Power Steering Rack Operation - Lotus Evora Service Notes

Lotus Service Notes
HI.1 - GENERAL DESCRIPTION
The power assisted steering system of the Evora comprises a column assembly, adjustable for height and
reach, connected via two universal joints and an intermediate shaft, to a rack and pinion steering gear mounted
on the chassis front subframe ahead of the front axle line. Power assistance is provided by an hydraulic pump
mounted on the LH side of the engine, driven by the auxiliary belt, with an hydraulic reservoir located at the RH
side of the engine bay. The pipes connecting the pump and reservoir to the rack assembly, are routed along
the outside of the RH main chassis rail, within the body sill.
The column assembly comprises three sections, upper, intermediate and lower, articulated with two
universal joints. The upper section is bolted to the chassis scuttle structure and provides for a steering wheel
reach variation of 40 mm, and a vertical adjustment range of 43 mm. A clamping lever is positioned below the
column, which is lowered to allow an adjustment to be made, and should be pushed fully upwards to clamp
the setting before the car is driven. The intermediate column, like the upper, is telescopic, and in this instance
allows for length compression of the column in the event of rack displacement in a vehicle collision. The short
lower column connects to the steering rack pinion shaft.
The steering rack assembly consists of an alloy pinion housing/valve body, and a steel tube through which
the rack is guided. The pinion housing features two lugs with solid alloy inserts, which bolt to an extruded alloy
bracket, itself bolted to the subframe. The non-pinion end of the rack tube is clamped via a split rubber sleeve
around the rack housing, to a second extruded bracket, again bolted to the subframe. Short ball jointed tie-rods
from each end of the rack connect via length adjustable track rods, to forward facing steering arms integral
with the forged steel front hub carriers, with outer pivot points positioned laterally to provide a 32% Ackermann
effect, and vertically for a toe-out on bump characteristic. The rack and pinion assembly is geared to provide
47.4mm rack travel for one steering wheel revolution, with 2.86 turns needed from lock to lock.
Steering power assistance is sourced from a belt driven, vane type hydraulic pump, mounted at the right
hand front of the engine. Pipework to and from the power rack assembly is routed within the RH body sill, with
a fluid reservoir mounted at the RH rear of the engine bay. Some cars destined for hot climates are equipped
with a befinned oil pipe cooler loop incorporated into the return line from rack to reservoir, and mounted ahead
of, and at the base of the engine coolant radiator.
HI.2 - POWER STEERING RACK OPERATION
The steering rack assembly comprises the following major components:
- a round section steel bar with rack teeth machined at one end.
- an alloy pinion housing to support and contain the pinion shaft and hydraulic valve body assembly, and
also support the pinion end of the rack bar.
- a steel tube pressed into the pinion housing to support the non-pinion end of the rack bar, and provide an
hydraulic cylinder for power operation of the rack.
The rack housing contains two seals, between which a piston on the rack bar operates to form two hy-
draulic cylinders, each of which is linked to the valve body by a steel pipe. By using a pumped oil supply and a
mechanism for creating a pressure differential between the two cylinders, a force can be applied to the piston
and rack bar to provide steering assistance in either direction.
The valve body, which is integral with the pinion gear housing, contains three main elements:
- an input shaft/valve rotor connected to the steering column;
- a valve sleeve fixed to the pinion gear.
- a torsion bar connecting the two;
Section HI
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