Ford Expedition gains independence

The Expedition's independent front suspension.

Topping the list of improvements Ford Motor Co. engineers made to the 2003 Expedition is the sport utility vehicle's all independent rear suspension system. The system provides a new level of ride and handling capability to the full-size SUV segment. Other improvements that augment the suspension system include a stiffer frame design and improved traction control.

A rugged, double-wishbone suspension-type is used in both the front and rear of the 2003 model. The rear suspension features an offset A-arm design for the lower control arm to uncouple ride tuning from handling tuning. This design channels jarring road forces into the rear control arm bushings, which are engineered to absorb bumps before they can reach the frame. Voids are cast into certain sides of the bushings to provide thinner, softer rubber walls that give under load and allow the wheels to momentarily move backward in response to a jarring force. The bushings also have been designed with asymmetric qualities, including metal reinforcements molded into the rubber of some of the parts to provide support in the areas that receive steering forces. Because these bushings are tailored to provide support in certain directions, the parts have been indexed to ensure they are installed in the correct orientation.

In addition to these bushings, Ford engineers developed a "grippy" bushing for the front and rear stabilizer bars. A flat area is forged into the stabilizer bar at its mounting points to match an oval-shaped bushing that prevents the bar from turning. This design provides quicker side-to-side response to cornering forces, reducing body lean while sharpening steering responsiveness. The responsiveness is enhanced further by the use of ball-type joints at the stabilizer bar end links.

The new suspension system also contributes to the vehicle's stability. The rear toe link design improves tracking during maneuvers by reducing the effect of roll on steering geometry. A robust crossmember that runs from the left to the right rear control arm mounts contributes to reduced frame flexing, further enhancing stability. The new suspension design also enabled engineers to lower the Expedition's roll centers by 58 mm (2.3 in) in the front and 211 mm (8.3 in) in the rear, reducing the leaning forces generated on the vehicle. The suspension system's resistance to body lean is also made apparent by the 50% improvement in roll stiffness achieved on the new design.

The independent rear suspension of the Expedition features an offset A-arm design for the lower control arm to uncouple ride tuning from handling tuning.

Engineers were also able to balance torque forces from the driveshaft and output shafts for "zero roll" geometry with a new rear differential mounting design. This balancing helps reduce halfshaft and CV joint angles under roll as well as any chance of binding. To reduce road vibrations transmitted to the cabin, the differential is mounted to a crossmember designed with a large cross section.

To improve the Expedition's suspension further, Ford Engineers increased the amount of aluminum used in the system's construction, enabling a reduction of unsprung mass by 50 kg (110 lb) compared to the live-axle design used previously. Six of the eight control arms are constructed from lightweight aluminum. The remaining two—the upper control arms for the front wheels—are made of forged, thin cross-section steel. By using steel for these two control arms, engineers afforded themselves room for packaging the coil-over-shock system in both the two-wheel and four-wheel-drive configurations.

Later in the model year, the Expedition will be offered with an optional four-wheel load-leveling air suspension, which employs air springs in place of the standard coil springs. The previous generation air suspension system used by Ford had air springs on the rear axle only, with high-pressure air shock absorbers fitted to the front.

Air springs provide the system with progressive resistance to suspension movement—there is very low resistance for about the first 25 mm (1 in) of movement, followed by higher resistance to prevent bottoming out. The progressive resistance of the air springs allows the suspension to react more quickly to small road imperfections, providing a softer ride than the traditional coil springs.

Later in the model year, the Expedition will be offered with an optional four-wheel load-leveling air suspension, which employs air springs in place of the standard coil springs.

In addition to the air springs, the system also employs Bilstein shocks, which feature a monotube design that allows the use of a larger hydraulic piston. The design provides increased flexibility in tuning response over conventional twin-tube shocks, improving both on- and off-road performance.

Another feature of the air suspension system is that it can be automatically lowered by 25 mm (1 in) when the transmission is shifted into park and the ignition is shut off, allowing easy ingress and egress of the vehicle. When the four-wheel-drive system is activated, the system automatically raises by 25 mm (1 in) to improve ground clearance for off-road operation.

For easy loading and hauling of heavy cargo, ride height sensors automatically adjust the pressure in the rear air springs to compensate for cargo weight. This adjustment keeps the vehicle level from front-to-rear and maintains the correct suspension geometry throughout a range of cargo weights.

In addition to the independent suspension system, Ford engineers sought to improve the 2003 Expedition's ride quality with a stiffer frame. Hydroforming, a metalworking technique that uses hydraulic pressure to create bends without weakened stress points, enabled engineers to develop a frame that was 67% stiffer in bending and 70% improved in torsional rigidity. The frame, which measures 3 m (10 ft) long, was formed by using hydraulic pressures up to 120 MPa (17.4 ksi), which expanded the tubular metal stock inside the frame mold. The process provides equal amounts of pressure to all sides of the part, reducing the tendency for variations in thickness that often occur with metal stamping.

Because the wall thickness can be more tightly controlled, the frame can be engineered with comparatively thinner metal overall and a larger cross section. Sections of the frame vary from 2 mm (0.08 in) in thickness to 4.7 mm (0.185 in) to match the specific loads at each location.

Another benefit from the hydroformed frame was the elimination of 12.2 m (40 ft) of weld lines and the overlapping material used to join the two C-channels that make up the frame.

A second-row bench seat with a sliding center section provides easy access to a child from the front passenger seat.

Front and rear sections of the frame are designed as fully boxed sections that can be replaced individually in the event of a crash. In addition, engineers designed larger porthole cutouts in the frame for the rear axles. The vertical oval-shaped holes, which are reinforced with a steel tubular tunnel, provide more room for the axles to move upward and downward as the independent rear suspension tracks uneven terrain. The Expedition's independent rear suspension is capable of a full 229 mm (9 in) of wheel travel.

Improved traction control is made possible in the Expedition through two types of systems—AdvanceTrac and ControlTrac. AdvanceTrac monitors driver intent, road surface conditions, and slip at all four wheels to deliver torque exactly where needed. The system features its own computer controller, enabling it to respond even before slip begins. AdvanceTrac uses electronic braking to transfer torque side-to-side, while the ControlTrac transfer case divides torque front-to-rear. Even with two wheels completely off the ground, the new Expedition can continue to move. Using the brakes rather than mechanical limited-slip devices inside the differential enables quicker response and more seamless performance.

The electronic traction control system also enables the driver to reduce overall engine power in some situations and access full power when needed. This can be particularly helpful for icy road conditions when reduced engine power can be used to prevent wheelspin.

Other improvements made to the Expedition for MY2003 include an improved third row seat with a lowered floor to provide a more natural seating position, a second-row bench seat with a sliding center section that can be moved forward for easy access to a young child, a new safety system, a safety canopy, and an enhanced tow rating of 8900 lb.

- Frank Bokulich