Saab 9-3: Chassis, brakes, steering

Sporty Dynamics Engineered for World-Class Handling

• Sporty handling to challenge class benchmark
• Saab ReAxs – unique rear-wheel-steer characteristic
• Braking system includes electronic driver aids
• Excellent steering response
• Saab-tuned Electronic Stability Program, ESP

“This car will put a smile on the face of anyone who appreciates a fine handling chassis. It’s a premium sedan with the responses of a sports car!”
– Ekkehard Schwartz, Chief Chassis Engineer

The new Saab 9-3 fulfils its role as a fun-to-drive sport sedan by displaying exceptional handling characteristics that not only set new standards for Saab, but also promise to set a new class benchmark. The result of a rigorous development program, the most exhaustive ever undertaken by Saab, the 9-3’s exceptional handling is designed to combine the benefits of front-wheel drive – predictability and excellent driver feedback – with new levels of chassis control for a more rewarding driving experience.

The new 9-3’s exceptionally stiff body and all-new suspension provided Saab test drivers with a solid foundation on which to achieve outstanding chassis dynamics, exemplary ride refinement and impressively low levels of noise, vibration and harshness. The chassis’s innate talents are backed up with a full alphabet of standard electronic driver aids, including ABS (Antilock Braking System), TCS (Traction Control System), EBD (Electronic Brake-force Distribution), Cornering Brake Control (CBC), Mechanical Brake Assist (MBA) and ESP (Electronic Stability Program).

Sporty handling
The top priority of the new 9-3’s chassis development program was to achieve best-in-class handling characteristics. This was an essential requirement for a car targeted at drivers whose priority is a satisfying, high-performance driving experience. Saab’s chassis engineers are confident that anyone who drives the new 9-3 will immediately appreciate its quick responses to steering, throttle and braking inputs, together with its impressive levels of grip, driver feedback and body control. In all respects it behaves as well as, or better than, its class rivals.

Compared to the previous model 9-3, the key dimensions of new car offer several changes that contribute to improvements in handling and grip. The front and rear tracks (59.8 in. and 59.3 in.) are, respectively 2.8 in. and 2.5 in. wider. Combined with a 2.7 in. increase inwheelbase, a .4 in. (10 mm) reduction in ride height, a lower center of gravity and an inherently stiffer sedan bodyshell, the dynamics of the new chassis are more competent in every respect.

ReAxs – passive rear-wheel steering
The adoption of a four-link rear suspension layout has allowed engineers to dial in a unique, passive rear-wheel-steer characteristic (Saab ReAxs) by carefully tuning both inboard and outboard suspension bushings.

When cornering, the kinetics at the rear axle induce a very slight deflection of both rear wheels in the opposite direction to the steering input, ie. toe-out for the outer wheel and toe-in for the inner wheel. Depending on the radius of a bend and the consequent loadings at the rear axle, one degree of movement at the front wheels would typically produce a small but significant response of about one hundredth of a degree at the rear.

This is sufficient to prevent excessive understeer, where the driver is forced to apply progressively more steering input to turn the front end of the car, increasing the scrub angles of the front tires. The ReAxs characteristic overcomes this “crabbing” effect, helping the tail of the car follow the direction of the front wheels, instead of its nose. For the driver, this gives the car a better balance, helping it turn in and respond more precisely to steering inputs.

Suspension design
The front suspension includes McPherson struts with de-coupled top mountings, separating the damper and spring loadings. The lower control arms are attached to a hydroformed (hollow section) front sub-frame, including a hydraulic bushing on the rearmost mounting each side for optimum damping. For good directional stability and ride comfort, the layout is designed to be largely impervious to lateral forces and compliant longitudinally.

The independent rear suspension is also mounted on a sub-frame and incorporates three transverse and one longitudinal link. Compared to the rigid, torsion beam used for the previous model, the new layout has far more longitudinal compliance. Like the front suspension, bushings are stiff for lateral forces and relatively soft longitudinally. The new suspension allows greater scope for optimizing both handling and ride characteristics, as well as helping to prevent road noise entering the cabin.

New for Saab is the adoption of shear bushings in the rear shock absorber top mountings. These effectively dissipate vibrations from the road by absorbing small vertical up and down movements before they are transmitted directly into the mounting and the structure of the car. This feature improves ride quality and also plays an important role in further attenuating road noise.

Reducing unsprung weight
To deliver the high standard of ride quality expected in this class of car, Saab engineers carried out a thorough program to minimize unsprung weight. As a result, all four wheel carriers (hubs) and brake calipers, lower control arms in the front suspension and the toe and lower links in the rear suspension are made of aluminum.

Elsewhere, weight-saving materials are used wherever stiffness is not compromised. Both front and rear anti-roll bars and the shock absorber piston rods are therefore made from hollow section steel.

