Existing systems thrusters rear wheelsinstalled on some modern cars and large trucks, the answer to the question of interest to us will not be given. They are just driving, not driving. The front wheels play the main role. At the same time, there are enough vehicles in the world that are driven exclusively by rear wheels. For example, all kinds of forklifts: from warehouse forklifts to career giants... The increased maneuverability provided by the rear steering wheels is a must for them. So what is worse in this sense, passenger transport?
One of the first explanations of such "injustice" among others that come to mind is the power of tradition. As it was customary "from time immemorial" to make the front axle, so it goes. But, you see, it sounds rather weak. How many years has been familiar and traditional, for example, rear drive... But as soon as they came up with a more comfortable front one, the whole world immediately gave up on the "tradition" and reoriented to the front-wheel drive type of passenger car. The second version, explaining the prevalence of front steer wheels, is technological. The driver sits in the front of the car, so the steering wheel is also in the front of it. In such conditions, "pulling" the mechanism of the steered wheels to the rear axle would greatly complicate the design for the sake of completely unobvious advantages.
In short, it's not worth the candle. This version seems to be quite viable. The main reason why the steerable wheels of most cars are front is completely different. A clue here can be the very high of the same loaders, turning the rear wheels capable of turning almost on the spot. The point is that turning rear wheels report vehicle oversteer. At speeds of 5-10 km / h, it is a blessing that provides excellent maneuverability. But when it comes to even a little more, each turn of the rear wheels will lead to a skid of the stern of the car.
Imagine the same forklift driving along a city street at a typical "automobile" speed of 50-60 km / h. A passenger car at this speed will easily fit into a smooth turn of the road. And our conditional loader, in best case, will turn sideways and, most likely, also turn over. Now let's imagine what will happen to a car driving backwards, about 100 km / h, and even in the rain, when the road is slippery. The slightest rearrangement - and it will spin like a top. That is why, by the way, on all modern passenger cars equipped with a steering rear suspension, on high speeds the rear wheels turn in the same direction as the front wheels - so that the car moves to the side almost sideways, and does not turn across the general direction of travel.
A punctured wheel is usually indicated by extraneous noise outside, as well as loss of speed and deterioration in handling. If the car is pulled to the side, and in order to keep it on the road, you have to make an effort, then most likely there is a puncture on one of the front wheels. If the back of the car starts to enter, then the problem is there.
When this happens, you will know exactly what the matter is. Don't panic at all. Level the vehicle and gradually reduce speed and pull over to the side.
How to change a wheel
1. Park on the side of the road
You cannot continue driving with a punctured tire, but stopping in the middle of the road is also not the case. Therefore, do not be afraid to drive a couple of tens of meters and choose a flat, dry place on the side of the road.
Car drivers with mechanical box gears, it is imperative to engage the first gear, and the owners of the automatic equipment must move the lever to the parking position (P).
And in any case, you need to put the car on the handbrake.
2. Install warning triangle and prepare tools
Park your car in safe place, don't forget to include alarm and install a warning triangle in the trunk. IN settlements it is placed 20 meters behind the car, and on the highway - 40 meters.
In the same place, in the trunk, find a spare tire and a jack with a balloon wrench. Usually the manufacturer places all this in a special niche under the floor, which can be reached by lifting the bottom panel.
It is good if you have a pump and a pressure gauge with you to check the pressure, and wheel chocks... And, of course, gloves do not hurt, since you still have to get your hands a little dirty.
3. Remove the wheel
After taking out all the tools and a spare wheel, place them next to the punctured wheel and ask all passengers to get out of the car. Even if it is outside or pouring rain, safety comes first.
Despite the handbrake and the engaged gear, before installing the jack, you must additionally secure the wheels with stops. For them, however, any stones or pieces of brick will go.
If the rear wheel needs to be replaced, the stops are placed on both sides of the front wheels, and vice versa.
Now you can start removing the wheel. First release the disc from the plastic cap and loosen the bolts with the wheel wrench. It will take a lot of effort to move them, which can be provided by the weight of your body simply by pushing the key with your foot. It is not necessary to completely unscrew the bolts: it is enough to unscrew them one turn.
After that, you need to raise the car with a jack. In no case should you install it anywhere. Especially for this purpose, there are small reinforced areas on the underbody, which are usually located behind the front wheel or just in front of the rear. The manufacturer designates them with triangles or cutouts at the bottom of the thresholds. If the weld seam is covered with plastic pads, they will be interrupted at the jacking points.
