While purists might consider anything but full-frame vehicles a splinter in the eye of automotive engineering, the fact is that monocoque (French for "single shell") chassis have been around nearly as long as the automobile itself. While the unibody (aka "unit body") didn't reach a state of any real respect until fairly recently, it made up for the time lapse by quickly becoming the predominant chassis type today.
Unibody construction got its start not on the streets, but in the skies. You can think of a unibody chassis like a football; it's essentially hollow on the inside, deriving all of its structural rigidity from its skin and outer body. Frame chassis are more like a house, where the frame is the foundation and everything on top is just there to make the foundation useful.
Frames come in three basic varieties. Perimeter frames are the most common, consisting of two long framerails that connect the front and rear axle assembly. A central or "spine" frame connects the front and rear with one central truss running along the middle of the car; you'll often find these under lightweight sports cars where centralized mass is a priority. Tube frames are the third basic type, and you'll generally find them underpinning top-echelon race cars. A tube frame is basically a cage that serves both as a structural member and as a roll cage to protect the driver. Frame construction is almost unilaterally stiffer than comparable unibody construction, but it's also far heaver and often more expensive to produce. However, this extra weight and cost is an acceptable trade-off for vehicles that regularly see heavy loads or hard use, since constant twisting will weaken and eventually break the welds holding a unibody together.
Unibody construction didn't really take off until the 1980s and 1990s, when computer simulation technology allowed engineers to precisely calculate stresses on the chassis. Prior to computer simulation, unibody construction was a matter of trial-and-error or guess-and-hope. Unibody cars aren't typically monocoques in the sense that an airplane is. Airplanes may or may not have any kind of internal construction, so removing any one outer panel could actually cause the whole plane to disintegrate. Unibody cars generally have a front subframe that holds the engine and front suspension and a rear subframe that holds the rear suspension. These subframes are in fact sub-assemblies that bolt to the car's chassis, and those sub-assemblies act as major supporting members in the chassis. In that sense, unibody cars are basically perimeter-frame chassis that use the car's body to join the front and rear subframes.
There's a lot of crossover between unibody and frame-type construction, enough so that separating the two types may prove difficult. Many new unibody cars use some sort of internal reinforcing tubes hidden in the door posts, front and rear extremes and roof pillars. Even though those reinforcing beams are technically part of the body, the way that they distribute load in the chassis makes the arrangement more functionally similar to a tube frame than a true unibody. Strut tower braces that reduce chassis flex at the strut towers effectively act as a partial tube frame, while subframe connectors -- long trusses that run under the car and connect the front and rear subframes -- effectively turn a unibody car into a type of full-frame vehicle. Bolt-in roll cages often do more than protect the driver, they help to keep the chassis from twisting under load. After bolting in a roll cage and a pair of strut tower braces, a unibody only needs a set of bars to connect the two in order to act as a full tube-frame assembly.
- Photo Credit Kim Steele/Photodisc/Getty Images