Although all Subaru all-wheel drive systems have the same designation and name, today there are several different implementations of Subaru AWD all-wheel drive.
All Subaru models, with the exception of the Subaru BRZ rear-wheel drive coupe, are fitted with Subaru's standard AWD symmetrical all-wheel drive. But despite the common name, there are at least four different all-wheel drive systems in use today.
Standard all-wheel drive system based on center self-locking differential and viscous coupling (CDG)
This is the system that most people associate with all-wheel drive. Found in most Subaru vehicles with a manual transmission. It is the most symmetrical of all all wheel drive configurations, with torque under normal driving conditions being split 50:50 between the front and rear axles.
Subaru cars like the Subaru WRX 2011 with a manual transmission have an all-wheel drive system based on a self-locking center differential and a viscous coupling
When slippage is detected either front or rear, the center differential can send up to 80 percent of the torque to the axle with the best traction. The center differential uses a viscous clutch that operates without computer control and responds to mechanical differences in wheel grip.
This type of AWD system has been around for a very long time, and its appearance on the 2015 Subaru WRX means it's probably not going anywhere anytime soon. This simple, reliable system is the workhorse of Subaru's AWD system. The system ensures safe, sporty driving by always making the most of available traction.
An all-wheel drive system based on an interaxle self-locking differential and a viscous coupling can be found on the Subaru Impreza 2014 2.0i configuration, on the XV Crosstrek 2014 with a 5-speed manual gearbox; on 2014 Subaru Outback, Subaru Forester with 6-speed manual transmission and 2015 WRX with 6-speed manual transmission.
All-wheel drive system withvariable torque distribution for vehicles with automatic transmission (VTD)
Subaru has recently begun transitioning most of its vehicles from standard torque-converting automatic transmissions to continuously variable transmission (CVT),
Legacy, Outback, and Tribeca with a powerful 3.6-liter engine use the Variable Torque Distribution All-Wheel Drive system for cars
but there are still cars using this system.
A symmetrical all-wheel drive version using variable torque distribution (VTD) is used on Legacy, Outback, Tribeca with a 3.6-liter six-cylinder engine and a five-speed automatic transmission. In this case, the default torque distribution is 45:55 with a shift towards the rear axle, and instead of a viscous center differential, a hydraulic multi-plate clutch is used in combination with a planetary type center differential.
When slippage is detected, based on signals from sensors that measure wheel slip, throttle position and braking force, the electronically controlled clutch can lock in a 50:50 split (torque) between the front and rear axles where maximum traction is required (wheels with the road).
While a purely mechanical viscous coupling is simpler and perhaps more flexible, an electronically controlled VTD system has the advantage of being active rather than reactive, moving torque between axles faster than a mechanical system can.
All-wheel drive system with active torque distribution (ACT)
With the transition to CVT, Subaru models such as the XV Crosstrek are also moving to AWD systems with a slight offset towards the front axle.
Newer "Subs" equipped with a CVT system already use the third version of the all-wheel drive (AWD) system. This all-wheel drive system is similar to the VTD system described above - both use an electronically controlled multi-plate clutch to manage torque, but the CVT systems distribute torque in a 60:40 ratio with an offset towards the front axle.
This all-wheel drive system is also called AWD with Active Torque Sharing (ACT). Subaru's original electronically controlled multi-plate torque converter adjusts front-to-rear torque distribution in real time according to driving conditions.
The use of this system improves the efficiency and stability of the vehicle. You can find this system on XV Crosstrek models, the new 2014 Forester, the new 2015 WRX and WRX STI, and older models such as the 2014 Legacy, 2014 Outback.
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.
Directly below the SI-Drive handle is a switch that allows WRX STI drivers to change the balance between the two center differentials.
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 either of the center differentials using the electronic control system Driver Controlled Center Differential (DCCD) - "Driver Controlled Center Differential".
Torque distribution for DCCD systems is 41:59 offset towards the rear axle. This is a performance-oriented all-wheel drive system for serious sports.
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 (i.e. non-locking) differential, but more powerful models (such as the WRX and Legacy 3.6R models) are often equipped with a limited slip differential on the rear axle, to improve the grip of the wheels on the rear axle when cornering.
The WRX STI also features a limited slip differential on the front axle to maximize all-wheel traction, and the latest 2015 WRX and 2015 WRX STI also feature brake-based torque distribution systems that brake the inside wheel when cornering to ensure power is transferred to the outside. side when turning and reduce the turning radius.
