The Hyperlogos

The Body of an automobile is what gives it shape, style, and structure. When we think of a car, the part of the car with which we identify is the body. It holds all the parts together and provides hopefully-aerodynamic surface for minimizing drag, assuming that is what you are aiming for. It provides security and comfort for passengers, again, assuming that is a design goal.

Types of Auto Body

There are basically two primary types of auto body, although the argument could be made for a third, and there are numerous examples of hybrids. The two major types are full frame and unibody.

Full Frame

Full frame vehicles are so-named because they have an underframe known as a chassis running the length of the vehicle. This is the only commonly used method of construction for half-ton and larger trucks, and in fact nearly every pickup truck made has been full-frame. Cars started to go unibody in the 1970s, although there are some examples of older vehicles (notably Nash and Volkswagen cars) which went to a unibody before that.

Full frame vehicles are heavy and cumbersome, and they provide new and interesting problems with body part alignment because their various constituent parts are anchored by the frame. However, they can provide a rigidity not often seen in unibody vehicles, and are generally easier to fix. When a unibody is damaged it may actually transmit some of the force to more or less any area of the car, and thus a relatively small accident can warp the entire body. Full-frame vehicles not only typically do not have this problem, but the frame is easier to straighten and individual pieces of the car can be moved around, shimmed, and otherwise adjusted to fit a tweaked frame.

Unibody

Cars which are made with unibody technology are so-called because the entire frame and body of the car is a single welded unit. Today this is generally made out of 100% high strength steel, but this trend did not really pick up until the late 1980s. The use of high strength steel and the monocoque design of a unibody means that you can make cars much lighter. There are of course drawbacks to unibody vehicles; a unibody is much harder to repair than a full-frame vehicle and the force of a serious impact may leave no part of the car undeformed.

Unibody cars are however typically much safer than full-frame vehicles because they generally are designed with crumple zones which absorb the force of an impact because they are more willing to deform than the rest of the vehicle. By spreading the force of the impact out over time, the maximum force exerted at any given time is reduced.

Variations and other models

In between the full frame and the unibody is the stub frame. It can be used in the front or the rear of a vehicle, but is usually used in the front. In a stub frame, the center of the vehicle is a unibody, but ordinary frame "rails" extend from the unibody, usually in front. A traditional full-frame sort of front end (consisting of inner and outer fenders, a firewall, a core/radiator support, and a hood) is attached to the stub frame.

Another option is the backbone chassis. Primarily used by Lotus for small, light performance vehicles, this design uses a boxed monocoque "backbone" member down the center of the car. The driver and passenger essentially sit in "tubs" with a groove between them, which straddles the backbone.

Finally, many racing vehicles use a tubular frame. This basically looks like a convoluted rollcage, and provides all of the structure for the car. Aerodynamics are provided by wrapping this in a shell of some sort. Most racing cars use a fiberglass and/or carbon fiber shell, though those which tend to bump (like American Super Modifieds) usually use flat planes of sheet metal.

Parts and Terminology

Auto body has a whole set of obscure terminology all its own. It may help to divide the car into three sections; hood, body, and trunk. Naturally, not all vehicles have all of these areas.

Major Auto Body Components: 1: Bumper Cover; 2: Hood; 3: Fender; 4: A-Pillar; 5: B-Pillar; 6: C-Pillar; 7: D-Pillar and Sail Area; 8: Rear Bumper Cover; 9: Torque Boxes

Taking a look at a typical unibody sedan, with four doors, first of all, the entire welded portion of the car is known as the unibody. This typically includes everything but fenders, hood, doors, trunk, and bumpers, which bolt on. Taking the area including the hood, we call it the "front end" for reasons which should be obvious. The very first part of the typical car is either the bumper, or bumper cover. Vehicles which look like they don't have a bumper in fact DO have a metal bumper underneath a bumper cover, typically made out of polyurethane plastic. Behind the bumper and cover is a vertical piece of metal called the radiator support or core support. The radiator, headlights, and other front-end equipment is mounted to the core support, and it has various holes in it to accomodate them. The core support lines up the fenders in front. Most vehicles in the world have a piece of styrofoam between the bumper cover and the bumper; in the US the vehicle is required to survive a direct bumper collision with a total of 5 mph of speed without damage.

Moving backwards, we get to the main body of the car which contains the cockpit. This begins with the firewall, which is a vertical separation between the engine compartment and the cockpit. Above this is the cowl, which is where the vents in front of the windshield are located. When the vehicle is moving, air pressure builds up against the windshield and forces air into the vent system (if it is open.) Below the radiator support, and angled back towards the rear of the car is the toe pan. This is the place where the pedals are located. The floor of the car is the underpan. The roof of the car is, well, called the roof.

A typical sedan has three pillars supporting the roof. The ones to the left and right of the windshield are known as the A-pillars. This is incremented as you go toward the back of the car no matter how many pillars it has. The one in between the doors is the B pillar, and the one at the back is the C pillar. If the C pillar is a wide area, as it usually is, this is known as the sail area. At the bottom of the first and last pillars we have a structure called a torque box which is where the pillar meets the frame rail built into the unibody. This is the most structurally sound portion of the vehicle.

The tops of the doors, also the bottom of the windows, are located at the belt line. The creased line which almost always appears below this on the side of the car is a style line and it not only gives the body more shape and thus style, but also makes the body rigid along that line, preventing vibration. The roof is just made out of curved sheet metal, and supporting it are several roof bows which run from side to side. There are usually several of them.

1966 Chevy Nova II (underside): You can clearly see the kick-up area above the rear axle in this picture, just in front of the fuel tank (black).

The lid on the trunk of the vehicle is called just that, the trunk lid. Above the wheels on the inside of the vehicle you can see the humps of the spaces built to accomodate the wheels. This is called the wheel house. In a vehicle with a fixed rear axle (either live or dead) or a torsion bar rear suspension, there is a raised area between the wheels known as the kick-up area. The outside area of the auto body above the wheels is not called the rear fenders, although on some older vehicles (like the VW beetle) there actually are rear fenders, marked by the fact that they can be removed without a cutting torch (or a can opener.) On unibody vehicles without rear fenders, the area is known as the quarter panel. Finally, the rear bumper and bumper cover work out the same way as the front.