Four wheel steering system (4WS). Four-wheel drive - how does it work, what is the benefit and in which car models can it be seen? Features of the all-wheel drive transmission
When drivers are driving a normal car, they turn the steering wheel, and in this movement, the front wheels change their direction - while the rear wheels are constantly pointed 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 if 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 figure above the text) start from one place (green) to make a slow sharp turn. By turning the rear wheels, the 4 KR machine turns steeper than the 2 KP machine and, therefore, 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 КР 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 car's 4 CR does not have a fishtail effect.
Steering wheel and 4 WS system
Sensitive sensors in the 4 KR system monitor how much the steering wheel is turned and, therefore, the front wheels at any given time (red line in the figure). When the steering angle is small (the first two columns), the 4KR system leaves the rear wheels straight or turns slightly towards the front wheels. On sharper turns - when the steering wheel makes more than one full turn (fourth column) - the 4 KR system turns the rear wheels in the opposite direction.
43 44 45 46 47 48 49 ..Renault Duster 2016. TRANSMISSION: 4 wheel drive (4WD)
Please note that driving off-road has nothing to do with driving on a well-equipped road.
To use the full potential of your vehicle, it is recommended that you complete specific 4x4 (4WD) driving training.
Your own safety and the safety of passengers depends on you, your skills and your attentiveness when driving off-road.
Mode switch 4x2 (2WD), 4x4 (4WD)
Depending on the road conditions, you can select one of the following modes by turning switch 1:
AUTO mode
To enable this mode, turn switch 1 to the AUTO position.
Operating principle
In AUTO mode, torque is automatically distributed between the front and rear wheels depending on road conditions and vehicle speed. This position improves vehicle stability on the road. Use this mode on any road (dry, snow, slippery road, etc.) or when towing a trailer, caravan, etc. There is no indication on the instrument panel for this mode.
2WD mode
To enable this mode, turn switch 1 to the 2WD position. A warning light on the instrument panel
Lamp 2WD
Operating principle
In the "2WD" mode, only the front wheels are used as driving wheels. Use this mode on dry roads with good grip.
To turn off this mode, turn switch 1 to the AUTO position. The warning light on the instrument panel goes out.
Lamp 2WD
4WD Lock Mode
To activate this mode, turn switch 1 to the 4WD Lock position. Then the switch returns to the AUTO position. The 4WD Lock warning lamp lights up on the instrument panel
Operating principle
In 4WD Lock mode, torque is distributed between the front and rear wheels in a proportion optimal for off-road driving. This mode should only be used off-road (examples: mud, steep slopes, sand).
To turn off this mode, turn switch 1 to the 4WD Lock position again. The warning light on the instrument panel goes out. When the engine is stopped, 4WD Lock remains on for one minute.
After a minute, the system switches to mode 2 \ LU or AUTO, depending on the position of the switch.
Note: when the speed is above 60 km / h in the "4WD Lock" mode, the system automatically switches to the "AUTO" mode.
4WD Lock warning light goes out
Features of the all-wheel drive transmission
The vehicle may make a lot of noise when AUTO or 4WD Lock is on. This is not a malfunction. If the system detects a difference in the dimensions of the front and rear wheels (for example, insufficient tire pressure, pronounced tire wear on one of the axles, etc.), it automatically switches to 2WD mode.
The warning lamps on the instrument panel come on and continue
Drive at a moderate speed and contact the manufacturer's dealer as soon as possible.
The solution to this problem is to replace the tires. Always use four identical tires (of the same make and design) with the same wear rate.
If the wheels slip too much, the mechanical components may overheat.
If this happens:
Warning light flashes first
4WD LOCK
4WD Lock is still on, but it is recommended to stop as soon as possible and let the system cool down (until the lamp stops flashing);
If wheel slip continues, the system automatically switches to 2WD mode to protect mechanical components.
The 4WD LOCK warning light flashes. At
This selection of another mode is not possible until the indicator stops flashing.
