X-ray spec: Jaguar D-type

Coventry's endurance racer broke new ground, both in structure and aerodynamic. Keith Howard talks tot the man who developed a legend

Down the decades numerous cars have claimed to draw inspiration from the aerospace industry, but few have done so with the conspicuous success of Jaguar's D-type. hi the wider car industry of the 1950s, aeroplane influences would become associated with silly styling extravagances on American road cars whereas — for all its undeniable beauty —Jaguar's racing successor to the C-type was designed by Malcolm Sayer on strictly Bauhaus principles. Form followed function — but it so happened that the form determined by aerodynamic principles turned out to be ravishing to the eye.

The D-type had its origins in the one-off C/D prototype that Jaguar built in 1953 as a first step towards creating a successor to its C-type. The latter had won Le Mans for a second time that year, not least because it pioneered the new technology of disc brakes. Jaguar understood that its replacement would have to continue the theme of technological advancement, but this time its most obvious manifestation would be in the way the car was constructed.

In the words of Norman Dewis, Jaguar's chief development driver whose recollections of the D -type appear over the page, "When we moved away from the C-type — the D was different in its centre of gravity, its steering geometry and things like that, as well as its construction — we moved into a new era of sportscar design. The concept of the D entirely breaks away from the old tradition of a complete chassis frame: it had a shorter wheelbase, was lower and, because of its structure, had much better handling than the C. You could feel it straight away."

Sayer's innovation was to use an elliptical-in-cross-section, stressed-skin cockpit tub built mostly in 16swg (1.6mm) aluminium alloy, the rear bulkhead of which carried the rear suspension pickup points. Running forward from this bulkhead was a substantial W-shaped triangulated frame. In the C/D interim vehicle this was constructed from magnesium alloy using argon arc welding, but this method was later abandoned when the welds proved to be porous, casting doubts on the structure's longevity.

Ahead of and behind the cockpit tub, the sumptuously curved body panels were shaped purely for aerodynamic efficiency, the C-type edict that the car should resemble Jaguar's road-going models having by now been rejected. When a fin was added behind the headrest to improve directional stability in crosswinds, the car's image as an aircraft on wheels became obvious even to the casual observer.

The D -type's record at Le Mans — three wins in the four years from 1954 to '57, albeit only one of those by a works car — is eloquent justification of Sayer's excellent chassis and aerodynamic design in particular, but also of Jaguar's racing programme in general. Any successful racing team depends as much on the meticulousness of its preparation as the inspiration of its designers, and Jaguar was no different. The D -type won races not just because of its fine engine, slippery aerodynamics and advanced construction, but because Jaguar had also made the car as bomb-proof as possible.

As residents living near MIRAs test track south of Nuneaton could testify, the car ran there day and night, clocking up the development miles that would ensure it was as durable as it was fast. Had it not been for this, the D -type's history might have been very different Instead of its forward thinking being a source of admiration, traditionalists would have shaken their heads knowingly and delivered aphorisms on the theme of sticking to the tried and tested.

But Sayer and Dewis, under the patronage of Sir William Lyons, were proved right. In the 50 years since the D-type first graced Europe's race circuits, this extraordinary car has cemented its place in history.

X-ray spec: Jaguar D-type

For 1955, the body gained a longer nose than shown here, in order to reduce the car's aerodynamic drag. The engine was also modified with new valve angles and a revised inlet manifold, changes which Dewis recalls, "gave us 26-28bhp more, which put us up to about 298bhp. In '55, I pulled 192mph down the Mulsanne whereas the previous year we did something like 180mph. So there was a big difference." Although Norman can't recall the improvement in drag co-efficient, we can estimate it from those power and speed figures. To achieve 180mph with 270bhp the car's CdA must have been about 0.60m sq; to make 192mph with 298bhp it must have been reduced to about 0.54m sq, and that's roughly a nine per cent improvement.

