To date, there are many all-wheel drive systems for cars. Consider the two most common versions using the example of Subaru cars, because some of them have a common name and designation. There are several different versions of the Subaru AWD all-wheel drive implementation.
All such models (except rear-wheel drive Subaru BRZ coupes) have standard AWD symmetrical all-wheel drive. The name is common, but four of its modifications of all-wheel drive systems are used.
Standard all-wheel drive system based on center self-locking differential and viscous coupling (CDG)
Most people believe that this category of systems is associated with all-wheel drive. It is very common in cars of a similar brand with a manual transmission. This model is a symmetrical all-wheel drive configuration, under normal conditions, the torque is in the ratio of the front and rear axles 50 to 50.
When the car slips, the differential, which is located between the axles, is able to send up to 80% of the torque to the front axle, this function ensures good tire grip with the roadway. A viscous coupling is used by such a differential so that it can respond to a mechanical difference in tire grip with the road without the participation of a computer.
You can see the cdg all-wheel drive type on the Subaru Forester, which has a six-speed gearbox.
Such a drive has been used for a long time, and the appearance of a new version next year only means that it will not disappear soon. The model is a reliable and simple all-wheel drive system that can provide a very safe driving when using the available traction.
It should be noted that the cdg type of all-wheel drive can be seen on the 2014 Subaru Impreza cars with a two-liter engine, as well as on the XV Crosstrek, which has a five-speed manual transmission, on the Ouback and Forester, which have a six-speed gearbox.
All-wheel drive system with variable torque distribution for vehicles with automatic transmission (VTD)
It is very important to note that Subaru has started to convert most of its vehicles from standard automatic to continuously variable transmission (CVT). At the same time, now you can still find cars with such a system.
Symmetrical all-wheel drive, which involves the use of variable torque distribution, can be found on the Tribeca (with a 3.6i engine and 6 cylinders, as well as a 5-speed gearbox), Outback and Legacy. Here there is a shift of torque towards the rear axle in the proportion of 45 to 55. Instead of a center differential with a viscous coupling, a multi-plate hydraulic clutch will be used here, which will be combined with a planetary variant differential.
When slip is detected, signals will be sent from sensors that are installed to measure wheel slip, as well as braking force and throttle position located near the throttle. In this case, the torque will be distributed evenly along the axes (50 to 50) to ensure maximum adhesion of the wheels to the asphalt surface.
A fully mechanical viscous coupling is much simpler and more flexible. The VTD system has the advantage that it has an active rather than a reactive component, this achieves a high speed of torque transfer between the axes, a mechanical system cannot boast of such.
All-wheel drive system with active torque distribution (ACT)
Subaru's newer models are already using a third variant of all-wheel drive systems. In particular, it has many similarities with the previous version - it also implies the use of an electronically controlled multi-disc system in a ratio of 60 to 40 with a torque shift to the front axle.
All-wheel drive type act is used on Subaru Legacy 2014 models
Also, this AWD has an active torque distribution called ACT. Thanks to the original multi-plate electronically controlled torque transmission clutch, the distribution of torque between the axles in real time corresponds to the driving conditions of the vehicle.
Such an all-wheel drive system allows you to increase both the stability and efficiency of the machine. The act all-wheel drive type is used on the Subaru XV Crosstrek, Legacy 2014, Outback 2014, WRX and WRX STI 2015 models.
All-wheel drive system with multi-mode center differential (DCCD)
In addition to the all-wheel drive systems described above, Subaru used other variants of symmetrical all-wheel drive, which are no longer used. But the last system we will mention today is the system that is used on the WRX STI.
This system uses two center differentials. One is electronically controlled and gives Subaru's on-board computer good control over the distribution of torque between the axles. The other is a mechanical device that can respond more quickly to external influences than its electronic "colleague". The driver's benefit, ideally, is to use the best of the electronic proactive and mechanical reactive "world".
Generally speaking, these differentials naturally make use of their differences - being harmoniously combined by a planetary gear - but the driver can shift the system towards any of the center differentials using the electronic control system Driver Controlled Center Differential (DCCD) - "Driver Controlled Center Differential".
The torque distribution for DCCD systems is 41:59 offset towards the rear axle. This performance-oriented all-wheel-drive system is for serious sporting events.
Side torque distribution
So far, we have figured out how modern Subaru distribute torque between the front and rear axles, but what about the distribution of torque between the wheels, between the left and right side? On both the front and rear axles, you will usually find a standard open-type differential (i.e., not subject to locking). More powerful models (such as the WRX and Legacy 3.6R models) are often fitted with a limited slip differential on the rear axle to improve rear wheel traction when cornering.