In this case, it is recommended to stop as soon as possible and let the system cool down (until the lamp stops flashing); It may take about five minutes to cool down. When the system detects that the front wheels are slipping too much, it changes the engine operating mode to reduce the slip.
ABS operation in LOCK mode (vehicles with ABS)
When 4WD Lock is active, ABS off-road mode is activated. In this situation, cyclic blocking of the wheels is allowed so that they push the soil better, which reduces the braking distance on loose soils. If this mode is enabled:
Vehicle maneuverability in braking mode is limited. This operating mode is not recommended in very weak traction conditions (eg ice).
Some noise may appear. This is normal and not a malfunction.
Vehicle stability control and traction control when driving off-road (vehicles with ESC)
When driving on loose ground (sand, mud, deep snow), it is recommended to disable ESC by pressing the ESC switch.
In this case, only the separate wheel braking system works. This system brakes the skid wheel or wheels, which transfers torque to wheels with more traction. This is especially useful when all-wheel drive is on.
All ESC functions are reactivated when approximately 50 km / h is exceeded (60 km / h in 4WD Lock mode) or after restarting the engine, or after pressing the ESC switch again.
Malfunctions
When the system detects a malfunction, it automatically enters 2WD mode, and the warning lights come on and
Continue driving at a moderate speed and contact the manufacturer's dealer as soon as possible.
In case of some malfunctions, the system may not switch to "2WD" mode or "4WD Lock" mode. The "AUTO" mode remains on.
Contact the manufacturer's service station as soon as possible.
4-wheel drive system
Whichever mode is selected, do not start the engine unless all four wheels are resting on the ground, such as when using a jack or rolling stand.
Do not turn the mode switch while cornering, reversing, or when the wheels are skidding. Select 2WD, AUTO or 4WD Lock mode only when the vehicle is driving in a straight line.
Use only tires with the required specifications.
The "4WD Lock" mode is specially designed for off-road driving. Using this mode in any other case may limit the vehicle's maneuverability and cause damage to the mechanical components of the vehicle.
Always fit tires with the same characteristics (make, size, type, wear, etc.) on four wheels of your car. Using tires of different sizes on the front, rear and / or right and left wheels can have serious consequences for the tires themselves, as well as for the transmission, transfer case and rear differential gears.
The set of parts that supply torque directly to the drive wheels is called driven wheels.
The transmission of torque from the differential to the drive wheels, depending on the type of wheel suspension, is carried out using solid axle shafts or cardan drives. Half-axles are used in the drive of the leading non-steer wheels; cardan drives with simple cardan joints - in the drive of non-steer wheels with a sprung main drive.
Cardan drives with synchronous joints (equal angular speeds) are used in the drive of the steered wheels. The drive to the drive wheels must ensure that there is no torque ripple at full wheel travel allowed by the car's suspension.
The semi-axles of the driving axle with a rigid beam (Fig. 15), depending on the loads tested by the semi-axle, are conventionally divided into semi-unloaded (Fig. 15- a), three-quarters unloaded (Fig. 15- b) and completely unloaded (fig. 15- v).
Semi-balanced axle shaft(fig. 15- a) has an external support installed inside the beam 5 bridge (Fig. 16). At the same time, from the side of the wheel, the axle shaft perceives all the efforts and moments acting from the roads. Semi-unloaded axle shafts have the simplest design and therefore are widely used in passenger cars. Typically, such designs do not have a wheel hub; it is replaced by the axle shaft flange, to which the wheel disc and brake drum are directly attached.
Fig. 15. Diagrams of the semiaxis load of the leading unguided bridges:
a- semi-unloaded semiaxis; b- the semiaxis is unloaded by three quarters; v- fully unloaded axle shaft.
The outer end of the semiaxis rests on ball (Fig. 16- a) or roller tapered (fig. 16- b) bearings that transmit both normal and axial forces. When using ball bearings to transmit the axial force of one of the directions, a locking ring is pressed onto the axle shaft 6 (fig. 16- a).