Bring the jack under the bottom and start turning its handle clockwise. Be sure to make sure that the jack lifts evenly, does not tilt.
If, under the weight of the machine, the lower paw of the jack goes into the ground, you need to put something like a piece of board or brick under it.
It is not worth lifting the wheel too much. It is enough to stop 5 cm from the ground. After that, you can completely unscrew the bolts and remove the punctured wheel from the hub. It is better to push it under the car as a safety net, and put the bolts somewhere on a rag so as not to get lost.
4. Install and check the spare wheel
It remains to put a spare tire instead of the punctured wheel. To do this, align the holes on the disc with the holes in the hub, put on the wheel and tighten the bolts by hand tightening them completely.
It is important to install the nuts securing the wheels to the hub with the semicircular side to the disc, and not outward.
Remove the punctured wheel from under the car, lower the jack and finally tighten the bolts. This must be done correctly. For wheels with four or six holes, opposite bolts are tightened in pairs. If there are five holes, then you need to pull in this order, as if you were drawing a five-pointed star.
It remains to assemble the tool, remove the jack and stops, and also check the pressure in the installed wheel and, if necessary, pump it up. If the pump is not at hand, you can ask for help from passing drivers.
If you use a small-sized spare wheel, the so-called stowaway, then do not forget about caution: usually you can move on it at a speed of no more than 80 km / h and a distance of no more than 100 kilometers.
And, of course, try to fix a punctured wheel as soon as possible in a specialized tire shop so as not to tempt fate and not drive without a spare wheel.
When japanese cars were considered the most advanced, legends reported that in the Land of the Rising Sun there are cars that turn all four wheels. Then, in the bustle of new things, those times were somehow forgotten. The turbulent beginning of the nineties has passed, and only the most necessary technical solutions of that time remained in mass production. But now interest in fully controlled chassis is growing again, however, already on a different technical level, without additional steering shafts and with a significantly simplified rear suspension.
And it would be okay only on a Porsche 911 GT3 or Lamborghini Aventador - but in the usual Renault Espace, turning rear wheels are also introduced. What is the meaning of such a technical solution, and why did the manufacturers go to such difficulties? And why has technology been forgotten until recently?
Why manageability
Adjusting handling has always been considered a very difficult job, and machines with perfect balance were among the best. Chassis modern machines, at first glance, has changed little in comparison with the eighties, but there is a difference. And it shows itself perfectly if you look at the speeds reached by cars in the "rearrangement" maneuver or on the race track.
The modern family hatchback is capable of outperforming most of the thirty-year-old supercars at the circuit, not least due to the fine tuning of handling and excellent chassis "tenacity". Of course, both rubber and the elasticity of motors also play a role, but now let's talk about geometry first.
No, it's not about a school subject at all - I'm talking about chassis geometry. This is a set of parameters describing the changes in the position of the chassis elements when the load changes. The essence of the focus is that when cornering, the car tilts, and the road has its own profile. With the correct calculation of the chassis geometry parameters, the tires always have optimal contact with the road for the given conditions.
This is not about the maximum downforce, but about the ratio of the coefficient of adhesion of the wheels of the front and rear axle, right and left wheels, and the ability of the wheel to perceive the load in three directions at any moment.
The task of increasing the contact area of \u200b\u200bthe wheels with the road is not as easy as it seems.
Of course, you can "clamp" the pendants and make the movement less. This is useful from many points of view, and is often done, but displacement can be used for a good cause. For example, so that the wheels turn themselves in a turn. If it is difficult to calculate the movements, then you can play along with them a little by placing steering and onto the rear axle, creating a fully driven machine.
And you can set the movement using a sophisticated suspension - for example, a multi-link, which allows you to adjust the geometry of the wheel movement within a very wide range and maintain these parameters when the elements are worn for a long time.
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If you are not a racer, this does not mean that handling is not important to you. It's just that in your case, this term means a completely different set of preferable parameters than the ideal accuracy and speed of reactions. Actually, active safety a car largely depends on its handling, and therefore, car designers work a lot and productively on these parameters. What does this have to do with chassis geometry?
