it is referred to as "negative deck." If the piston is above the deck it is called "positive deck."
detonation : Detonation is a secondary combustion event, after the spark plug has created normal combustion. Detonation occurs when end gases or pockets of unburned mixture are ignited by intense heat or pressure within the combustion chamber. This causes a sharp spike in cylinder pressure which makes the engine resonate, often creating a knocking or pinging sound.
diffuser : The megaphone-like section of a tuned pipe, just after the header. The diffuser can alter the intensity and duration of the peak powerband. Shorter and steeper angled diffusers will create intense but narrow RPM range power, while long diffusers and lesser angles broaden the RPM range and reduce peak output.
displacement : The volume displaced by the piston(s) traveling from BDC to TDC. Displacement is usually expressed in cc for scooters.
divergent cone : See diffuser.
DOHC : See dual overhead cam.
DOT : An acronym for "Department of Transportation".
drive belt : The belt that connects the front and rear pulleys in a CVT.
dry clutch : A clutch not lubricated or cooled by fluid. Most CVT transmissions in scooters use dry clutches.
dry shot : When referring to nitrous oxide systems, a dry shot injects only nitrous oxide. Dry shot systems require additional fuel to be delivered through a method other than the nitrous kit. The simplicity of dry kits makes installation faster and easier, but tuning can be more difficult.
dual overhead cam (DOHC) : An engine that uses two camshafts (per head) mounted in the head(s) to control the valves. Usually one camshaft will operate the intake valves and the other camshaft will operate the exhaust valves.
duration : The length of time, expressed in degrees of crankshaft rotation, that something happens. Commonly used for port and valve open times.
dwell : See belly.
dyno : Slang for dynamometer. A device that measure horsepower and torque. 2 common types of dynos are engine and chassis dynos. Engine dynos measure power output at the crankshaft or flywheel. Chassis dynos measure power output at the wheels.
E
effective stroke : The distance between TDC and the top of the exhaust port in 2 stroke engines.
EFI : See electronic fuel injection.
EGT : An acronym for "exhaust gas temperature."
electronic fuel injection : A system for delivering fuel that uses a computer, fuel injectors, sensors, and other components rather than a carburetor.
electronic throttle control : A method of controlling throttle opening using voltage or an electronic signal rather than a cable or linkage.
enricher/enrichener : A device that adds additional fuel to the mixture to create a rich condition. Not to be confused with a choke which blocks airflow to create a rich condition. Enricheners are primarily used to make cold starting easier.
ETC : See electronic throttle control.
F
fairing : A body panel, typically plastic, that covers the frame of a scooter. Fairings can be used for decorative and/or aerodynamic functions.
feeler gauge : A thin metal strip of a precise thickness that is used to measure small clearances between two objects.
fixed half : The half of a CVT pulley that does not move other than to rotate along with the belt. The half of the front pulley (variator) that remains in a fixed position relative to the crankshaft. Sometimes called a variator fan, although many racing versions have no cooling fins so they do not act as a fan. The fixed half of the rear pulley is the rear section that the torque driver rides on.
forced induction : A method of engine aspiration that forces air or the air/fuel mixture into the cylinder(s) rather than having it drawn in by engine vacuum. A compressor, such as a turbocharger or supercharger, is used to push a greater amount of air into the cylinder.
four-cycle : See four-stroke.
four-stroke : An internal combustion engine that creates combustion once in every two revolutions of the crankshaft, which is four strokes of the piston(s). Read this for more information.
four valve head : A cylinder head that uses two intake valves and two exhaust valves to increase cylinder filling and evacuation capabilities.
front transfer ports : See main transfer ports.
G
gudgeon pin : See wrist pin.
H
header : The first section of an exhaust system that attaches to an engine's exhaust port or outlet.
HID : See high-intensity discharge.
high-intensity discharge (HID) : A type of headlights that produce a more intense and whiter light than conventional halogen bulbs. HID lights usually draw less power to make the same or more light than a halogen bulb.
hit the pipe : A phrase used by 2 stroke enthusiats when a tuned pipe reaches it's peak powerband. Some tuned pipes reach their powerband suddenly and intensely and "hitting the pipe" is clearly felt, while others come on gradually enough that you may never know they "hit the pipe." May also be referred to as "getting on the pipe". Example : "This high end engine combo doesn't even hit the pipe till 10,000RPM, and then it goes hard."
hybrid : A vehicle that uses both an electric motor and internal combustion engine. Hybrids typically offer very high fuel efficiency by shutting off the gasoline engine and using the electric motor whenever possible.
I
J
jug : Slang for the cylinder of an engine.
K
keeper : A clip or other device used to keep a valve spring in position. Keepers usually fit into a groove on the valve stem in some manner.
L
leakdown test : A method of testing an engine's cylinder sealing capabilities. While a compression test will show the engine's ability to create cylinder pressure, a leakdown test will show an engine's ability to maintain cylinder pressure. A leakdown test involves a special tool, a leakdown tester, which allows air to be injected into a cylinder. The leakdown tester monitors the pressure of the inlet air and the pressure maintained in the cylinder so you know what percentage is leaking out. A leakdown tester allows you to listen for leaks in the intake track, exahust system, or crankcase as well to make diagnosis easier.
lean : Having too little of something. An engine that is running lean does not have enough fuel in the air/fuel mixture. Technically, a lean mixture is any mixture with less fuel than what is considered stoichiometric. When speaking of two stroke engines, lean could refer to a low amount of oil present in the fuel.
M
main spring : See contra spring.
main transfer ports : The transfer ports located nearest the exhaust port in a two-stroke cylinder.
moveable half : The half of the front pulley that moves. Some call the movable half a variator while others call the whole front pulley the variator.
muffler : A device used to quiet an engine's exhaust. Sometimes called a silencer.