10.05.2006
After the 4WD schemes used in Toyota were examined in some detail in previous materials, it turned out that there is still an information vacuum with other brands ... Let's first take the four-wheel drive of Subaru cars, 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, all Subaru on the mechanics have 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 obtained by combining 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 of the Impreza STi, there is also an advanced manual transmission with an "electronically controlled" center differential (DCCD), 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.
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1.1. Active AWD / Active Torque Split AWD |
Permanent front-wheel drive, without center differential, connection of the rear wheels with an electronically controlled hydromechanical clutch
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1 - torque converter lock-up damper, 2 - torque converter clutch, 3 - input shaft, 4 - oil pump drive shaft, 5 - torque converter clutch housing, 6 - oil pump, 7 - oil pump housing, 8 - transmission housing, 9 - speed sensor turbine wheel, 10 - 4th clutch, 11 - reverse clutch, 12 - 2-4 brake, 13 - front planetary gear set, 14 - 1st clutch, 15 - rear planetary gear set, 16 - 1st brake gear and reverse, 17 - transmission output shaft, 18 - "P" mode gear, 19 - front drive gear, 20 - rear output shaft speed sensor, 21 - rear output shaft, 22 - shank, 23 - clutch A- AWD, 24 - front drive driven gear, 25 - freewheel, 26 - valve block, 27 - sump, 28 - front output shaft, 29 - hypoid gear, 30 - impeller, 31 - stator, 32 - turbine. |
E this option has long been installed on the vast majority of Subaru (with automatic transmission type TZ1) and is widely known from the Legacy model of 89. In fact, this four-wheel drive is as “honest” as the fresh Toyota Active Torque Control - the same rear-wheel drive and the same TOD (Torque on Demand) principle. There is no center differential, and the rear-wheel drive is activated by a hydromechanical clutch (friction package) in the transfer case.
The Subar scheme has some advantages in the working algorithm over other types of plug-in 4WD (especially the simplest ones, like the primitive V-Flex). Albeit small, but the moment during A-AWD operation is constantly transmitted back (unless the system is forcibly turned off), and not only when the front wheels slip - this is more useful and efficient. Thanks to hydromechanics, the force can be redistributed a little more accurately than in an electromechanical ATC. In addition, A-AWD is structurally more durable. For cars with a viscous coupling for connecting the rear wheels, there is a danger of a sharp spontaneous “appearance” of the rear drive in a turn, followed by an uncontrolled “flight”, but in A-AWD this probability, although not completely excluded, is significantly reduced. However, with age, as wear and tear, the predictability and smoothness of the connection of the rear wheels decreases significantly.
The algorithm of the system remains the same throughout the entire release period, only slightly corrected.
1) Under normal conditions, with the accelerator pedal fully released, the torque distribution between the front and rear wheels is 95/5..90/10.
2) As you press on the gas, the pressure supplied to the clutch package begins to increase, the discs gradually tighten and the torque distribution begins to shift towards 80/20 ... 70/30 ... etc. The relationship between gas and line pressure is by no means linear, but rather looks like a parabola - so that a significant redistribution occurs only when the pedal is pressed hard. With a fully recessed pedal, the friction clutches are pressed with maximum effort and the distribution reaches 60/40 ... 55/45. Literally, "50/50" is not achieved in this scheme - this is not a hard lock.
3) In addition, the speed sensors of the front and rear output shafts installed on the box make it possible to determine the slip of the front wheels, after which the maximum part of the moment is taken back regardless of the degree of gas supply (except for the case of a fully released accelerator). This function is active at low speeds, up to about 60 km/h.
4) When forced into 1st gear (selector), the clutches are immediately pressed with the maximum possible pressure - thus, as it were, "difficult all-terrain conditions" are determined and the drive remains the most "permanently full".
5) When the "FWD" fuse is plugged into the connector, no overpressure is supplied to the clutch and the drive is constantly carried out only on the front wheels (distribution "100/0").
6) With the development of automotive electronics, it has become more convenient to control slippage using standard ABS sensors and reduce the degree of clutch blocking when cornering or ABS is triggered.
It should be noted that all passport distributions of moments are given only in statics - during acceleration / deceleration, the weight distribution along the axes changes, so the real moments on the axes are different (sometimes "very different"), just like with different coefficients of wheel adhesion to the road.
