Platform and Chassis



BEAUTY IS MORE THAN SKIN-DEEP

An In-Depth Look At The New Saab 900 Platform and Chassis

NORCROSS, Ga. -- Like all Saabs, the new 900 is designed to serve as a consistent communication link between road surface and driver, providing continuous feedback and accurate information about ever-changing driving conditions. An integral part of the Swedish automaker's advanced chassis philosophy is a unique "signal-to-noise" ratio, which is designed into every Saab and keeps the driver in touch with his or her surroundings and road surface changes at all times. Saab believes that drivers must feel road surface feedback, but not uncomfortable harshness, and hear sound without irritating noise, to make clear and concise driving decisions for utmost performance. This is especially vital during emergency situations and is the basis of Saab's legendary fun- to-drive persona.

The most suitable starting point for the development of the 1994 Saab 900 was its new platform. This carefully-chosen structure, optimized to provide characteristic Saab handling and feel, balances a new level of improved comfort with utmost stability and steering precision.

Each section and panel joint of the Saab 900 floorplan was strengthened and reinforced through careful selection of material gauge. Additional joint welds were included to improve the torsional rigidity of the body structure, and to further add to the integrity of the integrated passenger compartment safety cage.

High-strength steel is specified for the door sills, which were integrated with a new, intermediate reinforcement and a special reinforced rear beam incorporated in the seat pan pressing. This beam is shaped to resist submarining and is iocated to help reduce side impact intrusion.

Compared to the previous 900, this floorplan is significantly stronger and stiffer, which makes it ideally suited to accept Saab's advanced chassis dynamics.

The entire front sub-frame assembly is galvanized for improved corrosion resistance. In addition, this hot-dip process introduces a direct chemical bond between each flange joint, naturally reinforcing the normal welding procedure and stiffening the assembly even further.

MacPherson Strut Front Suspension

The 1994 Saab 900 features MacPherson strut front suspension with special upper mounts that decouple the spring and damper attachments. As a result, the stiffness of their rubber bushings could be carefully optimized for more accurate chassis control. An additional, triangular tubular brace is bolted between the two upper strut towers to add to the lateral stiffness by a factor of three. This rigid sub-system improves steering precision and handling response, without compromising ride comfort.

The coil spring for each front strut bears against an insulating rubber ring, mounted in a steel collar. This ring also houses a ball bearing and bears against a secondary rubber insulator. The secondary pad complements the function of the primary insulator for better isolation of the stiffer support structure from road shock. Long bump stops with a special profile progressively limit suspension travel. The upper ends of piston rods for the gas-pressurized telescopic shock absorbers are mounted in an inner bushing which is bonded directly to the strut's upper aluminum housing. This composite design has been extensively developed to contribute to Saab's unique "signal-to-noise" ratio.

The front struts are clamped at their lower end to cast steering knuckles, which are integrated with the hub carriers and bearing housings. Conventional lower suspensions arms or wishbones are replaced by unique two-piece lower links, forged in aluminum alloy to provide high strength and stiffness, with reduced weight and low acoustic noise transmission. They help dampen the noise path to the interior as close as possible to its source (the road) which is more effective than adding insulation. In addition, this design reduces unsprung weight for improved ride quality. The two-piece links also add a kinematic steer effect that compensates for toe-steer caused by torque changes at the front wheels under hard acceleration, and increases stability and traction.

Instead of double horizontal bushings for the inboard wishbone pivots, a combination system with a vertical rear bushing is used. The normal horizontal front bushing resists braking and acceleration torque, while the special rear unit controls compliant wheel displacement over bumps, for a smoother, quieter ride. It also allows the wheel to steer itself against any imbalance in braking force. Consequently, stability is maintained by using negative scrub-steer offset to counter any unequal frictional forces at either front wheel.

Under high acceleration forces, normal compliance in the bushings causes the front wheels to turn slightly, reducing the traction available in a powerful front-wheel drive application and increasing tire wear. To compensate for this natural effect, which is particularly noticeable on cars with high start-up torque-like the V6-powered Saab 900 SE, the two-piece lower links also include an extra bushing which serves as an integrated ball joint connection. This extra degree of lateral freedom virtually eliminates torque-steer when accelerating, so full traction is maintained.

The extra bushing also increases the longitudinal compliance of the lower strut assembly, improving ride comfort over sharp-edged obstacles such as potholes, railway crossings, and road section joints. Shock absorber valving has been tuned specifically for the Saab 900, giving stiffer control at low piston speeds to improve the control of body movements and provide a flatter ride quality.

