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Designing the XJ13

XJ13, Building The Legend, Neville Swales, Jaguar

On the 3rd June 1965 an internal "Instruction to Proceed (XJ13 Car)" was issued by Jaguar’s Bob Knight – it started, “Build one prototype competiton car …”. Responsibilities for all aspects of the car’s design were allocated – the responsibility for the body being given to Malcolm SayerPhil Weaver and Bob Blake.

The body was to be, “Light alloy skin on monocoque structure. Comprising of three main sections”. These three sections were:

  • Body Front Structure” (main skin, front bulkhead, headlamp diaphragms, air-ducts for radiator/brakes etc, internal structure to suit 1964 Jaguar Lightweight E-Type independent front suspension and a boot lid).
  • Body Centre Structure” (floor & sills, fuel & oil tanks, seat back bulkhead, doors and windscreen).
  • “Body Rear Structure” (main skin, engine cover, spare wheel, cooling ducts for transmission & brakes, rear lid and rear valance).

Other responsibilities were allocated as appropriate. Rather telling was the comment “For the first car only” which does confirm the prototype XJ13 was planned to be one of many.

By the June of 1965, the quad-cam V12 engine project for the XJ13 was well underway – with the emphasis very much on racing. The first V12 engine to be fitted to any car was my engine (engine number 2) which was fitted to a car codenamed “XJ5/5” – XJ5 being the code name for the Mk10 successor. This engine was fitted to this sable-coloured “test mule” in April 1965 – complete with Lucas Mechanical Fuel Injection and modified dry sump - some two months before the XJ13’s “Instruction to Proceed” was issued. The engine in the “test mule” was built to the same specification as the first engine (number one) which was to be installed in the XJ13 a year later in April 1966.

Overall responsibility for the shape was given to the late Malcolm Sayer – the man already responsible for the Jaguar C-Type, D-Type and later to be responsible for the iconic designs of the E-Type and XJS.

XJ13, Building The Legend, Neville Swales, Jaguar
Malcolm Sayer, 1916-1970

Malcolm Sayer was a student of aerodynamics at Loughborough University’s Department of Aeronautical and Automotive Engineering in 1938. He was one of the first designers to apply the principles of aerodynamics to cars with his scientific calculations, creating some of the most beautiful forms of the era. Sadly he died in 1970, at the relatively young age of 54.

After graduating from Loughborough he joined the Bristol Aero Company where he worked on various projects including their radial engine. One of Sayer’s colleagues at Bristol was Phil Weaver who was later to work to take charge of Jaguar’s Competition Department and work alongside Sayer on the XJ13. In an interview with Phil Weaver, Paul Skilleter (well-known Jaguar Historian and Author) recorded Weaver’s recollections of his time with Sayer at Bristol.

“Sayer was very highly thought of at Bristol and had the idea of reverse cooling of radial engines. You would think that if you were forcing an aircraft through the air that the engines, which were air-cooled, would benefit from being pushed through the air. But Sayer proved to them that’s not so, because a lot of the air couldn’t get out, having been forced in through the baffles and cowlings. The air used to compress and didn’t flow. Now Sayer actually worked out a theory that you had to reverse the air flow and suck the air out into the hub of the airscrew. When we were given a BMW radial engine by the Ministry at the end of the war, we found that they were doing the same thing, but Malcolm was the first to do it in Britain.”

Malcolm Sayer joined Jaguar in 1950 and his talent was soon recognised. One of his first tasks was to design a suitable body for Jaguar’s XK120C (the “C-Type”). The chassis had already been designed by Jaguar’s Technical Director Bill Heynes. Sayer worked alongside Bob Blake who had been given the responsibility of producing a body. Sayer brought his aerodynamic skills to bear on the project and added a large element of science to the body design. He was one of the first to use wind tunnels in automotive design and photographs exists of the various small-scale models he had made to investigate the aerodynamic characteristics of his various designs.

Norman Dewis, Jaguar’s renowned Chief Tester, joined Jaguar not long after Malcolm Sayer and recalled how Sayer worked:

“Sayer would produce a model first. At the time we didn’t have a wind tunnel at MIRA, but Sayer had very good connections with Farnborough where they had excellent facilities. So he used to take the model down there and do the wind tunnel tests, changing anything that needed changing to get the best shape he could.

That was then related to a full body size shape, and then with the full size body we used to do all the final aerodynamics trim with tufts of wool.

We used to call in at a wool shop on the way to MIRA and buy a ball of wool. We would then cut lots of 3” lengths of wool and with a roll of Sellotape stick the wool all over the car where we wanted them – over the front, going up to the windscreen, over the back, going up the sides. Then I used to drive the car round the outer circuit at MIRA and Sayer would be driven in another car. He would sit in the back and from that position he would look out of the side and from the back.

We used to have a hand-signalling device to manoeuvre me to where he wanted me and the at, say, 80/90mph, he would observe the wool tufts. Anywhere there was turbulence, he would make changes. We would then try it again until we got a good flow.”

A number of contemporary sources cite Sayer’s habit of drawing a full-size car on the walls of his office or even with chalk on the floor. I don’t doubt that some of his initial designs for the XJ13 were done in this way. He had at least one small-scale model made up for testing before Bob Blake began the task of clothing the chassis/monocoque.

