History of the emergence of rear-wheel steering.
Improving such an important parameter as drivability remains one of the priorities in the development of new cars. Modern steering systems are doing a good job, and developers in the pursuit of controllability, often go in the direction of creating additional devices not related to the steering .
Traction control systems and computer control systems of directional stability can be attributed to these devices.
However, even before the mass introduction of microprocessors in the control systems of the car, there were developments which allowed to improve controllability. These include tailwheel steering.
Examples of equipping moving equipment with thruster rear wheels can be found as early as the beginning of the twentieth century. This principle has been used for a long time in forklift trucks, working in the confined spaces of warehouses, factory floors, etc. The articulated rear suspension was used on tractors and off-road vehicles even before the war, such as the pre-war Mercedes Kübelwagen G5.
Rear wheel steering and the theory of cornering Even with the most advanced suspension design, such as a multilever suspension, when driving at high speed, the inertia of the straight-ahead rear wheels that resist cornering becomes a serious factor that affects steering. When the steering wheel turns and the front wheels begin to move to the left or right in the direction of the turn, the rear unsteered wheels try to stay on the same trajectory.
Types of Thruster Rear Suspension and Patterns of Operation In the earliest systems – for example, on tractors of the twenties – the thruster angle was large, up to 15 degrees. As top speeds increased, such large angles had to be abandoned. In today’s vehicles, the steering wheel systems provide a maximum of 5-8 degrees of steering.
Rear Thruster suspension is divided into two types – active and passive.
Active Thruster Suspension If a vehicle is equipped with an active Thruster rear suspension, all four wheels turn at once, responding to the movement of the steering wheel. In modern systems, the power from the steering wheel to the rear wheels is not transmitted mechanically through a system of levers, but through the command of an electronic control unit and retractor relays, otherwise known as actuators. These move the rear steering rods, similar to those used in the main steering system.
The active steering suspension works in two modes. When driving at low speed, such as in a parking lot or when pulling into a garage, when the front wheels are turned to the right, the rear wheels are turned to the left, and vice versa. This makes it possible to reduce the turning radius by twenty to twenty-five percent.
At high speed, the operating pattern changes. When the front wheels turn to the left, the rear wheels steer in the same direction, but by a smaller angle. The electronic control unit monitors the precise angle of steering, taking into account the readings of the angular acceleration sensor, speed sensor and others, forming an optimal algorithm for passing the bend. The most famous are the systems of tail swinging suspension of Japanese manufacturers. For example, the company Honda began installing as an option thruster rear wheels on the sports coupe Prelude back in 1987. In 1988, the same option appeared in Mazda for 626 and MX6 models. American manufacturers also experimented with rear wheel steering. The General Motors Quadrasteer was installed as an option on the Suburban, Yukon, and Silverado SUVs. Nissan’s HICAS, in its early days, was hydraulically actuated and was combined with power steering into a single unit. The system was installed on Nissan and Infiniti rear-wheel drive cars. In the mid-nineties, the complex and not too reliable hydraulic system was abandoned in favor of a drive from electronically controlled actuators. In 2008, the corporation Renault-Nissan introduced a new system of steering suspension Active Drive in cars Ranault Laguna.
European manufacturers are also not left out. For example, the modern system of rear wheel steering in BMW cars is called Integral Active Steering.
Passive Thruster Suspension Many modern vehicles use a simplified rear wheel steering system that counteracts the inertia of a straight line by using suspension elements with certain physical properties. This type of thruster suspension is called a passive suspension. In vehicles with passive steering, the rear suspension is constructed according to a special geometry, and usually with the use of a moving Watt link. The system is designed so that, when cornering at high speed, the rear wheels tend to steer in the same direction as the front wheels due to the redistribution of forces in the suspension. In addition to the geometry, the effect is enhanced by the selection of silent blocks of a certain shape and elasticity. Such a design significantly improves the stability of the car when turning. Passive system of rear wheel steering was equipped, for example, Ford Focus cars of the first generation.
Thrust arms. How the rear steering suspension works
Improvements to the steering system of a car have been ongoing throughout the history of automotive engineering.
To create comfortable and safe driving, modern cars are equipped with all kinds of power steering, directional stability systems, ABS anti-skid devices and many other things that actively or passively participate in maintaining the stability of the car on the road.
This goal is also served by the rear underride suspension, which has a history of more than a dozen years – for the first time this design applied to the pre-war jeep Mercedes Kubelwagen G5.
How it works
The principle of operation of the system, designed to improve maneuverability, based on the fact that at the time of turning the front wheels to one side, the rear steer in the opposite, thereby skidding, the rear of the machine and reducing the turning radius.
At high speeds, the rear wheels can turn to one side in sync with the front wheels to cut the car’s trajectory, or they can turn in the opposite direction, skidding the rear of the car in the process. In both cases, the goal is the same – to improve the stability of the car on the turn, by reducing the tipping moment caused by the significant side load on the rear wheels during this maneuver.