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1.2. VTD AWD |
Permanent four-wheel drive, with center differential, electronically controlled hydromechanical clutch lock
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1 - torque converter lock-up damper, 2 - torque converter clutch, 3 - input shaft, 4 - oil pump drive shaft, 5 - torque converter clutch housing, 6 - oil pump, 7 - oil pump housing, 8 - transmission housing, 9 - speed sensor turbine wheel, 10 - 4th clutch, 11 - reverse clutch, 12 - 2-4 brake, 13 - front planetary gear set, 14 - 1st clutch, 15 - rear planetary gear set, 16 - 1st brake gear and reverse, 17 - countershaft, 18 - "P" mode gear, 19 - front drive gear, 20 - rear output shaft speed sensor, 21 - rear output shaft, 22 - shank, 23 - center differential, 24 - center differential lock clutch, 25 - front drive driven gear, 26 - overrunning clutch, 27 - valve block, 28 - sump, 29 - front output shaft, 30 - hypoid gear, 31 - impeller, 32 - stator, 33 - turbine . |
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 all-wheel drive is really permanent, with an asymmetric center differential (45:55), which is blocked by an electronically controlled 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).
Subaru usually attaches a fairly advanced VDC (Vehicle Dynamic Control) system to the VTD, in our opinion - a system of exchange rate stability or stabilization. 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.
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1.3. "V Flex" |
Permanent front-wheel drive, no center differential, viscous coupling for rear wheels
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.
We have already said that in English under the concept of LSD everyone gets self-locking differentials, but in our tradition this is usually called a system with a viscous coupling. But Subaru used a whole range of LSD differentials in different designs on their cars ...
2.1. Old style viscous LSD
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In the LSD differential, the right and left side gears are "connected" through a viscous coupling - the right splined shaft passes through the cup and engages with the clutch hub (the differential satellites are mounted cantilevered). The clutch housing is one piece with the gear of the left axle shaft. In a cavity filled with silicone fluid and air, there are discs on the splines of the hub and body - the outer ones are held in place by spacer rings, the inner ones are able to move slightly along the axis (for the possibility of obtaining a "hump effect"). The clutch works directly on the difference in speed between the right and left axle shafts.
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During rectilinear motion, the right and left wheels rotate at the same speed, the differential cup and side gears move together, and the moment is equally divided between the axle shafts. When there is a difference in the frequency of rotation of the wheels, the housing and the hub with the disks fixed to them move relative to each other, which causes the appearance of a friction force in the silicone fluid. Due to this, in theory (only in theory), there should be a redistribution of torque between the wheels.
2.2. New viscous LSD
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- Impreza WRX manual transmission until 1997
- Forester SF, SG (except FullTime VTD + VDC versions)
- Legacy 2.0T, 2.5 (except FullTime VTD + VDC versions)
Working fluid - transmission oil class API GL-5, viscosity according to SAE 75W-90, capacity ~0.8 / 1.1 l.
2.3. Friction LSD
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The next in line of appearance is the friction mechanical differential, used on most versions of the Impreza STi since the mid-90s. The principle of its operation is even simpler - side gears have a minimum axial play, a set of washers is installed between them and the differential housing. When there is a difference in the speed between the wheels, the differential works like any free one. The satellites begin to rotate, while there is a load on the gears of the axle shafts, the axial component of which presses the pack of washers and the differential is partially blocked.
The cam-type friction differential was first used by Subaru in 1996 on turbo Imprezas, then it appeared on the Forester STi versions. The principle of its operation is well known to the majority from our classic trucks, shishigs and UAZs.
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. The separator rotates together with the differential case, the keys (or "crackers") fixed on the separator can move in the transverse direction. The protrusions and cavities of the cam shafts, together with the keys, form a transmission of rotation, like a chain.
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Scope (on domestic market models):
- Impreza WRX after 1996
- Forester STi
The working fluid is an ordinary gear oil of API GL-5 class, viscosity according to SAE 75W-90, capacity ~ 0.8 l.
Eugene
Moscow
[email protected] website
Legion-Autodata
Information on car maintenance and repair can be found in the book (books):
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.
Subaru celebrates 40th anniversary of its 4x4 vehicles
Fuji Heavy Industries Ltd. (FHI), the manufacturer of Subaru vehicles, announced that 2012 marks the 40th anniversary of the debut of Subaru's all-wheel drive vehicles, the first of which, the Subaru Leone Estate Van 4WD, was introduced in Japan in 1972.
To this day, FHI remains a pioneer in all-wheel drive passenger cars. Subaru's total production of *1 4x4 vehicles reached 11,782,812 (January 31, 2012), representing approximately 55.7% of the brand's total sales.
Subaru's all-wheel drive system ensures efficient distribution of traction to all four wheels. Thanks to the combination of symmetrical all-wheel drive (SAWD) and the horizontally opposed Subaru Boxer engine, the power unit is located symmetrically relative to the longitudinal axis of the car, and the transmission is shifted back within the wheelbase. This arrangement optimizes the longitudinal-transverse mass balance and provides stable traction on any surface in different driving conditions. In addition, excellent high-speed stability and excellent steering and handling characteristics are achieved, making SAWD the core technology behind Subaru's philosophy of safety combined with driving pleasure.
