Audi, Porsche, Mercedes, and Volkswagen Repair, Maintenance, and Performance Related Drivetrain Glossary Portland Oregon

All-season tire: Tires with special tread designed to improve traction on snow or ice (generally provides 37% higher average snow traction compared with non-all-season tires), while providing acceptable noise levels on smooth road surfaces.

All-wheel drive (AWD): A vehicle drivetrain with a center differential having all four wheels under power at all times.

Annulus: Any type of ring gear, including the ring part of the ring and pinion in the rear end, and the gears in the planetary gear set of an automatic transmission.

Aquaplaning: A tire unable to remain in contact with the ground or pavement in wet weather that rides on the water itself. Also known as hydroplaning or, more simply, planing.

Aspect ratio: Also known as tire profile. A measurement of a tire; the percentage of the tire's height to the width.

Automatic transmission (AT – A/T): A transmission in which gear ratios are changed automatically.

Automatic transmission cooler: A device, often found in the radiator, through which automatic transmission fluid circulates to be cooled by surrounding air or engine coolant.

Automatic transmission fluid (ATF): A red, petroleum-based fluid used to transfer power and control, lubricate, cool, and clean the automatic transmission.

Axle boot: The flexible cover that retains grease and/or oil in a transmission or a constant velocity joint.

Axle hop: The tendency of a live axle housing to rotate with the wheels slightly and then snap back during hard acceleration. This action may be repeated several times, creating a loss of traction until the driver releases the accelerator.

Axle shaft: Alloy steel shaft that transfers torque from the differential side gears to the drive wheels. This shaft also supports vehicle weight on most passenger cars.

Beam axle: A shaft that does not transmit power but provides a means of fastening wheels to either, or both, ends.

Cheater slick: A tire that is constructed of the same sticky rubber as a racing tire, but has a shallow tread cut into it to make it street legal.

Close ratios: A transmission, usually used in drag racers, with close spacing between the speeds of the gears, allowing for minimum engine rpm reduction when shifting.

A device for connecting and disconnecting the power flow between the engine and standard transmissions, used during starting, shifting, and stopping.
A device used to connect two collinear shafts to a driving mechanism such as a motor, engine, or line shaft, and to disconnect them at will.

Clutch cable: A cable that actuates the clutch fork of a manual-transmission system.

Clutch disc: Circular-shaped component, with a friction facing on each side, that transfers power from the flywheel and pressure plate to the splined clutch shaft.

Clutch packs: A series of clutch discs and plates, installed alternately in the clutch housing, to act as a driving or driven unit.

Clutch pedal: A pivoting component inside the vehicle that the driver depresses with his/her foot to operate the clutch.

Clutch plate: A pressure plate that forces the clutch disc against the flywheel.

Clutch-release bearing: A component, attached to the clutch-release fork, that contacts and then moves the release levers when the clutch pedal is depressed.

Clutch-release fork: A pivoting clutch housing component that transfers motion from the free-play adjusting rod on the clutch linkage to the attached clutch-release bearing.

Clutch slippage: A term given to the condition when, although engine speed is increased, increased torque is not transferred through to the driving wheels.

Constant-velocity joint: Two universal joints, closely coupled, so their acceleration-deceleration effects cancel each other out.

Contact patch: The area of contact of a tire with the road surface when the tire is supporting the vehicle weight.

Continuous variable transmission (CTV): A stepless transmission that uses a sheave clutch to transmit engine torque.

Differential: The section of the rear-axle assembly that provides three functions it allows the wheels to revolve at different speeds during turns, provides the final gear reduction, and changes the angle of drive 90 degrees.

Differential assembly: The mechanism that relates to front- or rear-driving axles that permits unequal travel distances and speeds between the vehicle's driving wheels.

Differential case: The housing of the differential that contains the side and pinion gears and the pinion shaft, and also serves as a mounting place for the ring gear.

Differential drive gear: A large, circular helical gear that is driven by the transaxle pinion gear and shaft and that drives the differential assembly.

Differential housing: Also known as rear-axle housing. A cast iron assembly that houses the differential unit and the drive axles.

Differential lock: A toggle or push-pull type air switch that locks together the rear axles of a tractor so they pull as one for off-the-road operation.

Differential pinion gears: Small, beveled gears located on the differential pinion shaft.

Differential pinion shaft: A short shaft locked to the differential case. This shaft supports the differential pinion gears.

Double clutch: A driving technique to minimize gear clash when shifting gears with a manual transmission.

Drive line: Assembly of various parts such as the driveshaft, universal joints, and connecting yokes that transmit torque from the transmission to the differential.

Drive shaft: An assembly of one or two universal joints connected to a shaft or tube used to transmit power from the transmission to the differential.

Drivetrain: All of the components required to deliver engine power to the road surface.

Final drive: The pinion, ring, and differential gears that provide power to the drive wheels.

