After more than 100 years of engineering, you'd think there would be little new under the sun in the world of automotive suspensions. Yet just like powertrains, new developments and refinements continue to surprise, delight, and sometimes amaze consumers and engineers alike.
Two new systems in particular are getting noticed: the four-link front suspension on the '96 Audi AG A4 and the SLA setup on Ford Motor Co.'s new '97 F-series pickup, which went into production last November. While these designs are being lauded for significantly improving the ride and handling of their respective vehicles, a new material -- plastic -- is beginning, to play an important role in a number of new suspension design marking the beginning of a new era this front as well.
Described by some auto critics as "a new benchmark for front-drive suspension engineering," the new A4's four-link front suspension is designed to drastically reduce torque steer and improve road feel for traditionally numb front-drivers. Borrowed from Audi's top-line, aluminum-bodied A8 sedan and returned for the A4, Audi engineers say the new multi-link suspension concept is based on sound physics. It seeks to create a "virtual steering axis" in order to nullify the undesirable effects of driving forces on a vehicle sending power through the front wheels.
In traditional designs, the front wheel is supported by single pivot points at the top and bottom of the wheel carrier. This simple design allows the wheel to be steered, but requires that there be a long offset, or space, between the axis on which the wheels steer and the actual area where the tire makes contact with the ground to turn.
This offset can act as a kind of lever that transmits and amplifies forces such as engine torque back through the steering system. This translates into the tugging at the wheel during hard acceleration known as torque steer and also interferes with feedback from the road surface during normal driving, causing a lack of what driving enthusiasts call "steering feel."
By reducing this offset to only 0.4 ins. (10mm), the four-link design practically eliminates the leverage that transmits undesired forces through to the steering wheel. The "virtual steering axis" is created by the geometry that results from locating individual pivot points at each of the four links, allowing the wheels to be steered almost directly from their center points, rather than at the end of lever arms (see illustration).
Audi claims the advanced suspension all but eliminates torque steer and provides "a very precise steering feel with supreme agility." Although not all test drivers are quite as enthusiastic, most agree the design is a significant breakthrough for front-drive cars.
The twin-forged upper short and long arm (SLA) front suspension design on the new Ford F-150 full-size pickups is not nearly as new or dramatic, yet its painstaking execution deserves much of the credit for the new truck's award-winning ride and handling.
It also may prove to be one of the most durable designs of its kind: nitpicking engineers added a few small touches that promise to greatly reduce wear and tear over the long haul.
Although some marketing folks still are in love with the much-advertised rugged image of the old F-150's "Twin I-beam" front suspension, the new F-150 product team decided early on that the old suspension -- in use since 1965 -- had to go. While it was indeed durable, the Twin I-beam design was difficult to align and provided steering often described as vague and imprecise. Critics say the I-beam setup was notorious for heavy tire wear and continual alignment hassles.
Ford officials acknowledge the trouble in a back-handed way. They say tire life could double with the new suspension.
Besides, the SLA design already is well-proven in numerous cars and trucks and offers several important advantages.
By eliminating the big I-beam cross arms underneath the engine, for instance, designers were able set the engine lower in the chassis. That enabled them to lower the hood and create a more modern, aerodynamic shape -- and a better view of the road for the driver.
"From a technical standpoint, we thought we could execute a little better in tailoring the suspension to the truck," adds Bob Westphal, F-Series chassis manager-Ford Light Truck Vehicle Center Engineering. While Ford's arch-rival Chevrolet uses a similar-type SLA on its C/K pickups, Mr. Westphal and Carl Boyd, leader of the team that developed the F-series steering, suspension, wheel and tire package, say their execution is much different. Steering, for instance, is designed to be faster and more responsive than either the Chevy or the old F-150.
Spring rates also are softer than the previous generation Ford to provide a more comfortable ride, and a much larger stabilizer bar is used to provide tauter handling. The rear Hotchkiss suspension also has more understeer than the old F-150, and that helps it track better with the front, Mr. Westphal says. Bigger standard tires improve the new truck's ride and handling, too, Mr. Boyd points out.
While reviewers have duly noted how much better the new F-150 rides and handles, not many have mentioned the small details that owners will likely appreciate far down the road. For instance, Mr. Westphal says extra-large shock absorbers with bigger-than-normal valve springs and heavier protective coatings are used so they'll last twice as long as normal, or about 75,000 miles. "This is the first truck we know of that has ball-joint seal shields," he adds. Most trucks have similar seals, but they frequently are ruptured by stones or other road debris. That makes them more vulnerable to wear and means they may have to be lubricated more frequently. The plastic shields on the F-150 protect the ball joint seals from harm, and makes them nearly maintenance free, Mr. Westphal says.
On the materials front, plastics are picking up momentum as automakers gain confidence with using them in suspension applications. Ford started using plastic suspension components in the U.S. on the Taurus/Sable several years ago, but the trend is a bit more pronounced in Europe.
Material suppliers such as DuPont Automotive say cost savings of up to 10% and weight savings ranging from 20% to 46% can by achieved by making stabilizer-bar linkage rods out of high-tech resins instead of steel.
Stabilizer bars, also known as anti-roll bars, resist a vehicle's tendency to roll outwards during cornering. They improve the weight distribution on the wheels and make for better cornering and road handling. TRW Ehrenreich Fahrwerksysteme, which makes plastic linkage rods in Europe, says they offer several advantages. Besides being lighter and less costly to produce, they also offer better vibration damping characteristics than metals, TRW says.
Peugeot has been using linkage rods made of DuPont's Zytel nylon resin in its 605 model's rear suspension since 1991. In 1993 it started using resin rods on the front suspension of its 306 model and is eying Zytel for a future model. The Audi 80 used similar nylon linkage rods beginning in 1993, and it is under study for the new all-wheel-drive A4.