N
NASRA : An acronym for "North American Scooter Racing Association." NASRA is an organization formed to help promote scooter racing in North America. Now defunct.
naturally aspirated : An engine that uses only vacuum created by the piston(s) traveling from TDC to BDC to draw the air/fuel mixture into the cylinder.
nitrous oxide : Nitrous oxide is a chemical made up of two nitrogen molecules and one oxygen molecule. Nitrous oxide breaks down at roughly 570˚F, and releases the nitrogen and oxygen molecules. It also cools the intake air temperature which creates a more dense charge. This cool, dense, charge allows even more oxygen to enter the cylinder for combustion. All of the extra oxygen from the nitrous oxide releasing it's oxygen and the cooler intake charge delivering more oxygen allows more fuel to be injected and burned to create more power. When tuned correctly, nitrous oxide systems can provide dramatic increases in an engine's horsepower and torque output.
O
odometer : A gauge that measures the distance a vehicle travels.
OHC : See overhead cam.
OHV : See overhead valve.
oil control ring : A piston ring used to prevent oil from getting above the piston and into the combustion chamber. Oil control rings are usually located on the lower portion of the piston, below compression rings. Most oil control rings are made up of three smaller rings. Two thin outer rail rings and a wavy middle expander ring.
oil injection : A system that injects two-stroke oil into the fuel mixture. Oil injection systems often use a crankshaft driven pump to draw oil from a reservoir and deliver it to the carburetor.
oil pump : In a four-stroke engine an oil pump moves oil from the crankcase, through passages, and up to the cylinder head to lubricate the valvetrain. In a two-stroke oil injected engine an oil pump pushes oil from a reservoir into the carburetor to be mixed with air and fuel.
over range transmission : A pulley set for CVT transmissions with greater belt travel, often achieved with larger diameter pulleys, that will allow more effective gear ratios. Over-range kits are usually used to obtain higher top speeds and better takeoffs, and often keep RPM more consistent.
overhead cam (OHC) : A type of engine using a camshaft or camshafts installed in the head(s). OHC engines operate the valves via rocker arms or often directly by the lifters.
overhead valve (OHV) : A type of engine that uses a camshaft mounted in the engine block, below the head(s). The valves are operated via lifters, pushrods, and rocker arms.
oversquare : An engine with a cylinder bore larger than it's stroke is said to be oversquare. For example, an engine with a bore of 40mm and a stroke of 39mm is considered oversquare.
oversteer : When a vehicle's rear tire(s) slip before it's front tire(s) causing the vehicle to rotate at an angle greater than intended.
P
petcock : A device used like an on/off switch for fuel flow installed inline between the fuel tank and carburetor. Two common types for scooters are vacuum operated and manually operated. Vacuum petcocks allow fuel flow when manifold vacuum is present (engine starting or running). Manually operated units can be switched on or off by the user.
ping : A slang term for detonation. Detonation causes a sharp pressure spike that causes the engine to resonate, which often sounds like a ping or knock. See also detonation.
piston : A device that travels inside a cylinder used to transfer force. In an engine, a piston transfers the energy created from combustion to the crankshaft. The piston is also used to help draw in the air/fuel mixture, compress it, and expell it. Two-stroke engines also use the piston as a valve, opening and closing ports. In a brake caliper, a piston transfers force created from hydraulic pressure to a brake pad in order to press the pad against the brake rotor.
piston ring : An open ended ring used to create a seal between a piston and the cylinder wall. Some applications use a single piston ring, while others require multiple rings. Piston rings also transfer heat from the piston to the cylinder wall.
piston ring end gap : The distance between the two ends of a piston ring when it is installed in the cylinder. Proper end gap is necessary for cylinder sealing and preventing damage or excess wear.
piston to valve clearance : The amount of clearance between the intake and exhaust valves and the top of the piston at top dead center. Insufficient piston to valve clearance can lead to catastrophic failure.
plug chop : A method of checking spark plugs to use their condition as an indicator of engine properties such as fuel mixture ratios, oil supply, and detonation among others. Usually the engine is run at a certain RPM or throttle position for a length of time and then shut off. Ideally the transmission will be disengaged via the clutch or shifting to neutral, although this isn't really possible with most scooters, to allow the engine to shut off instantly and give the most accurate reading. The spark plug(s) would then be removed and inspected. For a true plug chop, part of the spark plug is cut or "chopped" away to allow better viewing of the insulator.
port : A passage in a cylinder or head that allows air, fuel, or a mixture to pass through it.
port map : A diagram showing a two-stroke cylinder's port layout. Port maps are often made by positioning and securing a piece of paper in the cylinder and then scribbling over the ports with a pencil to show their outlines.
port window : In two-stroke engines, the part of a port that opens into the cylinder.
powertrain : The system of devices used to deliver power to a vehicles drive tire(s). The engine, CVT transmission, and final drive transmission would all be parts of a typical scooter's powertrain.
pre-ignition : Igntion of the air/fuel mixture before the spark plug fires. Pre-ignition is often caused by hot spots like the tip of a spark plug or carbon deposits. When the mixture is ignited early it puts a huge load on the engine, becuase the piston that is trying to reach TDC is now resisting the forces of combustion. The heat and extreme forces can cause engine damage very quicky. Unlike detonation, pre-ignition usually creates no noise audible to the operator.
pre-mix : Mixing two-stroke oil into the fuel before it reaches the carburetor. Pre-mixing can be used as the sole method of lubrication or in conjuction with oil injection.
pushrod engine : See OHV.