A major contributor to D-type reliability was Norman Dewis' memory: "When we went over for the Le Mans test weekend I occasionally drove, and I was always a reserve for the race. Engine man Bob Knight said, 'Norman, give us the gear and revs you use for each corner' So after so many laps I'd come in and give him this info— I had to keep it in my head — then go out again, gather some more. So we built up a picture of what the car did for one lap. For the next year we set a target lap time, then built a histogram that told us we'd be running at, say, 2000rpm for so long at full throttle and so long at half-throttle, throughout the rev range. Then we simulated that on the dyno. If there was a failure they'd run the test again until the engine could run that histogram reliably. Other teams didn't know we were doing this and I don't think they did it themselves."

The D's straight-six XK was fitted with dry-sump lubrication. "That was a big change. With the wet sump [on the C-type] we had problems with oil surge exposing the oil pump's pick-up pipe so it sucked in air. A 1g stop would also throw all the oil to the front of the sump where the timing chains would pick it up and fling it into the head. We knew we had a problem there, but fortunately we just scraped through on it. Although we didn't have engine reliability problems, it could get worrying. We tried different baffle systems with flaps that would open and shut under differing conditions of cornering, braking and acceleration. It worked pretty well, but the dry sump system was much better"

Although not strictly necessary, synchromesh was fitted to first gear. "We didn't need it at Le Mans or Reims or Spa. But some of the drivers thought they might need bottom gear occasionally, so we made it all-synchro for that purpose. The ratio was so low you could almost start the car in second anyway." The 'box was criticised by some drivers, but Dewis defends it: "The engine and gearbox were two beautiful units that could stand hammering all day long. You could have gone 48 hours, never mind 24. In fact, I'd already done almost 48 hours to ensure we'd last 24!" But the transmission did have a habit of jumping out of second, eventually prevented by fitting a clutch interlock mechanism. "I never understood why Jock Thompson, who designed the gearbox, never got second gear right. The locking device was an afterthought, but it worked."

"The first D-type I had was fitted with the head fairing only, no fin. Towards the end of its development — we'd done all the wind tunnel and wool tuft tests — I was doing a six-hour endurance test at MIRA. As I was coming off the banking, doing about 165mph, suddenly the car twitched down the straight. I went to the control tower, looked at the wind direction, and noticed that on this straight we had a crosswind hitting me at about 45 degrees. When I got back I said to Malcolm Sayer, 'Are we sure about the car's stability in crosswinds?' We waited until conditions were more or less the same, took the car out again and found that it was the wind that was causing this effect. So Malcolm placed a fin behind the headrest. We tried various sizes: we started with a small one and increased it up to the size you see here."

A distinctive D-type feature was its Dunlop aluminium-alloy disc wheels. "They were lighter, yes, but the other big point was their stiffness. The wire wheels had always given us problems. In the early C-type days, all the spokes were loose after a race. We increased the number of spokes which was an improvement, but even then the odd one or two would come loose. That's all it takes for the rim to deflect and begin steering the car. Going to the disc wheel was a breakthrough — handling was much more consistent over a long race. But we had to be careful with brake cooling, one of the reasons we had stuck with the wires. The disc wheel had holes drilled near the rim to give us the same extraction. There's no pictures, but on the first set I tested those holes weren't there."

The rear axle, carried over from the C-type, was criticised by those who thought the car deserved better. Certainly it had its limitations, although Norman Dewis recalls it was well enough suited to the D-type's main role: "We wanted to win endurance races at big circuits like Le Mans and Reims. That's where we scored. On the short circuits we didn't do too badly, but the D-type was not the right car for them. We had traction problems in tight corners or any corner with a bump. There were a lot of bumps at Silverstone in those days and the car would jump from one to the next. Also we'd get inside-wheel lift. Today the car is beautiful on short circuits like Silverstone and Goodwood because they have billiard table surfaces. I warned Nigel Webb, who owns XKD 505, the car that won Le Mans in 1955, that the D-type wasn't ideal for this sort of track, but Win Percy drove it and said it was marvellous. He could get the back out and it was controllable because there were no bumps."