Why manageability
Configuring handling has always been considered a very difficult job and the car got a perfect balance among the best. The chassis of modern machines, at first glance, little has changed in comparison to the eighties years, but the difference is. And it perfectly manifests itself when you look at the speed achieved on the machine maneuver “perestavka” or on the racetrack.
Modern family hatchback is able to get ahead of most of the supercars thirty years on the circuit, and not least of all due to fine-tune handling and excellent “tenacity” of the chassis. Of course, the rubber elasticity and motors also play a role, but now mainly talk about geometry.
No, it is not about school subjects – I’m talking about the geometry of the chassis. This is a set of parameters describing the change in the position of the chassis elements when the load changes. focus point is that when cornering the car tilts, and the road has its own profile. With the proper calculation of chassis geometry parameters of the tires always have optimal conditions for data contact with the road.
The point here is not about the maximum downforce, and the relationship between the coefficient of friction of the wheels front and rear, left and right wheels, and the wheel’s ability to take in every moment of the load in three directions.
Of course, you can “pinch” the suspension and make travel less. This is useful in many ways, and often do just that, but then move can be used for a good cause. For example, to turn the wheel in the turning yourself. If it is difficult to calculate the movement, you can play along with them a little bit, put the steering and the rear axle, creating polnoupravlyaemuyu car.
And you can set the movement with the help of sophisticated suspension – for example, a multi-link, which allows you to adjust the geometry of wheel movement in a very wide range and store these settings in case of wear elements for a long time.
If you are not the driver, it does not mean that the control is not important to you. Just in your case, the term refers to a completely different set of preferred options than perfect accuracy and speed of reaction. Actually, the active safety of the machine depends on its manageability, and because of these parameters car designers work hard and productively. And how does this relate to the chassis geometry?
As the car turns
It would be easy: turned the front wheels – and the car turned. But in practice, all the more difficult. To begin with, even at standstill turn not only to the front wheels. Since the front axle is the angle of Castor, the front wheel when you turn the rise, each at its height. As far as – depending on the width and rubber hardness, the geometry of the suspension and so forth.
The machine will receive as a result of some bank, depending on the roll center height of front and rear suspensions and position the center of gravity at this point. The rear wheels or even cutting back bridge turn too – simply by virtue of the fact that for any change in the position of the body wheels are not just walking up and down, and too little, but the turn.
The dynamics of this parameter will be added to the heap heeling moment from the center of mass of the car and driving off the rubber. Among all the parameters that need to be calculated, for us the greatest value will be the instantaneous center of rotation and the turning radius of the front and rear axle and the center of mass. Instantaneous center of rotation does not coincide with the geometric, which is calculated according to the rule Ackerman – the point at which the centers of the circles are the rolling of the wheels. Moreover, the dynamics of such a point does not exist because of the slides. But the figures for an example, the situation is simpler to avoid inducing confusion.
At first glance, if dovorachivat rear wheels in the opposite direction from the front side, then decreases the turning radius of the machine. This is important from the standpoint of ease of use and maneuverability. The smaller the radius, the more convenient. But the car ride not only on trucks speeds in a shopping center, so you have to take into account other factors.
But if the wheels rotate in the same direction as the front? At first glance, nonsense: the car “go sideways,” by and large radius, if the rear wheels are turned through a smaller angle than the front. By itself, a larger turning radius means that would be less than the redistribution of loads between right and left wheels, and hence better traction and comfort.
But it seems that the same can be achieved by simply turning the steering wheel to the smaller angle? You can even do this automatically – the good, steering with variable pitch is not uncommon. But by turning the rear wheels in the direction of rotation is also reduced slip angle of the rear axle, and hence a tendency to oversteer. If it is absolutely simple – the car becomes more resistant to skidding occurs. At high speeds, it is extremely important.
A similar effect could be obtained by simply increasing the wheelbase. But machines are limited sizes – but by changing the rear wheel angle can get what you want, without increasing the size. And for korotkobaznoy machine is just saving: you can save a combination of stability on the road, typical for large machines without giving up a good understeer.
Not only control
For the stability of the rear wheel on the road in turn should be rotated in the direction of rotation of the front, and for better maneuverability – in the opposite direction. If maneuverability is not too much trouble, it can be used for self-centering of the wheels especially movement of the machine in turn. For example, the presence of roll. When you compress the suspension will dovorachivat wheel, and we will get what they want.
But there are two problems. Firstly, the suspension reacts the same way to the load change, and it would be desirable to control less dependent on the load and more from the actual roll and lateral forces. Secondly, on the rear-wheel drive car is very tempting to link the rotation of the wheels to the vector thrust.
If complicate suspension by introducing levers that affect the angles of the wheels at a certain load, we get a multi-link suspension. Yes, the same one that appeared on the Mercedes W201 and now applied to most of the C-class cars and above. And not only the rear axle but also the front.
This multi-link suspension yielded the same effect as the forced rotation of the rear axle, and to renounce the use of complex systems of forced rotation of a quarter of a century. arm suspension system defines a complex path wheel motion as a function of the longitudinal, transverse and vertical loads.
One can fairly accurately adjust the chassis geometry based on how the car will behave when a considerable lateral forces, with different ratio of the vertical and transverse loads. For rear-wheel drive cars it was a great help in the fight for better control from the start, and front-wheel Examples of such technology a little later, with the growth of mass loads and requirements already in their handling.
Why reappear polnoupravlyaemye chassis
The most obvious answer to this question – a price reduction on drive mechanisms and electronics control and development of stability and security. The new technological level declined from the rear steering linkage and rails. Multi-link suspension provide already sufficient angle resetting wheels to implement the desired effect. It remains to equip them with the lever in place, in charge of the complete rotation of the wheel, an active electric or hydraulic actuator.
Electronics which precisely defines what is happening at the moment with the machine, allows the use of large-angle resetting, and also cheaper to set up than a complex suspension. And as an additional factor – the most improved understeer at low speeds. You can tighten the wheel in the opposite direction and to improve maneuverability in the narrow streets.
I do not be surprised if a similar system will soon be introduced in large quantities on the machines from the C-class and above, and in conjunction with a simplified rear suspension geometry – for example, with mnogoryichazhka and with a torsion beam. The economic meaning of this definitely is, because you can get control as the more expensive machines, at a lower cost. And one more complicated and expensive the wear assembly “plus” will not. For auto manufacturers seem to have made a commitment to make the car disposable.