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.
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):
Quick jump to sections
The world premiere of the Subaru XV crossover, created on the basis of the Subaru Impreza model, took place in 2011 and today this car has firmly established itself in the ranks of urban SUVs.
There is never too much ground clearance, especially in our conditions.
Therefore, it is worth getting acquainted with the crossover, and which has the maximum ground clearance. This is the new Subaru XV, which has a ground clearance of 220mm. This car, like the Subaru Forester, is built on the platform of the new Impreza. It is slightly smaller than the "forester", but its ground clearance is exactly the same. Plus the mandatory all-wheel drive. It's a Subaru!
Why does a car need such an impressive distance between the road and the body? Ask those who live outside the city and every day overcome kilometers of not the best roads. Also, this question will be answered by those who live in the city, but on those streets where there is no asphalt.
Alternative option
However, ground clearance is not the only criterion when choosing a versatile vehicle. After all, if this were the case, then there simply wasn’t an alternative to an equal SUV, but there is such an alternative. Subaru XV in terms of off-road capabilities can give odds to many framers, and as for behavior on asphalt and fuel consumption, almost any comparison will be in favor of a crossover.
In order to better understand the dimensions of the Subaru XV, we present the data of the Forester. XV is 15 cm shorter and 12 cm lower, but they have almost the same wheelbase. In fact, no one will feel the difference of 5 mm in practice, and therefore the interior of the Subaru XV is almost as spacious as that of the Forester.
Specifications
- Length: 4450 mm
- Width: 1780 mm
- Height: 1615 mm
- Wheelbase: 2635 mm
- Curb weight: 1415 kg
- Ground clearance: 22 cm
- Trunk volume: 310 / 1210 liters
The difference in length is noticeable only in the volume of the trunk. If the Forester has 505 liters, then the Subaru XVI has only 310. On the other hand, for most compact five-doors it is quite a normal figure. Of course, the trunk can be quadrupled if the rear seats are folded down. For a car with all-wheel drive, there is always overall luggage with which you need to make an excursion to nature.
Yes, the backs of the rear sofa are not adjustable in terms of the angle of inclination. But the landing here is lighter than on the Forester, and this allows you to move on asphalt with more confidence. This Subaru is capable of cornering at speeds worthy of the finest premium car brands.
The fact that the car has a ground clearance of 22 cm is absolutely not felt. And it's understandable why. The boxer engine traditionally allows you to make the center of gravity lower than other cars. Plus, permanent all-wheel drive and a very well-tuned system of exchange rate stability.
As for engines, we have Subaru XV available with two engines, both petrol. The volume of the base unit is 1600 "cubes". It has 114 hp.
But much more interesting, of course, is a two-liter engine, in which one and a half hundred autohorses. With it, acceleration from standstill to the first hundred takes 10.5 seconds, and fuel consumption in the combined cycle is less than 8 liters per 100 km. And here's what's interesting: this indicator for the version with automatic transmission is better than for a car with a 6-speed manual.
Engines:
- 1.6 liter petrol
- Power 114 hp
- Torque: 150 Nm
- Maximum speed: 179 km/h
- Acceleration time to 100 km/h: 13.1 sec
- 2 liter petrol
- Power 150 hp
- Torque: 198 Nm
- Maximum speed: 187 km/h
- Acceleration time to 100 km/h: 10.7 sec
- Average fuel consumption: 6.5 liters per 100 km
Features of the variator
The reason is simple: here, as on the new generation Forester, it’s not a classic automatic, but a Lineartronic CVT. That is, there is no gear shifting, as such, but there is constantly unrelenting traction in almost the entire rev range. There is some howling characteristic of the variator, but it is drowned in the specific pleasant sound of the boxer engine. Especially if this motor is spinning.
By the way, if desired, the variator provides the ability to shift gears in manual mode, moreover, not only with a selector, but also with paddle shifters. Although, to be honest, the CVT does a great job without the driver's prompts.
By the standards of the class, the Subaru XV has a fairly spacious interior. Especially when compared with crossover competitors. Here you immediately feel the advantage that the car is built on the basis of a passenger car. And the landing is more comfortable, and the controls are all at your fingertips.
