In its pure form a car's space frame should look like this --- the need is to resist relative rotation, created mostly by the suspension, between the points in the front plane and those in the parallel rear plane

In its pure form a car's space frame should look like this --- the need is to resist relative rotation, created mostly by the suspension, between the points in the front plane and those in the parallel rear plane. If working with tubing this is best done by a rectangular box formed by twelve tubes, with a diagonal across each of its six flat planes. Torsion in the structure as a whole resolves itself into tension and compression forces in the various tubes.

There's a lot of barbs aimed at the Locost (and Lotus Seven) frame for not being proper space frames. In truth, it's hard, if not impossible, to find a true space frame in any car chassis. The power plant and occupant's needs just require too many compromises to retain all the needed elements to make a proper frame. Looking at our frame if we cut out the top diagonal, so it can be occupied by a motor or person, when torsion is applied the upper corners are free to move. If you need to see this effect just take a shoebox and twist the ends.

Like most car frame designs the Locost is a "three box" or maybe four, if you see the front suspension bracing as creating a separate box. The frame in front of the cockpit can be closed up with "Y" or removable diagonals, but the cockpit is harder to deal with. Not that there haven't been those who have tried. One of the most successful attempts was the 1954 Mercedes Benz W196. http://georgecushing.net/w196.JPG

This 80 pound chassis had a torsional stiffness of 4000lbft/ deg. Compare this to the 1200-1400lbft/deg. Stiffness of the Locost. Note the triangular cockpit side boxes and the six tubes bracing the top of the foot well. Similar framing is found on the MB300SL "Gullwing". http://georgecushing.net/300SL.JPG

The '54 300SL got its nickname from its unique doors which hinged upward from the roof's centerline. The reason for the doors' design was the wide and high doorsills necessary to preserve the torsional stiffness of the space frame around the cockpit opening. The sill appears to be higher than that of the Locost and is certainly more complex. The foot boxes and engine bay are stiffened by a structure that has 12 tubes merging in one point! The SL was succeeded by the 300SLR (roadster) and things got a little more complex as the sills had to be lowered for the door openings.

It looks like MB replaced some of the complex foot box tubing with sheet metal boxes as the truss-like dash structure of the 300SL has disappeared. http://georgecushing.net/300SLR.JPG

The Lotus 25 F1 car chassis was essentially a Type 24 with the tube chassis replaced by the stressed skin "monocoupe" chassis. The 25 only had 2400lbft/deg. torsional stiffness, but compared to the 24's 1000lbft it was quite a improvement. Within a couple of years a F1 monocoupe was up to 11,000lbft. http://georgecushing.net/Lotus25.JPG

Notice the ring frame at the mid-point of the cockpit. This structure was introduced in the 1960 Type 18 F1 car to eliminate tubing at the rear bulkhead of the front space frame box. Here it was carried over to the monocoupe chassis. A similar ring frame forms the rear bulkhead. These frames were made by welding sheet metal to inner and outer rings of tubing.

The photo shows such a frame in the Type 30 sports racer, which used an Elan type backbone frame. In turn design elements of the 30 showed up in the later Europa.

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