Braking system
The powerful steel disc brakes (ventilated at the front) are a match for the performance of the new 9-3. The Linear model will feature discs that measure 11.8-in. (300 mm) at the front and 11.4-in. (290 mm) at the rear. The Arc and Vector models will feature even larger 12.3-in. (312 mm) front discs.

High speed braking stability sets new standards for Saab thanks to the ReAxs control of the rear suspension geometry. This effectively preloads the rear tires by applying some lateral force through toe-in as braking commences.

An automatic brake boost feature, Mechanical Brake Assist (MBA), is introduced for the first time. This increases braking pressure from the master cylinder when the driver stomps hard on the brake pedal – such as during a panic stop. Activating ahead of the car’s ABS, this is designed to stop the vehicle more quickly.

A four-channel ABS (Anti-lock Braking System) and TCS (Traction Control System), with additional throttle control, is standard on all variants. A series of electronic brake control functions also utilize the ABS wheel-speed sensors and variable valving at each wheel.

Electronic Brake-force Distribution (EBD) is designed to ensure that optimum braking force is always applied at both axles for maximum stability under heavy braking.

New for Saab is the introduction of Cornering Brake Control (CBC), which is activated when the car is cornering during heavy braking. Brake pressure is individually varied between all four wheels to help keep the car stable, designed to minimize the possibility of snap oversteer or understeer characteristics.

All four brake discs have full size dust shields to help ensure braking performance is not impaired by large amounts of snow, mud or dust jamming the calipers.

Precise Steering
The power steering on Saab cars is widely acknowledged for achieving just the right balance between assistance and feel. In the straight-ahead position, there is virtually no power assistance, in order to give the driver as much direct control as possible. Even at low speeds, the driver’s feel for the road surface is not impaired by excessive power assist.

The rack-and-pinion steering system for the new 9-3 continues this tradition. The hydraulic pump is camshaft-driven and the rack is mounted low down at the back of the front sub-frame. The geometry and tuning of the front and rear suspensions also combine to give the steering its distinctive feel, linearity and responsiveness. At just 2.97 turns from lock to lock, it is the quickest steering yet fitted to a Saab car.

As a general indication of the tautness of the new 9-3’s chassis, tests show its lateral acceleration response time – the speed at which all chassis response to a steering input is completed – is about 30 percent faster than average for this size of car.

Advanced Electronic Stability Program (ESP)
The fine chassis dynamics of the new 9-3 is supplemented by an advanced, Saab-tuned ESP system which is designed to gently counteract over-exuberance or misjudgments from the driver.

The ESP has been honed by Saab technicians using dedicated software to match the behavior of the 9-3 chassis. The “intelligent” system is extremely progressive in use and is designed to function almost imperceptibly, avoiding harsh interventions when the chassis has already reached the limits of adhesion.

A yaw sensor in the center of the car detects movement around the vertical axis and a sensor on the steering column measures the steering angle being applied. Inputs from these sources are then correlated with the speed of the car.

The system is extremely flexible and it is programmed to execute whatever actions will most effectively assist the driver in controlling the vehicle. Depending upon the chassis’s disposition, it is possible for braking to be applied via the ABS system to any or all of the four wheels, as well as control of the electronic throttle.

New Saab 9-3 In Focus – Making the Right Noises
Stefan Svedhem’s brief in the development of the new 9-3 was a simple one: turn noise into sound. His role was to ensure that the improvement in levels of noise, vibration and harshness (NVH) was at least equal to the progress made in the chassis’s ride and handling qualities. While that meant eliminating unwanted noises, it also required sophisticated “noise management” or, as Stefan prefers to say, “turning noise into sound.”

“Noise is usually something irritating and unwanted,” says Stefan, manager of Saab’s Noise and Vibration Center, “whereas sound is something we appreciate and feel positive about.” The work of Stefan and his team included everything from eliminating squeaks and rattles and checking “click” quality from the switchgear to making sure noises from the road and powertrain were the “right sounds.”

In addition to making hundreds of hours of sound recordings from different positions inside the cabin, the team also used a powerful, finite element computer model. With over 400,000 elements, this was the main tool in combating structure-born noise and vibration inside the car.

“We have made a lot of progress with low frequency noise, typically the bump/thump from the road transmitted through the suspension,” says Stefan. “And we have balanced this with improved insulation for high frequency sound, such as tire and wind noise.”

Good NVH does not necessarily mean eliminating noises altogether because sounds, such as engine or road noises, can be provide relevant signals for the driver. It would be disorientating to be placed in a complete “noise vacuum.” “Our work with noise,” adds Stefan, “usually involves what I call the ‘three I’s’. Information, where the driver should hear what he expects, such as when turning the starter or operating a switch.

Impressiveness, where noises should become nice sounds, such as when the engine is being revved up or when the door closes with a nice thud. Lastly, there is irritation, and these are noises we don’t want to hear at all, such a poor door seal and, of course, squeaks and rattles.”