- UN
- Indicates "unified" screw threads to "inch" dimensions used in the U.S. as distinguished from metric dimensions.
- Unbalance
- A condition of wheels which are not Balanced.
Also see
- Unburnt hydrocarbons
- Pollutant (basically petrol vapor) released into the atmosphere as an exhaust gas, due to incomplete combustion
- UNC thread
- Acronym for Unified National Coarse thread; one of the screw threads used on British cars.
Also see
- Underbody
- 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.
Also see
- 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 structure
- The structural members and flat panels of the bodywork on the chassis. Also called substructure
- Undercoating
- 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.
- Undercure
-
- A condition in which the vulcanization or curing has not been adequate.
- Degree of cure less than optimum. May be evidenced by tackiness, softness, off-color, or inferior physical properties of an adhesive.
- 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.
- Underdrain
- 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.
- Underfloor mid-engine
- A design, introduced on the Toyota Previa minivan, where the engine is located below the passenger compartment, between front and rear
- Underinflation
- Insufficient air pressure in a tire for the amount of load carried. Tires will wear on both outer sides of the tread.
- Underpowered
- 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
- Underride
- 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.
- Underseal
-
- A rustproof coating with sound-deadening properties applied to the underbody. Also called Underbody sealing compound
- To apply underseal.
Also see
- Undershield
- 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
- 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
-
- 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.
- 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.
- Underslung frame
- A frame design of the pre-war era whose characteristic feature is that the frame members run below the axles
- 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.
- Understeer
- 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.
Also see
- Undertray
- The bottom panel of multi-layer or complex underbody panel assemblies, which faces the road surface
- 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.
- 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)
- Unicycle
- 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 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
- Union Cycliste International
- (UCI) International governing body of professional and amateur bicycle racing
- Uni-servo brake
-
- A servo brake with one single-end wheel cylinder and two self-energizing brake shoes
- 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.
- 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
-
(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
Also see
- Universal spark plug socket
- A spark plug socket with universal joint for reaching hard-to-get-at spark plugs
- UNJ
- 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.
- 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.
- 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
- Unobtanium
-
- A coined word describing a bicycle or accessory made from expensive, high-tech material. A play on unobtainable and titanium.
- 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.
- UNR
- Indicates "unified" screw threads to "inch" dimensions used in the U.S. as distinguished from metric dimensions.
- 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.
Also see