MY08 Saab 9-3 – Chassis, brakes and Steering

THIS IS THE PRESS RELEASE THAT COVERS THE XWD SYSTEM!!

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Sporty Dynamics come as standard

• Sporty handling to challenge class benchmark
• Saab ‘ReAxs’ – unique rear wheel steer characteristic
• All-wheel-drive (Saab XWD) offered on Aero variants
• Three brake sizes according to engine power

The Sport Sedan and Sport Combi fulfil their roles as fun-to-drivecars by displaying exceptional handling characteristics that not only set new standards for Saab but also challenge class benchmarks. They are the result of a rigorous development program, the most exhaustive ever undertaken by Saab, 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.

An exceptionally stiff body provided Saab engineers 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 arsenal of electronic driver aids, including ABS (Antilock Braking System), TCS (Traction Control System), EBD (Electronic Brake force Distribution), Cornering Brake Control (CBC) and an optional ESP® (Electronic Stability Program).

Sporty Handling with Saab ReAxs – passive rear wheel steer
The top priority of the Saab 9-3 chassis development program was to achieve best-in-class handling characteristics. This was an essential requirement as the car, in either sedan and wagon format, is targeted at owners who value the enjoymnent of a satisfying, high performance driving experience.

Anyone who drives the Sport Sedan or SportCombi will immediately appreciate its quick responses to steering, throttle and braking inputs, together with its impressive levels of grip, driver ‘feedback’ and body control. A low center of gravity and the exceptionally stiff bodyshell also play an important role.

The adoption of a four-link rear suspension layout, with toe-links and the use of ball joints instead of rubber inboard and outboard suspension bushings, has allowed engineers to dial in a unique, passive rear wheel steer characteristic, Saab ReAxs.

When cornering, the elasto-kinematics at the rear axle induce a very slight deflection of both rear wheels in the opposite direction to the steering input, i.e. 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 lock to turn the front end of the car, increasing the scrub angles of the front tyres. 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 closely to steering inputs.

Suspension Design
The front suspension is by MacPherson struts with de-coupled top mountings, separating the damper and spring loadings, gas shock absorbers and a direct-acting anti-roll bar. The lower control arms are attached to a hydroformed (hollow section) front sub-frame, including a hydraulic bush 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 an isolated sub-frame and incorporates three transverse and one longitudinal link with an anti-roll bar. This layout allows a high degree of wheel movement control with good longitudinal compliance. Like the front suspension, bushings are stiff for lateral forces and relatively soft longitudinally. There is great scope for optimizing both handling and ride characteristics, as well as helping to prevent road noise entering the cabin.

Shear bushings are used 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.

To deliver the high standard of ride quality expected in this class of car, unsprung weight is reduced by using aluminum for the front wheel hubs and brake calipers, the lower control arms in the front suspension and the toe and lower links in the rear suspension.

Saab XWD
An active all-wheel-drive system (Saab XWD) is designed to optimize handling and stability in all driving conditions. It is offered as an option on Aero variants, together with an uprated 280 hp, 2.8V6 turbo engine. This state-of-the-art system includes innovative developments in wheel slip management the use of an electronically-controlled, rear limited-slip differential. It comes with a choice of six-speed manual or automatic transmission.

Saab XWD is a fully automatic, on-demand system capable of sending up to 100% of engine torque to the front or rear wheels whenever necessary. While offering sure-footed handling in low-grip conditions, its sophisticated operation also adds a further sporty dimension to the driving experience in all road conditions. Fine balancing of the drive torque between the front and rear axles raises the threshold at which ESP throttle and braking interventions are triggered, providing more scope for closer driver involvement.

The system is governed by its own electronic control unit, which functions in harness with the engine, transmission and ABS/ESP control modules. The hardware consists of a Power Take-off Unit (PTU) in the front final-drive that transmits engine torque through a prop-shaft to the Rear Drive Module (RDM). This incorporates a Torque Transfer Device (TTD) and an optional electronically-controlled Limited Slip Differential (eLSD). Both are wet, multi-plate clutch units from Haldex.

At take-off, the TTD is initially activated when the clutch plates are forced together under hydraulic pressure, thereby engaging the RDM. This pre-emptive function is a valuable improvement in current technology, which requires the detection of wheel slip before the TTD is activated. For the driver, the enhanced functionality gives maximum traction immediately for smooth, strong acceleration from rest without the possibility of any initial hesitation.

On the open road, drive torque is seamlessly and continuously varied between the axles by the control of a valve in the TTD, which increases or reduces the pressure on the wet clutch plates. When cornering, Saab XWD rewards the driver by providing enhanced, more finely balanced chassis dynamics. Data from the vehicle’s ABS/ESP sensors – measuring wheel speed, yaw rate and steering angle – is utilized and careful programming of Saab XWD enables the application of rear drive to balance oversteer and understeer characteristics,, improving stability and roadholding.

