Turning the steering wheel means rotating the front wheels about their steering axes, overcoming the friction generated between the tread and the tarmac. That resistance grows with the weight bearing on the front axle and with the width of the tyres, and reaches its maximum value in the worst possible condition: with the vehicle stationary or nearly so, when the wheel is not rolling but scrubbing on the spot. With purely mechanical steering the only way to make the effort bearable would be to lengthen the steering ratio, that is to increase the number of turns of the wheel needed to go from one lock to the other, with an obvious loss of response. This is where power steering comes in, a device that adds to the driver’s muscular effort a force generated by an external energy source, without ever taking the driver’s place: the link between the steering wheel and the wheels remains mechanical and the car stays controllable, albeit with considerable effort, even if the assistance fails. Over the course of their evolution, power steering systems can be grouped into three families:
- Hydraulic power steering
- Electro-hydraulic power steering
- Electric power steering
Hydraulic power steering
This is the solution historically adopted on production cars, and it remained dominant until the early 2000s. The system consists of a pump (typically a vane pump), an oil reservoir, the relevant pipework and a rack and pinion steering box into which a piston and a distribution valve are integrated. The pump is driven from the crankshaft by a belt and keeps the circuit constantly pressurised. The heart of the system is the rotary valve: it is made up of two coaxial elements connected to one another by a torsion bar. When the driver turns the steering wheel the wheels resist and the torsion bar deforms elastically through an angle proportional to the effort applied. This small relative rotation opens the ports that connect the pump delivery to one of the two chambers of the cylinder formed inside the steering box, while the other chamber is connected to the return to the reservoir. The piston, which is integral with the rack, thus receives a thrust that adds to the one applied by the driver. Once the manoeuvre is over, the torsion bar returns to the neutral position and the valve closes the ports again.
The merit of this layout is its progressive nature: the hydraulic thrust is a direct function of the torque applied to the steering wheel, and the inertia of the fluid itself gives the steering a damping effect appreciated by many enthusiasts, who credit these systems with a particularly communicative contact with the road. The main flaw, on the other hand, is one of energy. The pump is driven by the engine at all times, even when travelling in a straight line with no assistance needed, and it has to be sized for the most demanding condition, namely a manoeuvre from rest with the engine idling: the result is a parasitic power draw present for the whole duration of the journey. To this must be added the space taken up by the pipework, the weight of the fluid and the need to check its level and for leaks from time to time.
Electro-hydraulic power steering
Electro-hydraulic power steering represents the intermediate step between the two philosophies. The hydraulic layout remains essentially unchanged — the rotary valve, the cylinder and the rack are the same — but the pump is no longer belt driven: it is operated by an electric motor, generally combined with the reservoir and the control unit in a single compact assembly installed in the engine bay.
The advantage is immediate: the pump’s flow rate no longer depends on the speed of the combustion engine but can be regulated electronically. The control unit, reading vehicle speed and steering activity, runs the pump at full flow only during manoeuvres and keeps it at reduced output, or practically at a standstill, when travelling in a straight line. The energy saving compared with a conventional belt-driven system is substantial, with reductions claimed by manufacturers of up to around 75% of the energy absorbed. It also becomes possible to offer a first form of speed-sensitive assistance, along with greater freedom of installation, since there is no longer any need to provide a pulley and a dedicated belt run. The limitations of hydraulics remain, however: the fluid, the seals and the periodic maintenance. That is why the solution, widespread above all between the late 1990s and the first decade of the 2000s, is today mainly used on commercial vehicles and heavy-duty applications, where the forces involved are high.
Electric power steering
In electric power steering, or EPS, hydraulics disappear altogether. Assistance is provided by an electric motor acting on the steering transmission through a reduction gear, typically a worm-and-wheel pair or a recirculating ball assembly driven by a toothed belt. In place of the rotary valve there is a torque sensor, which measures the twist of the same elastic bar; the control unit combines this signal with vehicle speed and steering angle and determines the assistance torque to be delivered moment by moment.
The main architectures are distinguished by the position of the electric motor: on the column (C-EPS), a cheap and compact solution typical of small cars; on the pinion of the steering box (P-EPS), also in a dual pinion version; and directly on the rack (R-EPS), adopted on heavier and more powerful cars because it allows greater forces to be transmitted without loading the column.
The advantages are numerous. Energy is only drawn when the wheel is turned, with benefits in fuel consumption that can be quantified in a few percentage points; the assistance is freely programmable in software, so it can be generous during manoeuvres and progressively reduced at high speed so as not to make the steering nervous; and the system also works with the engine switched off, an essential requirement for cars with stop-start and for electric cars. Above all, since the electric motor is an actuator the control unit can command on its own, EPS is the technical prerequisite for modern driver assistance systems: lane keeping, automatic parking and emergency evasive manoeuvres all work by applying a torque to the steering. The price to pay, and one these systems are often reproached for, is a more filtered feedback from the road than the old hydraulic systems offered, even if control algorithms have gradually narrowed the gap.
The latest step in this evolution is steer-by-wire, which does away with the mechanical link between the steering wheel and the wheels, replacing it with electrical signals and actuators: a solution already available on production models such as the Lexus RZ, and made possible precisely by the maturity the electric technology has reached.