First, let's review some basics.

Axle: This is a term that has several related but distinct meanings. An axle is any set of two or more of a vehicle's wheels that rotate around a common axis. The beam or assembly connecting those wheels is also called an axle. Of course, many modern vehicles have independent suspension, with no direct physical connection between the wheels. In such vehicles, the term axle is still applied in a more abstract sense, to mean the imaginary line between the centers of the left and right wheels.

Wheelbase: The distance between the center of a wheel on the car's front axle and the center of the corresponding wheel on the rear axle. On passenger cars, that distance is usually -- but not always -- the same from right to left. The wheelbase quoted in a car's specifications is measured with the car stationary and unladen; depending on the suspension geometry, the actual wheelbase may change as each wheel moves through its range of travel. A car's wheelbase is a critical dimension in many respects, affecting ride quality, maneuverability, and passenger space. In general, a longer wheelbase benefits ride quality (because it lowers the frequency of ride motions) and passenger room (by allowing more legroom), but reduces maneuverability (by spreading the car's mass over a longer distance, thus raising its polar moment of inertia). Conversely, a short wheelbase provides better maneuverability at the cost of a choppier ride and less useful interior room.

Overhang: Front overhang is the distance between the front axle and the front end of the car; rear overhang is the distance between the rear axle and the rear end of the car.

Overall length: The distance from the front of the vehicle to the rear, including bumpers. A vehicle's overall length is equal to the sum of the wheelbase, the front overhang, and the rear overhang.


In the illustration above, A is the front overhang; B is the wheelbase; and C is the rear overhang. Naturally, overall length = A + B + C.

Track or Tread Width: A vehicle's tread width (track) is the distance between the horizontal centers of the left and right wheels on each axle. The track widths of the front and rear axles are often slightly different, so a car's specifications will usually list front and rear track separately. (The terms "track" and "tread width" are often used interchangeably, but "track" has become more common, to avoid confusion with the width of each tire.) As with the wheelbase, the track widths shown in a car's specifications are static measurements; depending on the suspension layout, track may change as the car's wheels move through their suspension travel.

Couple: In automotive design terms, couple is the distance between the driver's hip joint when seated (which designers call the "H-point") and the rear axle. A close-coupled car is one in which this distance is very short. The modern BMW Z4 roadster, which has the cabin pushed so far back that the driver is practically sitting on the rear axle, is a close-coupled design.

Next, let's define a couple of terms the describe how components are positioned inside the vehicle.

Transverse or Longitudinal: Naturally enough, transverse means sideways, while longitudinal means lengthwise. In a car, if the component is parallel to the axles when seen in plan view (that is, when viewed from overhead), it is transverse. If it is perpendicular to the axles, it is longitudinal. With automotive engines, a transverse engine is mounted so that its crankshaft is parallel to the axles, while a longitudinal engine (also called a north-south engine) is mounted so that its crankshaft is perpendicular to the axle.

Leading or Trailing: The edge of a component nearest the front of the vehicle is called the leading edge; the edge furthest from the front of the vehicle is the trailing edge. This can be a little confusing when applied to suspension components, because they're described in terms of their relationship to the body, rather than the wheel. A suspension link that connects to the wheel behind the point where it connects to the body is called a trailing link (or trailing arm). A suspension arm that connects to the wheel ahead of the point where it connects to the body is called a leading link.

Now let's look at some terms related to the body.

Body panels: The outer skin of most cars is usually separate from the internal structure, consisting of panels of metal or plastic bolted or glued to the structure. Since the body panels of most modern cars are sheet steel, a car's skin is often generically called sheet metal, although aluminum and composite panels are becoming increasingly common.

Although each body panel may appear to be a single piece, it is often made up of several smaller pieces, which are then welded or bonded together. Designers often use chrome or plastic trim to disguise the resulting seams.

The panel gaps between body panels, incidentally, are sometimes called shutlines.

Hood: Since we are American, we describe the panel covering a car's engine compartment as the hood; English-speaking readers elsewhere in the world call this the bonnet.

Deck: When we're talking about a car's body, the deck is the section of the body behind the rear window. On most front-engined vehicles, the deck area contains the cargo area (trunk, to our American readers, boot to others). Stylists call the trunk lid the decklid.

In an engine, the deck is the upper surface of the engine block, where the cylinder head mates to the block. Customizers also use "deck" as a verb, referring to the removal of all exterior trim from the deck and decklid.


