Although Chapman didn't invent the monocoque structure, he definitely did successfully adapt it to the single-seater race car and initiated the "monocoque era" in Formula I - which continues to be the design standard in today's Grand Prix cars. Rather than being a traditional chassis, the monocoque was more a stressed-skin fuselage of the type used by the avaiation industry.

But Chapman wasn't thinking of airplane fuselages when he designed his first monocoque chassis. Instead, he was trying to adapt the backbone chassis of the prototype Elan to a single-seater race car. He reasoned that it would be possible to separate the sides of the backbone far enough apart for the driver to sit between them. These sides would be boxes themselves and could double as fuel tanks. The chassis would resemble a tub structure with the two D-shaped sides composed of monocoque pontoons linked by a curved undertray and three bulkheads - at the driver's bay/engine bay interface, an instrument panel frame, and at the front suspension mounts. The rear extensions of the monocoque pontoons provided engine mounts to which the V-8 Coventry-Climax was bolted. Photo: Clark & Chapman & some of the monocoque's components.

The bare monocoque chassis weighed just 65 lbs and had 1000 ft-lb/deg of twist as compared to the space frame Lotus 24 with 700 ft-lb/deg and weighing in at 82 lb. However, with engine installed the torsional stiffness of the car - the Lotus 25 - rose to 2400 ft-lb/deg, about twice that of rival space frame Formula I cars like the Ferrari and Brabham. On the fast curves the monocoque car was not appreciably quicker that the other cars, but on medium and tight corners, the monocoque's stiffness, which allowed for a softer suspension, resulted in much higher speeds. With Clark driving in 1963, the Lotus 25 won 7 of the 10 Grands Prix, with 1 second place and 1 third place. Chapman considered this car to have the cleanest lines of any car he had produced. A fully-dressed Lotus 25 is shown in the photo below.

Given the performance superiority of this design, the other Formula I manufacturers soon adopted this technology. Lotus and McLaren are credited with advancing the monocoque concept by replacing aluminum as the construction material with composite materials (e.g. kevlar) in 1977. These provided a stronger chassis with less weight. And that is the current state of chassis design today - composite monocoque structures.