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It's light -- but no lightweight

Jaguar's aluminum-bodied XJ8 sedan is faster and stronger and still offers the best fuel mileage in its class. Call it an unalloyed triumph.

October 22, 2003|DAN NEIL

With all the shrillness at their command, SUV zealots seized upon last week's report by the federal government that correlated lower vehicle weight in passenger cars with higher fatality rates. One conservative lobbying group, the Competitive Enterprise Institute, gloated that the report showed that fuel economy standards were "even deadlier than previously thought."

The study indicates no such thing, and the institute's spin on the topic is the stuff of magic mushrooms. It does say that drivers of lighter vehicles have tended to come out on the short end of the stick, mortality-wise, in collisions with heavier vehicles. As the owner of a small vintage sports car, I do not take this conclusion, um, lightly.

At issue here is the relationship between crashworthiness and fuel economy. Given that the Bush administration plans to revise mileage standards for light trucks -- meaning sport utility vehicles, pickups and minivans -- a key question is: Would a drive toward lighter and more fuel-efficient vehicles cost lives? The answer: not necessarily.

Smarter design can compensate for a lighter vehicle's loss of Newtonian leverage.

Consider the Jaguar XJ8. For model year 2004, the venerable dame of British sedans has been reincarnated as a high-tech showpiece, its most salient feature being its all-aluminum, lightweight construction. The chassis is an aluminum monocoque, an assembly of alloy panels, beams, castings and extrusions that are riveted and glued together rather than welded like a steel unibody. Rival Audi's A8 L is an aluminum chassis car also, but the construction technique found in the XJ8 is more like that used in the Aston Martin Vanquish, the Lotus Elise and the Panoz Esperante (if the last two seem obscure, it only goes to show how exotic this method is).

The result is that the new XJ8 -- which is bigger, faster, more powerful and more richly appointed than its predecessor -- is also 200 pounds lighter than its forerunner and quite a bit lighter than many of its competitors. The aluminum diet helps the XJ8, with a 4.2-liter, 294-horsepower V-8, return the highest Environmental Protection Agency-rated gas mileage of any luxury sedan, a conscience-clearing 28 miles per gallon highway and a combined rating of 22 mpg. That's a full 10% higher than the previous model.

Oh, and by the way, it romps to 60 mph in 6.3 seconds (half a second faster than before), carries four sets of golf clubs in the trunk and seats five people in a vibe of deep civility and leather-bound cool.

But is it any less safe for being lighter? I don't think so.

The government study, issued by the National Highway Traffic Safety Administration, is judicious on the matter of vehicle weight and safety, pointing out that although the conservation of momentum favors occupants in heavier cars, mass alone does not make a vehicle safer. Under the heading "crashworthiness," the study says that heavier cars, "with their longer hoods and extra space in the occupant compartment provide an opportunity for a more gradual deceleration of the vehicle, and the occupant within the vehicle" in a collision.

What if you could build a big car that also was lighter? The report goes on to say: "While it is conceivable that light vehicles could be built with similarly long hoods and mild deceleration pulses, it would probably require major changes in materials and design."

The aluminum Jaguar XJ8 embodies such changes. This car is 200.4 inches long -- nearly 17 feet -- and the revised design retains the pip-pip classicism of the older XJs, with the long, graceful hood swept back in fluted contours from the quad headlights. Under the hood, Jaguar has engineered a bolt-on front structure that can sustain impacts of up to 10 mph without deforming the remaining structure, which is itself protected by extruded aluminum "crush tubes," sacrificial members that deflect crash energy away from the cabin. The 2004 XJ8 has not yet been crash-rated by NHTSA or the Insurance Institute for Highway Safety, but I'd be very surprised if it doesn't get full marks in crash protection.

Another factor that has historically favored heavier vehicles in collisions is structural integrity. The beefier steel members in heavy cars, around doors and along roof rails, can fend off impacts from other vehicles. However, the Jaguar's aluminum structure -- 60% stiffer, or more rigid, than the steel-bodied car it replaces -- seems to suggest that the choice between weight and safety is a false one. The cabin openings are ringed with high-strength alloy extrusions, and similar structures gird the floor, roof and bumpers. It all looks as if it can take a pretty good lick.

NHTSA's study also notes that bigger cars, not necessarily heavier cars, are more dynamically stable. Longer-wheelbase cars have less of a tendency to spin; wider vehicles have less of a tendency to roll over. The Jag's wheelbase is 119.4 inches, 5% longer than before. And yet it's 5% lighter. Go figure.

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