| db |
An
abbreviation for decibel. A measurement of sound intensity typically
used to measure wind and road noise in the passenger compartment. |
|
|
| Dead
axle |
An
axle that supports a set of wheels, runs the width of the vehicle
but does not drive the wheels. Commonly found on the rear end of front
wheel drive vehicles. Also referred to as a Beam
Axle. |
|
|
| Deceleration |
The
slowing down of a vehicle, such as coasting or braking. Opposite of
Acceleration. |
|
|
| Decelerometer |
An
instrument used to measure deceleration. Often used by engineers when
measuring braking. |
|
|
| Decibel
(db) |
A
logarithmic unit used to measure sound pressure level. The dbA scale
is used to measure sound inside the passenger compartment of a car. |
|
|
| Deck |
Another
term used to describe a Trunk Lid or Rear Deck Lid of
a passenger car. The body panel that covers the trunk compartment. |
|
|
| Deck
lid |
The
lift-up body panel at the rear of a notchback passenger car that covers
and allows access to the trunk area. On some rear engine cars, it
provides access to the engine. Also referred to as the Trunk Lid,
Deck, or Rear Deck Lid. |
|
|
| de
Dion axle |
A
hybrid rear axle design that combines characteristics of a solid Live
axle and independent suspension system. Typically, a differential
is mounted to the vehicle's frame, and universal-jointed half shafts
lead to the outer assembly which consists of trailing arms which support
a hub. This system provides the ruggedness of a live axle system but
unsprung weight is reduced because the center differential is separate
from the axles. This design also allows the application of inboard
brakes (further reducing unsprung weight). |
|
|
| Deer
whistles |
Aftermarket
devices which can be mounted on the front of vehicles which manufacturers
claim use airflow to produce a tone or whistling noise at frequencies
inaudible to humans, but which are audible and annoying to animals
such as deer. When the deer hear the approaching vehicle, they scatter
thereby reducing deer/auto collisions. Also called Ultrasonic warning
devices. |
|
|
| Deflection |
The
amount a tire's radius is reduced due to the weight of a vehicle.
|
|
|
| Deflection
steer |
Undesirable
changes in a vehicle's steering or suspension geometry caused by compression
and twisting of rubber bushings and joints in the steering. Deflection
steer becomes worse when suspension parts such as ball joints and
bushings wear. |
|
|
| Defrost |
A
setting on a vehicle's heating and ventilation system that directs
warm air to the windshield and side windows to reduce and eliminate
condensation from forming on the glass surfaces. Also useful for melting
ice on the exterior surfaces of the glass. Many vehicles have a rear
window defroster. A rear defroster typically uses thin wires imbedded
in the glass, or adhered to it's surface. When electricity is passed
through the wires, they become warm and thus warm the glass, thereby
reducing condensation and melting ice. |
|
|
| Department
of transportation (DOT) |
A
federal government agency created in 1966 that oversees all aspects
of the U.S. transportation system. The National Highway Traffic Safety
Administration (NHTSA) is part of the U.S. DOT. Most individual states
have their own 'state' department of transportation system as well.
|
|
|
| Departure
angle |
The
maximum angle of a hill a vehicle can descend without getting it's
rear bumper hung up. Used to measure off-road worthiness of a vehicle.
Determined by drawing an imaginary line from the base of the rear
tire past the lowest point of a vehicle's rear overhang. The rear
bumper is usually the lowest point. A vehicle's departure angle can
be increased by installing larger diameter tires, a lift kit, custom
bumper (or no bumper), or bobbing (shortening) the rear. Also see
Approach angle, Breakover angle, Lift kit and Bobbing. |
|
|
| Depth
gauge |
See
Tire depth gauge. |
|
|
| Detonation |
An
undesirable combustion condition where some of the air/fuel mixture
auto-ignites resulting in a "pinging" or "knocking"
sound. Under normal combustion conditions the air/fuel charge is ignited
by the spark plug and burning occurs evenly throughout the combustion
chamber. During detonation, part of the compressed air/fuel charge
autoignites separately and in a different area of the combustion chamber
at about the same time normal ignition (from the spark plug) occurs.
Pressure waves from the two exploding charges clash inside the combustion
chamber creating the pining sound normally associated with detonation.
Detonation is often caused by high excessive combustion temperatures/pressures,
using low octane fuels and carbon deposits in the combustion chamber.