Q
R
rear transfer ports : See secondary transfer ports.
reed block : The structure that reed valves are installed on. A reed block usually has one or more port that is covered when the reed valve is closed and open when the reed valve is open to control air and fuel flow into the crankcase.
reed valve : A valve in the intake track of a two-stroke engine. Reed valves are opened and closed by pressure changes inside the crankcase to allow air and fuel into the crankcase or to seal off the crankcase. The use of reed valves creates less emissions, better fuel economy, and more power.
restrictor : Any device that limits power output or speed by limiting flow or travel. Common restrictors include exhaust and intake restrictors that limit airflow, variator restrictors that limit belt travel, and throttle restrictors that can prevent the throttle from being opened all the way.
retainer : A device used in conjunction with a keeper or keepers to hold a valve spring in place.
revolutions per minute (RPM) : The number of revolutions the crankshaft will make in one minute at it's current rate of rotation.
rich : Having a large quantity or too much of something. Saying an engine is running rich usually refers to the amount of fuel in the mixture being too great. When dealing with two strokes it may be referring to a greater than ideal amount of oil in the gasoline.
rocker arm : A lever with one end positioned on the tip of a valve stem and the other end on a camshaft or pushrod. When the camshaft or pushrod lifts and applies pressure to one end of the rocker arm, the other end applies pressure to the valve to open it.
roller weight : Often called "rollers." Roller weights are small cylindrical weights inside the variator. Roller weights are forced toward the outer edges of the variator as centrifugal force from the crankshaft rotating the variator increases. As the roller weights move outward, they force the variator to close and move the belt outward to create higher gear ratios. Lighter weights resist centrifugal force more and keep the gear ratios low longer. Heavier weights move outward at lower RPM to achieve higher gear ratios faster.
RPM : See revolutions per minute.
S
secondary transfer ports : The transfer ports located nearest the boost port in a two-stroke cylinder.
single overhead cam (SOHC) : An engine that uses one camshaft (per cylinder head) mounted in the head(s). Each cam controls both the intake and exhaust valves.
sleeper : A performance modified vehicle with no obvious external signs that it has been modified. A sleeper may have a racing oriented engine with a stock or quiet exhaust and no visual modifications.
sliding weight : Usually referred to as "sliders." Sliding weights do the same job as roller weights. As their name implies, they slide in the variator rather than roll because they have one or more flat edges rather than being cylindrical like roller weights.
Snell : A non-profit organization that sets strict safety standards for helmets to conform to.
SOHC : See single overhead cam.
spark knock : A slang term used for detonation. Detonation can cause a knocking or pinging sound. See also detonation.
speedometer : A gauge that measures vehicle speed.
sprung weight : Any weight supported by the suspension. The frame and body are examples of sprung weight.
square : An engine with a bore equal to it's stroke is said to be square. For example, and engine with a cylinder bore of 40mm and a stroke of 40mm is considered square.
squish band : A section of the cylinder head cut an an angle designed to help force the air/fuel mixture into the center of the combustion chamber.
squish clearance : The distance between the top of the piston and the lowest point on the cylinder head in the cylinder.
stinger : The end section of an un-muffled tuned pipe, or the section between the baffle and the muffler on muffled pipes. The stinger allows exhaust gases to escape from the expansion chamber. It's diameter and length can create varying backpressure that affects exhaust gas temperature and subsequently affects power output and engine cooling.
stoichiometric : Sometimes abbreviated "stoich." A chemically balanced mixture considered to be ideal. 14.7:1 is considered the a stoichiometric mixture of air/fuel for gasoline engines. Other fuels have different stoichiometric ratios.
stroke : The distance a piston travels from top dead center (TDC) to bottom dead center (BDC). Usually measured in millimeters(mm) when referring to scooters.
stroker : An engine or crankshaft with a larger stroke than stock.
supercharger : A crankshaft driven compressor designed to increase manifold pressure and force the air/fuel mixture into the cylinder(s) for increased cylinder filling and power output.
swept volume : See displacement.
T
tachometer : A gauge that measures engine RPM.
TDC : See top dead center.
three valve head : A cylinder head that uses two intake valves and one exhaust valve.
throttle : A system for controlling engine fuel and air intake via a butterfly valve or slide valve inside a carburetor or throttle body.
top dead center (TDC) : The highest point of a piston's travel.
torque driver : A part of the rear pulley that controls the movement of the pulley halves when opening and closing.
torque spring : See contra spring.
transfer port : A port that allows air/fuel mixture to transfer into the cylinder from the crankcase of a two-stroke engine.
tuned length : The distance from the exhaust side of the piston to the mid point of the baffle of a tuned pipe for a two-stroke.
tuned pipe : An exhaust system designed to increase power on 2 stroke engines. When properly designed, a tuned pipe will help to evacuate exhaust gases as well as aid in cylinder filling.
turbocharger : An exhaust driven compressor designed to increase manifold pressure and force the air/fuel mixture into the cylinder(s) for increased cylinder filling and power output.
twin cam : See DOHC.
twist 'n' go : A term for scooters with automatic CVT transmissions. Twist the throttle and go.
two-cycle : See two-stroke.
two-stroke : An internal combustion engine that completes combustion in every revolution of the engine, or two strokes of the piston. Read this for more information.
two valve head : A standard cylinder head that uses one exhaust valve and one intake valve.
U
undersquare : An engine with a stroke longer than it's cylinder bore diameter is said to be undersquare. For example, an engine with a 40mm bore and a 45mm stroke is considered undersquare.
understeer : When the front tire(s) slip or skid before the rear tire(s), causing the vehicle to rotate at an angle lesser than it's intended angle.
unsprung weight : Any weight not supported by the suspension, such as wheels and tires.
