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U

U Bolts: U shaped bolts used to fix leaf springs to the axle or other similar tasks such as fixing a steering rack to the body.

U/J: Universal Joint.

Ultra Violet: A frequency of light that is above the visible spectrum.

Unburned Hydrocarbons: Engine or fuel tank emissions in the exhaust gas, fuel line vents or lubricating oil vapours. Harmful to health. Also HC or THC (total hydrocarbon content of exhaust gas).

Understeer: Steering condition of a turning vehicle when the vehicle attempts to turn by a lesser degree than the steered position taken by the driver. This condition is usually provided by vehicle design in order to improve the driveability of the vehicle.

Underwritten: Sign and accept liability (legal).

Unequal Wishbones: Suspension type with an upper wishbone shorter than the lower wishbone. The geometry of this layout keeps the wheel camber reasonably constant during suspension movements. Also SLA suspension.

Unidirectional: Move in one direction only.

Unidirectional Clutch: A clutch, used in starter motors for example, to only allow drive to pass in one direction.

Union Nut: A hollow nut used to secure hydraulic pipes.

Unit-body Construction: Vehicle body and chassis constructed as a single unit. As distinct from a separate chassis and body construction. Also unitary body construction.

Unitary Body Construction: Vehicle body and chassis constructed as a single unit. As distinct from a separate chassis and body construction. Also unit-body construction.

Universal Joints: Rotating shaft coupling that permits angular axial displacement. Some types will also operate with linear misalignment of axes.

Unladen: Not carrying a load.

Unleaded Petrol: Petrol without any added lead to improve the octane rating.

Unsprung Mass: The part of the vehicle not supported by the suspension springs. The road wheels, hubs, brakes and part of the suspension system.

Unsprung Mass (Weight): The part of the vehicle not supported by the suspension springs. The road wheels, hubs, brakes and part of the suspension system.

Up Draft: A carburettor where the choke tube (venturi) is fitted vertically and the air enters from the bottom. Upwards movement of air.

Upper Beam: The upper of the two headlamp beams. Also driving beam and main beam.

Upper Control Arm: The upper and shorter wishbone in an independent suspension system.

UV: Ultra violet.