Saab 900 – Chassis

The month of 900 loving continues and here we have the next part of the Saab 900 Engineering brochure from back in 1985.

This was supplied and scanned by 1985 Gripen and gives a great insight into what’s become the epitome of classic Saab.

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Saab 900 – Chassis

Saab ChassisThe Saab 900 incorporates the same basic design features which have made earlier Saabs widely acclaimed for their roadholding properties. The basic elements include front-wheel drive. 60-40 weight distribution, lightweight rear axle, l5-inch wheels, rack-and pinion steering and pivot-mounted springs.

Before the 900 series was launched in 1978, designers devoted massive work to refining the chassis geometry. The design specification was severe. The Saab 900 was to have excellent directional stability and consistent behaviour:

– on different types of road and on a variety of surfaces,
– when carrying a range of loads – including maximum load,
– throughout the speed range,
– in cornering, regardless of whether the driver decelerates, accelerates or brakes the car.

Saab Chassis

In recent years, the automotive industry has displayed a preference for smaller wheel sizes. But Saab has remained faithful to large wheels. The latter offer greater ground clearance, better comfort and better traction on loose surfaces, such as on sand and snow. Larger wheels also provide space for larger brakes, and the brakes are easier to adjust. The Saab 900 has hub-centred wheels. This provides the best conditions for true circularity of the combination of wheel and tire.

The geometry and bushings have been adjusted so that the rear axle exerts no self-steering effect when the car rolls and does not affect the lateral and braking forces. Computers and advanced electronic measuring equipment, designed by the Saab Scania Aerospace and Computer Divisions, were used to optimize the chassis design.

Conventional optical wheel-geometry measuring equipment was simply not accurate enough to satisfy the ambitions of the engineers. The front-wheel coil springs are pivot-mounted – an unusual refinement on standard cars. Owing to the pivot mounting, the springs always remain straight.

Saab chassisThe rear axle is in the form of a lightweight, rigid tube which maintains the rear wheels parallel at al times. This eliminates track variations – a typical disadvantage of the split rear axle and independent suspension for the rear wheels. The fuel tank is located between the rear wheels – the safest place in the event of a collision.

The terms understeer, oversteer and neutral steering are often used when the road behaviour of a car is discussed. When cornering at high speed, a car with understeer will run at a wider radius than that corresponding to the steering wheel displacement. In the same situation, a car with oversteer will run at a tighter radius, whereas a car with neutral steering has a behaviour which lies between these two alternatives. On a car with rear-wheel drive (which normally has a certain amount of understeer) the rear wheels cannot resist lateral forces to the same extent as the front wheels when the car is accelerating. High acceleration may therefore cause the normal understeer of the car to change to oversteer. The car may then be difficult to manoeuvre, particularly on a slippery surface – such as when overtaking on a slushy road or across a ridge of snow in the centre of the road.

Saab Chassis

On cars with front-wheel drive, the rear wheels maintain the directional stability of the car, since they are not subjected to
driving forces. On the Saab 900, the rear axle, weight distribution and design of the brakes also contribute to the rear wheels being capable of withstanding relatively high lateral forces, even during braking.

Saab ChassisThe Saab 900 normally has a certain amount of understeer and retains this characteristic, even under extreme conditions. When carrying the driver and full tank of fuel, the approximate weight distribution of the Saab 900 is 60% on the front wheels and 40 % on the rear wheels. Fully laden, the distribution changes to 5l% – 49%. The Saab 900 thus still has a slight amount of understeer and has consistent behaviour, with good directional stability, even when fully laden.

As opposed to this, a car with a curb weight distribution of, say, 54% on the front wheels and 46% at the rear wheels will have the reverse distribution when fully laden. The road behaviour and characteristics of the car will therefore change – from understeer to oversteer. This may be risky on a slippery surface.

The Saab 900 has disc brakes all round, to ensure the shortest possible braking distance and consistent braking characteristics on all wheels, even under extreme conditions. The handbrake acts on the front-wheel discs.

When the brakes are applied, the loading on the front wheels increases. As a result. the harder the brakes are applied, the higher should be the braking effort exerted on the front wheels in relation to the rear wheels. The Saab brake system is designed so that the braking effort distribution on gentle braking or on a slippery surface is about 60% to the front wheels and 4O% at the rear. On heavy braking on dry roadways, the distribution changes to 80% – 20%.

Saab Chassis

The Saab 900 has a diagonally split, dual-circuit brake system. As a result, half of the braking effort will still be available, even if one of the circuits should fail.

A good steering system must be easy to operate, although without giving rise to ‘wander’. lt must have no backlash and must provide a good ‘feel of the road’. And it must not transmit shocks to the steering wheel when the car runs over irregularities in the road surface. Finally, the steering wheel travel must not be affected by movement of the suspension springs.

The Saab 900 has rack-and-pinion steering. This system has no backlash and a minimum of ‘springiness’ – without the need for periodic maintenance. (On the other hand, a worm-and-pinion steering mechanism requires periodic adjustment to eliminate the backlash.)

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6 Comments

  1. Man, I love reading these things!
    Makes me wonder though, does IRS offer any real benefit to a FWD car? ReAxs and bragging rights are the only things I can think of. XWD is obviously another matter. Could that be the real reason it’s been IRS all along, i.e., planning on XWD?

  2. Sorry to have to correct you Swade, but this Engineering Features booklet was from 1983 (pre-16-valve and pre-9000), not 1985. The other scan I sent you was from the 1985 U.S. 900 Sales Brochure so maybe that’s where the confusion comes from.

    Great that you’re posting this stuff for everyone to enjoy. Thanks.

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