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Underfill

A depression on the weld face or root surface extending below the adjacent surface of the base metal.

Underinflation

A condition with respect to tire inflation that decreases the rolling diameter and increases the contact area, which results in excessive sidewall flexing and tread wear causing excessive heat buildup, ultimately leading to severe tire damage.

Undersize (US)

A part of item that is smaller than that specified.

Underslung Suspension

A suspension system in which the spring is positioned under the axle.

Understaging

A vehicle placed at the beginning of a drag race, behind the normal starting point.

Understeer

A condition where the front of the vehicle tends to break out and slide toward the outside of a turn.

Undertread Compound

A blend of synthetic rubbers used in the material under the tire tread.

UNEP

An acronym for United Nations Environment Program.

Unequal A-Arms

A suspension system where one arm is larger than the other.

Unequal Length Control-arm Suspension

Front suspension-control arms of unequal length to compensate for jounce and rebound.

UNF

An acronym for Unified national fine. Identifying a standard thread pitch and count.

Unfair Advantage

Not really "unfair", the term given for that extra margin realized by a racing team with careful planning and attention to detail.

Unglued

An engine or transmission that has blown and is destroyed.

Unibody

A short term for unit body.

Unicast

A component or part cast as a single unit.

Unicast Rotor

Type of rotor used on man late-model cars having the rotor and hub cast as one integral component, often from gray iron rather than cast iron.

Unidirectional Rotor

A rotor with cooling fins that are curved or formed at a angle to the hub center to increase cooling airflow. Because the fins only work properly when the rotor turns in one direction, unidirectional rotors cannot be interchanged from right to left .

Unidirectional Tread Pattern

A tire tread pattern that can be rotated only in one direction. Thus left to right rotation cannot be done.

Uniform Pitch Spring

A valve spring having uniformly spaced coils.

Uniform Tire Quality Grading

Information required by the Department of Transportation to be molded into the sidewall of each tire to provide information relating to tread wear, traction, and temperature ratings.