V
vacuum : Negative manifold pressure. A pressure that is less than atmospheric pressure (14.7psi).
valve cover : A cover that attaches to a cylinder head which protects the valvetrain.
valve lash : The clearance between the rocker arm and the tip of the vavle stem. Valve lash is measured while the valve is fully closed with the other end of the rocker arm on the cam's base circle.
valve spring : A spring used to apply pressure to a valve. The spring returns the valve to a closed position and keeps the valve seated firmly.
valvetrain : The parts used to control the operation of an engine's valves. The camshaft, pushrod(s), rocker arm(s), etc... would all be part of the valvetrain.
variator : The front pulley in a CVT transmission consisting of a moveable half and a fixed half. The moveable half is often referred to as the variator. The variator is able to change it's effective diameter by being pushed closed via centrifugal force.
variator fan : The fixed half of the front pulley in a CVT transmission. The variator fan has fins on the outside to move air and aid in cooling CVT components. The belt travels on the rear half of the fan.
volumetric efficiency : A comparison of the volume of air that is drawn into an engine during induction versus the volume of air that would fill the engine with the piston at BDC at atmospheric pressure. Volumetric efficiency is expressed as a percentage. Typical engines range from 60% to 120% volumetric efficiency. Percentages greater than 100% are usually achieved through the use of forced induction.
W
wet clutch : A clutch that is lubricated or cooled by a fluid. Most scooters or motorcycles that have wet clutches use the engine oil to cool or lubricate them. Those models must use a different type of engine oil, specifically intended for wet clutches, or the clutch slippage may occur.
wet shot : When referring to nitrous oxide systems, a wet shot injects both nitrous oxide and fuel. Ths can make tuning easier and more precise, but wet shot systems do have additional solenoids, lines, and other components that can make installation more difficult.
WFO : An acronym for "wide freaking open" usually referring to the throttle being all the way open (full throttle). See also WOT.
wheelbase : The distance between the center of the front wheel(s) and the center of the rear wheel(s). Bikes with shorter a shorter wheelbase tend to turn more quickly, while bikes with a longer wheelbase are generally more stable.
WOT : An acronym for "wide open throttle."
wrist pin : A hollow metal rod used to attach a piston to a connecting rod.
wrist pin bearing : A needle bearing that fits around the wrist pin, like a sleeve, inside the connecting rod.
X
Y
Z
#
2T : Acronym for 2 takt. Takt is a German word, meaning cycle. See two-stroke.
4T : Acronym for 4 takt. Takt is a German word, meaning cycle. See four-stroke.
A
ABS : See anti-lock brake system.
aftermarket : Parts or accessories not from the original equipment manufacturer.
air/fuel ratio : A ratio expressing the mass of air versus the mass of fuel present in a mixture. For example, an air/fuel ratio of 12:1 means there would be 12 pounds of air for every 1 pound of fuel.
anti-lock brake system (ABS) : A brake system designed to prevent locking up the brakes and skidding the tires for safety and stability. ABS can be computer controlled, but scooters commonly use a pressure relief valve in the brake line(s) to help prevent lock-up.
ATGATT : An acronym for "All The Gear, All The Time." This refers to wearing the proper safety equipment, such as a helmet, gloves, an armored jacket, riding pants, and boots, on every ride.
auxilliary transfer port : The transfer ports located between the main and secondary transfer ports of some two-stroke cylinders.
B
baffle : The section of a tuned pipe just after the belly and just before the stinger. The baffle is a reverse-megaphone that reflects pressure waves and helps to push any fuel mixture that has made it's way into the exhaust port or header pipe back into the cylinder for combustion. Shorter baffles and steep angles help to produce a more instense but narrow powerband, while longer baffles or lesser angles produce a wider but less intense powerband. May also be part of a muffler/silencer assembly.
BBK : See big bore kit.
BDC : See bottom dead center.
belly : A section of pipe between the diffuser and the baffle in a tuned pipe. Altering the length of the belly can narrow or widen the RPM range in which the pipe makes it's power. Shorter bellies create a more narrow powerband and longer bellies broaden the powerband.
big bore kit : A package that usually contains a cylinder with a larger bore, a larger piston to accomodate the new bore, piston rings, and sometimes other items such as a wrist pin, cylinder head, gaskets, or a variety of other add-ons. Big bore kits create more displacement through the use of the large bore.
blow-by : When hot gases from compression or combustion escape past the piston rings and into the crankcase.
BMEP : See brake mean effective pressure.
boost : Positive manifold pressure. A pressure greater than atmospheric pressure, which is 14.7psi. If an engine makes 15psi of boost, this does not mean it is only 0.3psi more than atmospheric pressure. Boost is expressed only as the amount of pressure above 14.7psi, so 15psi of boost would equal a pressure of 29.7psi.
boost port : A port or ports located opposite of the exhaust port in a two-stroke cylinder. Boost ports aid in cylinder scavenging through the use of steep cylinder entry angles that help to push pockets of stale mixture out of the cylinder. Boost ports can also help deliver additional air/fuel mixture, which is especially beneficial at high engine speeds.
bore : The diameter of the cylinder, usually measured in millimeters(mm) when dealing with scooters.
bottom dead center (BDC) : The lowest point of a piston's travel.
brake mean effective pressure (BMEP) : A theoretical measurement of the average amount of pressure that would be necessary to produce force measured by a dynamometer. BMEP is useful for benchmarking any engine's ability and effectiveness at producing horsepower.
bridged port : A port with a center divider or support. Bridged ports are used in many aftermarket two-stroke cylinders for the exhaust port, because a single large port would not support the piston ring(s) properly and could cause rings to snag and break.
burnout : Spinning the drive tire(s). In racing this cleans and heats the tire compound for better traction. Burnouts are often done for fun, just to make some smoke.
C
cage : Slang used by scooterists and motorcyclists for a vehicle that confines the driver within the frame and/or body panels such as a car, truck, or SUV.
cager : A slang term used by riders of two-wheeled vehicles referring to opearators cars and trucks. Usually used in a derogatory manner.
capacitor discharge igntion (CDI) : CDI ignitions provide powerful, but short, sparks by charging a high voltage capacitor and then disscharging it quickly. There are two common types of CDI systems, AC-CDI and DC-CDI. AC(Alternating Current)-CDIs use power from the stator/alternator to charge the capacitor. DC(Direct Current)-CDIs use battery power to charge the capacitor.