The WRX STI is also equipped with a limited slip differential on the front axle to maximize all-wheel traction. The latest 2015 WRX and 2015 WRX STI also use brake-based torque distribution systems that brake the inside wheel when cornering to ensure power is transferred to the outside when cornering and reduce the turning radius.
After the 4WD schemes used on Toyota were considered in some detail in previous materials, it turned out that there is still an information vacuum with other brands. Let's start with all-wheel drive cars Subaru, which many call "the most real, advanced and correct."
Mechanical boxes, by tradition, are of little interest to us. Moreover, everything is quite transparent with them - since the second half of the 90s, Subaru mechanics have had an honest all-wheel drive with three differentials (the center differential is blocked by a closed viscous coupling). Of the negative sides, it is worth mentioning an overly complicated design, resulting from the combination of a longitudinally mounted engine and the original front-wheel drive. As well as the refusal of the Subarovites from the further mass use of such an undoubtedly useful thing as a downshift. On single "sports" versions, there is also a highly advanced manual transmission with an "electronically controlled" center differential, where the driver can change the degree of its blocking on the go ...
But let's not digress. There are two main types of 4WD used in automatic transmissions currently operated by Subaru.
1. Active AWD
This option has long been installed on the vast majority of Subaru (with automatic transmission type TZ1). In fact, this "all-wheel drive" is as "honest" as Toyota's V-Flex or ATC - the same plug-in rear wheels and the same TOD (Torque on Demand) principle. There is no center differential, and the rear-wheel drive is switched on by a hydromechanical clutch in the transfer case - it goes back from ~ 10% of the force under normal conditions (if this is not attributed to internal friction in the clutch) to almost 50% in the limit state.
Although the Subar scheme has some advantages in the working algorithm over other types of plug-in 4WD. Albeit small, but the moment during A-AWD operation (unless the system is forcibly turned off) is still transmitted back constantly, and not only when the front wheels slip - this is more useful and efficient. Thanks to hydromechanics, it is possible to redistribute the force (although it is too loudly said to "redistribute" - just select a part) more accurately than in the electromechanical ATC - A-AWD is able to work out slightly both in turns and during acceleration and braking, and it will be structurally stronger. The probability of a sharp spontaneous "appearance" of the rear drive in a turn with subsequent uncontrolled "flight" has been reduced (there is such a danger for cars with a viscous coupling for connecting the rear wheels).
To improve the "all-terrain" qualities, Subaru often installs an automatic locking mechanism (viscous clutch, "cam differential" - see below about it) in the rear differential of models with A-AWD.
2. VTD AWD
The VTD (Variable Torque Distribution) scheme is used on less mass-produced versions with automatic transmissions such as TV1 (and TZ102Y, in the case of the Impreza WRX GF8) - as a rule, the most powerful in the range. Here, everything is in order with "honesty" - the four-wheel drive is really permanent, with an interaxle differential (blocked by a hydromechanical clutch). By the way, since the mid-80s, Toyota 4WD has been working on the same principle on the A241H and A540H boxes, but now, alas, it has remained only on the original rear-wheel drive models (FullTime-H or i-Four all-wheel drive).
Every VTD flyer states that "Torque is split 45/55 between the front and rear wheels." And wow, many are actually beginning to believe that they are driven forward along the track by 55% rear-wheel drive. You need to understand that these figures are an abstract indicator. When the car moves in a straight line and all wheels rotate at the same speed, the center differential, of course, does not work out, and the moment is clearly divided between the axles in half. What do 45 and 55 mean? Just gear ratios in the planetary gear set of the differential. If the front wheels are forcibly stopped completely, then the differential carrier also stops, and the gear ratio between the rear drive drive shaft and the transfer case input shaft will just be the same 55/100, that is, 55% of the torque developed by the engine will go back (the differential will work as an overdrive ). If the rear wheels freeze, then 45% of the torque will go forward through the differential carrier in the same way. Of course, the presence of blocking is not taken into account here, and indeed ... In reality, the distribution of moments is a constant floating value and is far from unambiguous.
