Steering Linkage
Steering Linkage
With a rack-and-pinion steering gear, the rack is connected by linkage directly to the steering knuckle. Recirculating ball-type steering gears require a more complicated linkage to change the rotary output of the sector shaft to the back-and-forth movement of the wheels. A steering linkage consists basically of a steering gear Pitman arm, a center-link, and a tie rod assembly connected to each
other by ball sockets. The Pitman arm is splined on the steering gear sector shaft. When the sector shaft turns, the Pitman arm swings in an arc. The swinging end of this arm is connected to the center link.
The center link (also called drag link or relay rod) transfers the swinging motion of the gear arm to a linear or back-and-forth motion. It can also change the direction of the sector shaft arm motion, depending on the type of linkage. The center link is connected to the tie rods. These transmit movement of the relay rod to the steering arms. The steering arms are part of, or attached to, the steering knuckle spindle assemblies. When the steering arm moves, the steering knuckle assembly rotates on the suspension control arm ball joints.
Tie rod ends are used to connect the tie rods to the center link and to the steering arms. They are also used on the end of the sector shaft arm and the idler arm. Adjustment of the tie rod length is provided in threaded sleeves that are locked by clamps.
A tie rod end is a ball located in a socket. The ball is attached to a tapered stud. A spring or plastic spacer holds the ball in position in the socket. The tapered stud fits into a taper in a steering arm and is held in position by a threaded nut. The ball and socket allows up-and-down movement between the tie rod and the steering arm as the car goes over bumps. The ball and socket also allows back-and-forth movement as the driver turns the steering wheel. Grease is held between the ball and socket with a grease seal.
In most steering linkage arrangements, one end of the center link is supported in the Pitman arm. The other end is supported by a frame-mounted idler arm. The idler arm pivots in a support attached to the frame when the steering linkage moves back and forth.
WARNING: All steering linkage parts are manufactured from malleable materials and will bend, distort, or deflect rather than fracture under extreme shock loads. This toughness and malleability are necessary to avoid the complete loss of control that would occur if any part of a steering linkage were to break. Steering linkage parts must never be heated during a repair because this could cause them to lose their malleability and, as a result, fracture.
Rack-and-Pinion Steering Gear
Rack-and-Pinion Steering Gear
The recirculating ball steering gear has the disadvantage that it occupies a good deal of
space, usually in the engine compartment. The rack-and-pinion steering gear was first developed for compact cars in which the engine compartment space was limited. The rack-and-pinion system has worked so well that it is currently being used in both imported and American compacts and intermediate size cars.
The basic parts of a rack-and-pinion steering gear are shown below. The steering wheel and steering shaft are connected to a pinion gear. The pinion gear is in mesh with a straight bar that has gear teeth cut into one side. The toothed bar is called a rack. When the driver turns the steering wheel, the pinion gear turns, causing the rack to move. This movement, in turn, is connected to a linkage that moves the front wheels.
The rack-and-pinion gear is mounted in a rack housing assembly. The steering linkage consists of two inner tie rods and two tie rod ends. The inner tie rod ends are attached to the steering rack ends. The outer tie rod ends are attached to the suspension arms on the steering knuckles. Rubber boots are used to cover and protect the inner tie rod assemblies from road splash.
Recirculating Ball and Nut Steering Gear
Recirculating Ball and Nut Steering Gear
The larger and heavier the car, the more difficult it is to steer. Most large American cars are equipped with a recirculating ball-type
steering gear. This type of steering gear is very low in friction and provides a good mechanical advantage for a heavy vehicle.
The recirculating ball and nut steering gear consists of several parts contained in a steering gear housing. The steering gear shaft is connected to the steering wheel either directly or through some type of flexible joint. There is a worm gear on the end of the steering gear shaft. A cross (Pitman) shaft is mounted in the housing in a position 90 degrees to the worm gear. A ball nut rides on the worm gear and a gear on the cross (Pitman) shaft, called the cross shaft sector, is engaged with this nut.
Ball or roller bearings are used to support both ends of the worm gear and are adjustable to remove end or side play from the worm gear. The cross (Pitman) shaft is supported by bushings, needle bearings, or a combination of the two, and provision is made to control the worm and cross shaft clearance. All parts are enclosed in a cast housing that is partly filled with lubricant. Seals are used to prevent the entry of dirt or the loss of lubricant. Provision is made to bolt the steering gear housing to a rigid area, usually the frame.
The ball nut has internal threads that are meshed to the threads of the worm with continuous rows of ball bearings between the two. The ball bearings are recirculated through two outside loops, called ball guides.
