Periodically, we feel it’s worth taking the time to define some of the terms we throw around with which some readers may not be familiar. This week, we examine some of the terminology of automotive design.





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 horizontal distance between the respective wheel centers of the front and rear axles. 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 (or 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: Couple can mean a variety of different things in automotive engineering, but to designers, couple distance is the distance between the driver’s hip joint when seated (which designers call the “H-point”) and the rear axle line. 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.

Next, let’s look at some terms related to the body.

Body panels: The outer skin of most cars is usually separate from the inner body structure and consists of panels of metal or plastic bolted, welded, or glued to the inner structure. (It’s worth noting here that while each exterior panel may appear to be a single piece, it is typically an amalgamation of several smaller pieces that are welded or otherwise bonded together.) Since the body panels of most modern cars are sheet steel, a car’s skin is often generically called sheet metal, although some cars use plastic or carbon fiber for certain exterior panels, either to save weight or better resist parking lot dings. The spaces between body panels are called panel gaps or shutlines.

Hood: Since we are American, we describe the panel that covers a car’s engine compartment as the hood; English-speaking readers elsewhere in the world call this the bonnet. (Since “hood” in British usage refers to a convertible top, we sometimes elect to use the word “bonnet” to avoid confusing our non-U.S. readers.)

Deck: When we’re talking about a car’s body, the deck is the section of the body behind the rear window. On front-engined vehicles, the deck (if there is one) usually contains the cargo area, which our American readers describe as the trunk and British readers call the boot. If the deck incorporates an opening panel, that panel is known as a decklid. (The word “deck” also refers to 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, some 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.



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, some designers — and some buyers — were keen on side mounts: spare wheels mounted in the front fenders, behind the front wheels. Like the later Continental kits, side mount 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 II.



This 1936 Cadillac V-12 convertible shows off its neatly trimmed and decorated side mount spare wheel. Like many side mounts of this era, the mirror is clipped to the top of the wheel’s cover. A major drawback of side mounts 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 needed for it to turn and move up and down through its suspension travel.

Fender: The exterior body panel surrounding each wheel well. (Our British readers call this a wing; American designers sometimes refer to the rear fenders as quarter panels.) The original purpose of fenders 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 of the wheel opening. Many fender skirts are removable to facilitate 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.

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 shared its cowl with the contemporary Lincoln Continental.

Floorpan: The bottom of a vehicle’s body, generally beginning at the cowl and comprising the floor of the cabin and cargo area. On many mass-production cars, the floorpan is one big, complex 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 (or variations of it that can be made with the same machinery) for several different models.



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.

Sill: Sometimes called the rocker area, the sill is the outermost longitudinal section of each body side, running beneath the door openings from the trailing edge of the front wheelhouse to the leading edge of the rear wheel opening. The sills are important structural members, particularly in unitized or semi-unitized vehicles, and typically provide a great deal of the body’s total bending stiffness. Early unitized cars, particularly ones intended to offered in convertible form, often had large, complex sill structures.

Rocker panel: The outside of the rocker area, which may be either a separate body panel concealing the sill (and/or the frame rail beneath it) or simply the exposed, finished outer section of the sill itself. Some cars cover the rocker panel with a plastic or chrome rocker molding, either for decoration or to protect the rocker panel from stone chips and road salt.

Rub strip: A horizontal trim strip or molding running 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, although the absence of protective trim tends to result in an assortment of minor dings and scratches after a few years in the real world.

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. Center pillars located behind the driver’s seat but ahead of the rear seats (if any) 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 rearmost 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 respectively.



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 body panel covering the rearmost roof pillar.



In the sixties and seventies, there was a brief vogue for “flying buttress” roof designs, in which 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 (or fast) to describe the angle of the windshield. In the forties and fifties, customizers and hot-rodders also coined the word rake (sometimes California rake) to describe a vehicle whose tail is noticeably higher than its nose.

Greenhouse: The collective term for the roof, roof pillars, side windows, windshield, and backlight.

Beltline: The uppermost horizontal edge of the doors and body sides, below the greenhouse.

Tumblehome: In automotive terms, the angle of the sides of the greenhouse (when viewed from the front or rear) relative to the vertical plane. If the side windows are completely vertical, tumblehome is zero. Beginning around the late 1950s, American stylists became enamored of increasingly radical tumblehome as a way to make a vehicle look lower than it was. However, extreme tumblehome generally requires the use of curved glass for the side windows (which is more expensive than flat glass) and can cut significantly into passengers’ head and shoulder room.



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.

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 originally called “ventiplanes”). Note also the prominent chrome reveals around the side window and windshield.

Reveal: An exterior molding or decorative trim piece surrounding a window or other exterior body component.

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 in the interests of reducing 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 more often cosmetic than aerodynamically useful, lip spoilers are often functional. (We’ve noted that the tinier and more anemic a spoiler looks, the more likely it is to actually work, although this tongue-in-cheek “rule” obviously ignores the more complex aerodynamics involved.)

Scoop: An opening that allows outside air to pass through a body panel. A functional scoop may serve to channel air into the cabin’s ventilation system; the engine’s intake manifold, radiator, or intercooler; or the brakes. Some scoops have more than one plenum, allowing them to perform several of these functions at once, but it’s more common for a scoop to be strictly decorative.



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 provide additional airflow to intercoolers or brakes or allow hot air to leave the cabin or engine compartment. Nonfunctional fender louvers periodically become popular as cosmetic add-ons.



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 5,409 cc (330 cu. in.) DOHC V-8 with a Lysholm-type supercharger, making 550 horsepower (410 kW).

Belly pan: A smooth pan or tray mounted on the underside of the car. Since underbody components like the engine oil pan, frame rails, and exhaust pipes tend to be aerodynamically cluttered, the typical purpose of the belly pan is to reduce drag. (Many modern cars have a detachable plastic cover under the engine, which serves a similar purpose at less cost than a full pan.) On some modern exotic sports cars, the belly pan incorporates venturi to control the speed of air passing under the car, which can be used to provide negative lift (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 km/h).

FIN

NOTE

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