Jaguar XJR-15 - Design

Design

The mid-engine, rear-wheel drive sports car is powered by a 450 hp (336 kW), naturally aspirated 24-valve V12 engine of 5993 cc, with a Group C bottom-end and Group A top-end. The engine features an advanced electronically managed fuel injection system with a very advanced (for its time) 'fly by wire' throttle. Transmission is via a TWR 6-speed manual, unsynchronized transmission (a 5-speed, synchromesh transmission was also available as an optional extra).

The XJR-15's chassis and bodywork are composed of carbon fibre and Kevlar (XJR-15 was the first road-going car built entirely of Carbon and Kevlar composites, before the McLaren F1 used similar techniques in 1993). It was designed to comply with 1990 Group C regulations, being 480 cm long, 190 cm wide and 110 cm high. At 1050 kg, the XJR-15 weighed about the same as a contemporary VW Golf.

Suspension is fully independent, with non-adjustable Bilstein shock absorbers all round. Front suspension is by wide-based wishbones, working push-rods to spring damper units mounted horizontally across the centre of the car. TWR racing practice is also followed at the rear, with vertical coil-springs mounted in units with uprights within the rear wheels, allowing for the maximum possible venturi tunnels. The engine forms a stressed member for the rear-frame. The bottom of the car is completely flat, in line with Group C practice.

Steel disc brakes are fitted, with powerful AP four-pot callipers.

The XJR-15 has a 0–60 mph time of 3.9 seconds and a (gearing limited) top speed of 191 mph (307 km/h).

Although marketed as a racer, the car had been developed as a "road-going-racer" and as such, the ride height was somewhat higher than required to take full advantage of under-body aerodynamics. Additionally, the suspension was softer than would be found on the XJR-9 racer and - in a last-minute deal - Tom Walkinshaw switched tyre suppliers from Goodyear to Bridgestone just before the race series started. When interviewed by Autosport in 2011, Ian Flux recalled: "The worst thing was that Tom had done a deal with Bridgestone. At first, it was going to be on road tyres, but then they changed to slicks and wets. The fronts weren't a problem, but they didn't have moulds for the rears, so used F40 moulds instead. They went off very quickly and it was hard to judge how hard to push."

As Tiff Needell, who road-tested a development car at Silverstone early in 1991, put it: "the result is oversteer". However, once accustomed to the characteristics, he went on: "Through the very tight chicane, the XJR-15 showed excellent change of direction and I was able to pick up power early for the long right hander leading up to Beckett's. This gradually became a long right-hand power slide as my confidence increased." Users of the car as a racer in later years would lower the suspension, fit a larger wing and proper tyres to restore race-car dynamics.

As a road-car, the suspension was more softly set-up and with better tyre balance, testers were unanimous in their praise. Ian Kuah, writing in World Sports Cars in 1992: "Considering its racing pedigree, ride quality is pretty good - at low speeds, better than a Ferrari 348...Levels of grip are far beyond those transgressed by any sane man, except perhaps when exiting a tight corner in a low gear when the sheer grunt pushing you through can persuade the huge Bridgestones to relinquish some grip. Seat of the pants feel and communication is terrific and the steering nicely weighted so that smooth inputs are easy. When it comes to stopping, the huge AP Racing brakes - with softer pads for road use - wash off speed with steely determination."

Ron Grable, writing in Motor Trend in May 1992: "As the engine sprang into a muted rumbling idle, it was impossible to keep from grinning. Easing the unsynchronised six-speed into gear, I accelerated onto the straight. Many race cars are diabolical to get moving...not so the Jag, the smooth V-12 pulled cleanly away, nearly as docile as a street-car. On the track, the XJR-15 is a truly wonderful ride, the perfect compromise between racing and street. You can say the savage edge of a pure race car has been softened slightly, or conversely, that it's the best handling street car you can imagine. Being 100% composite, it's so light that every aspect of performance is enhanced. Relatively low spring and roll rates are enough to keep it stable in pitch and roll, as well as deliver a high level of ride compliance. The brakes are phenomenal and the acceleration fierce. And always, there's that V-12, a medley of mechanical noises superimposed over the raucous rise and fall of the exhaust."

The XJR-15 offers little in the way of practicality. Entry to the car, over a wide sill, requires the driver to step onto the driving seat. The gear-lever is mounted on the right-hand side of the driver (all cars are right-hand-drive), while the driver and passenger seat are extremely close together - almost central in the car. There is little in the way of sound insulation, so an in-car head-set system is fitted. There is virtually no storage space. However, considering the purpose for which it was intended, the interior was highly praised in contemporary road reports. Ron Grable again: "Aesthetically, the XJR-15's interior is breathtaking. Expanses of shiny black carbon fibre woven with yellow Kevlar are everywhere, all fitting together with meticulous precision. Instrumentation is detailed and legibly analogue. The shift leaver is less than 3 inches from the small steering wheel, and the motion between gears is almost imperceptible. The reclined seating position provides excellent forward visibility - over the top of the instrument panel you see only racetrack."