Steering (Steering Rack): Description and operation

Steering Rack

The steering rack is unique in its design having the track rods mounted at a central location on the rack. The track rods are very long and combined with the optimized steering geometry provide low levels of ’bump steer’.

The steering rack is mounted in the engine compartment on the lower part of the bulkhead. The rack is retained with two bolts through cast lugs near the valve unit and two bolts which secure a clamp over the opposite end of the rack. The lower bolt through the cast lug is fitted with a large washer. The steering rack is handed for left and right hand drive vehicles.

The rack requires 3.16 turns from lock to lock and the rack and pinion has an overall ratio of 19.6:1.

A valve unit is fitted to one end of the steering rack and connects, via a splined rotor shaft, to the adaptor of the steering column. The valve unit has four hydraulic connections; a pressure feed from the PAS pump, a return line and two pressure lines to the annulus and full area of the hydraulic cylinder.

A rotor shaft, which is located through the valve unit, is connected to a pinion gear which drives on a rack which, in turn is connected to the piston rod in the hydraulic cylinder.

At a central position on the rack are two threaded holes which allow for the attachment of the track rods. Rubber bellows cover the movement area of the rack.

Two hydraulic ports at each end of the hydraulic cylinder of the rack are connected by metal pipes to the valve unit. The ports supply hydraulic pressure from the valve unit to the annulus and full area of the cylinder. The cylinder end of the piston rod is fitted with a piston which houses a piston seal.

The track rods are fitted with rubber bushes which are hard enough to give positive feel to the steering but at the same time reduce unwanted feedback through the track rod. Each track rod is fitted with a track rod end. The track rod end is a ball joint which locates on an extended bracket on the suspension strut and secured with a lock nut. A turnbuckle is screwed into the track rod and the track rod end and allows for the adjustment of the steering alignment.

The track rods are located on a bushed slider which locates on the rack. Two spacers are located between the slider and the track rods. Two flanged bolts secure the track rods to the rack.

Valve Unit

The valve unit is an integral part of the steering rack. The principal function of the valve unit is to provide maximum power assistance (ie; when parking) with minimum effort required to turn the steering wheel.

The cast outer casing of the valve unit has four ports which provide the connections for pressure feed from the pump, return to the fluid reservoir and pressure feed from the valve unit to the annulus and full area of the cylinder. A non return valve and seal is fitted in the pressure feed port from the PAS pump.

The valve unit comprises an outer sleeve, rotor, torsion bar and a pinion shaft. The valve unit is coaxial with the pinion shaft which is connected to the steering column via the input shaft. The valve unit components are located in a housing which is screwed to a mating casting on the steering rack main body and secured with screws.