Audi A4 Dynamic Suspension and Steering System

The dynamic suspension underpinning the new Audi A4 sets higher standards than ever before, combining precision and dynamism with a high level of stability and control. It has been entirely redesigned, with suspension mounts, steering, wheels and brakes developed for top performance and many aluminium components used to keep unsprung weight to a minimum. Two innovative technologies complement this extensive redevelopment work: the Audi drive select adaptive regulating system and Audi dynamic steering.

The five-link front suspension makes a considerable contribution to the car’s dynamic character. By moving the differential to the front and the clutch to the rear, the engineers were able to reposition the front axle 154 millimetres further forward – a design approach that Audi introduced for the first time on the current A8 model. Together with relocation of the car’s battery in the boot, this optimises front and rear axle-load distribution.

The largest and most complex component in the entire running gear is the aluminium support beam for the engine and the front suspension assemblies. It is bolted rigidly to the front end of the car and thus forms part of the body structure. Thanks to its high rigidity, steering movements are carried out promptly and accurately. On the new Audi A4, aluminium is also used for the bearing mount that links the upper suspension control arms with the body, and for the pivot bearing. The latter is manufactured by a combined casting and forging technique that gives it very high strength.

Each front wheel is located by five suspension links – a support link and a control arm at the bottom and two control arms at the top. The fifth link – the track rod – connects the steering box and the pivot bearing. All these links are made of forged aluminium to keep unsprung weight low, provide ultra-precise wheel control and ensure a high level of crash safety. To save weight, the anti-roll bar is actually a high-strength tube.

The geometry of the upper and lower control arms enables the virtual steering axis – a value obtained from an imaginary extension of the control arms to the point where they intersect – to be close to the wheel centreline. This reduces the leverage exerted by driveline and external forces so that the driver does not feel them at the steering wheel.

The five-link suspension with its tapering control-arm layout handles longitudinal and lateral forces separately when the car is in motion. The bearings are rigid in a lateral direction, to promote precision and permit high cornering speeds. Longitudinally, however, they are softer and more supple, enabling the A4 to deliver outstanding handling without major sacrifices in ride comfort.

The steering system fitted to the outgoing A4 was mounted high up and well to the rear, above the gearbox and attached to the base of the radiator water chamber. It is now further forward and lower, on the subframe just below the front axle.

Since movement of the steering wheel is transmitted to the wheels in a very direct manner by the track rods, steering response is immediate, and feedback from the road surface precise and well differentiated. Friction-optimised and lubricated track rod joints add to this sensitivity.

The steering column has a torsionally rigid bolted joint at the cross-member below the windscreen and the bearing mount on the firewall, which also makes steering response more precise. The standard power steering has a rack and pinion mechanism in an aluminium casing. With an overall ratio of 16.1 : 1, the steering is responsive and direct, but with no hint of nervousness. A controlled-output vane-type pump supplies the necessary hydraulic energy; unlike conventional power-steering pumps, which circulate a large volume of oil internally, it delivers only as much oil as is needed in any given operating situation, helping to minimise power draw, and therefore fuel consumption.

A4 3.2 FSI, A4 2.7 TDI and A4 3.0 TDI models are equipped with an even more sophisticated servotronic power steering system, with the degree of power assistance dependent on road speed. This system makes parking easier by reducing the effort needed at the steering wheel, whereas at higher road speeds the necessary effort is increased to ensure maximum precision.

The rear suspension in the new Audi A4 is based on the toe-controlled trapezoidal layout chosen for the largest A6 and A8 model lines – with compact dimensions and excellent ride quality and comfort. The suspension kinematics for the A4, however, were calculated without reference to any other models.

The backbone of the rear suspension is a subframe resistant to torsional and bending loads. It is welded together from two longitudinal and two lateral tubes of high-strength steel, the longitudinal tubes being produced by the internal forming method using water at very high pressure.

This subframe is attached to the bodyshell by four large rubber mountings; these are extra-firm laterally for dynamic handling, but softer in the vertical and longitudinal directions in the interest of ride comfort. All the suspension control arms are acoustically decoupled from the axle subframe by means of elastomer bushings.

To minimise unsprung masses for the best possible ride comfort, the two trapezoidal links are warm-hardened aluminium castings, the wheel carriers are of chill-cast aluminium and the upper control arms and track rods are aluminium forgings. Their high rigidity ensures that toe and camber angles change very little when dynamic forces act on the wheels. The anti-roll bar, in fact a tube as at the front axle, also combines low weight with high rigidity.

The installation position of the suspension springs is entirely new: they no longer act on the trapezoidal links as on the previous model, but directly on the wheel carriers. The designers adopted this layout because it enables the new Audi A4’s handling capabilities to be exploited to the full without demanding compromises in ride comfort.

Separate spring and shock absorber mountings improve initial response, and as a further contribution to ride comfort, suspension travel has been increased by 20 millimetres. The suspension bushings use special rubber mixtures that also enhance ride comfort. A novel type of elastomer element in the wheel carrier has a damping effect on vibration generated by wheel rotation. The rear suspension kinematics also effectively control brake dive.

The new A4 S line is fitted as standard with lowered sports suspension, bringing firmer springs and shock absorbers and reducing the body’s ride height by 20 millimetres.

The compact trapezoidal-link suspension has notable packaging advantages: the floor of the boot in the new Audi A4 is low and flat, with a load-through width of 100 centimetres. Front-wheel-drive and quattro models have axles of almost identical construction, differing only in minor details: the subframes of quattro models, for instance, have an additional mounting for the rear-axle final drive.

Published 9 February 2008 Melanie Carter

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