LIGHT? FANTASTIC!
Tuesday 11th February 2014: Jaguar has been a pioneer in aluminium-bodied cars, bringing a lightweight, series-production car – the X350-series XJ – to market in 2003. While there were other commercially available aluminium-bodied cars before that, none of them were aimed at the mass-market and in most cases they were largely hand-built, low volume products.
The C-X17 concept crossover – freshly arrived in South Africa from an appearance at the Brussels Motor Show – showcases the progress that Jaguar has made with aluminium construction in the last decade and is the lodestone pointing towards what Jaguar’s engineers are planning for the future.
From the very beginning Jaguar realised the many advantages that came with lightweight alloy: recyclability, better crashworthiness, resistance to corrosion, manufacturing flexibility, and enhanced performance in every aspect of vehicle dynamics. Academic studies conducted over the years indicated that a weight saving of 100 kilograms translates into a fuel saving of about 0.35 litres of petrol per 100 km and a reduction in CO2 emissions in the region of 10 grams per kilometre.
Jaguar engineers realised that with aluminium there was the potential for a 50 percent weight saving compared to steel, without comprising overall strength and actually increasing torsional rigidity. One of the biggest challenges was making the manufacturing/assembly process commercially viable and with X350 Jaguar cracked this nut. Nearly 3 200 rivets and 100 metres of specialised adhesive bonded some 273 aluminium sheet stampings, 22 extruded aluminium components, and 15 aluminium castings for an end result which was 200 kilograms lighter than its predecessor – despite being longer, wider and taller. In addition, it was also 60 percent stiffer.
The design and fabrication concept behind the X350 was conceived to be suitable for high volume production – in excess of 100 000 units per year – and the body structure featured the first industrial use of the rivet-bonded joining technology, with self-piercing rivets and epoxy structural adhesive joining together the aluminium pressings, castings and extrusions.
The X350 wasn’t the first aluminium Jaguar however, and a decade earlier the company had employed Alcan’s Aluminium Vehicle Technology (AVT) structural bonding system on the XJ220 sports car, produced from 1992 to 1994. Just 275 XJ220s were made but it provided a wealth of useful information not only on aluminium but also on many other aspects of vehicle design, and working with alloys at the manufacturing phase.
The technology of the XJ saloon was taken to its next level in the XK, but because it was available both as a two-door coupe and two-door cabriolet/convertible there were a number of additional challenges: as well as the need for roofless version, there were more stringent packaging restrictions. Launched in 2006, it made even wider use of lightweight aluminium castings and extrusions as well as stamped aluminium panels.
More significantly, a combination of riveting and bonding – with the exception of a single, non-structural weld – mated the various components to one another. The results were sensational: the coupe is more than 30 percent stiffer than last-generation steel model; and in the convertible version, the torsional stiffness is actually increased by 50 percent.
Key to this (particularly in the case of the convertible) are the large, rectangular-section side sills. Thanks to the sophisticated design of a single extrusion which incorporates a number of structural castings, there was no need for the extra stiffening panels seen on many other convertibles.
These developments led to the production of the current (X351) XJ in 2010. It took lightweight vehicle technology to a new level, reducing part count and also pioneered the use of pre-bent and hydroformed (the use of hydraulic pressure to form material placed in a die) extrusions for certain parts. This method of forming is also well-suited for deformable components such as crash-structures that can be replaced in modular fashion. Aluminium castings, on the other hand, are ideally suited for use in components which are required to cope with high loads and complex geometries, thereby minimising weight (partly by eliminating the ‘stack-up’ issues that come with overlapping layers of steel) and improving packaging.
The bonded and riveted joining of the aluminium monocoque body structure was further refined, and the number of self-piercing rivets was reduced by 11 percent to 2 840 (compared to 5000 spot welds for an equivalent steel body) and the length of the adhesive bonding material was increased by to a total of 154 metres. Energy-intensive MIG (Metal Inert Gas) welding was thus eliminated from the assembly plant.
Almost 90 percent of the current XJ’s body-in-white is made from various grades of aluminium, the balance being made up of steel, and magnesium.
The C-X17 sports crossover concept introduces the next wave of Jaguar’s advanced aluminium architecture. This modular, scalable architecture will allow Jaguar to grow its product portfolio and target high-growth areas of the premium market, beginning with a new mid-sized C/D segment sedan in 2015.
The C-X17 concept is just one example of the diversity of vehicles that could be produced using the new architecture. As a sports crossover, the Jaguar C-X17 stretches the design possibilities of the segment by combining the character and driving experience of a sports car with increased presence and flexibility – all imbued with the sleek lines, sporting design and luxurious sophistication for which Jaguar is renowned.