Detonation, also called, Preignition, can cause serious engine
damage such as holes burnt through the tops of pistons. |
|
|
|
Detroit
Locker
|
A
brand name of a popular locking differential system. Most common open
differential systems send torque to only one wheel -- typically the
wheel with the least traction. A locking differential system sends
power to both wheels regardless of traction. The advantage is reduced
wheel spin in slippery conditions and improved acceleration. Lockers
are widely used by street rodders and racer as well as in off-road
vehicles. |
|
|
| Deutsche
Institut Fur Normung (DIN) |
The
German Institute for Standards. Many measurements widely used in Europe
are translated to conform to DIN standards. For example, engine power
rating (converted from SAE HP by a factor of 1.014). |
|
|
| DeVille |
French
for "of town". Also a name for a formal bodystyle used widely
by Cadillac. The term goes back to the days of horse and buggy. A
coach was labeled "de Ville" if it had a removable panel
over the front passenger compartment which could be opened to allow
increased ventilation and light. |
|
|
| Dextron
III |
A
type of automatic transmission fluid (ATF) designed and patented by
General Motors. Used by the majority of the world's auto manufacturers
in their automatic transmissions. Automatic transmission fluid performs
three fundamental tasks: lubricates parts, distributes heat, a working
(hydraulic) fluid which operates clutches and valves inside the transmission.
Dextron III superseded Dextron II in 1995. Older vehicles which are
designed to use Type F (a Ford Motor Company automatic transmission
fluid) should not use Dextron II or III. For more info, see Automatic
transmission fluid. |
|
|
| Diagnose |
The
act of examining and analyzing the facts [by a mechanic] to attempt
to understand or explain a problem. |
|
|
| Diagnostics |
The
ability of some modern electronic systems to acquire feedback from
sensors and determine or suggest problems within its own system. Many
modern vehicle use Onboard Diagnostics which have the ability to display
system problems on their own display without the need to hook up external
diagnostic equipment. Examples include some electronic climate control
systems, electronic ride control systems and engine management systems.
|
|
|
| Diaphragm |
A
flexible membrane, usually made of thin rubber, separating two cavities
that can be used to push fluid or air, or move mechanical components.
Diaphragms are commonly found in automobile brake boosters, fuel pumps,
and carburetors. Diaphragms are also widely used in Vacuum Servos
to move heating/air-conditioning vent doors and control some cruise
control and transmission functions. |
|
|
| Diaphragm
spring |
A
flat, disc-shaped springing mechanism consisting of long finger-like
projections commonly found on clutch pressure plates. |
|
|
| Dies |
Specially
formed, hardened metal blocks used by auto manufacturers in presses
to bend or stamp flat metal into objects. Dies are used to stamp out
door panels, fenders, brackets and most metal components on a vehicle. |
|
|
| Diesel
engine |
An
engine where combustion occurs through compression rather than spark
ignition. In a diesel engine, spark plugs are not used. Air is admitted
into the combustion chamber and compressed to very high pressures.
The high compression pressure raises the temperature of the air
to the point where diesel fuel ignites at. Diesel fuel is injected
into the hot compressed air and instantly begins burning. The diesel
engine is named after it's German inventor, Rudolph Diesel.
An
important distinction between a diesel engine and a gasoline engine
is that a gasoline engine's speed is regulated by controlling airflow.
Speed of a diesel engine is controlled by regulating fuel fuel flow.
Also see Internal combustion engine and four-cycle engine.
Also
see: Liner
|
|
|
| Dieseling |
A
conditions that occurs in a gasoline engine where an engine continues
to "run-on" after the ignition has been turned off. A
form of autoignition which sounds somewhat like a diesel motor,
hence the name. It was a common occurrence on early emission vehicles
in the 1970's where ignition timing at idle was retarded -- resulting
in high cylinder temperatures and manifold pressures. Also can be
caused by excessive carbon buildup and hot glowing carbon particles
inside the combustion chamber. Also called Run-on.
|
|
|
| Differential |
A
system of gears that transfers power from a rotating driveshaft or
transmission to turn axles or other output shafts, but allows for
differentiating speeds at each of the axles, hence the name. The gears
in the rear axle of a rear-wheel-drive vehicle are commonly referred
to as the differential. A differential allows a vehicle to receive
power at it's drive wheels, yet negotiate a turn where the wheel on
the outside of the turn makes more revolutions than the inner wheel.