Sayer’s final designs were “formalised” as side, front, rear and plan view documents which may have become internal “standards” for his designs and used for things such as centre-of-gravity studies etc. The detail shapes of compound curves etc were established mathematically using a technique peculiar to Sayer.

Examples of these final standardised documents are shown below:

XJ13, Building The Legend, Neville Swales, Jaguar
XJ13 “dimension summary”© Jaguar Heritage

XJ13, Building The Legend, Neville Swales, Jaguar
In late 1967, after he had designed the XJ13, Malcolm Sayer designed three more V12 mid-engined sports racing cars. The drawing above shows one of these designs in the form of a “dimension summary”.copy; Jaguar Heritage

It is interesting to note that Sayer’s original design as shown above differs in many respects from the rebuilt “original”. It was my aim to reproduce the XJ13 exactly as Sayer had intended and before the addition of “1970s wide wheels/wheelarches” and other “updates”. It is important to me to recreate the car as close to its original specification as possible – not only to satisfy requirements for potential racing against cars of the period, but also because the historical significance of the surviving original prototype engine demands this. After all, Jaguar had always intended to produce more than one car and I feel an authentic copy could be considered to be a “continuation” in line with Jaguar’s original intentions.

Malcolm Sayer was very much a man “ahead of his time”. There is much talk nowadays of Computer Aided Design (CAD) and Computer Aided Manufacturing (CAM) but it seems that as early as the 1950s Sayer had developed his own longhand version of similar techniques. He kept his calculations and means of representing complex shapes mathematically very close to his chest and there is little information on his methodology available today.
Paul Skilleter reported that Cyril Crouch, who worked in the Body Drawing Office in Sayer’s time, recalls him

“using Chambers seven-figure log tables to calculate all the shapes, as one would do on a computer now.”

Bob Blake had joined Jaguar from Briggs Cunningham and was a legendary body-builder. He was able to interpret Sayer’s data and successfully translated his drawings into the full-scale XJ13. He was joined in this task by Roger Shelbourne and Geoff Joyce. Peter Wilson also worked on the car and was responsible for making some components of the car and chassis. Wilson confirms that, contrary to popular belief, the original car wasn’t built by Abbey Panels – their involvement was limited to “the fabrication of skin panels to our formers, and roller-seam welding of the inner sill panels to the main floor and outer sill sections.”
Peter, in his book “Cat Out Of The Bag” goes on to say,

“As our surface table was not large enough, or indeed remotely suitable, Bob Blake, Geoff and Roger built a rigid wooden platform on which to build the XJ13 monocoque … First they constructed a perimeter frame from 6x4” timber, cross-braced at intervals along its length. This was topped with ¾”thick plywood sheet, which they then marked out with ‘10’ lines to enable accurate positioning of each of the myriad of construction reference points defined by Malcolm Sayer’s ‘drawings’”.

Much of this data has survived – including the precise location points of items such as suspension components, steering rack, anti-roll bars in 3D space. This data will be used in the construction of my 1966 XJ13 copy. The main difference being that technology allows us to carry out this operation on a computer screen before the actual car is manufactured.

To help achieve a faithful copy of the original design, the technique of 3D scanning coupled with digital techniques to incorporate data from period photographs, original technical drawings and eye-witness accounts is being used.
An example of how digital data can be built up from an original document (in this case an accurate drawing used in 1965 for a centre-of-gravity study) is shown below. The pictures show initial work on reproducing the original centre monocoque structure.

XJ13, Building The Legend, Neville Swales, Jaguar
© Neville Swales
Not to be reproduced without permission.

XJ13, Building The Legend, Neville Swales, Jaguar
© Neville Swales
Not to be reproduced without permission.

XJ13, Building The Legend, Neville Swales, Jaguar
© Neville Swales
Not to be reproduced without permission.

This digital data was supplemented and cross-referenced with original photographs, reports and original technical data – as well as the “original” prototype. Once the information had been captured, it was possible to estimate things such as weights, roll-centres, centre-of-gravity etc. It was also possible to virtually “trial-fit” components to reduce the possibility of an error during the actual build.

Once sufficient digital data had been assembled and the on-screen 3D images verified with photographs etc, the next step was to use the data to produce a physical scale model using CAM techniques including the use of a 3D printer. The scale model was painted in the correct shade of 1965 BRG (British Racing Green), handled and verified for accuracy. It also gave an opportunity to see how the light catches the car in comparison with period photographs and the current “original”. Once this had been accomplished, the digital data was used to manufacture millimetre-perfect full-scale formers and bucks – even to the extent of pre-marked or drilled rivet locations.

As well as being able to use these powerful techniques to faithfully reproduce the original car, they were also be used to reproduce unobtainable and unique parts such as cylinder heads etc. The following picture shows this scanning technique in action on the prototype engine:

XJ13, Building The Legend, Neville Swales, Jaguar
© Neville Swales
Not to be reproduced without permission.

The item to be scanned (anything from a small component to a full-size car) is covered in a non-reflective white powder. Small adhesive dots are applied across its surface and the item is laser scanned. The small dots allow the sophisticated software to locate specific points in 3D space. Internal passageways may be scanned using similar techniques using lasers on probes.

Manipulation of the resultant data and production of a final digital representation of the scanned item is a skilled operation. Once the item has been captured in this way, faithful clones can be reproduced using computer aided manufacturing techniques.

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