On some cars, there is a gap between the base of the rear window and the leading edge of the decklid; for obscure reasons, stylists call this area the "Dutchman."

Continental kit: A spare wheel mounted externally on a car's deck or rear bumper, often with a cover painted the color of the body and embellished with chrome. The term is derived from Edsel Ford's original 1939 Lincoln Continental, which established it as a styling feature after the war.


Buick did not offer a Continental kit as a factory option in the fifties, but various dealers and aftermarket companies offered accessory kits to add one.

In the twenties and thirties, buyers preferred sidemounts: spare wheels mounted in the front fenders, behind the front wheels. Like the later Continental kits, sidemount spares frequently had body-colored covers, for a more streamlined look. By the late thirties, they were considered increasingly archaic, and they fell out of favor just before World War 2.


This 1936 Cadillac V-12 convertible shows off its neatly trimmed and decorated sidemount spare wheel. Like many sidemounts of this era, the mirror is clipped to the top of the wheel's cover. A major drawback of sidemounts was that moisture would pool in the well under the wheel, promoting corrosion.

Wheel well or wheelhouse: The area in each corner of the car's body that contains the wheel -- and the clearance for it to turn and move up and down through its suspension travel.

Fender: A fender is the body panel surrounding each wheelhouse. (Our British readers call this a wing.) Its original purpose was to keep the wheels from throwing mud, water, or dirt onto the windshield and interior. A fender skirt is a trim panel that covers part or all of the wheel opening. It is usually removable, to allow the wheel to be removed for tire changes or other maintenance.


In the thirties and forties, automotive fenders became progressively more streamlined and integrated into the body. By the late fifties, they had almost completely "melted" into the surrounding body. Fender skirts, however, have never quite caught on, perhaps because they complicate tire changing.

Floorpan: The bottom of a car's body, which forms the floor of the cabin and the trunk. On most mass-production cars, the floorpan is one big steel stamping. Since this is usually the largest and most expensive single piece of the body, it's common for manufacturers to use the same floorpan for several different models, so they can amortize its production costs.

Cowl: The portion of the body structure below the base of the windshield, to which the front fenders are attached. In a front-engine car, the cowl incorporates the firewall, the panel that separates the engine compartment from the cabin. The cowl usually contains the car's heating and ventilation system (which on modern cars generally draw their air from a plenum at the base of the windshield). The cowl is a major structural element, and is typically the second-largest (and most complex) single piece of the body. Manufacturers will often share the same cowl structure between several vehicles. For example, the 1961-1966 Ford Thunderbird shares its cowl with the contemporary Lincoln Continental.


Until the early fifties, many cars had pop-up vents in the cowl, at the base of the windshield. Modern cars still take their ventilation air from this area, although usually through a grille just behind and below the trailing edge of the hood, in the same area as the windshield wipers.

Rocker panel: A longitudinal body panel on each side of the car, beneath the doors. The rocker panel usually conceals the outer frame rail. Some cars cover the rocker panel with a plastic rocker molding, either for decoration or to protect the rocker panel from stone chips and road salt.

Sill: The bottom area of each door opening, between the rocker panel and the floor of the cabin. The width of the sills has a significant effect on the ease of entry and exit. C4 Corvettes (the 1984-1996 generation) had particularly wide sills, which, combined with their low roof and low-set seats, made clambering in or out a cumbersome operation.



Rub strip: A horizontal trim strip or molding across the door and sometimes the inside of the front fender to protect against dings and scratches from the doors of other cars. In recent years, it has become popular to omit the rub strips for a cleaner appearance. (Cleaner it is, until you spend a few months in the real world, which usually leaves the doors of such cars sporting a sad array of battle scars.)



Roof pillars: The vertical pillars (or posts; the terms are used interchangeably) that support a car's roof. The forward pillars, which also support the windshield, are called A-pillars. If a car has center pillars, behind the driver's seat but ahead of the rear seats (if any), they are called B-pillars. The rear pillars, behind the cabin, are called C-pillars. If the car has an additional set of pillars behind the C-pillars, such as the rear pillars of a station wagon (estate car), they are called D-pillars. ("Post" may be used interchangeably with "pillar" in this sense.)



In the fifties and sixties, there was a great craze for pillarless hardtops, which had no B-posts. Some manufacturers offered hardtop coupes, sedans, and even station wagons. Even if a vehicle has no B-pillars, the pillars behind the cabin are still called C-pillars and D-pillars.