The ignition box is commonly referred to as a CDI.
cam chain : A chain that allows the crankshaft to rotate the camshaft via sprockets or gears attached to each of them.
cam sprocket : A sprocket attached to the camshaft that allows the cam chain, driven by a gear or sprocket on the crankshaft, to turn the camshaft.
camshaft : Sometimes simply referred to as a "cam." A shaft inside the engine with lobes (cams) machined into it to allow it to control the intake and/or exhaust valve(s).
cc : An acronym for "cubic centimeters". The most common unit of measurement for engine displacement in scooters.
CDI : See capacitor discharge ignition.
chamfer : A beveled edge. Exhaust ports in two-stroke engines are often chamferred and smoothed to reduce piston ring wear and promote flow.
chassis : The frame or structure of the scooter.
choke : A device that blocks airflow into an engine, which creates a very rich condition. Primarily used for easier cold starting.
CHT : An acronym for "cylinder head temperature."
circlip : A circular, or semi-circular, clip used to hold something in place. Circlips usually fit into a receiver groove of some sort.
combustion chamber : The area inside a cylinder, between the top of the piston and a cavity in the cylinder head, where combustion occurs. The cavity in the cylinder head is also called a combustion chamber, though the true combustion chamber is the area defined in the previous sentence.
compression ratio : A ratio expressing the difference between the volume of the cylinder and combustion chamber when the piston is at BDC and the volume of the combustion chamber when the piston is at TDC. For example, an engine with a 10:1 compression ratio has a total cylinder volume 10 times the volume of it's combustion chamber.
compression relief : A valve that opens to release cylinder pressure and aid in starting high compression engines.
compression ring : A piston ring used to prevent compression and combustion forces from escaping past the piston and into the crankcase.
compression test : A method of checking cylinder pressure created during compression. Low cylinder pressures can indicate many problems such as piston ring sealing issues, stuck valves, valve seating or sealing issues, and more.
connecting rod : A rod that connects a piston to a crankshaft. The end that the piston attaches to is often called the small end. The end that the crankshaft connects to is often referred to as the big end.
conrod : An abbreviation for connecting rod.
contact patch : The area of a tire that actually touches the surface of the road.
continuously variable transmission (CVT) : A type of automatic transmission that changes steplessly through gear ratios. The minimum and maximum effective gearing are determined by variables such as pulley diameter and belt travel on most scooters. Effective gears within the minimum and maximum limits are infinite.
contra spring : A large spring installed between the rear pulley assembly and the clutch in a CVT transmission. The contra spring puts pressure on the rear pulley to help keep the transmission in a low gear longer and return the transmission to a lower gear ratio when necessary. May also be known as main spring or torque spring.
convergent cone : See baffle.
countersteer : Turning the handlebars in the opposite direction that you wish to travel. For example, pushing the handlebars left to make a right hand turn. By turning the handlebars, and subsequently the wheel/tire, to the left the bike will actually lean right. The easiest way to watch this in action is to sit still and turn the handlebars to the left and right. You should notice that you will lean in the opposite direction. Countersteering should only be used at speeds above 5-10MPH. Steer normally below those speeds.
crankcase : The section of an engine that encases the crankshaft.
crankshaft : Often shortened to "crank." A shaft with one or more offset journals that rotates as the piston(s) travel in the cylinder(s). Journals on greater or lesser offsets create more or less stroke respectively. A crankshaft converts the reciprocating linear motion of the piston(s) to a rotational motion, often used to turn the variator and flywheel.
crown : The top of a piston.
CVT : See continuously variable transmission.
D
deck : The top surface of the cylinder.
deck height : The distance between the top of the cylinder and the top of the piston. If the piston is below the deck it is referred to as "negative deck." If the piston is above the deck it is called "positive deck."
detonation : Detonation is a secondary combustion event, after the spark plug has created normal combustion. Detonation occurs when end gases or pockets of unburned mixture are ignited by intense heat or pressure within the combustion chamber. This causes a sharp spike in cylinder pressure which makes the engine resonate, often creating a knocking or pinging sound.
diffuser : The megaphone-like section of a tuned pipe, just after the header. The diffuser can alter the intensity and duration of the peak powerband. Shorter and steeper angled diffusers will create intense but narrow RPM range power, while long diffusers and lesser angles broaden the RPM range and reduce peak output.
displacement : The volume displaced by the piston(s) traveling from BDC to TDC. Displacement is usually expressed in cc for scooters.
divergent cone : See diffuser.
DOHC : See dual overhead cam.
DOT : An acronym for "Department of Transportation".
drive belt : The belt that connects the front and rear pulleys in a CVT.
dry clutch : A clutch not lubricated or cooled by fluid. Most CVT transmissions in scooters use dry clutches.
dry shot : When referring to nitrous oxide systems, a dry shot injects only nitrous oxide. Dry shot systems require additional fuel to be delivered through a method other than the nitrous kit. The simplicity of dry kits makes installation faster and easier, but tuning can be more difficult.
dual overhead cam (DOHC) : An engine that uses two camshafts (per head) mounted in the head(s) to control the valves. Usually one camshaft will operate the intake valves and the other camshaft will operate the exhaust valves.
duration : The length of time, expressed in degrees of crankshaft rotation, that something happens. Commonly used for port and valve open times.
dwell : See belly.
dyno : Slang for dynamometer. A device that measure horsepower and torque. 2 common types of dynos are engine and chassis dynos. Engine dynos measure power output at the crankshaft or flywheel. Chassis dynos measure power output at the wheels.
E
effective stroke : The distance between TDC and the top of the exhaust port in 2 stroke engines.