Differentials are also commonly used in transfer cases of four-wheel
drive vehicles where front and rear driveshaft speeds may differ (from
turning). On rear wheel drive vehicles, the rear differential is sometimes
referred to as the Pumpkin. Also see Rear-wheel-drive, Rear
end, and Four-wheel-drive. |
|
|
| Digital
instrumentation |
Electronic
gauges that use lighted numbers, rather than analog indicators (i.e.
needles) to display important engine and automotive information such
as vehicle speed, engine RPM, temperature, oil pressure, etc.. Also
see Light Emitting Diode. |
|
|
| Dimmer |
A
switch and lever system that switches between low beam and high
beam headlights. On modern vehicles the lever is positioned on the
left side of the steering wheel. When the lever is pulled back (and
pushed forward on some vehicles) it controls an electrical switch
which switches current that powers the the bright and dim headlight
circuits when the headlights are turned on. Many vehicles have an
additional mode which turns on the lights momentarily when the lever
is pulled back -- even if the lights are not turned on, for purposes
of signaling to other drivers.
Automobiles
produced in the U.S. before the mid 1970's used a floor mounted
foot switch. Also see Also see Low beam and High beam.
|
|
|
| DIN |
See
Deutsche Institute fur Normung (above)
|
|
|
| Diode |
An
small electronic component that allows current to flow in one direction
-- like a one-way valve for electricity. Can also be used to change
current from AC to DC. Diodes are found on many circuit boards in
modern automobiles such as in radios, computers, voltage regulators
and Alternators. A diode is a class of semiconductors. |
|
|
| Dipstick |
A
thin strip of metal with graduations at it's lower end that is used
to measure oil level in engines, transmissions and power steering
systems. |
|
|
| Direct
current (DC) |
Electric
current that flows in one direction. Automotive car batteries and
electrical systems operate on DC current, except for the Alternator
which outputs AC [alternating] current. Generators, used on older
(pre 1965) vehicles put out DC current. Also see Generator and Alternator. |
|
|
| Direct
drive |
A
term typically used in the automotive world to indicate direct engagement
between the engine crankshaft and a driven component. Generally
referred to being in 4th gear where the crankshaft, transmission
and driveshaft all rotate at the same speed. Direct drive is expressed
as a ratio of 1:1.
Accessories
which are driven by a belt or chain are considered belt/chain driven.
|
|
|
| Direct
injection |
A
fuel injection system that sprays fuel directly into the combustion
chamber rather than upstream in another chamber (such as intake manifold).
Relatively new to gasoline engines, but is used widely on diesel engines.
Such a system improves fuel economy in diesel engines but also makes
them sound noisier. |
|
|
| Directional
tire |
A
tire with a one-way tread pattern. Directional tires must always be
installed so they rotate in a specific direction, usually indicated
by an arrow on the sidewall. Therefore, tires cannot be rotated in
an "X" pattern, only front to back. Most directional tires
are rain or other high- performance tires. In wet weather, the rain
tires do a superior job moving water away from the tread. |
|
|
| Disc
brake |
A
type of brake system that uses a metal disc which turns with the tire,
and a stationary caliper which straddles the disc. When the operator
pushes the brake pedal, hydraulic fluid enters the caliper which forces
brake pads (made of friction material) mounted in the calipers against
the rotating disc -- thereby slowing or stopping the vehicle. Disc
brakes operate more efficiently than drum brakes because they dissipate
heat better. Disc brakes are found on the front of all vehicles sold
in the US since the 1970's. |
|
|
| Discharge |
The
flow of current OUT of a battery (opposite of charge). The term is
commonly used by mechanics to describe a condition where a short circuit,
or some other condition is causing a slow leak of a car battery, making
it go "dead". When driving, a discharge can be caused by
the alternator not functioning properly relying on battery power to
operate the ignition and other accessories (eventually resulting in
a dead battery). Most battery discharges are the result of faulty
wiring, a bad alternator, or something as simple as a loose or broken
drive belt. |
|
|
| Dished
piston |
A
piston with a depression in it's top surface. Generally found in standard
and relatively low compression engines. By varying the amount of dish
or dome at the piston's crown, piston manufacturers can offer engine
builders and street rodders a range of pistons with which to increase
or decrease an engine's compression ratio (and performance). Also
see Compression ratio. |
|
|
| Displacement |
A
term commonly used to describe engine size. An engine's displacement
is the total volume inside the cylinder (when the piston is at the
bottom of it's travel), multiplied by the total number of cylinders.
