Glossary of Automotive Terms – U

Letter U – Dictionary of Automotive Terms

Abbreviation for Union of Automobile Importers in Bulgaria
Abbreviation for Universal Asynchronous Receiver-Transmitter
Abbreviation for United Auto Workers — An international union of workers in the automobile, aerospace, and agricultural implement industries in North America.
Ubbehohde viscosimeter
A suspended level apparatus for accurately determining the viscosity of a liquid.

U-shaped bolt with threads at both ends for attaching leaf springs to the axle or securing exhaust pipes, ring bolts, and shackles

U brake
Heavy-duty centerpull Mountain bike brakes that affix to frame posts.
Heavy-duty centerpull Mountain bike brakes that affix to frame posts.
U brakes
Heavy-duty centerpull Mountain bike brakes that affix to frame posts.
Heavy-duty centerpull Mountain bike brakes that affix to frame posts.
A flat piece of metal with two edges turned up at right angles in the same direction, i.e., in the shape of a U when seen from the end of the metal. Used in automobile frames to give strength. Sometimes two U-channels are nested together to create a box called a Box section construction
Abbreviation for Union Cycliste International the international governing body of professional and amateur bicycle racing
U-cylinder engine
Abbreviation for Underdrive



Abbreviation for universal integrated circuit
Abbreviation for Up-Integrated Direct Ignition
A term named after the Uinta mountains describing a natural black bitumen used in the manufacture of acid, alkali, and waterproof coatings. Also called Uintahite.

Abbreviation for Universal Japanese Motorcycle
U joint

Abbreviation for Universal joint.


Abbreviation for Association of Ukrainian Motor Vehicle Manufacturers.

Abbreviation for Underwriters Laboratories, Inc.
Abbreviation for Underwriter’s Laboratories.
Abbreviation for Ultra Large Crude Carriers. Tankers larger than 300,000 dwt.
Abbreviation for Ultra Low Emission Vehicle Standards
  1. The amount which a tank or vessel lacks of being full.
  2. Empty space present when container is not full. Opposite of innage
A U-shaped bicycle lock; called D-lock in Britain
Abbreviation for Ultra-Low Profile. A tire with a low aspect ratio
Abbreviation for Ultra-low sulfur diesel fuel
Ultimate cornering force
The maximum steady-state cornering or side force generated by the tires when a vehicle is driven around a Skidpad with the speed increased gradually until if it were increased further the vehicle would slide off course.
Ultimate Strength
The stress level that a pressure-bearing part can tolerate without rupture, fracture, deformation or other physical damage.
Ultra 7
A freewheel designed to allow seven Cogs to fit into the space normally taken up by six.
Ultralight tube
An inner tube with thin walls.
Ultra-Low Emission Vehicle

  1. A vehicle that meets the ULEV emission standards set by the California Air Resources Board. The ULEV standards are more stringent than the LEV standards.
  2. A vehicle meeting either EPA’s CFV ULEV standards or CARB’s California Low-Emission Vehicle Program ULEV standards. ULEVs produce fewer emissions than LEVs. Fleets who purchase CFV ULEVs may earn credits under the Clean-Fuel Fleet Vehicle Program. Manufacturers that sell CFV ULEVs may earn credits under the federal California Pilot Program.
Ultra Low Emission Vehicle Standards
(ULEV) State and federal governments set standards for automakers developing vehicles that will output lower emission levels. Each state determines their own levels as does the federal government. California and New York have some of the most stringent laws. Once the requirements are met for a specific vehicle, the automaker may call that car or truck a Low Emission Vehicle or LEV.
Ultra-low sulfur diesel fuel
(ULSD) Diesel fuel containing a maximum 15 parts per million (ppm) sulfur.
A fabric in clothing which works to keep you dry and comfortable in two ways larger yarns next to the skin pull moisture via capillary action to finer yarns on the face of the fabric, and hydrophobic polyester speeds evaporation.
Ultra 6
A freewheel designed to allow six Cogs to fit into the space normally taken up by five.
Ultrasonic welding
A high-tech welding procedure used for metals (e.g., in wire bonding of integrated circuits) and plastics. Pressure is applied to the parts to be joined and ultrasonic vibrations are transmitted through the materials; frictional heat at the materials interface causes localized melting and coalescence; ultrasonic welding is fast and produces precise, strong welds
Invisible radiation waves with frequencies from 4 to 400 nanometres which is shorter than wave lengths of visible light and longer than X-ray.
Ultraviolet rays
Energy waves that emanate from the electrodes and the welding flames of such a frequency that these rays are in the ultraviolet ray light spectrum.
A windproof, waterproof, highly breathable fabric that is perfect for foul weather cycling gear. Ultrex fabric lets perspiration vapor out while keeping rain and wind from getting in.


