Platform chassis

A platform chassis is a form of vehicle frame / automobile chassis, constructed as a flat plate or platform, sometimes integrating a backbone or frame-structure with a vehicle's floor-pan.

A platform chassis is a separate chassis for a car or small vehicle. It is distinguished from other forms of chassis by being arranged as largely a single flat steel sheet, usually with additional box section stiffeners.

Platform chassis developed after the ladder chassis or cruciform chassis designs in the 1930s, once car bodies began to be pressed from large steel sheets, rather than the chassis assembled from rolled channel sections. As a semi-monocoque they are still a form of body-on-frame construction, rather than a monocoque or unibody where the bodyshell and chassis are integrated into one component. Although both body and platform chassis are each made from similar pressed steel panels welded together, they were often bolted as the final two units and so may still be separable after construction.

Achieving sufficient rigidity is difficult with a pure platform and so they are usually extended vertically with some form of box or tube section. This can be either a perimeter frame (e.g. Renault 4) or a central spine (e.g. VW Beetle). Longitudinal rigidity is more critical than transverse rigidity and so this stiffening is mostly as front to back girders, rather than crosswise. The diaphragm sheet is usually pressed into shallow stiffening ridges as well, but these are mostly to stop drumming noise and are too minor to provide major structural strength.

Platforms are usually the full width of the car, but shorter and only span the length between the wheels. Suspension attachments for the wheels and the weight of the engine and transmission are carried on additional subframes beyond this. These subframes may be formed of box section tubes welded on to the chassis, deep-drawn box sections formed from sheet, or as separate subframes that are bolted in place.

Platform chassis have been used for both rear-wheel drive (Beetle) and front-wheel drive (Renault) layouts. However they keep the engine at the driven end, with a transaxle, rather than using the propeller shaft of the Hotchkiss layout of front-engine, rear-drive that was universal with chassis designs beforehand. This avoided transmitting the propeller shaft torque through the platform. Where a propeller shaft has been used, these have been for rare examples with four-wheel drive.

Attaching the suspension to a platform chassis requires independent suspension and encourages the use of suspension features such as torsion bars or trailing arms. As the chassis is in a single plane, it would be difficult to use a solid axle, without raising the entire platform above the axle line. It is also difficult to arrange spring attachments for springs such as half-elliptic springs requiring distance fore and aft of the axle line, or for coil spring or strut suspensions needing an attachment point raised vertically above the platform. Instead, springs such as torsion bars are used. Whether these are transverse or longitudinal, they lie in the horizontal plane of the platform and so can be worked in to such a chassis. Other torsion-based systems, such as twist-beam rear suspension, may be used as well for front-wheel drive cars with light rears.

The Volkswagen Type 1 Beetle used a platform chassis, with independent torsion bar suspension at each end. The 'Volkswagen', 'Peoples' Car' or 'KdF-Wagen' project began in 1933 and by 1934 Porsche had sketched out a design for a rear-engined, platform chassis car with four seats and torsion bar suspension. A central stiffening tube provided much of the strength of the chassis.