Expressed in cubic inches (ci), cubic centimeters (cc) , or liters
(l). |
|
|
| Distribution
block |
A
device that splits an input current or grounds into two or more output
currents or grounds. Used to wire up accessories. Can also contain
fuses. |
|
|
| Distributor |
An
ignition component used with older ignitions systems that distributes
energy (spark) to the spark plugs. The distributor basically consists
of a shaft within a housing with a gear on one end which is inserted
into the engine and driven by a gear on the camshaft. At the top of
the distributor shaft is either an electronic pickup coil, or a multi-lobed
device which opens set of contact points which cause current to be
induced in the ignition coil. Since the shaft is rotated by the camshaft
(which is rotated by the crankshaft) the distributor is timed to engine
speed. A distributor cap and rotor are mounted at the top of the distributor.
As current from the coil enters the top center of the distributor
cap, a rotating rotor (connected to the distributor shaft) passes
by a terminal on the outside of the cap and sends the spark to the
spark plug associated with that terminal. |
|
|
| Distributor
cap |
A
replaceable plastic device mounted at the top of an engine's distributor
that distributes energy to each of the spark plugs. Contains evenly
spaced metal pins for attaching spark plug wires and conducting current.
The metal conducting pins are usually made of brass or aluminum. |
|
|
| Dive |
The
tendency of a vehicle's front end to drop during braking. The condition
can become more pronounced when front brakes become weak or glazed
(more braking is done by rear brakes), or from worn out shocks or
struts. |
|
|
| Dodge |
An
automobile company, now part of the Dahlmer-Chrysler Corporation founded
in 1914 by John and Horace Dodge, two brothers who were bicycle parts
producers |
|
|
| Dogtrack |
A
condition in which the vehicle goes down the road at a slight sideways
angle under. Viewed from behind, during normal straight line driving,
the front wheel tracks to one side of the rear wheel, sometimes by
as much as several inches in severe cases. Caused by misalignment
of the axles or a bent frame. Most common occurrences is with a solid
axle, rear-wheel-drive vehicle where the leaf spring has become bent
(due to a collision) or the rear axle has crept forward on one side
of the leaf spring due to loose U-bolts or a worn or broken leafspring
locating pin. |
|
|
| DOHC |
Dual
Overhead Cam. A term used to describe an engine design where two camshafts
are used and both located in a single cylinder head. One camshaft
operates the intake valves in that head, the other it's exhaust valves.
The advantage is increased responsiveness. In a "V-shaped"
DOHC engine, a set of camshafts would be used for each head (four
in total). Also referred to as Double Overhead Cam, Twin Cam and Quad
Cam. |
|
|
| Domed
piston |
A
piston with a raised crown. Also see Dished Piston (above) and Compression. |
|
|
| Door
edge guards |
Thin
metal or plastic strips usually in the form of a channel which fit
over the rear edges of doors to protect against paint chipping when
the door contacts other objects (i.e. other car doors, walls in the
garage, etc.). May be shiny chromed plastic or polished stainless
steel or painted to match body color. Reducing chipped paint also
reduces corrosion from occurring at the chips. |
|
|
| Door
handle |
An
assembly on the interior or exterior of a car or truck door that operates
it's latching mechanism. Although many styles and types exist, most
exterior door handles are of the lift-up type, and most interior door
handles are pull-back levers recessed into the door panels. Rods run
from the handles to the door latch assembly which is nearly always
mounted at the back edge of the door near the striker (the post the
door latches to). Also see Striker. |
|
|
| Door
jam |
See
Jam. |
|
|
| Door
latch |
An
assembly mounted inside the rear edge of doors which fastens to the
striker bolt and holds the door securely shut. For safety, automobiles
sold in the U.S. have two-stage latches. The first stage, latches
with almost no closing effort. The second stage latches when the door
is fully closed. Also see latch. |
|
|
| Door
hinge |
A
hinge, usually made of metal that allows the doors to swing open and
close. |
|
|
| Door
hinge pin kits |
An
inexpensive replacement kit available from dealers and aftermarket
sources which includes door pins and bushings needed to rebuild a
door hinge. Door hinge pins and bushings wear out over time and use.