An oil deflector placed near the valve tip to throw oil from the valve stem area.


Indicates ‘unified’ screw threads to ‘inch’ dimensions used in the U.S. as distinguished from metric dimensions.
A condition of wheels which are not Balanced.

Unburnt hydrocarbons
Pollutant (basically petrol vapor) released into the atmosphere as an exhaust gas, due to incomplete combustion
An orderly, neat instrument panel that is free of irrelevant detail
UNC thread
Abbreviation for Unified National Coarse thread; one of the screw threads used on British cars.

Under-axle clearance
The distance from the ground to the bottom of each of the axles excluding the differential.
Under-belly clearance
The distance from the ground to the lowest part of the bottom surface of the vehicle excluding the axles and differentials.
The underside of a car. Commonly called the floor pan. Usually made up of several smaller panels joined together to form a single unit and reinforced on the underside by floor pan cross bars.

Underbody coating
A coating of the underbody of a vehicle, usually with organic coating materials (plastic, wax, bitumen-based products), to protect it from mechanical damage and corrosion
Underbody protection
A coating of the underbody of a vehicle, usually with organic coating materials (plastic, wax, bitumen-based products), to protect it from mechanical damage and corrosion
Underbody sealing compound
Underbody structure
The structural members and flat panels of the bodywork on the chassis. Also called substructure
Underbody treatment
To run farther into the turns than normal without applying much brake
Under Bucket
A system low on refrigerant, resulting in improper cooling.
  1. All of the products used to prepare the surface to receive color Coats (Primers, surfaces, putties, Sealers, etc.).
  2. A rustproof coating with sound-deadening properties applied to the Underbody
The material sprayed on the underside of the automobile, under the hood, trunk lid, etc. which is designed to deaden sound and prevent the formation of rust.