The power-assisted rack and pinion system is mounted on the firewall, reducing the possibility of damage in the event of a collision, and thereby allowing steering to be maintained. The system has been developed to improve on-center feel and features a new spring-loaded servo valve arrangement. This gives the new 900 a manual steering feel in the straight-ahead position, and power assistance only when needed to reduce turning effort. The Saab 900's turning circle is 36.4 ft. wall to wall, and 35.4 ft., curb to curb.

Twist-Beam Rear Axle

The new Saab 900 rear axle is designed to provide dynamic roll oversteer, which Saab engineers believe gives better straight-line stability and more positive turn-in control than roll understeer. This makes the Saab 900 more responsive, and at the same time more damped in its directional movements--it's a characteristic that has been a fundamental part of the Saab handling philosophy for many years.

The H-shaped axle side arms are of a heavy gauge metal, and additional welds reinforce the assembly. This design is lighter than its predecessor and provides more longitudinal compliance, to further improve ride comfort. The front ends of the side arms are connected to the body through large-diameter, well-tuned rubber bushings, with direct-acting compact, taper-wound coil springs. Separate gas-pressurized telescopic shock absorbers are attached behind the wheel hubs, while maintenance-free wheel bearings are mounted inside welded hub carriers.

For fine-tuning the roll stiffness at the rear, a separate stabilizer bar is mounted inside the open-channel cross tube of the rear axle assembly, with a second stabilizer bar bolted externally behind. Extra plates are welded to the rear axle mounts to improve lateral stiffness.

The Saab 900 S is fitted with 15-inch steel wheels and full wheel covers, while V6 models include 15-inch "Turbine-Spoke" alloy wheels. Both are designed for optimum cooling of the brakes, and allow ample space for large-diameter brake discs. Saab's traditionally large wheels also improve ground clearance over bumps and reduce tire wear. Pirelli P4000E tires are specified, in the popular 195/60 R15V dimension.

Four-Wheel Disc Brakes and ABS Standard on All Saabs

The Saab 900 has a dual-circuit, diagonally-split brake system which operates large diameter disc brakes (11.2 inch front, 10.2 inch rear) on all four wheels. The front brakes are ventilated, and use a single-piston sliding caliper arrangement, which eliminates the risk of hydraulic vapor lock during heavy thermal loading. The rear discs are solid, with pin-type calipers. Saab was the first auto manufacturer to introduce asbestos-free brake pads in 1982 and of course, the new 900 is so equipped.

All Saab 900 models are equipped with a standard lightweight and compact electronic anti-lock brake system (ABS). Normal high-performance braking is still available even if the ABS electronics should fail for any reason. The three-channel system measures all four wheel speeds by inductive sensors. On the rear wheels the sensors are integrated in a unique new Swedish SKF hub bearing.

When the car is travelling above a minimum speed, which can be as low as 2 mph, signals from the wheel sensors are evaluated and compared in an electronic control unit. As soon as the control unit detects that one of the wheels is approaching its limit of adhesion during braking, a command signal is applied to the relevant ABS valve to control the brake pressure being applied, thereby preventing wheel lock.

If the wheel sensor still detects a locking tendency on that particular wheel, brake pressure is reduced slightly more to prevent locking. Simultaneously, pressure is maintained at a sufficient level to ensure maximum braking effect for the available friction coefficient on the road surface at that instant. Should the wheel speed increase as the friction improves, the relevant sensor signals cause the electronic control unit to update its command signals to increase the brake pressure, and hence the braking forces.

This sequence of events takes place several times per second. The system includes two microprocessors which independently process the signals received from the sensors. A self-monitoring function checks the entire ABS system continuously. Should an error in the system be detected, the ABS function is automatically discontinued, a warning lamp on the instrument panel illuminates and the malfunction is stored in the electronic control unit. This information can then be retrieved during service to simplify the repair.

Traction Control System (TCS) Is Standard With V6 Engine

The V6 engine-equipped Saab 900 SE also comes standard with a Saab- developed Traction Control System (TCS). TCS prevents wheelspin on slippery surfaces by automatically reducing the throttle opening through electronic control, to the point of maximum available traction.

The TCS receives wheel speed information from the ABS system. As soon as the TCS detects a front wheel is about to start slipping, it reduces the throttle opening to achieve the best combination of tractive force and steerability. An on-off switch is provided on the instrument panel so the driver can temporarily override the system, as needs dictate.


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