In highway cruising conditions, when traction or optimum grip is not an issue, only 5% to 10% of engine torque is typically transmitted to the rear wheels. This helps provide the driver with a measure of greater stability, while helping to save fuel.

The ultimate ‘icing on the cake’ with Saab XWD is the eLSD option. In this segment of the market, it is the first application of an electronically-controlled, multi-plate limited slip differential in combination with all-wheel-drive. It is installed alongside the RDM and operates via pressurized clutch plates on a principle similar to the larger TTD. In icy or wet split-friction conditions, for example, it uses input from the rear wheel speed sensors and can transfer up to 40% of torque between the drive shafts, to whichever wheel has more grip.

The eLSD also gives the driver enhanced control when cornering hard or completing a high speed maneuver, such as a lane change, by momentarily applying more or less torque to either of the wheels to help the rear of the car more closely follow the direction of the front wheels. In this way, the eLSD can keep the car better balanced and more tightly controlled without requiring ‘outside’ intervention from electronic stability aides.

Installation of Saab XWD includes the fitment of a new rear sub-frame to carry the RDM, revised rear suspension geometry and new wheel hubs for the drive shafts. The three piece prop-shaft runs through two bearings with constant velocity joints for smooth running with minimal ‘wind-up’. Wheelbase and rear track dimension are unaltered.

Braking System
In EU specification, three brake sizes are fitted according to engine power. For all four cylinder engines, except the 210 bhp version, 285 mm front (ventilated) and 278 mm solid rear discs are fitted. For the 210 bhp engine they are 302/292 mm (both ventilated) and for the Aero 2.8V6 turbo and 1.9TTiD engines these are increased to 314/292 mm

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

A four channel ABS (Anti-lock Braking System) and a Traction Control System (TCS), is standard fitment for all variants. An automatic brake boost feature, Mechanical Brake Assist (MBA), increases braking pressure from the master cylinder when the driver stamps hard on the brake pedal, stopping the vehicle quicker and activating the ABS more effectively.

Other electronic brake control functions also utilize the ABS wheel speed sensors and variable valving at each wheel. Electronic Brake force Distribution (EBD) ensures an optimum braking force is always applied at both axles simultaneously for maximum stability under heavy braking.

Cornering Brake Control (CBC) is activated when the car is cornering under heavy braking. Brake pressure is individually varied between all four wheels in order to keep the car stable, minimizing any snap oversteer or understeer characteristics.

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

Advanced Electronic Stability Program®
The fine chassis dynamics of the Saab 9-3 can be supplemented by a new generation Saab-tuned ESP® system, which is designed to gently counteract over-exuberance or misjudgements from the driver.

The latest, driver-selectable ESP® has been recalibrated and honed using dedicated software to match the behavior of the 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 prevent the onset of a loss of control. Depending upon the chassis’s disposition, it is possible for braking to be applied via the ABS system to one, two or three wheels at once, or, if necessary, for the electronic throttle opening to be reduced.

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 assistance, in order the give the driver as much direct control as possible, and even at low speeds the driver’s feel for the road surface is never impaired

For variants with 1.8i naturally-aspirated gasoline and turbo diesel engines, an electro-hydraulic power steering system is fitted, which uses a pump driven by a computer-controlled electric motor as a further contribution to improved fuel consumption.

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

The hydraulic pump for the rack and pinion system is camshaft or belt-driven, depending on the engine variant, 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.

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2 Comments

  1. should I interpret this like there will not be a cross country version, just a XWD feature?

    If so I think that would be a huge misstake. The bigger plastic bumpers and slightly higher suspention attracts lots of wommen, being a good compromise between a combi and a suv.

    The old spy pics of the 9-3 XC promised well, but now it just have to go into production.

    Swade – thanks for a great coverage. I am glad that you have enjoyed it and that sweden showed itself from the best side!

  2. Yes cj, I think you are correct. There is no such version.

    I don’t think that SAAB has ever had a cross country version of any model.

    While the XC70 is one of the top selling Volvos in the US, I don’t think it is nearly as popular anywhere else. Also, I don’t think the 9-3 SportCombi is a good alternative for those who want a car the size of the XC70. The SAAB 9-3 SportCombi is much smaller and us in another market segment entirely. It’s not a full sized wagon/estate/combi by any stretch of the imagination.

    A more targeted competitor would be the 9-5 Combi. So if SAAB wants to compete with the XC70 (and the Audi Allroad) they would need to make the 9-5 four wheel drive and add all the cladding plus the high suspension.

    And just in my opinion, the XC70 is awful. But that’s my opinion. Seems lots of people like the car and it was a smart move from Volvo.

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