Subaru usually attaches a fairly advanced VDC (Vehicle Dynamic Control) system to the VTD, in our opinion - a system of exchange rate stability. At the start, its component, TCS (Traction Control System), slows down the slipping wheel and slightly strangles the engine (firstly, by the ignition timing, and secondly, even by turning off part of the nozzles). Classic dynamic stabilization works on the go. Well, thanks to the ability to arbitrarily slow down any of the wheels, VDC emulates (simulates) a cross-axle differential lock. Of course, this is great, but you should not seriously rely on the capabilities of such a system - so far, none of the automakers has even managed to bring the "electronic lock" closer to traditional mechanics in terms of reliability and, most importantly, efficiency.
3. "V-Flex"
Probably worth mentioning is 4WD, which is used on small models with CVTs (like the Vivio and Pleo). Here the scheme is even simpler - a permanent front-wheel drive and a rear axle "connected" by a viscous coupling when the front wheels slip.
About the cam differential
1 - separator, 2 - guide cams,
3 - thrust bearing, 4 - differential housing, 5 - washer, 6 - hub
We have already said that in English all self-locking differentials fall under the concept of LSD, however, in our tradition, this is usually called a system with a viscous coupling. The LSD rear differential often used on Subaru is built differently - it can be called "friction, cam type". There is actually no rigid connection between the drive gear of the differential and the semi-axes, the difference in the angular velocity of rotation is provided by slipping of one semi-axis relative to the other, and the "lock" is inherent in the very principle of operation.
The separator rotates with the differential housing. The "keys" fixed on the separator can move in the transverse direction. The protrusions and cavities of the cams (let's call them that) together with the keys form a transmission of rotation, like a chain.
If the resistance on the wheels is the same, then the keys do not slip and both axle shafts rotate at the same speed. If the resistance on one wheel is noticeably greater, then the keys begin to slide along the cavities and protrusions of the corresponding cam, still trying to turn it in the direction of rotation of the separator. Unlike a planetary type differential, the speed of rotation of the second half-axle does not increase (that is, if one wheel is stationary, the second will not spin twice as fast as the differential housing).
Whether or not a car with such a differential can “drive on one wheel” is determined by the current balance between the resistance on the axle shaft, the speed of rotation of the body, the amount of force transmitted back and the friction in the key-cam pair. However, this design is certainly not "off"-road.
Even at the very beginning of its history, Subaru made a bet on all-wheel drive versions of its models, a technology that at that time was available mainly on special vehicles. In 1972, Subaru introduced its first four-wheel-drive model, the Leone Estate Van 4WD, and since then, more than half of the company's sales have come from all-wheel drive vehicles. It is also important that Subaru's symmetrical all-wheel drive was not adapted to cars with a single axle drive, but was immediately created for use on cars with four-wheel drive. As for the all-wheel drive Subaru Symmetrical All Wheel Drive with axle shafts of the same length, coupled with a longitudinally located boxer Subaru Boxer engine and a transmission shifted to the wheelbase, this arrangement allows, in addition to close to ideal weight distribution along the axes, to ensure efficient implementation of engine power and a good grip balance wheels with the road on any type of surface. That is, the optimal distribution of torque between all wheels, and hence a high level of controllability.
Torque is optimally distributed to all wheels, resulting in near-neutral steering
Symmetrical all-wheel drive confidently counteracts both the drift of the front axle and the skid of the rear
There are four types of all-wheel drive Symmetrical AWD. The first of them, VTD, is not represented on the Russian market today, but was previously used on Legacy GT models 2010-2013, Forester S-Edition of the same period, Outback with a 3.6-liter engine 2010-2014, Tribeca, WRX and WRX STI 2011–212 This system uses a planetary type center differential, which is blocked by an electronically controlled multi-plate hydraulic clutch.
The original 45:55 torque distribution is constantly monitored by Vehicle Dynamic Control and automatically adjusted according to road conditions, road profile and topography. The second system is ACT with active torque distribution. Here, through a multi-plate electronically controlled clutch, the torque, depending on the condition of the road, is dosed to the front and rear wheels up to a ratio of 60:40 in real time. On the Russian market with this type of all-wheel drive, Forester, Outback and XV models with Lineatronic transmission are presented.
For mechanical transmissions, the CDG all-wheel drive system with a self-locking differential is designed. In its design, an interaxle differential with bevel gears, blocked by a viscous coupling, is used. At the same time, under normal driving conditions, the distribution of traction between the front and rear wheels occurs in a ratio of 50:50. This system is very well suited for sports driving, so it is not surprising that it was previously used on the WRX model with a manual transmission, and today the Forester and XV models with a manual transmission are on the Russian market. The fourth type of all-wheel drive Subaru - DCCD has in its arsenal an electronically controlled active limited slip differential, and it is fully focused on sports driving enthusiasts, those who love the Subaru brand for its cars with a racing character.