EFI : See electronic fuel injection.
EGT : An acronym for "exhaust gas temperature."
electronic fuel injection : A system for delivering fuel that uses a computer, fuel injectors, sensors, and other components rather than a carburetor.
electronic throttle control : A method of controlling throttle opening using voltage or an electronic signal rather than a cable or linkage.
enricher/enrichener : A device that adds additional fuel to the mixture to create a rich condition. Not to be confused with a choke which blocks airflow to create a rich condition. Enricheners are primarily used to make cold starting easier.
ETC : See electronic throttle control.
F
fairing : A body panel, typically plastic, that covers the frame of a scooter. Fairings can be used for decorative and/or aerodynamic functions.
feeler gauge : A thin metal strip of a precise thickness that is used to measure small clearances between two objects.
fixed half : The half of a CVT pulley that does not move other than to rotate along with the belt. The half of the front pulley (variator) that remains in a fixed position relative to the crankshaft. Sometimes called a variator fan, although many racing versions have no cooling fins so they do not act as a fan. The fixed half of the rear pulley is the rear section that the torque driver rides on.
forced induction : A method of engine aspiration that forces air or the air/fuel mixture into the cylinder(s) rather than having it drawn in by engine vacuum. A compressor, such as a turbocharger or supercharger, is used to push a greater amount of air into the cylinder.
four-cycle : See four-stroke.
four-stroke : An internal combustion engine that creates combustion once in every two revolutions of the crankshaft, which is four strokes of the piston(s). Read this for more information.
four valve head : A cylinder head that uses two intake valves and two exhaust valves to increase cylinder filling and evacuation capabilities.
front transfer ports : See main transfer ports.
G
gudgeon pin : See wrist pin.
H
header : The first section of an exhaust system that attaches to an engine's exhaust port or outlet.
HID : See high-intensity discharge.
high-intensity discharge (HID) : A type of headlights that produce a more intense and whiter light than conventional halogen bulbs. HID lights usually draw less power to make the same or more light than a halogen bulb.
hit the pipe : A phrase used by 2 stroke enthusiats when a tuned pipe reaches it's peak powerband. Some tuned pipes reach their powerband suddenly and intensely and "hitting the pipe" is clearly felt, while others come on gradually enough that you may never know they "hit the pipe." May also be referred to as "getting on the pipe". Example : "This high end engine combo doesn't even hit the pipe till 10,000RPM, and then it goes hard."
hybrid : A vehicle that uses both an electric motor and internal combustion engine. Hybrids typically offer very high fuel efficiency by shutting off the gasoline engine and using the electric motor whenever possible.
I
J
jug : Slang for the cylinder of an engine.
K
keeper : A clip or other device used to keep a valve spring in position. Keepers usually fit into a groove on the valve stem in some manner.
L
leakdown test : A method of testing an engine's cylinder sealing capabilities. While a compression test will show the engine's ability to create cylinder pressure, a leakdown test will show an engine's ability to maintain cylinder pressure. A leakdown test involves a special tool, a leakdown tester, which allows air to be injected into a cylinder. The leakdown tester monitors the pressure of the inlet air and the pressure maintained in the cylinder so you know what percentage is leaking out. A leakdown tester allows you to listen for leaks in the intake track, exahust system, or crankcase as well to make diagnosis easier.
lean : Having too little of something. An engine that is running lean does not have enough fuel in the air/fuel mixture. Technically, a lean mixture is any mixture with less fuel than what is considered stoichiometric. When speaking of two stroke engines, lean could refer to a low amount of oil present in the fuel.
M
main spring : See contra spring.
main transfer ports : The transfer ports located nearest the exhaust port in a two-stroke cylinder.
moveable half : The half of the front pulley that moves. Some call the movable half a variator while others call the whole front pulley the variator.
muffler : A device used to quiet an engine's exhaust. Sometimes called a silencer.
N
NASRA : An acronym for "North American Scooter Racing Association." NASRA is an organization formed to help promote scooter racing in North America. Now defunct.
naturally aspirated : An engine that uses only vacuum created by the piston(s) traveling from TDC to BDC to draw the air/fuel mixture into the cylinder.
nitrous oxide : Nitrous oxide is a chemical made up of two nitrogen molecules and one oxygen molecule. Nitrous oxide breaks down at roughly 570˚F, and releases the nitrogen and oxygen molecules. It also cools the intake air temperature which creates a more dense charge. This cool, dense, charge allows even more oxygen to enter the cylinder for combustion. All of the extra oxygen from the nitrous oxide releasing it's oxygen and the cooler intake charge delivering more oxygen allows more fuel to be injected and burned to create more power. When tuned correctly, nitrous oxide systems can provide dramatic increases in an engine's horsepower and torque output.
O
odometer : A gauge that measures the distance a vehicle travels.
OHC : See overhead cam.
OHV : See overhead valve.
oil control ring : A piston ring used to prevent oil from getting above the piston and into the combustion chamber. Oil control rings are usually located on the lower portion of the piston, below compression rings. Most oil control rings are made up of three smaller rings. Two thin outer rail rings and a wavy middle expander ring.
oil injection : A system that injects two-stroke oil into the fuel mixture. Oil injection systems often use a crankshaft driven pump to draw oil from a reservoir and deliver it to the carburetor.
oil pump : In a four-stroke engine an oil pump moves oil from the crankcase, through passages, and up to the cylinder head to lubricate the valvetrain. In a two-stroke oil injected engine an oil pump pushes oil from a reservoir into the carburetor to be mixed with air and fuel.
over range transmission : A pulley set for CVT transmissions with greater belt travel, often achieved with larger diameter pulleys, that will allow more effective gear ratios. Over-range kits are usually used to obtain higher top speeds and better takeoffs, and often keep RPM more consistent.
overhead cam (OHC) : A type of engine using a camshaft or camshafts installed in the head(s). OHC engines operate the valves via rocker arms or often directly by the lifters.
overhead valve (OHV) : A type of engine that uses a camshaft mounted in the engine block, below the head(s). The valves are operated via lifters, pushrods, and rocker arms.
oversquare : An engine with a cylinder bore larger than it's stroke is said to be oversquare. For example, an engine with a bore of 40mm and a stroke of 39mm is considered oversquare.
oversteer : When a vehicle's rear tire(s) slip before it's front tire(s) causing the vehicle to rotate at an angle greater than intended.