Replacing the worn parts restores the 'tight' feel when closing the
door and reduces sagging doors and eliminates air leaks around side
windows. |
|
|
| Door
panel |
A
vinyl, cloth or plastic covered interior panel which covers the inside
of the door and reduces interior noise. In addition, many door panels
contain an armrest and switches for power doors, windows and mirrors. |
|
|
| Door
skin |
A
stamped sheetmetal or a fiberglass panel fitted to a door frame
that makes up the exterior body panel of the door. Door skins are
folded over at the edges of the door frame and welded or glued in
place during the manufacture of the vehicle -- then painted body
color. The skins themselves are not very strong nor rigid until
attached to the door.
A
damaged door may be salvageable and only require a new door skin.
Although not replaceable with the ease of, say, a front fender,
a damaged door skin may be cut off the door frame by a skilled technician
and replaced with a new skin which can be purchased through most
manufacturer's parts departments. 'Skinning' a door is an acceptable
autobody procedure and can save owners hundreds of dollars in parts
and labor costs because door skins are relatively cheap, can be
installed without having to remove door and window hardware (unlike
replacing an entire door shell), plus, the technician doesn't need
to paint up the entire door, just the outer skin and part of the
door jam. Also see Door Jam (above).
|
|
|
| DOT |
See
Department Of Transportation |
|
|
| Double
A arm |
A
common front suspension design where two unequal length and non-parallel
suspension arms, shaped like a capitol "A" are used for
each front wheel. Produces a very rugged suspension system but one
that is heavier and with more parts than the MacPhearson strut design.
Double A-arm suspension design is widely used on pickup truck and
full-sized sedan front suspensions. |
|
|
| Double
clutch |
A
driving technique used frequently by racers where the driver presses
the clutch and shifts out of gear into neutral, then releases the
clutch about halfway and increases engine speed, then pushes the clutch
again and shift into gear. The purpose is to prevent transmission
gears from clashing, particularly gears which are unsynchronized.
This technique can be used when shifting up or down through the gears. |
|
|
| Double
cone synchronizers |
A
type of synchronizer setup used in some manual transmissions that
promotes smoother shifting and increased transmission life. Two synchronizers
are used on the synchronizer cone (rather than the typical 1). |
|
|
| Double
overhead cam |
Same
as Dual Overhead Cam. A term used to describe an engine design where
two camshafts are used and both located in it's cylinder head. One
camshaft operates the intake valves in that head, the other it's exhaust
valves, the advantage being increased responsiveness. In a "V-shaped"
DOHC engine, a set of camshafts would be used for each head (four
in total). |
|
|
| Double
wishbone suspension |
A
common independent front suspension design where two unequal length
and non-parallel suspension arms in the shape of a "Y" (resembling
a wishbone) are used for each front wheel. Similar to Double A
Arm. |
|
|
| Downdraft
carburetor |
A
carburetor where the air flows downward through the carburetor and
into the intake manifold. Another arrangement is the sidedraft carburetor
which mounts horizontally to an engine and draws the air in sideways
relative to engine position. Most carbureted vehicles use a downdraft
style carburetor. |
|
|
| Downforce |
Downward
force on a car caused by downward air pressure. Commonly associated
with race cars. The use of special aerodynamic designs and spoilers
is often used used to increase downforce which can be used to increase
a race car's traction, handling and cornering capabilities. |
|
|
| Downshift |
The
process of switching a transmission from a higher gear to a lower
gear. |
|
|
| D-pillar |
The
fourth vertical pillar from the front in an automobile that supports
the roof. Usually only found on the rear of station wagons and long
sport utility vehicles or minivans. Also referred to as a D-post. |
|
|
| D-post |
See
D-pillar (above). |
|
|
| Drafting |
A
racing technique where a race car follows directly behind another
race car to reduce drag and improve speed. By becoming aerodynamically
"one", both cars can reduce their aerodynamic drag compared
to what it would be if they were separated by several car lengths.
The lead car must "cut" through the air, but it benefits
from reduced draft by reducing turbulence normally produced at it's
tail end. The trailing vehicle benefits by not having to "slice"
through the air. Racers also use this technique to conserve fuel thereby
reducing their number of pit stops |
|
|
| Drag |
Air
resistance on a forward moving body -- such as your car or truck.
