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Lotus Evora. Manual - part 43

Lotus Service Notes

Section HI

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;

Lotus Service Notes

Section HI

The steering column is clamped to splines on the top end of the rack input shaft, the lower end of which

is machined to form a valve rotor, turning within the valve sleeve. The valve sleeve is fixed to the pinion gear

and contains hydraulic ports which are controlled by the position of the rotor relative to the sleeve. The rotor

and sleeve are connected by the torsion bar, the top end of which is secured inside the top end of the hollow

input shaft (rotor), and the bottom end splined into the pinion gear (sleeve). The degree of twist of the torsion

bar is determined by the force applied at the steering wheel, and is proportional to the loading at the front tyres.

These forces tend to be highest when manoeuvering at low speed (e.g. parking) and result in the greatest

angular displacement between valve rotor and sleeve.

The valve body incorporates 4 ports:

inlet from the engine driven pump;

outlet (return) to the reservoir;

connection to the right hand rack cylinder;

connection to the left hand rack cylinder.

Connection pin

Splines to

intermediate

shaft Input shaft/valve rotor

Torsion bar

Hydraulic seal

Roller bearing

Outlet to reservoir

Valve body To/from rack cylinder 1

Inlet from pump

Sealing rings

To/from rack cylinder 2

Valve sleeve

Torsion bar splined

to sleeve/pinion

Ball bearing/seal

Pinion gear

h61

Lotus Service Notes

Section HI

Power Steering Schematic (RHD shown)

PAS reservoir

Low pressure line from reservoir to pump

Engine driven pump

Supply line to valve body

Low pressure return line to reservoir

Supply/return to RH cylinder

Valve body Supply/return to LH cylinder

Hydraulic seal

Track rod

Rack & pinion gear RH rack cylinder LH rack cylinder

Sealed

piston

h62

Revise

Lotus Service Notes

Section HI

The schematic diagram below shows the principle of valve control, although in practice, six pockets are

machined in the rotor and sleeve, and the valve system is repeated three times. The valve sleeve is provided

with four sealing rings in order to divide the inlet and outlet feeds and connect with ports in the valve body,

whilst allowing 360°rotation of the valve assembly.

The engine driven pump supplies oil at a controlled rate. A progressive restriction of this oil flow will cause

the pressure to increase whilst the flow rate is maintained. Eventually, when the flow is completely restricted,

the pressure will rise to the relief valve setting in the pump, causing oil to flow through the relief valve and re-

circulate within the pump.

The hydraulic valve in the steering gear provides this restriction according to the force applied at the

steering wheel and the loading on the front tyres. The valve is configured as 'open centre' such that when no

torque is applied to the steering wheel, there is minimal restriction to the oil supply, resulting in low oil pressure

and the freedom to flow to both rack cylinders and through the outlet port back to the reservoir. The pressure

differential across the rack piston is zero.

When the steering is turned to the left against some resistance, the input shaft/rotor transmits the motion

to the pinion gear/valve sleeve via the torsion bar, which twists in proportion to the effort applied at the wheel

and the resistance at the tyres. Effort is high typically at slow vehicle speeds, or when parking. When the bar

twists, the angular position of the rotor relative to the valve sleeve alters, with the result that the ports to cylin-

der 1 become biased towards the pressurised supply, and the ports to cylinder 2 biased to the reservoir return

port at low pressure. Hence a pressure differential is created within the rack housing, with higher pressure in

cylinder 1 applying a force to the steering rack piston to assist the turn.

If the steering is turned hard against resistance, or the lock stop, the valve will completely close off the

return path and cause delivery pressure from the pump to rise until the pressure relief valve in the pump opens;

maximum assistance has been reached.

The ultimate degree to which the torsion bar is permitted to twist, is limited by mechanical contact between

the input shaft and the pinion gear. This mechanism prevents the torsion bar being over stressed, defines the

maximum level of assistance, and provides a safety back up in case of torsion bar failure; steering control would

be retained, albeit with a small amount of lost motion.

RHD steering being Pressurised oil to

turned to left Restricted return RH rack cylinder

path causes supply Valve sleeve - fixed to pinion

side pressure increase Angular position to rotor

displaced due to flex in

Return to torsion bar

reservoir

Oil supply

from pump

Valve rotor/input shaft

Torsion bar (connects (fixed to column)

valve sleeve to rotor)

Restricted return path

causes supply side

Oil returning from pressure increase

h40a

LH rack cylinder

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