Types of Thruster Suspensions
Thruster rear suspensions developed today can operate in both active and passive modes.
In the first case, the control of the rear wheels are engaged in electronic units that turn them simultaneously with the front, in the second – the wheels are everted by means of rods, levers and load changes on the wheels.
The design of the passive rear suspension steering system is ingeniously simple. It consists of four transverse rods (two for each wheel), attached to the body through silent blocks, and to the hub through ball bearings. The main role in turning the wheels is played by the levers attached to the front of the hub.
When turning at high speed, for example, to the right, due to centrifugal forces, the car body rolls on the left side – the distance between the bottom and the hub decreases, and since the length of the link remains the same, it simply pushes the left wheel out. On the raised side, on the contrary – the distance increases, and the pull rod pulls the right wheel inward. As a result, they change direction to the opposite side of the front wheels, thereby reducing lateral load and tipping torque.
This change may be insignificant, a few hundredths of a degree, but it is quite sufficient to reduce the machine’s tendency to tip over and thereby greatly improve the machine’s stability.
With active steering, all the wheels move at the same time.
The forces from the steering wheel are transmitted to the electronic control unit, and from it to the retracting relays, as they are called – actuators, which move the rear steering rods, similar to the front.
This system works in two modes:
- at low speeds of up to 40 km/h, it uses a maneuvering function where the rear wheels turn in the opposite direction to the front wheels, reducing the turning radius;
- at speeds above 40 km/h, optimized cornering mode is activated – based on the readings of sensors of angular acceleration, speed and many others, the best angle of the rear wheels is calculated, but in the same direction as the front ones.
Four on four: why modern cars need rear steering wheels
And okay only on Porsche 911 GT3 or Lamborghini Aventador – but in fact they introduce rotating rear wheels on usual Renault Espace too. What is the sense of such a technical solution, and why did the manufacturers go to such difficulties? And why they forgot about the technology until recently?
Why is the drivability needed?
Adjustment of drivability was always considered to be a very difficult work, and the cars with perfect balance were among the best. The chassis of modern cars, at first glance, has changed little compared to the eighties, but there is a difference. And it shows itself perfectly, if you look at the speeds achieved by the cars on the “re-positioning” maneuver or on the racetrack.
The modern family hatchback is able to outstrip most of the supercars of thirty years ago on the autodrome, not least due to the fine-tuning of the handling and excellent “tenacity” of the chassis. Certainly, both rubber and elasticity of motors also play their role, but now we shall speak about geometry first of all.
No, it’s not about a school subject at all – I mean the geometry of the chassis. This is a set of parameters describing changes in the position of the chassis elements when the load changes. The trick is that the car tilts when cornering, and the road has its own profile. With the correct calculation of chassis geometry parameters tires always have optimal contact with the road for these conditions.
It is not about the maximum downforce, but the ratio of the coefficient of traction of the front and rear axles, right and left wheels, and the ability of the wheel at each moment to take the load in three directions.
The task of increasing the contact area of the wheels with the road is not as simple as it seems.
Of course, it is possible to “clamp” the suspensions and make the displacements smaller. This is useful from many points of view, and it is often done, but the displacements can be used for a good cause. For example, to make wheels turn themselves in a turn. If it’s hard to calculate displacements, you can play with them a little by putting steering on the rear axle as well, creating a fully steerable car.
Or you can set the displacement with a sophisticated suspension – for example, multilever, which allows you to adjust the geometry of the wheel movement in a very wide range and keep these parameters when the elements are worn for a long time.
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If you are not a racer, it does not mean that handling is not important to you. It’s just that in your case, the term means a completely different set of preferred parameters than perfect accuracy and quick reactions. Actually, the active safety of a car depends largely on its controllability, which is why car designers work a lot and productively on these parameters. What does it have to do with chassis geometry?
How a car turns
It would seem that there is nothing easier: you turn the front wheels and the car turns. But in practice, everything is noticeably more complicated. For a start, even when the car is standing, not only front wheels will turn. Since the front suspension has a castor angle, the front wheels will lift, each to its own height, when turning. How much depends on the width and hardness of rubber, suspension geometry, and so on.
The car will get some roll as a result, depending on the height of the front and rear suspension roll center and the position of the center of mass at that moment. The rear wheels or even the uncut rear axle will also turn – simply by virtue of the fact that with any change in body position, the wheels do not just go up and down, but also turn a little.
In the dynamics, to this pile of parameters will be added the roll moment from the center of mass of the car and the rubber yaw. Among all the parameters that need to be calculated, the most important for us will be the instantaneous center of rotation and the turning radii of the front and rear axles and the center of mass. The instantaneous center of rotation does not coincide at all with the geometric one, which is calculated according to Ackerman’s rule – the point where centers of rolling circles of all wheels are located. Moreover, in dynamics such a point simply does not exist because of slips. But in the illustrations for the example a simpler situation is considered so as not to cause confusion.