There are three components that affect aerodynamics: Drag, caused
by a vehicles' shape and protruding objects such as mirrors, Air friction
on the vehicle's surface, and airflow through the car or truck (for
ventilation purposes). Vehicles with greater aerodynamics (less air
resistance) get better fuel economy and often benefit from improved
road manners. |
|
|
| Drag
coefficient (Cd) |
A
measure of aerodynamic efficiency. Factors such as a vehicle's shape,
protrusions such as mirrors and trim, and airflow through and around
the body all determine the amount of air resistance and aerodynamic
drag on a vehicle. Coefficient of drag (Cd) is measured in wind-tunnel
test or by a technique called coastdown (letting air resistance slow
a coasting vehicle and measuring distance). |
|
|
| Dressing |
A
chemical, usually a spray that can be applied to tires or an engine
compartment to make components shine and appear new again. Used by
auto dealers. |
|
|
| Dress-up
kit |
See
Chrome dress-up kit. |
|
|
| Drift |
A
term used in racing to describe a vehicle in a controlled four-wheel
slide. Race drivers apply the throttle and use careful steering to
drift around corners, thereby enabling them to negotiate the corner
faster. |
|
|
| Drip
molding |
A
decorative molding, typically made of stainless steel which fits over
the vehicle's drip rails. Typically found on older cars and trucks
with higher trim levels. |
|
|
| Drip
rail |
An
uplifted lip, or channel found on the edges of an older vehicle's
roof. The lip serves as a channel that captures water running off
the roof and channels it to the rear of the vehicle. Prevents water
from dripping on the side windows. Often absent from newer vehicles
to reduce drag and wind noise. Many newer vehicles use a groove in
the roof or specially designed rubber gaskets which capture and redirect
water running off the roof. |
|
|
| Driveability |
How
well an engine starts, runs and responds to throttle changes on the
highway. Special attention is given to how well an engine performs
when cold, at steady speeds and during acceleration. Should be smooth
and steady. |
|
|
| Drive
axle |
An
axle that transmits power to a drive wheel On most vehicle's one axle
is the primary drive axle, even though each axle has the ability to
transmit power. Also see Star wheel. |
|
|
| Drive
belt |
A
circular, flexible belt-like device typically made of fiberglass-reinforced
rubber threaded tightly around engine pulleys to drive engine accessories
such as the alternator, power steering pump, air-conditioning compressor
and emission devices. Older vehicles typically used several "V"
shaped belts, but these days manufacturers mostly use a single flat
"serpentine" belt which contains many ribs (grooves) to
drive all accessories. Many overhead cam engines use a flat (cogged)
belt to drive the camshaft off the engine's crankshaft. |
|
|
| Drive-by-wire |
An
electronic accelerator that uses electronic means to operate the accelerator.
Found on some modern vehicles, there is no linkage that runs from
the pedal to the throttle. Instead, a position sensor, mounted at
the accelerator pedal, sends a signal through a wire to the vehicle's
computer which sends a signal to a small electric motor placed on
the motor which opens and closes the throttle valve. |
|
|
| Driveline |
See
Drivetrain (below) |
|
|
| Driveshaft |
A
metal shaft containing universal joints used on rear-wheel-drive and
four-wheel drive vehicles to transmit power from the transmission
or transfer case to the drive axles. Also called the Propeller
Shaft or just Prop Shaft. |
|
|
| Drivetrain |
All
the power transmitting components of a vehicle used to transmit power
from the engine to the road. Includes clutch, transmission, driveshafts,
universal joints, differentials, axles, wheels and tires. |
|
|
| Driving
lights |
Lights
used on the front of a vehicle that are always operational when the
vehicle is operated. Used as a safety measure to signal other drivers
of their presence. Also, special high intensity lamps that can be
switched on at night that cast a further beam than the vehicle's high
beam lights. |
|
|
| DLR |
See
Daytime Running Lights (above) |
|
|
| Drop-down
pitman arm |
A
pitman arm, usually aftermarket, that allows correct steering geometry
to be retained after a suspension lift has been installed. When installed
to the steering box, the dropped pitman arm is further away from steering
box than the stock pitman arm. The lower distance compensates for
the distance the vehicle has been lifted. |
| Drop-step
bumper |
See
Step bumper. |
| Drum
brake |
A
type of brake system that uses a round drum-shaped device as one of
it's key components. In a drum brake system, the drum is mounted to
the axle or hub assembly and rotates with the wheel. Special curved
brake shoes fit inside the drum. When the driver pushes the brake
pedal, pressurized hydraulic fluid rushes into a wheel cylinder which
forces the brake shoes (special friction elements) outward against
the inside of the moving brake drum -- which slows or stops the vehicle. |
|
|
| Dry
sump |
An
oiling system used on race cars where the oil is not contained in
the engine's oil pan or crankcase, but rather in another container
outside the engine. Race engine often employ such a system because
it permits the use of lower profile oil pans allowing the engines
to be installed lower in the chassis, thereby reducing a car's center
of gravity and improving cornering. Also, the forces of extreme cornering
on a traditional oiling system can cause oil in an oil pan to slosh
to one side creating a condition where an engine may draw air rather
than oil into it's oil pump which would lead to a ruined engine. |
|
|
| Dual
braking system |
A
brake system found on all modern vehicles that incorporates two pistons
in the master cylinder that each control the braking of a pair of
wheels on separate hydraulic circuits. The system is designed for
safety, to prevent total loss of braking should a fluid leak develop
in one part of the brake system. Some vehicles use a diagonal system
where one front and rear wheel (on the opposite side) are hooked together.
