Loading on an automobile chassis/body can be quite complicated. There are weights to be supported like the body and chassis, engine, drive train, fuel tank, occupants and sundry other items not only in their static condition but also in their dynamic condition due to breaking, acceleration, and turning. There are suspension loads in the verticle, fore-aft, and side directions. There are also vibration and impact loads and decisions as to factors of safety based upon 1g and multiple g loads.
A monocoque structure can yield outstanding stiffness and strength for its weight but it must be modified to account for occupant space, aerodynamics, getting in and out, mechanical accessibility, and visibility, among other things.
The problem with monocoques is the resolution of point loads and in the case of a roadster, the open area of the cockpit and the necessary door apertures.
On a GRP (Glass Reinforced Plastic) structure, point loads, like suspension pickups, can be dealt with using localized strengthening, like bobbins, a localized increase in wall thickness, and if necessary the intersection of additional GRP panels, ribs, or substructures.
Overall chassis/body stiffness is more difficult. The body/chassis structure must resist bending up and down, side-to-side, and in torsion. Multiple closed, or triangulated sections, are the general strategy, where one makes each section of the structure as rigid as possible and combines them in a rigid way. There is the engine section, the cockpit, and the rear section, usually separated with bulkheads.
The problem with a monocoque chassis in an open roadster is stiffening of the cockpit area and the scuttle bulkhead where the occupant?s legs must go. The scuttle bulkhead is probably best dealt with by the construction of a hoop arrangement where a solid panel is modified with a centralized hole and additional details to make up for the hole's weakening effect. This substructure can be made quite light and stiff in plane. Race cars can use an outer tubular hoop and an inner tubular hoop concentrically joined together with a welded web. The requirement for doors in an open roadster will tax the structural engineer as the necessary cutouts will severely weaken a simpler structure in both bending and torsion. The Type-14 Elite addresses this issue with a structurally integrated top. This is not a perfect solution as it can?t be perfectly triangulated unless somehow tied in to a roll cage of some sort. Still, the large cross section helps a lot. An Elise addresses this issue with large cross section sills on both sides of the cockpit effectively creating a massive two beam structure.
A monocoque structure is stiff due to its geometry. When one deviates from the ideal geometry, band aids must be added. The Lotus 25 Grand Prix car has no doors but it does have an open cockpit, so an important part of the structure is missing. IIRC, this was overcome by two large section tubes on either side of the driver that were also gas tanks.
The most expedient method of achieving stiffness is a large cross section geometry. A hollow tube of large diameter will be much stiffer than a solid rod of the same weight. Similarly, a hollow box section beam will be much stiffer than a solid beam of the same weight. This is true in both bending and torsion. The idea is to put the material weight were it will do the most good, as far away from the neutral axis as possible. In engineering speak, it?s all about the moments of inertia, both in bending and torsion.
Some moments of inertia (stiffness) relationships to ponder:
???????????????...........................Ix (verticle bending)??Io (torsion)
For a solid rectangular beam:?.................bh^3/12???????????bh(b^2+h^2)
For a solid circular beam:???..................?r^4/4???????????? ?r^4/2
Where:
^=raised to the power Example ^2 is squared; ^3 raised to the 3rd power; ^4 raised to the 4th power
b=the base dimension of a rectangular cross section
h=the height dimension of a rectangular cross section
r= the diameter of a circular cross section
For hollow sections the technique is to subtract the hollow void value for a solid from the larger solid section value.
Example for a rectangular section: Ix solid-Ix cavity= Ix hollow tube ; The difference between solid and cavity dimensions is the combined wall thicknesses.
Note the h^3, b^2, r^4 terms. Small increases here yield big stiffness benefits.
Although dated now, the best book I?ve read on the subject is,
?Racing and Sports Car Chassis Design? by Costin and Phipps.
In the case of the Elan, I suspect the biggest problem was trying to obtain adequate chassis/body stiffness in a roadster. Except for the case of the coupe and +2, there wasn?t a good solution for bending and torsional rigidity available in the allowable development time frame. Likely, the solution for an all GRP chassis/body would have been a combination of a large cross section closed center tunnel and large cross section sills. Because of geometry constraints, the closed sections would likely have had thick and heavy walls. Another possible solution would have been to make the doors structurally rigid with structural, locating locks upon closure.
Because of time and likely financial constraints, Hickman and Chapman adopted the backbone chassis as pretty elegant solution to take out suspension point loads and stiffen the cockpit area of the chassis with a minimum weight penalty. It also isolated the occupants from the high frequency road harshness because the body was not rigidly coupled to the backbone chassis except in selected areas like the rear bulkhead to suspension towers, where the verticle wall forms an effective cross brace. I note that Dave Bean's 26R uses a cross chassis rod for much the same effect on the front suspension mounts.
Holed bulkheads in the chassis interior are effectively rigid hoops at the beginning and end of the central tunnel and limit tunnel geometry distortion under torsional and bending loads.
As a young puppy, I was thinking in terms of structural doors and locks when I envisioned building my own special roadster. This was a motivator for my return to college to get my mechanical engineering degree some years after I finished my stint in the army. I still have that dream. Maybe some day.
Chapman and Hickman were pretty crafty, creative folks.
Bill