A crossmember is a structural section of steel, usually boxed, that is bolted across the underside of a monocoque / unibody motor vehicle

Motor vehicle

A motor vehicle or road vehicle is a self-propelled wheeled vehicle that does not operate on rails, such as trains or trolleys. The vehicle propulsion is provided by an engine or motor, usually by an internal combustion engine, or an electric motor, or some combination of the two, such as hybrid...

, to support the internal combustion engine

Internal combustion engine

The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

 and / or transmission

Transmission (mechanics)

A machine consists of a power source and a power transmission system, which provides controlled application of the power. Merriam-Webster defines transmission as: an assembly of parts including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a...

. For the suspension of any car to operate as it should, for proper handling, and even to keep the body panels in alignment, the frame has to be strong enough to cope with the loads applied to it. It must not deflect, and it has to have enough torsional strength to resist twisting.

A "K" member is a crossmember in a vehicle with a longitudinally-mounted engine, contains the engine mounts.

In early Ford vehicles

An "X" crossmember can usually be found on the frames of vintage Fords, particularly those made between 1928-31 (Model A) as these cars had limber stock chassis.

Although it used more substantial rails than a Model T, the Model A

Model A

Model A may refer to:* Ford Model A , a model of car built by the Ford Motor Company* Ford Model A , a model of car built by the Ford Motor Company* One of the letter-series models of Farmall tractors...

frame was still just a simple "ladder" design. Unlike the '32 frame, which had a K-member, and '33 to '48 frames, which were equipped with a substantial X-member, the A-frame was only fitted with front, center, and rear crossmembers.