How the car turns
It would seem that what is simpler: turned the front wheels - and the car turned. But in practice, everything is much more complicated. For a start, even on standing car not only the front wheels will turn. Since the front suspension has a castor angle, the front wheels will rise when cornering, each to its own height. How much depends on the width and hardness of the rubber, suspension geometry, and so on.
As a result, the car will get some roll, depending on the height of the center of roll of the front and rear suspensions and the position of the center of mass at that moment. Rear wheels or even continuous rear axle they will also turn - simply due to the fact that with any change in the position of the body, the wheels not only go up and down, but also turn a little, but turn.
In dynamics, heeling moment from the center of mass of the machine and rubber slip will be added to this heap of parameters. Among all the parameters that need to be calculated, the instantaneous center of rotation and the radii of rotation of the front and rear axles and the center of mass will have the greatest value for us. The instantaneous center of rotation does not at all coincide with the geometric one, which is calculated according to the Ackermann rule - the point at which the centers of the rolling circles of all wheels are located. Moreover, such a point simply does not exist in dynamics due to slips. But in the figures, for example, a simpler situation is considered in order not to cause confusion.
At first glance, if you turn the rear wheels in the opposite direction from the front, then the turning radius of the car decreases. This is important for ease of use and maneuverability. The smaller the radius, the more comfortable it is. But cars don't just run at forklift speeds in the mall, so there are other factors to consider.
But what if you turn the wheels in the same direction as the front ones? At first glance, this is nonsense: the car will "go sideways" along a large radius if the rear wheels are turned at a smaller angle than the front ones. By itself, a larger turning radius means that there will be less redistribution of loads between the right and left wheels, which means better grip wheels with road and comfort.
But it seems that the same can be achieved by simply turning the steering wheel to a smaller angle? You can even do this automatically - fortunately, variable-pitch steering mechanisms are now not uncommon. But when the rear wheels turn in the direction of the turn, the rear axle slip angle also decreases, and hence the tendency to oversteer. Quite simply, the car becomes more resistant to skidding. On high speeds this is extremely important.
A similar effect could be obtained by simply increasing the wheelbase. But the size of the cars is limited - but by changing the angle of rotation of the rear wheels, you can get what you want without increasing the size. And for a short-wheelbase car, this is simply salvation: you can maintain the combination of road stability characteristic of big carswithout giving up good understeer.
Not only management
For stability on the road, the rear wheel should turn in the direction of the front turn, and for better maneuverability, in the opposite direction. If there are no special difficulties with maneuverability, then you can use the features of the car's movement in a corner to turn the wheels. For example, the presence of a roll. When compressed, the suspension will turn the wheel, and we get what we want.
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But there are two problems here. First, the suspension responds in the same way to changes in load, and I would like the handling to be less dependent on the load and more on the roll itself and lateral forces. Secondly, on rear-wheel drive cars, it is very tempting to tie the rotation of the wheels to the thrust vector.
If we complicate the suspension by introducing levers that act on the wheel alignment angles under a certain load, then we get a multi-link suspension. Yes, the same one that appeared on the Mercedes W201 and is now used on most cars of the C-class and above. And not only on the rear axle, but also on the front.
It was the multi-link suspension that made it possible to obtain the same effect as the forced rotation of the rear axle, and to abandon the use of complex forced rotation systems for a quarter of a century. The system of levers in such a suspension sets a complex trajectory of the wheel movement depending on the longitudinal, lateral and vertical loads.
It is possible to fine-tune the chassis geometry to reflect how the machine will behave when significant lateral forces appear, with different ratios of vertical and lateral loads. For rear-wheel drive cars, this turned out to be a serious help in the struggle for better handling from the very beginning, and front-wheel drive cars tried on similar technologies a little later, with an increase in weight, loads and requirements for their handling.
First fully driven cars
Cars with two steered axles were not created at all for excellent handling. These cars did not drive on the highway at all. high speedbecause they were all-terrain vehicles. For example, the famous Unimog - a universal chassis high cross-country ability has all four steerable wheels. Of course, in order to better drive off-road and maneuver in confined spaces.
Japanese cars of the early 80s, in terms of design complexity, left not far from them. On Honda prelude 1987 was the back steering rack and the shaft connecting it to the steering wheel, and the system worked depending on the angle of rotation of the wheels. At small angles of rotation, the rear wheels turned in the same direction as the front ones, and at large angles, in the opposite direction. Even in this form, the effect was sufficient for other Japanese manufacturers to introduce similar technology.