  1. A condition in which the vulcanization or curing has not been adequate.
  2. Degree of cure less than optimum. May be evidenced by tackiness, softness, off-color, or inferior physical properties of an adhesive.
A depression at the toe of the weld which is below the surface of the base metal.
Undercut Head
For short lengths of flat and oval head machine screws, the heads are undercut to 70 per cent of normal side height to afford greater length of thread on the screws.
A 4-inch plastic drain tile installed along the edge of pavement, about 40 inches deep. The drain tile is ‘daylighted’ every so often at low spots in the topography, usually into streams that cross the road or run near the roadway. The work is done in a ‘train’ approach. There is a machine that digs the trench and installs the tile, a machine that places the material to fill the trench, trucks to deliver the fill material, a piece of equipment for compaction, and a tractor-mounted rotary broom to clean up. Additionally, there will be a truck and backhoe crew that installs the outlets to the streams. This process usually takes a week or two for a particular road project, depending on the length of the road section.
Underfilm corrosion
Under Flexure
The underside of a car
Underfloor mid-engine
A design, introduced on the Toyota Previa minivan, where the engine is located below the passenger compartment, between front and rear
The underbody backbone of a car, both for unitary or chassis designs
Underhead collar
A washer fitted under the head of a bolt or screw
Underhood area
The engine compartment
Underinflated tire
A tire which has too little air in it, causing increased wear at the tread edge
Insufficient air pressure in a tire for the amount of load carried. Tires will wear on both outer sides of the tread.
Under Load
A description of a vehicle with an engine that provides less power than is necessary for acceptable performance; especially noticeable when accelerating, passing, or going uphill
Occurs primarily in rear-end collisions when the striking vehicle is wedges under the rear of the vehicle being struck.
Underride Guard
A rear impact guard installed at the rear or under the rear of the cargo body of a truck or trailer so that when the vehicle is struck from the rear, it can limit the distance that the striking vehicle’s front end slides under the rear of the truck or trailer. Most semitrailers have what are known as ICC bumpers, but a hydraulic liftgate at the rear of a box van can act as an underride guard. See ICC BUMPER.
  1. A rustproof coating with sound-deadening properties applied to the underbody. Also called Underbody sealing compound
  2. To apply underseal.
Underseal coating
Under seat panel
A body accessory panel made from tough, oil-resistant plastic for fitting on cars that do not have a standard wheel housing panel. It is designed to protect the upper inside area of the fender and the door hinge area from splashes of water and mud deposits; it is also used as standard equipment on some modern cars to replace the traditional steel wheel housing panel
The lower side of the underbody, i.e., the surfaces that face the road surface
Underside paneling
A smooth cover on the underside of a car for improving aerodynamics below the vehicle, e.g., on the Porsche 911 and 928
Under sill panel
A separate closing panel or section used on cars where the inner sill or side member consists of several separate parts; it forms the bottom of the inner sill and connects the sill area to the floor panel
Under-size Body
  1. The reduced body of a bolt or screw, the diameter of which may range from below the pitch diameter to the minimum major diameter of the thread. Such a body diameter is found on some bolts or screws having rolled threads.
  2. Where the shoulder of a fastener equals the pitch diameter or less, which means the shoulder is smaller than the outside diameter of the threads. It would indicate that a fastener was not extruded during its manufacture. Also called reduced body diameter
Undersize grind
A type of brake shoe arcing that produces a lining with a constant thickness ground to a radius slightly smaller than that of the brake drum.
Undersize tire
The use of a tire having insufficient carrying capacity for the load. Also replacing original-equipment tire size with a smaller size of sufficient capacity, thus reducing unladen weight.
Suspended below a supporting member, e.g., of a chassis suspended below the axles
Underslung frame
A frame design of the pre-war era whose characteristic feature is that the frame members run below the axles
Undersquare engine
Under square engine
An engine in which the Bore diameter is smaller than the length of the stroke. Also called Long stroke engine
Under-square engine
An engine in which the Bore diameter is smaller than the length of the stroke.
The tendency for a vehicle, when negotiating a corner, to turn less sharply than the driver intends. The front end wants to run wide in a turn. To correct this problem, the suspension needs to be stiffer in the front or softer in the rear. A front-engine vehicle has a natural tendency to plow straight ahead on turns (i.e., understeer) unless its suspension is adjusted to counteract it. Sometimes called push.

The bottom panel of multi-layer or complex underbody panel assemblies, which faces the road surface
A cushioning layer between the belt layers and the tread in radial tires.


Underwriters Laboratories, Inc. (UL)
An independent testing organization that sets safety standards for electric motor and other electrical equipment
Unequal-length wishbone suspension
A double wishbone suspension system in which the upper wishbone is shorter than the lower one, with both converging slightly at the wheel hub; reduces tire wear due to variations in track and camber angle when cornering
Unequal wishbones
A colloquial term for Unequal-length wishbone suspension
Unfinished oils
All oils requiring further processing, except those requiring only mechanical blending. Unfinished oils are produced by partial refining of crude oil and include Naphthas and lighter oils, Kerosene and light gas oils, heavy gas oils, and residuum.
UNF thread
Unified National Fine thread; one of the screw threads used on British cars.

Unglazed solar collector
A solar thermal collector that has an absorber that does not have a glazed covering. Solar swimming pool heater systems usually use unglazed collectors because they circulate relatively large volumes of water through the collector and capture nearly 80 percent of the solar energy available.
Unibody construction
A manufacturing process where sheet metal body parts are combined with stress-bearing elements to form the body and chassis as a single piece, as opposed to attaching body parts to a frame.