P
petcock : A device used like an on/off switch for fuel flow installed inline between the fuel tank and carburetor. Two common types for scooters are vacuum operated and manually operated. Vacuum petcocks allow fuel flow when manifold vacuum is present (engine starting or running). Manually operated units can be switched on or off by the user.
ping : A slang term for detonation. Detonation causes a sharp pressure spike that causes the engine to resonate, which often sounds like a ping or knock. See also detonation.
piston : A device that travels inside a cylinder used to transfer force. In an engine, a piston transfers the energy created from combustion to the crankshaft. The piston is also used to help draw in the air/fuel mixture, compress it, and expell it. Two-stroke engines also use the piston as a valve, opening and closing ports. In a brake caliper, a piston transfers force created from hydraulic pressure to a brake pad in order to press the pad against the brake rotor.
piston ring : An open ended ring used to create a seal between a piston and the cylinder wall. Some applications use a single piston ring, while others require multiple rings. Piston rings also transfer heat from the piston to the cylinder wall.
piston ring end gap : The distance between the two ends of a piston ring when it is installed in the cylinder. Proper end gap is necessary for cylinder sealing and preventing damage or excess wear.
piston to valve clearance : The amount of clearance between the intake and exhaust valves and the top of the piston at top dead center. Insufficient piston to valve clearance can lead to catastrophic failure.
plug chop : A method of checking spark plugs to use their condition as an indicator of engine properties such as fuel mixture ratios, oil supply, and detonation among others. Usually the engine is run at a certain RPM or throttle position for a length of time and then shut off. Ideally the transmission will be disengaged via the clutch or shifting to neutral, although this isn't really possible with most scooters, to allow the engine to shut off instantly and give the most accurate reading. The spark plug(s) would then be removed and inspected. For a true plug chop, part of the spark plug is cut or "chopped" away to allow better viewing of the insulator.
port : A passage in a cylinder or head that allows air, fuel, or a mixture to pass through it.
port map : A diagram showing a two-stroke cylinder's port layout. Port maps are often made by positioning and securing a piece of paper in the cylinder and then scribbling over the ports with a pencil to show their outlines.
port window : In two-stroke engines, the part of a port that opens into the cylinder.
powertrain : The system of devices used to deliver power to a vehicles drive tire(s). The engine, CVT transmission, and final drive transmission would all be parts of a typical scooter's powertrain.
pre-ignition : Igntion of the air/fuel mixture before the spark plug fires. Pre-ignition is often caused by hot spots like the tip of a spark plug or carbon deposits. When the mixture is ignited early it puts a huge load on the engine, becuase the piston that is trying to reach TDC is now resisting the forces of combustion. The heat and extreme forces can cause engine damage very quicky. Unlike detonation, pre-ignition usually creates no noise audible to the operator.
pre-mix : Mixing two-stroke oil into the fuel before it reaches the carburetor. Pre-mixing can be used as the sole method of lubrication or in conjuction with oil injection.
pushrod engine : See OHV.
Q
R
rear transfer ports : See secondary transfer ports.
reed block : The structure that reed valves are installed on. A reed block usually has one or more port that is covered when the reed valve is closed and open when the reed valve is open to control air and fuel flow into the crankcase.
reed valve : A valve in the intake track of a two-stroke engine. Reed valves are opened and closed by pressure changes inside the crankcase to allow air and fuel into the crankcase or to seal off the crankcase. The use of reed valves creates less emissions, better fuel economy, and more power.
restrictor : Any device that limits power output or speed by limiting flow or travel. Common restrictors include exhaust and intake restrictors that limit airflow, variator restrictors that limit belt travel, and throttle restrictors that can prevent the throttle from being opened all the way.
retainer : A device used in conjunction with a keeper or keepers to hold a valve spring in place.
revolutions per minute (RPM) : The number of revolutions the crankshaft will make in one minute at it's current rate of rotation.
rich : Having a large quantity or too much of something. Saying an engine is running rich usually refers to the amount of fuel in the mixture being too great. When dealing with two strokes it may be referring to a greater than ideal amount of oil in the gasoline.
rocker arm : A lever with one end positioned on the tip of a valve stem and the other end on a camshaft or pushrod. When the camshaft or pushrod lifts and applies pressure to one end of the rocker arm, the other end applies pressure to the valve to open it.
roller weight : Often called "rollers." Roller weights are small cylindrical weights inside the variator. Roller weights are forced toward the outer edges of the variator as centrifugal force from the crankshaft rotating the variator increases. As the roller weights move outward, they force the variator to close and move the belt outward to create higher gear ratios. Lighter weights resist centrifugal force more and keep the gear ratios low longer. Heavier weights move outward at lower RPM to achieve higher gear ratios faster.
RPM : See revolutions per minute.
S
secondary transfer ports : The transfer ports located nearest the boost port in a two-stroke cylinder.
single overhead cam (SOHC) : An engine that uses one camshaft (per cylinder head) mounted in the head(s). Each cam controls both the intake and exhaust valves.
sleeper : A performance modified vehicle with no obvious external signs that it has been modified. A sleeper may have a racing oriented engine with a stock or quiet exhaust and no visual modifications.
sliding weight : Usually referred to as "sliders." Sliding weights do the same job as roller weights. As their name implies, they slide in the variator rather than roll because they have one or more flat edges rather than being cylindrical like roller weights.