Flexplate: A lightweight flywheel with a starter ring gear around its outside diameter, used on engines equipped with an automatic transmission; also serves as the attachment point for the torque converter.

A heavy metal wheel with starter ring gear that is mounted at the rear of the crankshaft. It absorbs energy on power stroke(s), returns energy on other stroke(s), and transfers power to the clutch or torque converter.
Front-most part of a clutch assembly that is bolted to the engine crankshaft with a rear surface to provide a smooth friction area for the disc-front facing to contact during clutch engagement.

Flywheel ring gear: A gear, fitted around the flywheel, that is engaged by teeth on the starting-motor drive to crank the engine.

Friction disc: A flat disc surfaced with a friction material on one or both sides, such as a clutch disc.

Geared speed: A theoretical vehicle speed based on engine rpm, transmission-gear ratio, rear-axle ratio, and tire size, not accounting for slippage.

Gear lubricant: A type of oil or grease especially formulated to lubricate gears.

Gear oil: A thick lubricant, generally with an SAE number of 80 or above, used in standard transmissions or differentials. These often contain additives, such as an EP additive, to guard against being squeezed out from between gear teeth.

Gear oil, limited-slip: A lubricant specified for use in certain limited-slip differentials to prevent chattering during turns and/or abnormal wear to the parts.

Gear ratio: The speed relationship that exists between a driving (input) and a driven (output) gear. For example, a driving gear that revolves twice for each driven-gear revolution has a 2 to 1 (2:1) ratio.

Gears: Mechanical devices containing teeth that mesh that transmit power, or turning force, from one shaft to another.

A floor- or steering-wheel-mounted lever used to manually change gears in the transmission.
A linkage-type mechanism by which the gears in a transmission are engaged.

Half shaft: The axle shafts on either side of the differential that transmit power to the drive wheels.

Helical gear: A gear having teeth cut at an angle across its face, or diagonally to the rotational axis.

Limited-slip differential: A differential having special friction mechanisms tending to keep both axle shafts rotating at the same speed, regardless of unequal tire-to-road surface friction.

Limited-slip differential gear oil: A specially formulated gear oil required in limited-slip differentials because of the extreme pressures on the clutch cones or clutch plates and discs.

Lug nut: A hex-sided, threaded device used to hold the wheel on a vehicle having lug studs.

Manual transmission: A manually shifted gearing device in the power train that allows variation on the relationship between engine speed and road speed.

Modular wheel: A wheel made of different sections that are bolted or riveted together.

Negative offset: A wheel rim that has been placed inward from the center of its mounting flange.

Organic friction material: A friction material having organic binders substantially formulated with nonmetallic fibers.

Out of round: The condition of a circular part when it is not circular, as in slightly oval shaped.

Outside snap ring: A snap ring used on the outside of a shaft or part to hold the assembly in place.

Pilot bearing: The bearing at the output end of the engine's crankshaft that supports the transmissions input shaft.

Pinion bearing: A bearing that is used to support the pinion gear in the differential housing.

Planetary gear set: A group of gears named after the solar system because of their arrangement and action. This unit consists of a center (sun) gear around which pinion (planet) gears revolve. The assembly is placed inside a ring gear having internal teeth. All gears mesh constantly. Planetary gear sets may be used to increase or decrease torque and/or obtain neutral, low, intermediate, high, or reverse.

Planetary gear system: A gear set consisting of a central sun gear surrounded by two or more planet gears which are, in turn, meshed with a ring gear, used in overdrives and automatic transmissions.

Power control module(PCM): A module or computer used in an electronic transmission to aid in control of the shift solenoids.

Pressure plate: That part of a manual clutch that is mounted on and rotates with the flywheel and exerts pressure against the friction disc.

Pressure plate assembly: An assembly of various parts that operate to press the clutch disc against the flywheel, engaging power flow to the clutch shaft, or release the disc, disengaging power flow.

Pull-type clutch: A type of clutch whereby the release bearing is pulled toward the transmission.

Push-type clutch: A type of clutch in which the release bearing is not attached to the clutch cover.

Radial runout: Variations in tire diameter; the measured amount of out-of-roundness on rotating tires.

Rear end: The differential and final drive assembly on a rear-wheel drive vehicle.

Rear-wheel drive: A drivetrain layout that provides power to the rear wheels only.

Rigid transmission mount: A solid mount for attaching the transmission to the frame without any provisions for vibration dampening.

Ring and pinion: The combination of a ring gear attached to the differential and the pinion at the end of the drive shaft.

The gear around the edge of a flywheel.
A large, circular gear such as that in the final drive assembly.

Run out: The amount a rotating object may wobble out of its plane of rotation.

Sealed bearing: A replaceable bearing, such as those found on many axle shafts that is lubricated and permanently sealed by the manufacturer to contain the grease while keeping out contaminants.

Self-adjusting clutch: A mechanism that automatically takes up the slack between the pressure plate and clutch disc.

