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XJ13 - Building the Chassis/Monocoque - Choice of Materials

As the build of my first XJ13 monocoque/chassis progressed, I wanted to consider the materials that should be used for construction of the component parts of my re-creations. Jaguar themselves went through a similar exercise in 1964 when the XJ13 had reached an advanced design stage.

Before then, in the few years leading up to 1964, various studies/reports were made on things such as the shape of the overall body, the design of the underlying chassis structure and suspension design. One, quite advanced, design was for a rather more integrated monocoque design as shown in the following sketch from the November of 1963:

Jaguar, XJ13, Neville Swales, Eagle Cars, E-Type Jaguar, E-Type, Building The Legend, tera, V12, V12, quadcam, Jaguar XJ13
Early monocoque design for the XJ13.
© Image - reproduced with permission.

 However, the above design was not progressed further and, instead, a separate monocoque/chassis unit was developed which was to be clothed by a largely unstressed outer skin:

Jaguar, XJ13, Neville Swales, Eagle Cars, E-Type Jaguar, E-Type, Building The Legend, tera, V12, V12, quadcam, Jaguar XJ13

Representation of original XJ13 monocoque/chassis.

In essence, the final design for the monocoque/chassis consisted of three main elements:

  • sills/floor/front & rear bulkheads
  • front suspension structure (coloured yellow)
  • rear engine mounting and rollbar (coloured green)

The design of the front suspension went through a few different incarnations as the car was being assembled. Derrick White, Jaguar's talented Race-car Engineer argued for a cutting-edge design (for the time) using widely-spaced upper and lower wishbones. This would have given better handling and a greater possibility of maximising the benefit of the wider tyre widths which were increasingly being used in the mid-1960s. His persistent arguments were repeatedly blocked by Bill Heynes who favoured a more tried-and tested production-based suspension. Heynes eventually prevailed and a design, based on the 1964 Lightweight E-Type was adopted - albeit with coil-over shocks in place of torsion bars.

This decision was one of the things that led to Derrick White becoming increasingly frustrated and his eventual defection to Cooper - a great loss to Jaguar. White went on to design the GP-winning Cooper-Maserati of 1966. Heynes, at the time, had been given direct supervision of the XJ13 project and it has been argued that his enormous workload at the time contributed to the slow development of the XJ13. Fortunately, Mike Kimberley was eventually given day-to-day responsibility for the car and development then continued at a greater pace.

Meanwhile, in 1964 when the car had reached an advanced design stage - on paper at least - Jaguar conducted an investigation into the best materials of construction for the chassis/monocoque. They considered mild-steel, aluminium and titanium. The investigation concluded:

  1. " ... For a given rigidity the weights of chassis built from 22swg mild steel, 14swg aluminium, and 18swg titanium would weigh almost exactly the same.
  2. Chassis constructed to the same design from 22swg mild steel, 14swg aluminium and 18swg titanium would have safety factors (based on ultimate tensile stress) of 1.00, 1.43 and 1.71 respectively (relative to 22swg mild steel)
  3. In view of the difficulty of working and welding titanium and its cost, and because it shows no weight advantage for a given rigidity, it appears that the choice must be between mild steel and aluminium ..."

In the end, the chassis sections coloured yellow/green in the drawing above were fabricated from mild-steel. The main centre section was fabricated from aluminium.

According to Peter Wilson, who actually lent a hand in constructing the XJ13:

" ... the monocoque was constructed almost entirely from NS4 2 percent magnesium and 2 percent manganese, half-hard alloy sheet, mostly of 18 swg thickness (0.048 inches), together with some sheet steel pressings in areas of high and concentrated stress, such as the main engine mountings and front suspension attachment areas."

The modern equivalent, Aluminium 5251 (NS4), is available and will be used for the recreation along with steel pressings where appropriate. I must admit to some relief that Jaguar didn't choose titanium 

For those engineers amongst you, and those well-versed in the mysteries of things such as Young's Modulus (I certainly don't include myself here!), the following summarises some of the data presented in Jaguar's investigation into material choice:

Mild Steel
(UTS=30T/sq in)   
24.0220.89630 x 1061.270.78

(NSA half-hard; UTS = 15T/sq in)

22.0280.38010 x 1062.900.50
(UTS = 30T/sq in)
22.0280.65516 x 1061.811.00

As is well-known, there is no such thing as a chassis that doesn't flex, but some are much stiffer than others. The choice of material is critical in this respect. The range of chassis stiffness has varied greatly over the years from about 500 lbft/degree in the 1930s to more than 20,000 lbft/deg in a modern race car. I should be able to measure the stiffness of my completed chassis and it will be interesting to compare the all-steel "trial" chassis to the final version.

Different chassis designs each have their own strengths and weaknesses. Every chassis is a compromise between weight, component size, complexity, vehicle intent, and ultimate cost. And even within a basic design method, strength and stiffness can vary significantly, depending on the details. There can be no such thing as the "ultimate chassis" for every car, because each car presents a different set of problems. The XJ13 chassis gave a whole new challenge because of the intention to mount the engine as a fully-stressed member - with the whole of the rear suspension hanging off the engine/transaxle. I believe this would have beaten Colin Chapman's Lotus by a few years had the XJ13 actually raced. Jaguar carried out a number of theoretical investigations into how well the car should stand up to the torsional loads applied to the chassis because of this arrangement and the final rear chassis design took these anticipated loads into account. The front suspension arrangement bears many similarities to the E-Types with steel tubing attached to the front bulkhead and Jaguar will have built up much experience of this design.

It may seem that an aluminium chassis was always the logical choice, but this is not necessarily true. Aluminium is more flexible than steel or titanium. Indeed, the ratio of stiffness to weight is almost identical to steel, so an aluminum chassis must weigh the same as a steel or titanium one to achieve the same stiffness. Aluminium has an advantage only where there are very thin sections where buckling is possible. This certainly applies to the large sill and floor sections.

In the end, the "unintended crash test" crash at MIRA in 1971 demonstrated better than anything else the soundness of Jaguar's basic chassis design. Although there was considerable damage to the outer structure, the basic chassis/monocoque survived almost intact. More importantly, the legendary Test Driver, Norman Dewis, survived unscathed.

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