Snell : A non-profit organization that sets strict safety standards for helmets to conform to.
SOHC : See single overhead cam.
spark knock : A slang term used for detonation. Detonation can cause a knocking or pinging sound. See also detonation.
speedometer : A gauge that measures vehicle speed.
sprung weight : Any weight supported by the suspension. The frame and body are examples of sprung weight.
square : An engine with a bore equal to it's stroke is said to be square. For example, and engine with a cylinder bore of 40mm and a stroke of 40mm is considered square.
squish band : A section of the cylinder head cut an an angle designed to help force the air/fuel mixture into the center of the combustion chamber.
squish clearance : The distance between the top of the piston and the lowest point on the cylinder head in the cylinder.
stinger : The end section of an un-muffled tuned pipe, or the section between the baffle and the muffler on muffled pipes. The stinger allows exhaust gases to escape from the expansion chamber. It's diameter and length can create varying backpressure that affects exhaust gas temperature and subsequently affects power output and engine cooling.
stoichiometric : Sometimes abbreviated "stoich." A chemically balanced mixture considered to be ideal. 14.7:1 is considered the a stoichiometric mixture of air/fuel for gasoline engines. Other fuels have different stoichiometric ratios.
stroke : The distance a piston travels from top dead center (TDC) to bottom dead center (BDC). Usually measured in millimeters(mm) when referring to scooters.
stroker : An engine or crankshaft with a larger stroke than stock.
supercharger : A crankshaft driven compressor designed to increase manifold pressure and force the air/fuel mixture into the cylinder(s) for increased cylinder filling and power output.
swept volume : See displacement.
T
tachometer : A gauge that measures engine RPM.
TDC : See top dead center.
three valve head : A cylinder head that uses two intake valves and one exhaust valve.
throttle : A system for controlling engine fuel and air intake via a butterfly valve or slide valve inside a carburetor or throttle body.
top dead center (TDC) : The highest point of a piston's travel.
torque driver : A part of the rear pulley that controls the movement of the pulley halves when opening and closing.
torque spring : See contra spring.
transfer port : A port that allows air/fuel mixture to transfer into the cylinder from the crankcase of a two-stroke engine.
tuned length : The distance from the exhaust side of the piston to the mid point of the baffle of a tuned pipe for a two-stroke.
tuned pipe : An exhaust system designed to increase power on 2 stroke engines. When properly designed, a tuned pipe will help to evacuate exhaust gases as well as aid in cylinder filling.
turbocharger : An exhaust driven compressor designed to increase manifold pressure and force the air/fuel mixture into the cylinder(s) for increased cylinder filling and power output.
twin cam : See DOHC.
twist 'n' go : A term for scooters with automatic CVT transmissions. Twist the throttle and go.
two-cycle : See two-stroke.
two-stroke : An internal combustion engine that completes combustion in every revolution of the engine, or two strokes of the piston. Read this for more information.
two valve head : A standard cylinder head that uses one exhaust valve and one intake valve.
U
undersquare : An engine with a stroke longer than it's cylinder bore diameter is said to be undersquare. For example, an engine with a 40mm bore and a 45mm stroke is considered undersquare.
understeer : When the front tire(s) slip or skid before the rear tire(s), causing the vehicle to rotate at an angle lesser than it's intended angle.
unsprung weight : Any weight not supported by the suspension, such as wheels and tires.
V
vacuum : Negative manifold pressure. A pressure that is less than atmospheric pressure (14.7psi).
valve cover : A cover that attaches to a cylinder head which protects the valvetrain.
valve lash : The clearance between the rocker arm and the tip of the vavle stem. Valve lash is measured while the valve is fully closed with the other end of the rocker arm on the cam's base circle.
valve spring : A spring used to apply pressure to a valve. The spring returns the valve to a closed position and keeps the valve seated firmly.
valvetrain : The parts used to control the operation of an engine's valves. The camshaft, pushrod(s), rocker arm(s), etc... would all be part of the valvetrain.
variator : The front pulley in a CVT transmission consisting of a moveable half and a fixed half. The moveable half is often referred to as the variator. The variator is able to change it's effective diameter by being pushed closed via centrifugal force.
variator fan : The fixed half of the front pulley in a CVT transmission. The variator fan has fins on the outside to move air and aid in cooling CVT components. The belt travels on the rear half of the fan.
volumetric efficiency : A comparison of the volume of air that is drawn into an engine during induction versus the volume of air that would fill the engine with the piston at BDC at atmospheric pressure. Volumetric efficiency is expressed as a percentage. Typical engines range from 60% to 120% volumetric efficiency. Percentages greater than 100% are usually achieved through the use of forced induction.
W
wet clutch : A clutch that is lubricated or cooled by a fluid. Most scooters or motorcycles that have wet clutches use the engine oil to cool or lubricate them. Those models must use a different type of engine oil, specifically intended for wet clutches, or the clutch slippage may occur.
wet shot : When referring to nitrous oxide systems, a wet shot injects both nitrous oxide and fuel. Ths can make tuning easier and more precise, but wet shot systems do have additional solenoids, lines, and other components that can make installation more difficult.
WFO : An acronym for "wide freaking open" usually referring to the throttle being all the way open (full throttle). See also WOT.
wheelbase : The distance between the center of the front wheel(s) and the center of the rear wheel(s). Bikes with shorter a shorter wheelbase tend to turn more quickly, while bikes with a longer wheelbase are generally more stable.
WOT : An acronym for "wide open throttle."
wrist pin : A hollow metal rod used to attach a piston to a connecting rod.
wrist pin bearing : A needle bearing that fits around the wrist pin, like a sleeve, inside the connecting rod.
X
Y
Z