Steering a course to reduced mass

Nearly anyone in the auto game who's clued in enough about suspensions to know a good one from a bad one will agree that BMW AG makes some good ones. Maybe the best. And don't think for a minute that BMW isn't aware of its iron-clad reputation in this area.

Suspension is important to BMW. Brilliant suspension design is a prime component in the sometimes enigmatic package of qualities that define a BMW. So when it came time for BMW to ladle a dollop of advanced-materials knowledge onto its new 5-series, suspension was practically a de facto area of choice.

BMW 5-series engineer-manager Jans-Hurgen Branz says that early in the new 5's development, employing light alloy for a major subsystem like the suspension seemed to make more sense than, say, for sheetmetal -- an area where aluminum's payoff is more mercurial. Mr. Branz calls this fundamental choice "intelligent lightweight design."

Although concentrating on the suspension actually was second on BMW's lightweight priority list (following the obvious choice: engines), Mr. Branz believes an aluminum suspension, given its dramatic positive influence on reducing a vehicle's unsprung weight, is preferable to crafting bodywork from aluminum.

The suspension for both 5-series models -- 6-cyl. 528i and 8-cyl. 540i -- is aluminum through-and-through, not simply a few alloy pieces sprinkled here and there. In the front, steering knuckles, the steering-gear housing, front and rear transverse arms, even the strut tubes, are all aluminum. Steel is used only in the wheel bearings of both models and for the front suspension subframe on the heavier-engined 540i; the 528i's front subframe now is aluminum, too. In concert with the aluminum components, BMW says it developed new techniques -- essentially a type of compression forming -- for crafting them in alloy.

The story's similar at the rear. The new 5-series now is blessed with the superb 4-link "integral rear axle" initially developed for the high-dollar 8-series coupes, replacing the out-going 5-series' dated trailing arm design.

The intricate 4-link rear suspension's major components are now aluminum, as is the rear subframe for both models. BMW says a strapping 50 lbs. (23 kg) drops from the integrated rear axle with the switch to alloy.

Alloy use in both front and rear suspensions shaves 70 lbs. (32 kg) off the new 5-series curb weight, despite the new rear suspension's more complex design. What's more, the alloy suspension systems -- combined with a lightweight steel bodyshell -- help to offset weight increases in other areas. In all, the new 5s weigh slightly less than the former models, despite the inevitable addition of extra equipment like side-impact air bags. Nor will a new 5-series owner pay more for the privilege of enjoying the advantages of his all-aluminum suspension: On an equipment-adjusted basis, the new 5-series models are less expensive than the outgoing models.

Before the new BMW became the first modern volume-produced passenger car with all-aluminum suspension, alloy fetishists were forced to spend around $80 grand for an Acura NSX, the only other current production car with aluminum suspension.

Considering the brilliant tune of BMW's everyday suspension, it's difficult to quantify the on-the-road gains aluminum delivers, but test drives of the new 5-series models on demanding, high-speed desert roads prove the cars do enjoy a discernible, light-on-the-feet cornering and damping balance. In short, a finicky driver will appreciate the aluminum suspension's "feel."

But aluminum suspension componentry isn't strictly the province of the automotive aristocracy. The cost of basic aluminum feedstock is expected to be marginally reduced, and manufacturing techniques are reaching the point where aluminum has become a viable alternative for some mainstream-vehicle suspension bits.

Most of General Motors Corp.'s new or upcoming midsizers -- Chevrolet Malibu, Olds Cutlass, Pontiac Grand Prix and Buick Regal -- get around on aluminum steering knuckles. GM's new U vans use the alloy knuckles, too, and Buick's new Park Avenue/Park Avenue Ultra are fitted with aluminum control arms.

Steering knuckles are aluminum castings, and they're more than just parts that get dirty. The knuckles have critical symbolic importance to suppliers because they weigh anywhere from 5.5 lbs. (2.5 kg) to 8 lbs. (3.5 kg) apiece. That means a minimum of 11 lbs. (5 kg) per vehicle. That sort of heft in affordable cars means aluminum's moving toward more ubiquitous suspension applications.