Unicrown fork
A mountain bicycle fork blades which curve in at the top and are welded to the steerer instead of fitting into a fork crown (there is no fork crown on a unicrown fork)
A one-wheel cycle having pedals attached to the axle and a saddle attached to the upper end of the fork tubes.
Unidirectional flow scavenging
A scavenging process of two-stroke engines. One of the characteristic features is that the flow of fresh charge does not follow a path opposed to that of the burnt charge; scavenging and exhaust ports are not located at the same stroke end of the piston but rather are staggered, i.e., the fresh charge flows along the length of the cylinder without its direction being reversed. This process is widely used in large engines, e.g., for ships
Unified National Coarse thread
Unified National Fine thread
Unified Thread Standards
The basic American standards for fastening screw threads as agreed upon by standard bodies of Canada, the United Kingdom, and the United States. They are a complete and integrated system of threads for fastening purposes. Their outstanding characteristic is general interchangeability of threads achieved through the standardization of thread form, diameter-pitch combinations, and limits of size.
Uniflow scavenging
A scavenging process of two-stroke engines. One of the characteristic features is that the flow of fresh charge does not follow a path opposed to that of the burnt charge; scavenging and exhaust ports are not located at the same stroke end of the piston but rather are staggered, i.e., the fresh charge flows along the length of the cylinder without its direction being reversed. This process is widely used in large engines, e.g., for ships
Uniform corrosion
Corrosion occurring over the entire exposed surface. The opposite is Localized corrosion
A hydraulic coupling that connects two brake lines.
Union Cycliste International
(UCI) International governing body of professional and amateur bicycle racing
Union nut
A nut used to secure the connection between pipes or rods
Uni-servo brake
  1. A servo brake with one single-end wheel cylinder and two self-energizing brake shoes
  2. A drum brake that has servo action in the forward direction only.
Uni-servo drum brake
A servo-action drum brake that has servo action only when the vehicle is braked in a forward direction.
Unitary construction
A modern chassis layout with no separate frame, using the sheet metal parts of the vehicle body or floorpan as structural members which also carry all suspension parts; introduced by Citroën in 1934 and by GM/Opel Olympia in 1935. Also called Monocoque
Unitary system
A heating/cooling system factory assembled in one package and usually designed for conditioning one space or room.
Unit body
A type of automobile construction in which the body, floor plan, and chassis form a single structure. Such a design is generally lighter and more rigid than a vehicle having a separate body and frame.
Unit construction
Integrated unit consisting of the engine and gearbox within the same casing
United States Council for Automotive Research
(USCAR) an organization founded by Chrysler, Ford, and General Motors to strengthen the technology base of the domestic automotive industry through research and co-operation.
Unit engine
A term used for motorcycles where engine and gearbox are accommodated in one common housing
Unit Injector
An injector which is camshaft-driven and incorporates a plunger. The plunger works in conjunction with orifices in the injector body to determine the beginning and end of injection. The plungers has a helix and is rotated by a rack so the beginning and end of injection can occur closer together or farther apart, thus shortening or lengthening the injection period and changing metering. A special pump which supplies fuel through an orifice to the injector operates at a pressure which is precisely controlled depending on the changes in engine speed.
Unitized construction
A type of body construction that doesn’t require a separate frame to provide structural strength or support for the car’s mechanical components. A unitized body can employ Monocoque construction, or it can use strong structural elements as an integral part of its construction.
Unitized transmission
A transmission (also unit transmission) that is an integral part of the engines bottom end.
Universal cable
A shift or brake cable that is designed to fit all types of levers; on each end is a different lead end and you cut off the one you do not need.
Universal Japanese Motorcycle
(UJM) during the 1970s, the Japanese became so identified with the four-cylinder standard-style motorcycle that this term described them.
Universal joint
Universal jointUniversal joint

(UJ) A flexible double-pivoted Joint that allows driving power to be carried through two shafts that are at an angle to each other. It consists of two Y-shaped yokes and a cross-shaped member called the Spider. The four arms of the spider are assembled into bearings in the ends of the two yokes. With the normal cross-and-two-yoke universal joint there is some change in speed when the driveshaft and the driven shaft are at an angle to each other. The change in speed occurs because the driven yoke and driven shaft speed up and then slow down twice with every revolution of the drive line. The greater the angle between the drive and driven shafts, the greater the speed variation. To eliminate this speed variation, which results in increased wear of the affected parts, Constant velocity joints are used on many cars. In front drive vehicles, it is called the Constant velocity joint. Also called Hooke joint or Cardan joint.

