Click Image to Enlarge
Jaguar’s new XJ series is a stamped aluminum monocoque structure that also uses the occasional casting and extrusion. Forty percent lighter than an equivalent steel structure, the 2004 XJ is the first of what promises to be many aluminum-intensive Jaguar vehicles. The company has asked material suppliers to improve the dimensional capabilities of aluminum, improve its manufacturing robustness and potential joining technologies, and reduce its cost per pound relative to steel, especially for volumes greater than 100,000 units per year...
Mark White, manager, Body-In- White Design, Body & Trim Engineering Jaguar Cars Ltd. (Coventry, England), is an aluminum man. His group built several aluminum Jaguar S-Types in order to verify that this was the way to go for the body structure of the XJ series replacement (codename: X350). They conferred with their counterparts at Ford about the lessons learned from Ford’s AIV–Aluminum Intensive Vehicle–Taurus fleet and P2000 research vehicles. And every day the weather cooperates, White can be found behind the wheel of his Lotus Elise, a lightweight specialist sports car with an extruded aluminum spaceframe.
“Fuel is much more expensive in Europe,” he says. “It’s the equivalent of $7 per gallon in the U.K., and vehicles taxes are based on a vehicle’s CO2 emissions, which are directly related to fuel economy.” This, says White, was a major impetus to get the weight out of the new XJ, especially in light of the trends in luxury vehicle mass. “The weight of the typical luxury vehicle has increased by 10 kg. per year on average,” he says, “which means that cars in this sector are on average 220 lb. heavier today than they were in 1992.” Fueled by increases in luxury and safety equipment, this has caused automakers to put larger, more powerful engines under the hood in order to mitigate this increase, and to keep up with the demand for more performance.
“I didn’t want to have to scrimp on equipment or performance with the X350,” says White. “Nor did I want to have a vehicle so unique in terms of repair procedures that it wouldn’t be possible for a customer to get it fixed at his local Jaguar dealer.” This is an obvious reference to Audi’s A8, whose aluminum spaceframe structure requires that major accident repair be handled by select Audi-authorized repair centers. The X350 engineers determined that the best way to avoid this problem, and to learn what it takes to build aluminum-intensive vehicles in higher volumes, was to rely on a stamped aluminum monocoque structure held together mainly by self-piercing rivets (SPRs). “The structure uses 114 meters of adhesives,” says White, “and 3,195 SPRs. There are 15 aluminum castings, 35 extrusions and 284 stampings in the structure, and the body is built using three framing stations.” It also has a magnesium cross-car beam behind the instrument panel, and magnesium seat frames.
White says the structure uses an inside-out build process that allows the rivet guns full access to every joint on the structure. The skin is made of 6111 bake-hardenable alloy, and the doors make use of die castings, extrusions and stampings in their structures. A bolt-on front end structure minimizes low-speed crash damage, and–along with the split rear fender–helps keep insurance costs in check. And every Jaguar dealer has been trained in the repair protocols for the X350’s unique underpinnings. “We developed the repair techniques concurrent with the design,” says White, “and tried to make certain they didn’t stray too far from what was familiar.” This is another reason why SPRs were used in the structure. “Rivets are better in terms of fatigue strength,” he says, “and far easier than spot welding in that it’s something that can be done anywhere in the world. Spot welding aluminum needs to be very precise.” It also takes a lot of energy, and the ability to correct for changing temperature and humidity.
Metal gauge ranges from 1.0 to 2.8-mm., and the rear floor pan–which encompasses the spare tire well–is 903-mm.(22.9-in.) long and has a 19-mm (4.8-in.) draw. It’s one of White’s favorite panels on the car. “We worked very hard with the material supplier and our die makers to make sure this panel didn’t crack or wrinkle,” he says. “It may seem silly, but it was important that this piece be as flawless as the rest of the vehicle. If we could do this, getting the rest right would follow.”
Overall, the X350 structure is 40% lighter than an equivalent steel counterpart. White estimates a steel body-in-white would weigh 480 kg, while the painted alloy X350 structure tips the scale at a svelte 295 kg. Compared to the previous generation XJ series, the new car is 60% stiffer, while also being longer, taller and wider. “In addition,” White points out proudly, “in Europe we will offer a version powered by the 3.0-liter V6 that has the same performance as the outgoing V8 model, but with significantly improved fuel economy.” And at $7 a gallon, that’s important.