CAR CHASSIS DESIGN

Car Chassis Design



Car Chassis Design

The tunnel becomes a primary load bearing member. This is a potentially fine design, and if we were building a new car from scratch, we would seriously consider a backbone. But , this is not a new car, it's a replica of a classic! Because it is designed around the original Ford engines (and we wanted our customers to have several different transmission choices), the bulk of a compatible structural tunnel was unacceptable, especially considering the passenger compartment was a fairly narrow one to begin with. A backbone would make it impossible to maintain the look of the original interior and engine compartment. It would also create servicing difficulties.

A variation to the sheet metal backbone is one that uses small tubes to create the central structure. TVR's Griffith was built like that - with an enormous tunnel. The Shelby Daytona Coupe added a tubular backbone to the original 289 chassis. It probably added 50% to the overall stiffness of the car! See below.

Space frame: A true space frame has small tubes that are only in tension or compression - and has no bending or twisting loads in those tubes. That means that each load-bearing point must be supported in three dimensions. It is nearly impossible to build an efficient space frame around the Cobra body. The rockers are simply too shallow, and the tunnel shaped incorrectly to make a reasonably triangulated structure.

It had rockers 12 inches tall and 10 inches wide and the chassis used hundreds of separate tubes. It was difficult to build and a nightmare to fix. The "space frame" chassis that is currently built for another replica simply uses smaller tubes, many carrying bending and torsional loads. It may look impressive, but functionally it's a bad compromise. Simply more complication without improvement.

Consider - the bending stiffness of a tube increases the by the square of the diameter of the (equal-wall-thickness) tube, and the torsional stiffness by the cube of the diameter, while the weight goes up linearly. The bottom line is - sometimes you're better off with a large tube.

An airplane (with a stressed outside skin) is close to a true monocoque. In the automotive world, it's time to compromise again, but the street car that compromises the least is probably the 1958 Lotus Elite. The design was made possible by the use of large fiberglass panels - otherwise the tooling and construction costs would have been tremendous. In the real world, the interior panels are stressed, but many cars have an aerodynamic facade of 'glass or aluminum.

The original GT40 - and our ERA GT - have a semi-monocoque chassis. The heaviest (steel) main panel on our ERA GT is only .045" thick, and most panels are only .032"! Reinforcements are required at the suspension points where there are local high loads. With the rockers 10" high x 9" wide, the net result is an incredibly stiff structure. But you can't build a classic roadster like ...