Universal joint socket
A socket with universal joint to reach into cramped working areas
Universally jointed shaft
Universal motor
Electric motor which will operate on either ac or DC.
Universal spark plug socket
A spark plug socket with universal joint for reaching hard-to-get-at spark plugs
A type of threads originating around the 1950’s with a more rounded fit in order to prevent cracks, reduce loosening due to vibration and slightly increase strength. Possessing a tighter fit, UNJ thread depth is smaller that the usual UN standards with the minor diameter of external threads on screws and internal threads on nuts both increasing. UNJ is used in critical applications by the aerospace and automotive industries.
Vehicle carrying fuel, driver but no payload or other load.
Unladen weight
The actual weight of a vehicle or combination with no payload.

Unleaded gas
Gasoline which has no tetraethyl lead.

Unleaded gasoline
Gasoline which has no tetraethyl lead.

Unleaded midgrade gasoline
Gasoline having an antiknock index, i.e., octane rating, greater than or equal to 88 and less than or equal to 90. Note: Octane requirements may vary by altitude.
Unleaded premium gasoline
Gasoline having an antiknock index, i.e., octane rating, greater than 90. Note: Octane requirements may vary by altitude.
Unleaded regular gasoline
Gasoline having an antiknock index, i.e., octane rating, greater than or equal to 85 and less than 88. Note: Octane requirements may vary by altitude.
Unloaded radius
Unloader valve
Unmanned machinery spaces
Engine room or space fitted with sensors and controls for monitoring and responding to machinery operating conditions. This makes it unnecessary for personnel to be present in the space at all times
  1. A coined word describing a bicycle or accessory made from expensive, high-tech material. A play on unobtainable and titanium.
  2. An object that is in a place that is impossible to retrieve but continues to rattle around (e.g., a weld flashing that has come off inside a bicycle tubing)
Unpressurized vessel
A vessel for containment of fluids not subject to the definition of PRESSURE VESSEL.
Unprocessed gas
Natural gas that has not gone through a processing plant.
Indicates ‘unified’ screw threads to ‘inch’ dimensions used in the U.S. as distinguished from metric dimensions.
Unsprung mass
Unsprung weight
This refers to all parts of the vehicle that are not supported by the Suspension system, wheels, tires, etc. High unsprung weight makes suspension movement more difficult to control.

Updraft carburetor
A carburetor in which the air passes upward through the carburetor into the intake manifold.
Updraught carburetor
Abbreviation for Upholstery.
(Uph) The fabric used to cover the seats and other panels.

