Racing-derived like their wire-wheeled predecessors, this is the generic name for any non pressed-steel, metal wheel - the single most important piece of automotive jewelry.

They are cast or forged in aluminum alloy, with a protective, usually silver-painted finish, which replaced the unsuccessful polished and lacquered treatment of the 70s (illus.Citroen CX). Darker paint finishes are sometimes selectively machined back to the bare metal to add contrasting 'sparkle', while brake dust can also add a little useful detail definition. The simple use of distinctive gold paint on the Subaru Impreza and Renault's Clio Williams, guaranteed instant recognition.

The importance of wheels to a car's appearance and stance is not lost on the manufacturers, who always list a range of larger-diameter optional-extra alloys, and also fuels an enormous aftermarket industry.

Landmark, timeless - and now ubiquitous - designs include Bugatti's beautiful 1924 'Type 35' (whose 8-spoke castings brilliantly incorporated the brake drums) and the 1960s 'Minilite'. Notable designs also included the 1970s faux alloy 'Rostyle' (found on MGB models) and the American 'Wolfrace'.

An armature is sometimes called a 'buck', but this can get confused with 'seating buck'.

Clay is heavy, expensive and temperature-sensitive, and if applied too thickly, it is prone to cracking under its own weight. For these reasons, clay models of any scale are built up over a supporting armature of wood and foam. In full-size models, they will also have a steel ‘chassis' with axles - road wheels aiding both reality and maneuverability.

The armature's dimensions will be based on those taken from tape and package drawing information, making allowance for a sufficient layer of clay for sculptural maneuver.

Generally, sketches, which may initially take creative liberties (particularly with proportion), will eventually be required to acknowledge the constraints of the package drawing and of reality.

Concept sketches will be free of such inhibiting constraints. They will be used to stimulate creative design ideas and directions and to propose novel solutions. Alternatively, they may be sketches that demonstrate the conceptual ideas behind the design, its influences or form language.

They may also be commissioned for publication, and as such, are likely to have been 'worked up' in Photoshop, and might be considered to be relaxed, high-impact renderings.

See also sketch

Conventional doors have to obey certain rules: hinge points must be in a vertical plane to avoid having to open a door 'uphill' against its own weight, or being dragged 'downhill' by it. 'Double parallel' hinges enabled the huge doors of the Renault Avantime to be opened in 'normal-sized' spaces, but provided the manufacturers, Matra, with an assembly nightmare.

But there are other forms of door, some which allow much more visual freedom and delight.

Gull Wing Doors

Hinged along the roof and resemble a bird's wings when opened upwards, these first appeared on the 1952 Mercedes-Benz 300SL racing cars. They were a logical German engineering solution to the problem of providing access yet still preserved the integrity of the tubular spaceframe's necessarily high and wide sills. The resulting visual delight was purely incidental...

Top-hinged doors reappeared on the 1974 Bricklin and the infamous 1981 DeLorean, and with variations: the 1991 Toyota Sera and the 2003 Ferrari Enzo both have one hinge in the roof and the other at the base of the A-pillar, while the Mercedes SLR McLaren has 'swing wing' doors with both hinges on the A-pillar.

Doors hinged at their rear, or trailing, edge. In the 1920s and 30s, it was quite common for all doors to be hinged from the B-pillars. Occasionally forced open by air pressure created by the forward movement of the car lead to their name, these were subsequently banned.

Suicide doors have been recently re-introduced in the rear of 'extended cab' pick-up trucks, the Mazda RX-8 and the Honda Element. Like the 1937 Lancia Aprilia and 1961 'Kennedy' Lincoln Continental, the RX-8 has no B-pillar (legislation is met by clever and robust engineering). The completely open body sides offer unrivalled access to the rear seats of a coupe.

BMW has coined the more elegant term 'coach doors' for its current Rolls-Royce models.

These doors pivot expensively upwards on a single hinge, aided by a gas strut. Though they solve the problem of opening a very long door in a confined space, they create another - that of turning a low-slung sports car into a garage roof-scraping one.

Scissor doors first made production on the 1971 Lamborghini Countach, were retained for its replacement, the 1990 Diablo, and have become a Lamborghini trademark.