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.
Volkswagen Beetle chassis 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. This basic design proceeded through the pre-war
Porsche Type 60, and several wartime military vehicles, before finally the first post-war mass-production Volkswagen Beetles of 1945.
WWII military vehicles In 1936, Porsche adapted the developing KdF-Wagen 'domestic' car as a sporting car suitable for the
auto trials organised by the
NSKK. These were slow-speed competitions across off-road terrain and obstacles, which developed both driving skill and vehicle agility, in ways which were expected to be useful for military motoring. This
Porsche Type 62 was an open-topped, open-sided four seater, still with rounded bodywork. Although it performed well in military trials at
Münsingen, its appearance was considered to be 'too civilian'. Porsche continued to develop the Type 62 with such features as a rear
portal axle to give increased ground clearance and the ability to move with infantry at a walking pace. The rounded bodywork was also replaced with ribbed flat panels, with doors, and by the end of 1939 this had been accepted as the military
Type 82 Kübelwagen. The Kübelwagen was highly effective, in large part due to the light weight achieved by use of its platform chassis. It weighed only 1,510 lb, the same design weight as the road car, and was 200-300 lb lighter than its rival vehicles from
Opel,
DKW and
Adler. A
four-wheel drive version was also developed, again using the same platform chassis and basic layout, but under the civilian enclosed bodywork. This became the
Type 87 Kommandeurswagen (Porsche Type 287) and was intended for use by senior field commanders. The central tube of the chassis pan was enlarged to give space for a propeller shaft and a powered front axle provided, with similar driveshafts as the rear axle.
VW Beetle The Beetle
powertrain comprised a rear-mounted flat-four engine with a longitudinal
transaxle ahead of it. The rear suspension was by
swing axles. The first 1945 civilian Beetle used a platform chassis which stopped short of the rear wheels and which had a central stiffening tube. This tube extended rearwards as a Y-shaped fork, which passed each side of the transaxle and to the engine mounts. The rear swing axles were also held at their outboard ends by flat plate trailing arms, which were pivoted on the ends of a crosswise tube, containing the
torsion bars. At this time the rear shock absorbers were only single-acting,
lever-type. The front suspension was provided by pairs of short
trailing arms, again with transverse torsion bars mounted in two separate tubes, mounted ahead of the flat platform. These torsion bars were stacks of flat strips, and the number of leaves was changed to vary the suspension stiffness, over the production of the Beetle. An advantage for the production of the vehicle was that controls such as the driver's pedals and
brake piping, the
gear stick and the
handbrake which required connections to the engine or drive train could be installed onto the platform before the body was added. Even, until 1952, the engine
choke control and its
Bowden cable were mounted on the central tunnel. Unlike most contemporary chassis-on-frame designs though, the
steering column was mounted on the bodyshell and was not connected to the
steering box on the chassis platform until the two were joined. In the 35 year production history of the Beetle, the basic platform chassis remained largely unchanged, except for one change to the front suspension of the 'Super Beetle'.
Developments From 1970, the new
1302 'Super Beetle' adopted
MacPherson strut front suspension. This increased front luggage capacity by nearly 50%, also due to a slightly lengthened front bonnet, but the upper mounts of the struts now coupled suspension forces into the inner wing panels of bodyshell rather than the platform: the chassis was now no longer a pure platform. The 1303 'Super Beetle' continued in production until 1975, although the Economy Beetle continued in parallel with it, still using the trailing arms and platform chassis.
Beetle conversions The platform chassis, and the large number of available VW Beetles, encouraged the use of the Beetle platform as a donor car for building
kit cars. The most iconic of these was the
dune buggy: a stripped-down Beetle chassis, with the simplest
fibreglass 'bathtub' body on top of this. The first production dune buggy, the
Meyers Manx, used a shortened VW platform as a basis. The front of the chassis was cut off and replaced by a taller structure of welded square steel tube. This made the front end vertically stiffer and the suspension was replaced by one based on Chevrolet parts, although still with double trailing arms. The Meyers Manx was widely copied and most of these copies retained the full VW platform and VW front suspension. == Safety ==