Only in the following generations did the rear steering rack drive become electric, and the steering angle also depended on the speed at which the maneuver was performed. However, they did not think to get rid of the shafts and rails. The structures remained complex, massive, bulky and expensive. As a result, cars with them did not gain much popularity and were sold only in the domestic Japanese market. In the rest of the world, unconditional leadership was taken by multi-link suspensions.
Why fully steerable chassis are reappearing
The most obvious answer to this question is to reduce the price of drive mechanisms and control electronics and develop stability and safety systems. At a new technological level, the rear steering trapezoid and racks were abandoned. Multi-link suspension provide a sufficient angle of rotation of the wheels to realize the desired effect. It remains to equip them instead of the lever responsible for turning the wheel with an active electric or hydraulic drive.
Electronics much more accurately determines what is happening at the moment with the car, allows the use of large turning angles, and is also cheaper to set up than a complex suspension. And as an additional factor - the very improvement of understeer at low speeds. You can turn the wheels in the opposite direction and improve the maneuverability of the car in narrow streets.
I would not be surprised if such systems will soon be massively implemented on cars from the C-class and above, and in combination with a simplified geometry of the rear suspension - for example, not with multi-link, but with a twisted beam. There is definitely an economic sense in this, because you can get controllability, like in more expensive cars, at a lower cost. And one more complicated and expensive wear-out knot will not be "superfluous". After all, auto manufacturers seem to have made a commitment to make the car disposable.
When drivers are driving ordinary carthey turn steering wheel, and following this movement, the front wheels change their direction - while the rear wheels are constantly pointing straight ahead.
This is the standard system called "two-wheel steering" or 2 WS for short. However, some firms are now producing cars with four-wheel steering (4 WS). The 4 WS systems vary from manufacturer to manufacturer, but in most of them the rear wheels turn in the same direction as the front wheels when the car is cornering at high speed. At low speeds, the direction of rotation of the rear wheels at 4 KR is opposite to the direction of rotation of the front wheels. This feature allows, in particular, to make sharper turns, which is useful when driving around the city or when parking in tight spaces. On-road tests of the 4 WS systems have shown that such systems provide greater driving safety. Yet four-wheel steering has not yet become widespread. Due to the fact that the cost of the 4 WS system, in the opinion of drivers, does not justify the benefits obtained with its help.
Two wheels against four
In 2 CR cars (bottom left), only the front wheels turn. If the car changes direction of 4 KR, then all four wheels (on the right) can turn.
How 4 CRs turn the wheels
Let's say two cars: 2 KR (blue) and 4 KR (yellow in the picture above the text) start from one place (green) to make a slow sharp turn. Thanks to the turning of the rear wheels, the 4 KR machine turns steeper than the 2 KP machine and, therefore, it requires less space to turn.
If these two cars make a smooth wide turn (as shown in the right figure), then all the wheels of the 4 KR machine go, as they say, track in track, and thus more reliable adhesion of the wheels to the road surface is provided.
Lane change
If the driver changes to a different lane on the highway, then the 2 KR car exhibits a "fishtail effect": its rear part is skidding, because the rear wheels tend to go in the old direction. To correct this situation, the driver has to turn the steering wheel twice before changing lanes and turn it twice after changing lanes. The 4 CR vehicle has no fishtail effect.
Steering wheel and 4 WS system
sensitive sensors in the 4 KR system monitor how much the steering wheel and, therefore, the front wheels are turned at any given time (red line in the figure). When the steering angle is small (first two columns), the 4KR system leaves the rear wheels straight or turns slightly towards the front wheels. In sharper turns - when the steering wheel makes more than one full turnover (fourth column) - the 4 KR system turns the rear wheels in the opposite direction.
And when cornering, it largely depends on the direction of the rear axle following the front track. This is necessary to reduce the steering angle of the vehicle and the wear of its tires. The use of a steered rear axle allows to reduce lateral acceleration when turning the car, which increases its stability. significantly improve vehicle maneuvering:
- Firstly, the sensitivity of the car to the steering wheel is increased. Indeed, when driving quietly along city streets, it is better to have a "sharp" steering control, so as not to rotate the steering wheel a few turns with each maneuver. On the freeway, "sharp" steering can cause problems - the car will react too harshly even to small steering.
- Secondly, to improve the maneuverability of the car when parking or turning in tight urban conditions, that is, to reduce the turning radius.