Three-quarter unloaded axle shaft(fig. 15- b) has an external support between the wheel hub and the axle beam (Fig. 17). In this case, the bending moments from the reactions of the tangential traction force R R or braking force R τ and from lateral (axial) force Y, arising, for example, when the car turns, (see Fig. 15- b) are perceived simultaneously by the axle shaft and the beam of the bridge through the bearing.
Fig. 16. Connection of a semi-balanced axle shaft with a wheel:
1 - semiaxis; 2 - wheel hub; 3 - bearing; 4 - conical fastening of the wheel hub; 5 - drive axle beam; 6 - locking ring.
The proportion of loads on the axle shaft depends on the design of the bearing and its stiffness.
Fig. 17. Connection of a three-quarter-balanced axle shaft with a wheel:
1 - semiaxis; 2 - bearing; 3 - drive axle beam; 4 - axle shaft flange.
Lateral (axial) force Y loads the bearing with a torque that causes the bearing to skew and drastically reduces its service life. Due to these disadvantages, semi-shafts of this type are of limited use.
Fig. 18. Connection of a fully balanced axle shaft with a wheel hub:
1 - semiaxis; 2 - drive axle beam; 3 - hub; 4 - bearing; 5 - fastening the wheel hub.
Fully balanced axle shaft has an external support with a wheel hub mounted on spaced two roller or angular contact ball bearings (fig. 15- v and 18).
The axle shaft is theoretically only loaded with the torque transmitted from the differential to the wheels. However, due to the elastic deformation of the axle beam, the technological misalignment of the wheel hub and the gear of the differential axle shaft, the non-perpendicularity of the flange plane to the axle axis, the semiaxis bending deformation may occur. The resulting bending stress is 5-70 MPa.
In fig. 19 shows the design of the drive to the steered wheels of a passenger car with a semi-unloaded semi-axle and a cam joint. Drives to steerable wheels of a continuous bridge have become widespread, in which the semi-axle of the unloaded type has hinges of equal angular velocities.
Fig. 19. Drive to the driven steered wheels of a passenger car:
1 - wheel hub; 2 - bearing; 3 - semiaxis; 4 - spring; 5 - constant-velocity joint.
The half-shafts take significant variable loads. Usually they are performed with thickenings at the ends so that the inner diameter of the splines is not less than the main diameter of the semiaxis. To reduce the stress concentration, they seek to increase the radii of transitions from one diameter to another, to reduce the depth of the slots, which necessitates an increase in their number (from 10 for cars and up to 18 for trucks). The concentration of stresses decreases significantly during the transition to involute splines.
Car manufacturers are happy to write in catalogs about the possibility of purchasing various car models with all-wheel drive on all four wheels (4 x 4). Unfortunately, this definition often obscures systems that work in different ways. So, we propose to find out what the seller really means when he says "4-wheel drive".
4 driving wheels, this is ...
The first type of four-wheel drive (4x4) is the permanent drive of all four driving wheels, where the torque is always distributed on two axles. This distribution is provided by a central distribution mechanism. For example, the following models have permanent four-wheel drive: Audi Allroad, Mitsubishi Lancer Evolution and Pajero, Toyota Land Cruiser or Land Rover Discovery.
4-wheel permanent all-wheel drive can be further divided into symmetrical and asymmetrical. An asymmetric drive is found, for example, in the Land Rover Defender model, in which the torque is distributed along two axes in equal parts. With the asymmetric version, the torque is transmitted to the axles as required - this distribution is provided by a semi-axle distribution mechanism or a multiplex clutch.
Another type of all-wheel drive (4 x 4) is a mechanically connected 4-wheel drive. In this case, we are talking about the phenomenon when one axis is constantly driving, and the second axis can be connected by turning on the corresponding lever or pressing the corresponding button. The plug-in all-wheel drive can be seen, for example, in the Suzuki Jimmy, Jeep Wrangler or Nissan Patrol models, which have permanent rear-wheel drive, and the front axle can be connected by yourself. However, it is recommended to use this function only in off-road conditions. If driving under normal conditions, all 4 driving wheels will interfere more than help.
