Jaguar XK's chief engineer discusses the challenges, benefits of building with aluminum
By MARK RECHTIN | AUTOMOTIVE NEWS
AutoWeek | Published 05/17/06, 3:18 pm et
Advertisement
Click Here!
Russ Varney has worked on Jaguar coupes since 1992. But as chief program engineer on the Jaguar XK's 2007 redesign, Varney was presented with a new challenge. The 48-year-old Englishman had to develop the coupe and convertible versions in parallel. In the past, Jaguar has done the coupe first, chopped off the roof and then reinforced the convertible. And Varney had to do it in aluminum instead of steel. Varney spoke with Automotive News Staff Reporter Mark Rechtin at the XK's launch.
Is the XK's use of aluminum an advance over the XJ sedan?
The XJ was largely a steel process made in aluminum. Most of the XJ parts are stamped. It taught us how to rivet and bond. It also taught us about the problems -- the "springback" of stamped parts, where the pieces won't keep their exact shape.
The XK needed a different solution because a coupe is a more difficult package, and we were making a version without a roof. We are using more castings and extrusions, which were about 11 percent for the XJ, but about 25 percent of the XK. Castings let you put the amount of material thickness you need in the right place, and you don't need two or three different parts connected with joints, like you do with stampings.
What is special about the use of aluminum in the new XK?
The whole side sill from the A-post to the back of the car is a single extrusion. It's 8 inches tall and 8 to 10 millimeters thick. I think it is the largest automotive aluminum extrusion piece in Europe.
Castings and extrusions reduce the number of joints where metal pieces come together, and that adds strength to the body. As a result, the new convertible is as stiff as the old coupe.
Then there's the weight savings. The Mercedes SL is very capable but it weighs (4,090 pounds). The BMW 6-series convertible's body-shake performance isn't great, and it still weighs (4,200 pounds). The XK convertible comes in at (3,620 pounds), and that's a massive difference. It's two people and their luggage less weight.
What difficulties did you encounter with aluminum?
Nobody has ever done a mass-produced aluminum convertible in monocoque (unibody) form. BMW's Z8 was a spaceframe, which isn't the same. There isn't a CAD (computer-aided design) tool to do this. We had to learn some lessons on the way.
Such as?
In creating the "torque box" section of the body-in-white, we were trying to direct crash-load transfers from the front clip to the side sill, and that makes for a lot of mass in a confined area. Because of that mass, we found out that the adhesive didn't get hot enough to cure in the paint shop. So we had to develop a different adhesive that went off at a different temperature.
What does the XK provide for the driver that the German coupes do not?
Those cars are focused on driving German roads and being competent at going 150 miles an hour. But those cars don't have a "roundness" to them. Their dynamics have all these sharp edges to them. For instance, the Audi DSG transmission's downshift will jar you to prove to you how good it is. We've rounded things off a bit.