Upper A-arm

Upper ball joint
A mechanical pivot located at the end of the upper suspension links.
Upper Coupler
Load bearing surface on the underside of the front of a semi-trailer. It rests on the fifth wheel of a tractor or dolly and has a downward-protruding kingpin which is captured by the locking jaws of the fifth wheel.
Upper-cylinder lubrication
A method of introducing a lubricant into fuel or intake manifold in order to permit lubrication of upper cylinder, valve guides, etc
Upper dead center
Upright mount
A bicycle mount for a roof rack where both wheels remain on the bike. The bike is clamped on the frame, and both wheels are held down by straps.
Upright sitting position
Sitting straight up on a motorcycle with raised handlebars, where your hands are level with the bend in your elbows. It is the easiest and most comfortable position in which to ride a bike because no part of your body is faced with undue stress based on position. On motorcycles where the rider is leaning forward or backward, backs and hands start to ache.
  1. A widening of the diameter by pounding or compressing the ends of a pipe or rod.
  2. Turning a vehicle over because of an accident.
  1. A metal working operation similar to forging
  2. The process of axial flow under axial compression of metal, as in forming heads on rivets by flattening the end of wire.
Shifting to a higher gear. In Britain it is called upward change.
Upside-down forks
Telescopic forks on a motorcycle in which the lower section, on which the wheel is mounted, telescopes into the fixed upper tube. They are sometimes called inverted telescopic forks on earlier motorcycles
The ascending stroke of a piston, from BDC to TDC
The upward force produced when an object is immersed in a liquid or gas.
Upward change
A British term for shifting to a higher gear. In North America it is called upshift
Uranium Reactor
Urban driving
Driving in cities and towns where the common speed limit is 50 kph (30 mph) with traffic lights, etc. Also called Metro driving
Urban test cycle
A driving cycle that simulates driving in a town
Urethane foam
Type of insulation which is foamed in between inner and outer walls of a container.
Abbreviation for Uniform Road Improvement Design Standards
Abbreviation for Urban Road Maintenance District. The voters in the urban unincorporated areas of the county formed URMD in 1987 and approved funding for it in 1994. URMD’s main purpose is to provide road maintenance services for all public local and neighborhood streets within the District boundaries.
  1. Abbreviation for United States Auto-Sound Challenge
  2. Abbreviation for United States of America Cycling
  3. Abbreviation for United States Army Corps of Engineers
Abbreviation for U.S. Army Corps of Engineers
USA Cycling
(USAC) The US governing body of professional and amateur bicycle racing
Abbreviation for Universal Serial Bus.
Abbreviation for United States Council for Automotive Research
Abbreviation for U.S. Army Corps of Engineers
U.S. Department of Energy
(DOE) A department of the United States federal government, established by the Carter Administration in 1977, to consolidate energy-oriented programs and agencies. The DOE mission includes the coordination and management of energy conservation, supply, information dissemination, regulation, research, development and demonstration. The Department includes the Office of Transportation Technologies, the umbrella of the Office of Alternative Fuels.
U section
A sheet metal section often used on underbody structural members; it differs from a box section in that the fourth wall of the box is missing, i.e., its cross section more or less resembles the letter U
Used car dealer
A proprietor who sells used cars. In some instances it is part of a new car dealership
Used Car Guide
Used vehicle
Vehicle that has been previously owned or driven. Also called preowned vehicle.
Useful thermal output
The thermal energy made available in a combined-heat-and-power system for use in any industrial or commercial process, heating or cooling application, or delivered to other end users, i.e., total thermal energy made available for processes and applications other than electrical generation.
U.S. Petroleum Gallon
231 cubic inches at 15.5°C
U.S. Environmental Protection Agency
(EPA) A government agency, established by the Nixon Administration in 1970, responsible for the protection of the environment and public health. EPA seeks to reduce air, water, and land pollution and pollution from solid waste, radiation, pesticides and toxic substances. EPA also controls emissions from motor vehicles, fuels and fuel additives.
An Australian term for a light pick up style vehicle. The ute has the tray as part of the overall body whereas a pick up, that looks similar, has the ability to remove the tray. Utes are based on the passenger car frame.
  1. A privately, publicly, or cooperatively owned line, facility, or system for producing, transmitting, or distributing communications, cable television, power, electricity, light, heat, gas, oil, crude products, water, steam, waste, or any other similar commodity which directly or indirectly serves the public. Additionally, the privately, publicly, or cooperatively owned company that owns the line, facility, or system.
  2. The trade name of a trailer manufacturer.
Utility Trailer
Normally refers to light duty trailers, esp. flatbeds.
Utility truck
A pick-up truck
Utility vehicle
UV absorber
Any substance that absorbs ultraviolet radiation, then dissipates the energy in a harmless form; used in plastics and rubbers to decrease light sensitivity
UV stabilizer
Any substance that absorbs ultraviolet radiation, then dissipates the energy in a harmless form; used in plastics and rubbers to decrease light sensitivity