- And thirdly, to increase directional stability during sharp maneuvers at high speed.
Turning the rear wheels in the same direction as the front wheels allows you to maintain the direction and speed of the vehicle's center of mass, but significantly increase the instantaneous turning radius. At the same time, the effects on the car are reduced and, as a result, directional stability is increased.
When driving at low speed, the rear wheels turn in antiphase with the front ones, and the instantaneous turning radius decreases, and when driving at high speed in a fast bend or when changing lanes on a motorway, the rear wheels, on the contrary, will turn at a small angle to that same side as the front. For example, a car, making a maneuver on a freeway, will not seem to turn, but move from row to row parallel to the lane markings. In this case, the car will move along an arc of lesser curvature and a greater radius. The moment that turns the car around the vertical axis will be less - therefore, the risk of loss of directional stability and the development of a skid of the rear axle will also decrease.
Figure: Turning radius of a conventional vehicle (MCP - instantaneous turning center) and a vehicle with all steering wheels (4WS)
In this regard, some manufacturers introduce rear axle control into the vehicle design. Mitsubishi was one of the first to present such a mechanical control of the rear axle.
Figure: Rear axle mechanical steering:
1 – oil pump; 2 - receiver; 3 - power steering gear; 4 - steering wheel; 5 - spool; 6 - pressure reducing valve; 7 - rear axle oil pump; 8 - power cylinder
IN common system car controls include a steering gear with a power steering cylinder) front axle control 3, oil pump 1, oil pump for rear axle control 7, rear axle control valve with spool 5 and pressure reducing valve 6, power cylinder for controlling the rear axle 8, steering rods for turning the front and rear axles.
When the front wheels are turned, the control pressure of the power cylinder of the front wheels is transmitted to the power cylinder of the rear wheels. This takes into account system pressure, steering speed and front axle side load. The control pressure acts on the rear axle valve spool. Depending on the acting pressure, the spool moves open by a certain amount oil channelsby which working fluid is fed to the rear axle power cylinder. The piston of the power cylinder, moving, acts on the steering rods of the rear axle, turning the rear axle by the required angle.
As the electronic systems They began to be used in steering the rear axle (4WS). An example is the electronically controlled rear axle of a Toyota Aristo, which in 1991 changed the mechanical one, the general view of which is shown in the first figure, and the actuator diagram in the second figure. A similar system is also used in BMW vehicles.
Figure: General view of a steered rear axle with an electromechanical actuator
Figure: Electromechanical rear axle swing actuator:
1 - rotor (hollow shaft); 2 - stator; 3 - planetary box gear; 4 - spindle nut; 5 - satellite; 6 - sun gear; 7 - spindle (screw); 8 - splined part of the spindle shaft; 9 - spindle rotation fuse; 10 - planetary carrier
The rear wheels are turned here by means of a special electric steering mechanism built into a rather complex rear suspension... And it is controlled by a special electronic unit that receives information from several sensors about the speed of the car, about the steering angle, front and rear wheels, etc.
The actuator consists of an electric motor (stator and rotor), a planetary gear and a spindle shaft acting on the steering rods of the rear axle. The electric motor is controlled from electronic unit control that receives signals from various steering sensors. Depending on the magnitude and time of voltage supply to the electric motor, the speed and rotation time of the rotor of the electric motor change. To increase the torque and pushing forces of the spindle, a planetary gear is used in the actuator.
When voltage is applied to the electric motor, the hollow shaft of the rotor 1 starts to rotate. On the rotor shaft there is a sun gear 6, which, through the satellites 5 and the planetary carrier 10, drives the spindle nut associated with it into rotation 4. The spindle shaft installed inside the hollow rotor shaft through the screw 7 begins to reciprocate, acting on the steering rods of the rear axis. To prevent the spindle shaft from spinning, a special fuse 10 is provided.
The 4WS system works in two modes. At low speed, the rear wheels turn in the opposite direction to the front wheels, and when maneuvering the same curvature, the steering wheel will need to be rotated at a smaller angle. This increases steering sensitivity and makes the vehicle more agile. For example, when turning, the front wheels will be turned all the way to the left, and the rear wheels will be turned to the right at an angle of up to eight degrees. At the same time, the turning radius will be reduced by 15% compared to a conventional car and will be only 4.7 meters.