The sliding ball nut has tapered teeth cut on one face that mate with teeth on the sector. As the steering wheel is rotated, the nut is moved up or down on the worm. Because teeth on the nut are meshed with the teeth on the sector, the movement of the nut causes the sector shaft to rotate and swing the steering linkage connected to it.
The recirculating ball construction results in a friction-free contact between the nut and the worm. When the steering wheel is turned to the left, the ball bearings roll between the worm and the nut and work their way upward in the worm groove. When the ball bearings reach the top of the nut, they enter two ball guides and are directed downward into the worm groove at a lower point. When the steering wheel is turned to the right, the ball bearings circulate in the opposite direction.
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Suspension and Steering - (Click on the images for a larger illustration)
Front Suspension
The NG900 has independent front suspension incorporating MacPherson struts, which are fitted between the steering swivel members and the body and are therefore direct-acting on the wheels. Besides being of compact, robust and reliable design, MacPherson strut suspension also provides good isolation from road noise. The long stroke of the strut makes for excellent roadholding and a comfortable ride. The spring and damper act in the ratio of 1:1 to the wheel.
Front Dampers and Springs
As stated above the front suspension on the NG900 utilises MacPherson struts. This is a suspension system that consists of a combination coil spring and damper in one compact unit at each wheel. With this "independent" suspension design, road shocks at one wheel are not transferred to the opposite wheel. MacPherson struts use fewer parts, meaning a reduction on weight and fewer elements that could wear out. The strut consists of a coil spring and interior damper with welded on spring cup, steering arm mount and steering swivel member. The steering swivel member is the main component in the wheel bearing assembly and consists of a bearing housing, which is integrated with the strut, and a ball joint mounting.
The front springs are of the cylindrical compression type with the top and bottom turns of the coil springs narrower than the other turns, providing greater deflection in a given working space. They are fitted with rubber supports at the top. The top spring support also acts as a compression stop for the spring and is held in place by the tension of the partially compressed spring. The stop for the spring on extension is incorporated in the damper.
The front dampers are of the gas-filled, twin-tube type. The space between the tubes serves as an expansion chamber. The damper is not an integral part of the MacPherson strut and can be replaced separately. The piston rod is protected against dirt and moisture by a gaiter to ensure a long service life. The compression stop and the protective gaiter form a single unit.
Front Suspension Arms
One end of each suspension arm (1) is attached to the subframe via a rubber bush and the other end to the steering swivel member via a ball joint. A support arm (2), secured to the rear of the subframe via a rubber bush, braces the suspension arm longitudinally. The anti roll bar (3) is attached to either the suspension arm or the support arm depending on the year of the car. This change was put in place in 1995. Cars from VIN serial numbers S2012272 and S70414773 onwards have the anti-roll bar mounted on the suspension arm. Earlier models have the anti-roll bar mounted on the support arm. There is a kit available from Saab to upgrade the earlier models if desired.
Track rods
The track rods are screwed to the middle of the steering gear via rubber bushes. The track-rod ends are secured to the track rods on both sides by means of an adjustment screw with both left-hand and right-hand threads which is used for toe-in adjustment. Each track-rod end is secured to the steering arm of the steering swivel member by means of a self-locking nut. The track-rod ends cannot be dismantled but they are self-adjusting to compensate for moderate wear.
Steering column assembly
The steering column assembly is bolted to the bulkhead. The steering column shaft is mounted in two needle bearings suspended in rubber mountings in the steering column assembly. A jointed intermediate shaft connects the steering column to the power-assisted steering gear. For reasons of safety the steering column assembly incorporates a collapsible steel cage, a telescopic steering column shaft and an intermediate shaft with a deformation zone designed to crumple progressively in the event of a head-on collision. In addition, the joint configuration is such that the shaft will be directed away from the driver in a collision. Adjustment of the position of the steering wheel spokes is achieved by adjusting the toe-in on both sides of the car.
Rear Suspension
The rear suspension consists of a semi-rigid trailing rear axle. Since the car has front-wheel drive, an extremely light and simple rear-axle construction can be used, which ensures that unsprung weight is kept to a minimum. The rear axle consists of two spring links connected by an extruded section acting as a torsion bar. There are two anti-roll bars, one outer bar and one inner bar which is inside the axle. Both anti-roll bars have a diameter of 15 mm.
Rear Dampers and Springs
The rear springs are miniblock helical compression springs and are provided with spring supports at the top and bottom. The top supports are made of polyurethane and the bottom ones of rubber. The top spring support also acts as a bump stop. The spring supports are held in place by the tension of the partially compressed spring. The stop for the spring on extension is incorporated in the damper. The rear dampers are of the double-acting, twin-tube, gas- filled type - the gas pressure is used to maintain pressure on the damper fluid. This reduces the foaming tendencies of the fluid and also the formation of air bubbles that can result in noisy operation of the damper.