Through continuous research, adapting Subaru's all-wheel drive system to the character of each model, FHI has perfected its technologies in this area - from technology that can provide handling on rough roads to unique technology that guarantees high stability in rain, snow or driving conditions. high speed. The latest developments include four-wheel traction control, which creates a reliable four-wheel traction at all times.
Additional Information
Subaru symmetrical all-wheel drive systems
- VTD all-wheel drive system*2: Sports version of electronically controlled all-wheel drive, which improves steering characteristics. The compact all-wheel drive system includes a planetary center differential and an electronically controlled multi-plate hydraulic lock-up clutch*3. The torque distribution between the front and rear wheels in a ratio of 45:55 is continuously adjusted by a differential lock using a multi-plate clutch. Torque distribution is controlled automatically, up to a 50:50 ratio between the front and rear wheels, taking into account the condition of the road surface. This provides excellent stability, and by distributing torque with emphasis on the rear wheels, steering characteristics are improved for aggressive, sporty driving.
Current models (Russian specification)]
On the Russian market Subaru Legacy GT, Forester S-Edition, Outback 3.6, Tribeca, WRX STI with automatic transmission - All-wheel drive system with active torque distribution (ACT): An electronically controlled all-wheel drive system that improves economy and stability. Subaru's original electronically controlled multi-plate torque converter adjusts front-to-rear torque distribution in real time according to driving conditions. In normal modes, the system distributes torque between the front and rear wheels in a ratio of 60:40. It maximizes the benefits of all-wheel drive, providing stable and safe handling in any driving situation, regardless of the driver's skill level.
On the Russian market Subaru Legacy/Outback 2.5 with Lineartronic transmission, Forester (with automatic transmission), Impreza and XV with Lineartronic transmission. - All-wheel drive system with center self-locking differential with viscous coupling (CDG): Mechanical four-wheel drive system for mechanical transmissions. The system is a combination of a center differential with bevel gears and a viscous coupling based lock. Under normal conditions, the torque between the front and rear wheels is distributed in a ratio of 50:50. The system ensures safe, sporty driving by always making the most of available traction.
[Current models (Russian specification)]
Subaru Legacy, Forester, Impreza and XV with manual transmission. - All-wheel drive system with multi-mode center differential (DCCD *4): A performance-oriented all-wheel drive system for serious sporting events. The all-wheel drive system with an electronically controlled active limited-slip center differential uses a combination of mechanical and electronic differential locks when changing torque. Torque is distributed between the front and rear wheels in a ratio of 41:59, with an emphasis on maximum driving performance and optimal control of the vehicle's dynamic stability. The mechanical interlock has a faster response and works before the electronic one. Working with high torque, the system demonstrates the best balance between sharpness of control and stability. There are preset differential lock control modes, as well as a manual control mode, which the driver can use according to the traffic situation.
[Current models (Russian specification)]
Subaru WRX STI with manual transmission.
*1 including production of vehicles with plug-in all-wheel drive
*2 VTD: Variable Torque Distribution
*3 Controlled limited slip differential
*4 DCCD: Active Center Differential
Symmetrical AWD
Symmetrical all-wheel drive
Since its introduction in 1972, Symmetrical AWD (All-Wheel Drive) technology has been continuously improved. Complemented by a horizontally opposed Subaru BOXER engine, it ensures perfect symmetry of the design. This results in maximum efficiency of engine power delivery, a high level of grip and vehicle stability, as well as ideal weight distribution. Absolute control over the car is maintained in almost any driving conditions, turning every kilometer of the distance traveled into pleasure.
Engine torque is constantly transmitted to all four wheels and provides maximum grip and, therefore, maximum vehicle handling, therefore, the better the grip of the wheels with the road, the more confident you feel behind the wheel of your car. This advantage is your guarantee of success in extreme conditions, whether it be bad weather or an emergency situation, when the score goes by a fraction of a second.
Advantages
The best balance
When you turn, the centrifugal force pushes the car to the edge of the road. How far the car rolls depends on the center of gravity. If it is located high, it takes more time to regain balance and control of the car. If it's low - like a Subaru - there's less body roll and less yaw, giving the car more stability.
Improved grip strength
Permanent four-wheel drive has special advantages over 2-wheel drive (2WD) - especially when cornering. By transmitting power through all four wheels, the car handles naturally and neutrally around corners, avoiding sluggishness or oversteer that can lead to instability and crashes.
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.
