Every week, Exhaust Notes will pick out an industry, technical or obscure term from the automotive world and give you the lowdown on what it means and how it’s used. Already know the answer? Congratulations, you’re ahead of the game.

If you’d like an explanation of an automotive term, e-mail us at autosblog@live.com

This week’s term: Unibody.

Unibody refers to a type of vehicle construction where the vehicle's chassis and body are integrated into a single unit. It is related to -- but not exactly the same as -- monocoque construction, where the structural load is supported by the external frame of the vehicle. However, both unibody and monocoque behave similarly, and both differ from the other major type of construction, called body-on-frame.

The advantage of having the body and chassis as a single unit is greater stiffness and torsional strength (the ability to remain sound during the stress of, say, taking a curve at high speed). In a body-on-frame car, where the frame is bolted to the chassis, the various points of weakness (i.e., in between bolts) cause vibrations throughout the vehicle; this is not the case with a unibody car. For this reason, unibody/monocoque cars tend to have much better performance capabilities and fuel economy. 

However, in some cases, such as towing or supporting heavy loads, or off-road driving, such lack of flexibility is not desirable; hence, many pickup trucks and true off-road vehicles will be of body-on-frame, rather than unibody, construction. And while unibody vehicles tend to be safer in crashes (the strong external frame creates a "crumple zone"), they are also more difficult to repair, as various damaged components must be cut off and rewelded, rather than simply unbolted and replaced, as with a body-on-frame vehicle. For the same reason, rust is also a greater problem for unibody vehicles.

Another disadvantage, from a carmaker's perspective, is the inability to modify a unibody vehicle. With a body-on-frame vehicle, a manufacturer can take an existing chassis and add a different frame, making, in the eyes of consumers, a totally new car. An example of this is the new Buick Regal, which is simply a newly designed body and interior on an existing Opel Insignia chassis.