In a worse-case scenario, the wheels on the other part of the brake
system would be unaffected and provide braking should a leak in another
part develop. |
|
|
| Dual
exhaust |
An
exhaust system frequently used on V-engine equipped vehicles where
two exhaust pipes and mufflers are used rather than one. With most
exhaust systems, the exhaust from each bank of cylinders is routed
into one exhaust pipe. In a dual system, the exhaust from each bank
of cylinders goes into a separate pipe with separate mufflers. The
advantage is reduced back pressure and increased power. |
|
|
| Dual
overhead cam (DOHC) |
A
term used to describe an engine design where two camshafts are located
in a cylinder head. One camshaft operates that head's intake valves,
the other it's exhaust valves, the advantage being increased responsiveness.
In a "V-shaped" DOHC engine, a set of camshafts would be
used for each head (four in total). Also referred to as Double Overhead
Cam, Twin Cam and Quad Cam. |
|
|
| Dual
point ignition |
A
type of mechanical ignition system used in some high-performance applications
where two sets of ignition points are used in a distributor, rather
than one set, which is the most common arrangement. The result is
more precise timing control. Also used in engine that contain two
spark plugs for each cylinder. |
|
|
| Dually |
A
trade name used by General Motors to describe it's heavy-duty pickups
with dual rear wheels. Used colloquially to describe any pickup with
dual rear wheels and flared wheelwells or bedsides. |
|
|
| Dumb
irons |
A
term applied to a part of vintage automobiles. The portion of the
frame that protruded in front of the body where the leaf springs attached
to. |
|
|
| Durability |
The
length of life, quality, or sturdiness of a component. How well something
wears, or holds up over time. |
|
|
| Duration |
The
length of time that a rotating camshaft lobe holds an intake or exhaust
valve open. Typically measured in degrees at the crankshaft. Longer
duration allows more time for air or exhaust gasses to enter or exit
the combustion chamber. Camshaft manufacturers offer camshaft which
have different degrees of duration for high-performance applications.
|
|
|
| Dwell |
An
ignition timing term associated with older vehicles that used traditional
point-type ignition systems. The length of time, expressed in degrees
of distributor rotation that the breaker points are held closed. |
|
|
| Dwell
angle |
See
Dwell (above) |
|
|
| Dykes
ring |
An
L-shaped piston ring. The design reduces the phenomenon known as "flutter"
-- slight movement of a conventional piston ring on a piston caused
by engine acceleration and deceleration. Flutter allows some compression/combustion
gas leakage between the ring and the piston. Also see Piston ring,
Compression ring, Oil ring and Ring groove. |
|
|
| Dynamic
balancing |
The
act of balancing a part such as a tire or crankshaft while in rotation.
Dynamic balancing is a more precise way to remove vibration than static
balancing. Also see Spin balancing, Bubble balancer and Blueprinting. |
|
|
| Dynamometer |
A
machine that measures the power and torque of an engine at it's flywheel.
A chassis dynamometer measures the power of a vehicle at it's drive
wheels. The vehicle is actually driven up onto the machine and it's
wheels turn a drum which measures the power. Often called a Dyno. |
|
|
| Dyno |
See
dynamometer (above) |