The third type of all-wheel drive (4 x 4) is an auto-connect drive. This solution is an intermediate option between permanent 4-wheel drive and mechanically connected drive. We will see such a drive in the following cars: Mitsubishi Outlander, Toyota RAV4, Volvo AWD, Suzuki SX4, Audi A3 or BMW X5. Here, the drive is permanently and directly to one axle, and thanks to the multiplex clutch, if necessary, it is also possible to automatically move the drive to the other axle.
Advantages and disadvantages of 4x4 all-wheel drive
It is true that in general, a four-wheel drive vehicle is more versatile than one-wheel drive vehicles, both in terms of surface quality and weather conditions. With all-wheel drive (4 x 4), you can probably go further than without it. However, this does not mean that we will achieve any goal at any time. Of course, 4x4 all-wheel drive provides better traction than single-axle drive, but if the car becomes uncontrollable and starts to slip, because it is possible, it will be very difficult to cope with this situation. Because in cars with four drive wheels, the rear part can begin to slip, and then the front part becomes uncontrollable.
It is also impossible to hide the fact that 4x4 vehicles are significantly more expensive than vehicles with a single drive axle. The maintenance of such vehicles is also more expensive. You will have to drive into gas stations more often, especially if your car has permanent four-wheel drive with all wheel drive.
Thanks to this system, your car becomes heavier, and therefore fuel consumption will be higher. You will also pay more for possible repairs. You will have a complex system in which, like any other part of the car, sooner or later a malfunction will occur.
Therefore, a car with two drive axles should not be bought for economic reasons. Such a vehicle is likely to be useful for those people who often drive on rough roads, for example, live or work in the mountains or forests, or tourists who regularly visit high-lying ski resorts.
Four-wheel drive vehicles (4 x 4) on the market
Finally, let's take a look at our selection of new 4x4 vehicles. Find one of the most popular Lithuanian classifieds sites. We select vehicles with all four driving wheels:
- Audi A5, A6, A7, Q2, Q3, Q5 and Q7;
- BMW, 4, 5 and 7 series, X1, X3, X4 and X5;
- Mercedes C-, E- and S-class, as well as models: CLA, GLC, GLE and GLS;
- Volkswagen Amarok, Golf, Multivan, Tiguan, Touareg.
- Subaru Impreza, Forester, Outback and XV;
- Mini Clubman, Cooper S, Countryman;
- Jeep Grand Cherokee.
If we change the filter parameters so that we are shown cars with an automatically connected second axle, we will see the following models: BMW, Audi, Volvo, Volkswagen, Mercedes, Porsche and Honda. The smallest number of offers for the sale of vehicles with a mechanically connected second axle. After setting such a request, we will see Nissan, Mitsubishi, Toyota, Suzuki, Jeep, Isuzu and Hyundai on the screen.
In general, there is plenty to choose from. Of course, there is no shortage of interesting models in the used car market. But remember that it is better to avoid used 4x4 vehicles offered for several thousand zlotys. A low price usually means that a lot of money will have to be left at the service station soon after the purchase.
Drive parts from Volkswagen Touareg transfer case
Nowadays, it is difficult to catch someone off guard with a question about a "four-wheel drive car". You will immediately be shown a passing SUV, the benefit of such equipment on the streets of our cities is more than enough. And those who are versed will also add that ordinary cars are also four-wheel drive (most often Audi and Subaru are mentioned). And that four-wheel drive can be "permanent" and "plug-in".
The question "Why?" meets, as a rule, one answer: "For better cross-country ability." However, regular readers of the automotive press are still aware of "better stability on slippery roads."
All this, as they say, is true, but not entirely. Therefore, today we will try to systematize our knowledge of all-wheel drive. More precisely, we will begin to cite, because this topic, like the entire modern car, is practically inexhaustible.
Divide by more
What drives the car? The engine turns the wheels, and they are already pushing off the road - just as we do when we take another step forward. Where the tire comes into contact with the road (let's call it the “contact patch”), the torque generated by the engine is converted into traction from the wheel. However, if the traction force is greater than the adhesion of the tire to the road, the wheel will slip - slip.