Shifter: A floor- or steering column-mounted lever on a motor vehicle used to select and/or shift the transmission gears.

Shift forks: A Y-shaped component located between the low/reverse, first/second, and third/fourth gears on the main shaft of a transmission that causes the gears to engage or disengage via the sliding clutches.

Shifting rods: The linkage of a manual transmission that connects the shifter to the shift forks.

A lever used to change gears in a transmission.
A lever that moves the starter drive pinion in and out of mesh with the flywheel in some applications.

Sidewall information: Certain information required by the Department of Transportation to be imprinted on each tire, such as size, load rating/inflation pressure maximums, generic name of each cord material in the sidewalls and tread areas, actual number of plies in the sidewall and tread area, the words tubeless or tube type, as applicable, the word radial, if applicable, and the manufacturing code to determine who made the tire, where it was made, and when it was made.

Snap ring: A circular retaining clip used inside or outside a shaft or part to secure a shaft, such as a floating wrist pin.

Speed rating: A tire rating that indicates the maximum safe vehicle speed that a tire will withstand.

Speed sensor: An electrical device that can sense the rotational speed of a shaft or member and transmit this information to another device, such as a readout. (ie wheel speed sensor)

To upshift a manual transmission without releasing the accelerator.
The art of upshifting a manual transmission without using the clutch.

Spider: A set of gears in the differential that allow the rear wheels to rotate at different speeds as the vehicle is cornering.

Splines: The internal or external longitudinal grooves in a gear or shaft that mesh when assembled, causing the gear to turn with the shaft, but allow lateral movement.

Stepped flywheel: A flywheel having a ledge to which a pressure plate is attached.

Straight cut (spur) gear: A transmission or differential gear having teeth cut straight across its face, parallel to the rotational axis.

Synchromesh: A type of manual transmission where the synchronizer is used to bring a selected gear up or down to the speed of the main shaft.

Tire balancing: The procedure of using special equipment for identifying the lighter portions of a tire and adding weights until opposite tire sections weigh the same.

Tire contact area: The footprint, patch, or patch area of the tire that is in contact with the road surface when the tire is supporting the vehicle weight.

To cushion the vehicle ride to provide a comfortable ride for the occupant; reduces jolts to the vehicle caused by road irregularities.
To support the vehicle weight.
To develop traction to drive and steer the vehicle under a wide variety of road conditions.
To contribute to the directional stability of the vehicle, and absorb all the stresses of accelerating, braking, and centrifugal force in turns.

Tire inflation: Air pressure inside the tire, measured in pounds per square inch (psi) or kilopascals (kPa), checked when tire is cool or has been driven less than one mile.

Tire profile: The profile of a tire is based on the width and height of a cross section of the tire determined by an aspect ratio equal to the section height divided by the section width.

Tire rotation: Swapping tires around to various positions, as prescribed by the tire manufacturer, on the vehicle to equalize tire wear.

Tire scrub: The sliding of a tire at an angle to the direction that it is pointed.

Tire sidewall: That area of the tire between the bead and the tread pattern made from an abrasive-resistant blend of rubber, which absorbs shocks and impacts from road irregularities and prevents damage to the plies.

Tire-speed rating: A tire rating that specifies the speed at which the tire can be safely used and does not suggest that a vehicle can be driven safely at the designated speed, because of various road and weather conditions that may be encountered. Also the condition of the vehicle may affect high-speed operation.

Torque converter: Unit that transfers power from the engine to the transmission input shaft by directing and redirecting fluid flow.

Transaxle: A combination transmission-and-axle assembly, common in front-engine, front-drive vehicles.

Transfer case: A small transmission used on four-wheel-drive vehicles to transfer engine torque to the front and/or rear axles.

Transmission: A gearing device of a vehicle that provides variable ratios between the engine output and the differential input.

Transmission oil cooler: Heat exchanger located in the radiator outlet end section through which transmission fluid flows for cooling purposes on most automatic transmission cars.

Tread: That portion of a tire that comes into contact with the surface of the road and is designed to allow the air flow to cool the tire and to channel water during wet weather.

Tread-wear indicator: Projections of rubber, 1/16 inch (1.59 mm) high at the bottom of the tread grooves, spaced around the tire to identify a tire worn to the recommended safe limit.

Tread-wear rating: A tire rating indicating the wear capabilities of the tread that allow consumers to compare tire life expectancy.

Tripod: The central part of certain CV joints having three arms or trunnions with needle bearings and rollers running in grooves or races in the assembly.

Tubeless tire: A tire having an integral inside rubber liner to retain air; mounted directly on the rim.

Viscous coupling: A coupling device having input and output shafts fitted with thin, alternating discs in a closed chamber filled with fluid.

Wheel (rim) offset: The distance between the wheel centerline and the mounting face of the wheel. Usually a negative number, the more negative the number (-50et), the flatter the wheel face (FWD fitment). The more positive the number (-25et), the deeper of a dish the wheel face exhibits (RWD fitment).