2.1 Design analysis  (Page 2/5)

There were several generations of bonnet used. Initially two were supplied depending on the choice of engine. Lotus Twin Cam powered cars had a flat bonnet with a bulge on the left hand side ( [link] ) to accommodate the twin Weber carburetors and the associated air box ( [link] ). Cars fitted with the Ford 1300 cc or 1600 cc crossflow Kent motors used a single down draft Weber carburetor, and their bonnets needed a bulge on top for clearance of the air filter ( [link] ). Later on a generic bonnet was designed that allowed either type of engine to be used.

While the Series 4 Seven’s front wings were of the clamshell type first seen on the Seven America, as opposed to the cycle wing style favored by the Series 1 and 2 cars, the front wings extended all the way to the rear. The advantage of this design was that the passenger on right hand drive cars (see [link] ) or the driver on left hand cars (see [link] ) did not risk burning their ankles on the hot exhaust while entering or exiting the car. The front wings were attached via bolts to the steel side panels. Support for the wings was by a curved steel stay.

The body panels were originally made from self-colored (pigmented) resin fiberglass composite. The color pigment was mixed with the gel coat. While this resulted in a simple finish it did limit the available colors and meant that many S4 Sevens were (or have been) repainted. The original colors included: white, red, light blue, yellow, bright orange, and the very seventies lime green. It should be noted that the self-coloring route predated the use of the technique in the Lotus Esprit of 1976.

The overall look of the Alan Barrett designed body was a cross between the traditional seven look and the type of beach buggy that was in vogue in the 1970’s. Traditionalist bemoaned that the Seven had gone soft. However, one positive result of the new body was that the truly horrible aerodynamic efficiency of the Seven was improved. For example, the rear wings (fenders) were elongated giving a more egg-shape profile. The Series 2 and 3 Sevens (and the subsequent Caterham derivatives) are among the cars with the highest drag coefficients (C _D = 0.6). To put this in perspective, a family saloon routinely achieves a C _D of 0.3. While exact numbers are not known for the S4 it is considered a significant step in the right direction.

The suspension and steering

The front suspension

The front suspension of the Seven S4 uses the links, anti-roll (sway) bar, and bushes from the Lotus Europa. This had two advantages:

1. The parts were cheaper and readily available in the factory parts bin.
2. Abandoning the Lotus 12 type front suspension (in which the upper wishbone was actually comprised of a single top link and the anti-roll bar) meant that the Seven had a true double wishbone front suspension for the first time.