From the moment the skin came off Paul's Super Seven, I was surprised by how flimsy the space frame appeared. It was poorly triangulated, had some suspension pick-up points that applied bending loads to tubes, lacked significant structure around the cockpit and indeed broke nearly every rule for space frame chassis design that my years at Autodynamics has taught me.
The others in the shop with Formula Ford experience noticed it too. Most of them had not been there during the halcyon pro-racing times, but they knew what a modern space frame looked like. I began making sketches. We all decided to build a crude fixture to measure the torsional rigidity of the original Seven.
We bolted Vallier's bare frame down to a 10 foot square cast iron surface plate by the rear suspension pickups and attached a 10' steel lever arm to the front pick-ups. Adding weight to the end of the bar enabled us to determine with some accuracy the torsional stiffness of the Seven chassis between the pick-up points (where it mattered). When it proved to be only a few hundred pounds for the first degree, I began to conjure up modifications to bring it up to speed. We tacked in tubes (in compression loading) to determine where we could gain the most rigidity with the least weight. Some of the modifications came from that exercise, others from good formula car practice I had observed over a decade of building tube-chassis race cars and alloy monocoques. We later measured a DSK chassis and got over 1600 lbs per degree. That permitted us to design a suspension that gave a softer, less jarring ride with even greater steering precision, lateral acceleration, neutrality at the limits and resistance to upset on rough surfaces.
The floor pan adds some chassis rigidity to the stock Seven, but the rest of the aluminum skin is purely cosmetic. It is not tight to the chassis, only held on by flimsy pop rivets around the edges and made of a soft grade of aluminum with no structural rigidity. It is the wrong stuff, shaped and attached the wrong way to add any strength. I do not believe that Chapman and the boys had designed the skin to take loads at all.
The suspension of Paul's Super Seven also came under scrutiny by that knowledgeable crew at Barnard Street and criticism filled the air. The front suspension pivot points were so misaligned that even without the springs installed, the rubber bushings pushed the suspension to the static, ride-height position when deflected and released. The suspension was essentially bound up in position and free to move only by deforming the rubber mounting and connecting bushings. The rear suspension was clever, but also put heavy twisting loads on the bushings, especially the center lower A-frame bushing, such that it failed frequently. I determined to redesign both suspensions, but retain the original pick-up points so that owners could retrofit the new system on the original cars without costly fabrication work.