| U-bolts |
Bolts
in the shape of a "U" with threads on each end. Used to
attach the rounded axle shafts to leaf springs |
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| U-joint |
Shortened
for Universal joint. See Universal joint. |
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| Ultimate
cornering force |
The
maximum cornering side force produced by the tires when driven around
a skid pad at the fastest speed without losing traction and skidding
out. |
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| Ultrasonic
warning devices |
See
Deer whistles. |
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| Undercoating |
A
protective coating sprayed underneath automobiles to prevent rust
from forming. Undercoating is generally a thick rubberized or
tar-like substance that is applied to areas such as the floor
pans, inside fender wells and along rocker panels where rust occurs
most. Also used inside doors and other body panels to reduce noise
in the passenger compartment. Can be applied at the factory by
the manufacturer or afterwards by dealers or autobody shops.
In
some cases undercoating can actually promote rust. When it becomes
old and peels away or begins to flake off it can trap moisture
and road salt (snow belt regions) between it and the very metal
it is supposed to protect.
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| Under
hood light |
A
small lamp assembly mounted beneath the hoods of many vehicles.
A tilt switch is usually integrated into the lamp to switch it on
when the hood is raised, and off when lowered. |
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| Underslung |
A
design used on vintage automobiles where rear leaf springs attached
to axle from below the axle housing. Allowed the vehicle to be lower
to the ground yet still allow good leaf spring articulation. |
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| Undersquare |
A
term that describes an engine where the cylinder bore is smaller
than the length of the piston stroke. Also see Oversquare. |
| Understeer |
A
handling term that describes an automobile that resists turning;
wants to "push" straight ahead through a turn and thus
requires additional steering as speed is increased around a corner.
Too much understeer is undesirable, but some understeer is built
into the suspension and steering of all automobiles because it lends
to more predictable handling. Also see Oversteer and Slip angle.
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| Unequal-length
A-arm suspension |
A
popular suspension design that consists of two unequal-length non-parallel
A-shaped arms at each wheel. The wide end of each A-arm attaches
to the chassis and the narrow end attaches to a steering knuckle
via ball joints. This allows suspension flex while providing good
lateral support. A coil spring can be mounted either above the upper
A-arm or above the lower A-arm. This was the predominant front suspension
design used on virtually all U.S. vehicles since the 1950's until
front-wheel-drive and MacPhearson strut technology became more popular
in the 1980s. |
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| Unibody |
See
Unitized construction (below). |
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| Unidirectional
tires |
Tires
designed to rotate a specific direction. Unidirectional tires have
a tread pattern designed for optimum grip, water channeling and
handling when run in only one direction. Cannot be rotated to different
sides of a vehicle. Also see Directional tire. |
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| Uniform
Tire Quality Grading (UTQG) |
A
tire quality grading system where the U.S. Government requires
manufacturers to test tires under specific conditions and label
them so consumers can easily compare tire quality between different
manufacturers. Test include tread wear, traction and resistance
to heat generation.
Traction:
There are three traction grades A (highest), B and C. Traction
grades represent the tire's ability to stop on wet pavement. Tests
are conducted under strict, controlled conditions on specified
test surfaces. Cornering is not tested.
Tread
wear: The rate of tread wear as determined from tests run on a
special test course under controlled conditions. Number such as
100, 150 and 200 would mean that a tire with a rating of 200 would
wear twice as well, or last twice as long as a tire with a rating
of 100.
Temperature:
There are three temperature grades A(highest), B and C. Represent
a tires resistance to heat generation and how well it dissipates
heat. Tested on a special test device indoors under controlled
conditions.
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| Unitized
construction |
An
automobile chassis which is made of many sheetmetal parts welded
together in such a way to make one complete, rigid structure which
doesn't require a separate frame. Unitized frame construction is
typically cheaper to produce and lighter and more rigid than a separate
body-on-frame design. However, problems resulting from rust damage
can be more of a problem than with a traditional frame. |
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| Universal
joint |
A
type of movable joint commonly used on drive shafts that allows
power to be transmitted to shafts that run at different angles.
Necessary for driveshafts to allow the driveshaft to move during
suspension flex. A universal joint consist of a cross-shaped heavy-duty
metal "spider" with bearing caps on each end. The bearing
caps fit into yokes on the driveshaft and differential mount. The
downside of universal joints is they cannot be run at large angles.
During each revolution, the driven shaft and driven yoke (mount),
speed up and slow down twice with each rotation. That is why lifted
trucks exhibit driveline vibration. Constant velocity joints can
be run at greater angles and are used on front wheel drive cars
and rear drive shafts when short driveshafts are used and wheel
travel is greater. Most universal joints require periodic greasing. |
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| Unleaded
gasoline |
Gasoline
which does not contain tetraethyl lead or any lead additive. See
Lead-free gasoline. |
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| Unsprung
weight |
The
weight of parts of a car which are not supported by the car's
springs such as wheels, brake parts, rear differential and axles,
suspension parts, springs, anti-sway bars and shocks.
Low
unsprung weigh is desirable when designing an automobile because
softer shocks can be used and bumps will not upset handling as
much. Plus it allows engineers to improve handling characteristics.
Ways to reduce unsprung weight include using independent rear
suspension which reduces the weight of suspension members and
allows the differential to be mounted to the body (sprung), alloy
tires, torsion bar suspension, using alloy brake calipers or moving
brake inboard.
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| Updraft
carburetor |
A
seldom used carburetor design where the air/fuel mixture flows upward
into the intake manifold and cylinders. |
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| Upshift |
Changing
transmission gears from a lower gear (numerically higher ratio)
to a higher gear (numerically lower ratio). |
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| Urethane |
A
type of plastic which is commonly used for suspension bushings
and other applications an automobile. Urethane suspension bushings
are typically used in high-performance applications because they
are stiffer than rubber bushings which increase handling and responsiveness
but also lead to increased vibration and road noise transmitted
to the passenger compartment.
Urethane-based
paints are the latest and most widely used type of automotive
paint due to it's increased durability and resistance to chipping
and oxidation over lacquers and enamels.
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| Utility
box |
An
aftermarket assembly with many doors and storage compartments that
can be installed in place of a pickup truck box. Used by electricians
and utility companies to store tools and supplies. Often, when companies
replace trucks, the utility boxes are removed and installed on the
new trucks. |
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| UTQG |
See
Uniform Tire Quality Grading (above). |
