Recently media talk about new vehicles with four wheel steering, also known as 4WS, AWS, RWS, which must not be confused with 4WD, AWD (acronym of four wheel drive/all wheel drive). Many car manufacturers have developed their own system, using different names as Renaut 4Control. Anyway the operating principle is the same: when the driver starts to steering, both front and rear wheels change its steering angle.
The driver can not change the rear wheel steering angle arbitrarily, but is demanded to the electronic management units mounted inside the vehicle that work together. The wheels are steered by electromechanical actuators.
What is the advantage of this system compared with front wheel steering system? To give a better answer to this question it is appropriate distinguish two driving conditions: low speed and high speed. There are other particular driving conditions, like line change.
The “secret” of this application is the possibility to change the instant centre of rotation “C” in a wider range than in the front wheel steering can reached.
To better understand the following discussion is useful tu deepen how instant centre of rotation works.
Instant centre of rotation
This is the point around which the vehicle turns and its position changes every instant. Graphically is the point of intersection between the straight lines orthogonal to the velocity vector of every point of the rigid body.
In the figure above is represented a four wheels vehicle scheme with front wheels steering. Velocity vectors of the four wheels with the corresponding orthogonal lines are represented, and their intersection is the instant centre of rotation.
The distance between C point and the vehicle centre of mass is named as centre of curvature of the trajectory. The shorter is this distance the tighter is the curve that is possible to ride.
Slow speed turns 4WS
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In this case rear wheels turn in counter-phase than the front ones, so if front wheels turn toward left, the rear ones turn toward right and vice versa.
In the picture above a simplified vehicle model is represented, named as single track model, where the two wheels for each axle are condensed in one. A model FWS and 4WS are superimposed.
With kinematic analysis is possible to understand that the point C has a different position in both cases, in particular the curvature radius in 4WS case is smaller than the FWS one because in the first case the distance between C and vehicle centre of mass is shorter. Is possible demonstrate the concept mathematically.
Ok…and now? What does it mean? Using 4WS is possible ride narrower turns making easier also the simplest parking manoeuvres, or in the traffic.
High speed turns 4WS
This case is a little bit more complex. If rear wheels turn in counter-phase make the vehicle more oversteer during turn-in, unfavourable condition for stability and security reasons.
In this condition rear wheel steering is in-phase, so rear wheels turn (a little bit) in the same direction of the front ones. This leads to many advantages:
- More lateral force is developed;
- more understeer during turn-in;
- Yaw velocity oscillations are well damped, making the vehicle more stable during transient conditions.
This classification is real but is too general, because the rear wheels steering angle and phase compared to the front ones depends on driving conditions and the needs to mantain the vehicle stable if it tends to spin or to go out of track due to excessive understeer. As said before everything is controlled by Electronic Control Units that works together.
Another case is line change. The 4WS system allows a faster manoeuvre, with a faster response to the driver input on the steering wheel: at the same time after the driver input, the lateral acceleration of the vehicle is higher than a FWS system.