It is with this type of drive that we have presented the Subaru WRX STI car. This design is a symbiosis of electronic and mechanical center differential locks that respond to changes in torque. First, a faster mechanical interlock is activated, then the electronic interlock is activated. The torque between the front and rear wheels is distributed in a ratio of 41:59, and the operation of the entire system is focused on the optimal use of maximum driving characteristics. The design of the differential provides for the possibility of "preload", that is, the mode of pre-setting its characteristics. By quickly realizing high torque, such a system strikes a good balance between sharpness and precision of control and vehicle stability. Of course, in this type of drive, a manual transmission control mode is also provided.
The low center of gravity of a compact boxer engine, symmetrical all-wheel drive with equal drive lengths and transmission variations... All this ensures excellent handling on any kind of surface.
And in conclusion, a few well-known postulates about the benefits of all-wheel drive. In this case, the symmetrical all-wheel drive Subaru Symmetrical AWD. Thanks to the fact that the torque is distributed to all four wheels, the car demonstrates stable behavior both on the turning arc on asphalt pavement and when driving on uneven roads. The advantage of an all-wheel drive car is especially noticeable when driving on winter roads. Secondly, an all-wheel drive car is more prone to neutral steering than its two-wheel drive counterparts. Thus, his driver is much less likely to go past the turn. And, of course, a four-wheel drive car, as a rule, has good acceleration dynamics: the torque transmitted to all four wheels allows you to better realize the capabilities of high-power engines.
The question is interesting, especially since last year the Japanese brand celebrated the 40th anniversary of the moment the first four-wheel drive car, the Subaru Leone Estate Van 4WD, rolled off the assembly line of the enterprise. A little statistics - for forty years, Subaru has produced more than 11 million copies of cars with all-wheel drive. To this day, all-wheel drive from Subaru is considered one of the most efficient transmissions in the world. The secret of the success of this system is that Japanese engineers use a symmetrical torque distribution system between the axles and between the wheels, which allows the machines on which this type of transmission is installed to effectively cope with off-road conditions (Forester, Tribeca, XV crossovers), so and feel confident on sports tracks (Impreza WRX STI). Of course, the system's effect would not be complete without the company's signature Boxer horizontally-opposed engine, which sits symmetrically along the car's longitudinal axis while the all-wheel drive system is pushed back toward the wheelbase. This arrangement of the units provides Subaru vehicles with stability on the road due to low body roll - since the horizontally opposed engine provides a low center of gravity, and the car does not experience oversteer or understeer when cornering at speed. And constant traction control on all four drive wheels allows you to have excellent grip on the road surface of almost any quality.
I note that the symmetrical all-wheel drive system is just a common name, and Subaru has four systems themselves.
I will briefly point out the features of each of them. The first, commonly referred to as sports all-wheel drive, is the VTD system. Its feature is to improve the turning characteristics of the car, which is achieved through the use of an interaxle planetary differential and a multi-plate hydraulic locking clutch, which is controlled electronically. The basic distribution of torque along the axles is expressed as 45:55, but at the slightest deterioration in the condition of the road surface, the system automatically equalizes the torque between both axles. This type of drive is equipped with models Legacy GT, Forester S-Edition, Impreza WRX STI with automatic transmission and others.
The second type of symmetrical all-wheel drive, used on the Forester with automatic transmission, Impreza, Outback and XV with Lineatronic transmission, is called ACT. Its peculiarity is that its design uses a special multi-plate clutch that corrects the distribution of torque between the axles depending on the condition of the road surface. By default, the moment in this system is distributed in a ratio of 60:40.
The third type of all-wheel drive transmission from Subaru is the CDG, which uses an interaxle self-locking differential and a viscous coupling. This system is designed for manual transmission models (Legacy, Impreza, Forester, XV). The torque distribution ratio between the axles in a normal situation for this type of drive is 50:50.
Finally, the fourth type of all-wheel drive in Subaru is the DCCD system. It is installed on the Impreza WRX STI with "mechanics", distributes the torque between the front and rear axles in a ratio of 41:59 using a multi-mode center differential, which is controlled electrically and mechanically. It is the combination of mechanical, when the driver himself can choose the moment of locking the differential, and electronic locks that makes this system flexible and suitable for use in racing under extreme conditions.