At first glance, if you turn the rear wheels in the opposite direction from the front wheels, the turning radius of the machine is reduced. This is important in terms of ease of operation and maneuverability. The smaller the radius, the more convenient it is. But cars don’t just drive at forklift speeds at the mall, so you have to consider other factors.
What if you turn the wheels in the same direction as the front wheels? At first glance, it doesn’t make sense: the car will “go sideways” on a larger radius if the rear wheels are turned at a smaller angle than the front wheels. By itself, a larger turning radius means that there will be less redistribution of loads between the right and left wheels, and therefore better traction and comfort.
But it seems that the same can be achieved simply by turning the steering wheel to a smaller angle? It is possible to do it even automatically – thank goodness, steering gears with variable pitch are not rare nowadays. But when you turn the rear wheels towards the turn, the angle of departure of the rear axle also decreases, and hence the tendency to oversteer. Quite simply, the car becomes more resistant to skidding. At high speeds, this is extremely important.
The similar effect could be received simply by increasing the wheelbase. But the size of cars is limited – but by changing the angle of rotation of rear wheels, you can get what you want without increasing the dimensions. And for a short wheelbase car, it’s a lifesaver: you can retain the combination of roadholding characteristic of large cars without giving up good turning ability.
Not just handling
For stability on the road, the rear wheel in the turn should turn in the direction of the turn of the front, and for better maneuverability – in the opposite direction. If there are no particular difficulties with maneuverability, you can use the peculiarities of the car’s movement in a turn for additional wheel steering. For example, the presence of roll. When compressed, the suspension will retorque the wheel, and we will get what we want.
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But there are two problems here. First, the suspension responds the same way to load changes, and I would like the handling to depend less on the load and more on the roll and lateral forces themselves. Secondly, on rear-wheel drive cars, it is very tempting to tie wheel steering to the traction vector.
If we complicate the suspension by introducing levers that act on wheel angles under a certain load, we get a multilever suspension. Yes, the very same which appeared on Mercedes W201 and now is applied on the majority of C-class and higher cars. And not only on the rear axle, but also on the front one.
It was the multilever suspension that made it possible to obtain the same effect as the forced steering of the rear axle, and to abandon the use of complex forced steering systems for a quarter of a century. The wishbone system in this suspension sets a complex trajectory for the wheel depending on longitudinal, transverse, and vertical loads.
It is possible to fine-tune the chassis geometry quite precisely, taking into account how the car will behave when significant lateral forces appear, at different ratios of vertical and transverse loads. For rear-wheel drive cars, this proved to be a serious help in the struggle for better handling from the very beginning, and front-wheel drive cars tried such technologies a little later, as the weight, loads and requirements to their handling already increased.
The first all-wheel-drive cars
The cars with two steering axles were not designed for excellent drivability. These cars could not be driven at high speed on the highway because they were cross-country vehicles. For example, the famous Unimog is a universal all-terrain chassis with all four steering wheels. Of course, in order to drive better off-road and maneuver in tight spaces.
In the photo: Mercedes-Benz Unimog U 1000
Japanese trucks of the early 80s could not go far from these trucks by design complexity. The 1987 Honda Prelude had a rear steering rack and shaft linking it to the steering wheel, and the system worked depending on the angle of the wheels. At small steering angles, the rear wheels would turn the same direction as the front wheels, and at larger angles, the opposite direction. Even in this form, the effect was enough for other Japanese manufacturers to implement similar technology.
On the photo: 1987 Honda Prelude
Only in the next generations, the rear steering rack drive was already electric, and the angle of rotation depended on the speed at which the maneuver was performed. However, they couldn’t get rid of the shafts and the rack. The designs remained complicated, massive, bulky and expensive. As a result, these trucks were not very popular and were sold only on the Japanese market. In the rest of the world the multilever suspension became the unquestionable leader.
Why the all-steer chassis are appearing again?
The most obvious answer to this question is to reduce the price of drive mechanisms and control electronics and the development of stability and safety systems. A new level of technology has eliminated rear steering trapeze and rack-and-pinion steering. Multilever suspensions already provide sufficient angle of wheel’s extension for realization of the necessary effect. It remains to equip them with active electric or hydraulic drive instead of lever, which is responsible for resetting the wheel.
Electronics defines much more precisely what is going on with the car at the moment, allows using large angles of follow-through, and is also cheaper in tuning than a complicated suspension. And as the additional factor – the improvement of steerability at low speeds. It is possible to turn wheels to the opposite side and to improve maneuverability of the car on narrow streets.
I would not be surprised if such systems will be massively implemented on the cars of the C-class and higher in the near future, and in combination with simplified geometry of a rear suspension – for example, not with multilevers, but with a twisted beam. There is definitely an economic sense in it, in fact it is possible to receive steerability, as at more expensive cars, at smaller expenses. One more complicated and expensive wear and tear knot won’t be “superfluous”. In fact, manufacturers of the car seem to undertake an obligation to make the car disposable.