Although true pillarless hardtops are rare today, many cars still strive for a hardtop look by concealing the B-post behind a darkened section of the rear quarter window. This early-nineties Dodge Stealth shows off a typical "blackout pillar" treatment. Note that the front windows have no frames, adding to the effect.

Sail panel: The sail panel is the body panel covering the C-pillar.


In the sixties and seventies, there was a brief vogue for "flying buttress" roof designs, where the sail panels extended further back than the roof pillars themselves. This is still occasionally seen on mid-engine cars, although it has fallen out of favor for most ordinary passenger cars.

Rake: Stylists use the term rake to describe the angle of the windshield. In the forties and fifties, customizers and hot rodders also coined the word rake (or California rake) to describe a car whose tail is noticeably higher than its nose.

Tumblehome: In automotive terms, the angle of a car's side windows when viewed from the front or rear. If a car's side windows are completely vertical, its tumblehome is zero. Since the late 1950s, American stylists have been enamored of radical tumblehome because it makes the roof look lower. Radical tumblehome poses a number of practical problems, however. It generally requires the side windows to use curved glass, which is more expensive, and it can significantly reduce head and shoulder room for passengers.


Chrysler's early-seventies "fuselage styling" theme emphasized some of the industry's most extreme tumblehome. The sides curve inward so sharply that the top edge of the side window sits practically above the driver's shoulder.

Beltline: The upper edge of the doors and body side, just below the side windows.

Greenhouse: The car's upper body, above the beltline, including the windows, roof pillars, and the roof itself.



Light: Stylists sometimes refer to a car's windows as "lights." For example, small windows between the roof pillars and the car's side windows are sometimes called quarterlights (or quarter windows). The rear window is called the backlight. A car with three windows on each side (for example, two regular windows and a quarter window) is called a "six-light" design, while one with only two windows per side is called a "four-light" design.


A 1967 Pontiac Firebird shows off its front quarterlight (which GM called "Ventiplanes"). Note also the prominent chrome reveals around the side window and windshield.

Reveal: The exterior trim or molding around a car's windows is called the window reveal.

Drip rail: A molding U-shaped channel at the outer edges of the roof, above the doors, that channels rain water away from the side windows.

Modesty panel: A body panel below the front or rear bumper that hides the ends of the frame or body structure.

Airdam: A modesty panel under a car's front bumper that controls the flow of air under the car. The main purpose of the airdam is to force air to flow around the car, rather than under it, to reduce aerodynamic drag and lift. On many modern cars, the airdam also channels air into the radiator, the engine's air intake, and sometimes the front brakes.


Most modern cars have airdams, but few are as aggressive as that of the Mitsubishi Lancer Evo. The extended lip also functions as a spoiler.

Spoiler: A device that improves a car's aerodynamics by "spoiling" aerodynamic drag, lift, or both. Small lip spoilers are commonly incorporated into the airdam and mounted on (or molded into) the rear decklid. Although spoilers and wings are frequently tacked-on decoration, lip spoilers are often functional. (As a general rule, the tinier and more anemic a spoiler looks, the more likely it is to actually work.)

Scoop: An opening that channels outside air into the car. Functional scoops may serve to channel air into the engine's intake or intercoolers, the cabin's ventilation system, or brakes. Of course, fake scoops are a common form of decoration.


Mid-engine cars, like this Lotus Elise, typically have a formidable array of functional scoops to channel cooling and intake air into the engine.

Louver: A slot or vent that either admits or exhausts air. Working louvers may admit air to intercoolers or brakes, or exhaust hot air from the engine compartment or cabin. Nonfunctional fender louvers have recently become an annoyingly popular styling feature.


The Ford GT shows off its array of engine cooling louvers. The GT has a great deal of heat to dissipate; under the cover is a 5.4 L (330 cu. in.) DOHC V8 with a Lysholm supercharger, making 550 horsepower (410 kW).

Belly pan: A smooth pan or tray mounted on the underside of the car. The belly pan's typical purpose is to cover underbody components in order to reduce drag. On some modern sports cars, it also incorporates venturi to control the speed of air under the car. By speeding up underbody air flow, the venturi lower the air pressure under the car, creating downforce.


The slots under the rear bumper of this Ferrari F430 are diffusers for the underbody venturi. Although the F430 has no wings and only a modest lip spoiler under the nose, the underbody channels enable it to produce more than 600 lbs (2,670 N) of downforce at 185 mph (300 kph).

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