It is clear that if the car has two driving wheels, then all the force created by the engine is distributed between the two contact spots.
And if four? Then between four. The more driving wheels, the less traction force is applied to each wheel, to each contact patch. And this means that with the same tire grip, we can develop a much greater total traction force, that is, accelerate faster, drive on steeper hills, tow a heavier trailer. Or vice versa - with the same (or even greater) traction force, we will be able to confidently move on a much more slippery surface.
In general, simple physics. And it is clear that all this can be useful for a road car just as much as for a cross-country vehicle.
Resilience has a lot to do with all of this. After all, thanks to the adhesion of the tires to the road, the car not only accelerates, but also stops, changes direction, and in general stands on the road, and does not lie in a ditch after the first turn. However, the greater the longitudinal force acting in the contact patch, the lower the lateral force will be sufficient to break the wheel into side slip. And the skidding wheel practically does not perceive the lateral load.
And, of course, you can imagine many different situations when the practical use of all-wheel drive is manifested simply in the fact that any wheel is driving. For example, several wheels suddenly found themselves in very poor grip conditions - on snow, ice, mud. Or generally "hang out" in the air (and this happens when driving over rough terrain).
In this case, we can only rely on the fact that the wheels, which retain grip on the ground, are also driving.
However, the advantages of all-wheel drive have to be paid for - the complication (and rise in price) of the design, the increase in the mass of the car (and hence the fuel consumption), and a decrease in the usable space allocated for passengers and cargo. After all, in order for the wheels to become leading, torque from the engine must be supplied to them. This means that additional units will appear - transfer cases (at least one), main gears with differentials (one for each drive axle), drive shafts. And therefore, for most of the 20th century, all-wheel drive became widespread mainly only where it was simply impossible to do without it - in off-road vehicles.
But in most of them, four-wheel drive was used only from time to time - only in difficult conditions. The rest of the time, idle units fiddled with themselves like a useless load, only worsening the dynamics of the car and increasing fuel consumption. Why?
His majesty the differential
Even at the dawn of the era of self-propelled carriages, when the drive wheels were fixed on a common rigid axle, the designers were faced with the fact that a sharp turn became an insurmountable obstacle for the car. Indeed, when cornering, the "outer" wheel travels a longer distance than the "inner" one (for the same time), which means it must rotate at a higher speed. Or the inner wheel should slip, which the low-power early engines could not provide - and simply stalled. And if there was enough engine power, then the car constantly skidded when cornering, the tires wore out very quickly, and the axles broke because of the resulting loads. And therefore, rather quickly, the single axle of the driving wheels was replaced by two semi-axles, between which a differential appeared, a planetary mechanism that provides the right and left wheels with equal torque, but allowing them to rotate at different speeds.
But the fact is that the front and rear wheels also cover different distances when turning.
Moreover, in real traffic conditions, they can travel different distances on a straight line, because there are irregularities on the roads. And this means that if we make a car with all-wheel drive, then another differential must be provided in it - between the front and rear axles. Otherwise, the tires will wear out quickly, and the loads that have arisen in the drive will render it unusable.
Of course, the center differential is the complication and rise in the cost of the design and, again, extra weight. And without it, in principle, you can do, but on one condition: we will use all-wheel drive only on rather slippery surfaces and at low speeds, when there are no serious troubles for the tires and the drive. And on a hard road, you will have to leave only one drive axle.
At the beginning and middle of the last century, this approach was fine. The all-wheel drive scheme without a center differential (with a rigid connection in the transfer case and disabling one of the driving axles) was popular on off-road vehicles until the end of the 20th century. Actually, she has survived to this day, modernizing as much as possible.
Now, to connect the "additional" drive axle, you do not need to stop (in the English language literature this is called "shiftonthefly"). Today, the plug-in front axle drive is used in the Isuzu Trooper with a manual transmission, in the Jeep Wrangler, in the Mitsubishi Pajero Sport and many other vehicles.
Always complete!
