The backbone chassis was probably the easiest chassis to draw, however it was a major departure from tradition. It is similar to a ladder chassis in that the body and associated mechanical components are bolted to it. The main difference is shown in the naming of the chassis – a ladder chassis looks like a ladder and a backbone chassis looks like a …. backbone!
Rover were the first production cars to have the backbone chassis with their 8hp model in 1904 and then other manufacturers picked up on the idea. There are a lot of web sites that claim that Colin Chapman of Lotus was the first, however Rover beat him by about 50 years! Most vehicles with this type of chassis are now heavy trucks because all the bodywork is mounted above the chassis and the mechanical components slung below. However it did become popular with kit car manufacturers because it was easy to bolt a fibreglass shell to the frame – Marcos amongst others, supplied kits with a backbone chassis.
Over the years the backbone grew in size as more power and therefore more strength was required – the original chassis were tubular or square steel sections and by the 1960s many had grown to be made of lightweight steel girders. In Italy, De Tomaso used a backbone chassis for the Mangusta which was repurposed from a racing car, the Sport 5000 (also known as the P70). However this chassis appeared to be very popular with the German and Czech manufacturers: Lloyd, Tatra, Volkswagen and Skoda all used it at some point before unitary construction became the norm. It seems to be Tatra who used it the most on both light cars and heavy trucks.
Lotus also used this type of chassis for the Elan and other cars such as the Europa with fibreglass or composite bodywork, hence people thinking it was Chapman who invented the backbone. For the Esprit, Lotus would glue the top and bottom halves of the fibreglass resin shell and then bolt it to a rolling chassis. Even TVR used a backbone chassis for their “S” Series in the 1980s – this car could be described as a compliment to the Lotus Elan – 25 years apart yet with the concepts that Colin Chapman popularised.
There have also been some “hybrid” chassis where a unitary body has been bolted to a backbone chassis. This actually helps one of the negative features of a backbone chassis – torsional twist. A human backbone is similar with the muscle structure, it can be completely rigid, yet with some movement can twist either side. Larger cars would twist a chassis like this quite quickly and so many manufacturers ended up putting spars on either side to help alleviate this issue. For small lightweight sports cars like a Lotus or TVR this wasn’t such a problem.
Advances in design, lightweight concepts and materials has meant that the backbone is not used very much these days – space frame chassis are lighter, stronger and more efficient for performance and handling.