Body in white
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Body in white or BIW refers to the stage in automobile manufacturing in which a car body's sheet metal components have been welded together. BIW is termed before painting & before moving parts (doors, hoods, and deck lids as well as fenders), the engine, chassis sub-assemblies, or trim (glass, seats, upholstery, electronics, etc.) have been assembled in the frame structure.
The name derives from manufacturing practices before steel unibody monocoques — when automobile bodies were made by outside firms on a separate chassis with an engine, suspension, and fenders attached. The manufacturers built or purchased wooden bodies (with thin, non-structural metal sheets on the outside) to bolt onto the frame. The bodies were painted white prior to the final color.
A folk etymology for Body in White suggests the term derives from the appearance of a car body after it is dipped into a white bath of primer (undercoat paint)— despite the primer's actual gray color. This could also refer to when car bodywork would be made of timber - all timber products, furniture etc, are considered to be "in the white" when at the stage of raw timber before finishing/varnishing.
In car design, the Body in White phase refers to the phase in which the final contours of the car body are worked out, in preparation for ordering of the expensive production stamping die. Extensive computer simulations of crash worthiness, manufacturability, and automotive aerodynamics are required before a clay model from the design studio can be converted into a Body in White ready for production.
Factories may offer BIW cars to racers, who then may replace up to 90% of the car with aftermarket parts. Frequently racers must apply to purchase one of these cars.