But “just off-road vehicles” is one thing. Their consumers were quite satisfied with the main advantages of the scheme with a disconnected bridge - relative simplicity and, accordingly, cheapness, and they were not worried about high-speed movement on asphalt. Quite different - when the four-wheel drive car is not a "conqueror of meadows and deserts", but a vehicle for everyday use (and mostly on normal roads). In this situation, disadvantages come to the fore. Firstly, the impossibility of constantly using the advantages of all-wheel drive (after all, when driving on hard surfaces, only one axle remains the leading one). Secondly, the increased requirements for the driver's qualifications: he must correctly assess the situation and make a decision whether to turn on the additional bridge or not turn it on. And mistakes are fraught with unpleasant consequences: the transformation of the car into an all-wheel drive instantly changes not only the cross-country ability, but also the handling.
So, in recent years, permanent all-wheel drive with an interaxle differential has been used much more often. This is the case for most four-wheel drive cars and latest SUVs (all Audi quattro, except A3; all BMW iX and X5; Hyundai Santa Fe; Jaguar XType; all Mercedes-Benz 4matic, M and G-class; Mitsubishi Pajero - in in general, the full list can take up the entire volume allocated for the article).
However, the "differential" drive is not without its drawbacks.
First, on a slippery surface, the differential may fail. Have you ever watched a car skidding in snow or liquid mud from the side? Then you should have noticed that while the skid wheel is spinning wildly, the other makes almost no attempts to budge. The differential is to blame. And the center differential will behave in the same way when the wheels of one of the axles are on a slippery surface. To prevent this from happening, all-wheel drive vehicles (especially with high cross-country ability) have to be equipped with differential locks. It is clear that this does not make the drive system simpler and cheaper.
In addition, the transfer case and additional drive shafts continue to weigh down the machine and take up a lot of space. And if for large cars with powerful engines all this is not so important, then for cars, especially compact ones, dynamics, efficiency and capacity are seriously affected.
As needed
Not without the "help" of compact cars, another concept of all-wheel drive was born, which is used on many modern cars. In Western literature, it is called "torqueondemand" (or simply "on demand") - "moment of need."
The idea is to add an automatic device to a simple (without center differential) drive with a disengageable bridge, which connects it if necessary (say, when the "main" driving wheels slip). And even better - transmitting to the "additional" axle exactly as much torque as necessary.
Of course, such a scheme is inferior to a permanent all-wheel drive, but it is structurally simpler, and most importantly, it is very convenient in order to make a small car with an all-wheel drive.
After all, when the engine is in front and the "main" driving wheels are front, you can even refuse a separate transfer case - just make a simple power take-off to the rear axle, and install the same automatic device in the front. Such a drive is compact and rather lightweight, and therefore is very popular among passenger models (Audi A3; Volvo AWD and XC; Volkswagen Golf 4Motion, etc.), as well as models of "intermediate" classes (Ford Maverick, Honda CRV; Nissan X -Trail; Volvo XC 90, etc.).
The first “on demand” systems were created on the basis of a viscous friction clutch (until recently it was still retained on all-wheel drive Volvo V70s, it is still installed on Chrysler Voyager AWD, Land Rover Freelander and some Mitsubishi Pajero Pinin). Later, several more relatively simple hydraulic-mechanical devices were proposed that work without any outside interference. We propose to devote separate materials to their designs and principles of operation.
But all simple couplings with "internal automatism" have significant drawbacks. Firstly, they are triggered already upon slipping, which may be too late. Secondly, their characteristics (response speed, dependence of the transmitted torque on the slip speed, etc.) are determined by the design and cannot be changed without disassembly (which is often possible only in the factory). And this means that there is no need to talk about adaptation to specific traffic conditions.
And since microprocessor technology has dropped significantly in recent years, computer-controlled devices are increasingly used in on demand systems. They regulate the torque transmitted to the "additional" bridge not only depending on the current situation, but also on the basis of the forecast of its development. The possibilities of electronically controlled systems are very wide. And therefore, they are increasingly being used instead of the center differential in transfer cases of large powerful models (Chevrolet Tahoe and TrailBlazer; Infiniti FX, etc.).