The interior, of course, is not as elegant as that of the Forster, but the quality of the finishing materials is also at its best. Front panel made of soft plastic. The seats, although they seem ordinary, are actually very tenacious to keep the driver and passengers in corners.
Audio system, climate control, power windows - all this is already "in the database". But keyless entry to the cabin, engine start button, leather seat upholstery, rain and light sensors, as well as dual-zone climate control, rely only on the top-end configuration. In it, the place of a monochrome display will also be taken by a multi-functional color one, the same as on the Forester, with a dynamic picture and a plug-in rear view camera.
All-wheel drive system
Subaru XV is only all-wheel drive. True, the “four by four” scheme here can be different. It all depends on the engine and transmission. The most off-road, oddly enough, version with a 1.6-liter engine and a manual transmission. It has an interaxle self-locking differential and a downshift is provided. So, if you plan to take real mud baths more or less regularly, it is better to opt for this version.
Cars with a CVT have their own symmetrical all-wheel drive scheme, with active torque distribution. By default, 60% of the drive is sent to the front wheels and 40% to the rear wheels. But for better grip and better handling, this ratio can change almost instantly and very flexibly. This is precisely the reason for the feeling of confidence that every driver gets behind the wheel of a Subaru.
Mandatory for all versions of the XV is the stability control system. By the way, in all configurations, except for the most basic, Subaru XV is equipped with front side and curtain airbags. In European tests, this crossover received the highest rating - five stars. Moreover, it was this car that was named "the safest for passengers' children."
The Subaru XV is truly a versatile machine that can handle just about every challenge our vehicles face in our environment equally well. It is comfortable in the city, rulitsya chic on the highway and is not afraid of moderate off-road.
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.
There are currently three types of drive used in conventional vehicles: front-wheel drive (FWD), rear-wheel drive (RWD), and all-wheel drive (4WD).
Already at the beginning of its history, Subaru made a bet on all-wheel drive, which at that time was used only for special cars. In this chapter, we will explain the benefits of Subaru's proprietary all-wheel drive system. For a better understanding, consider the influence of each type of drive on the dynamic qualities of the car. Since these qualities are largely dependent on the properties of the tires that are responsible for the connection between the car and the road surface, you should first familiarize yourself with the characteristics of the tires.
In addition to providing ride comfort by absorbing road bumps, tires perform three other important functions:
Since traction and braking forces cannot occur simultaneously, in the illustration on the right, the force acting on the tire is represented by two components. These are two elemental forces, the magnitude of which is limited by the general properties of the tire, which means that there is no possibility of control if the tire has exhausted the supply of properties for acceleration.
Imagine a car moving in an arc. In this situation, a lateral force acts on all four tires, balancing the centrifugal force that occurs during the turning of the car. And although only the front wheels are steerable, forces act on all four wheels of the car, tending to push it outward, out of the trajectory of the turn. If the vehicle speed continues to increase, the force acting on the tires and providing a given trajectory of movement will reach its limit, after which the car will deviate from the given trajectory. In such a case, if one of the tires is loaded with positive or negative (braking) torque, it will reach its grip limit before the rest of the tires. Depending on the type of drive (FWD/RWD/4WD), this phenomenon may affect the behavior of the vehicle in one way or another.*
The characteristics of tires are highly dependent on their material and construction, as well as the condition of the road. In addition, they are affected by the applied vertical load (the greater the load on the tire, the greater the force in contact with the road it can realize). The tire is able to maintain a given trajectory only during rotation. If the wheel is completely blocked, the car becomes uncontrollable.
- Centrifugal force
- Side reaction of the tire
- Maximum adhesion force
- Traction force
- Target trajectory
* The behavior of the car is affected not only by the type of drive system. Most vehicles, regardless of drive type, are designed with little understeer on normal dry roads for safety reasons. The most obvious features of behavior depending on the type of drive are manifested in limiting modes or on a slippery road.
Front-wheel drive
Rear drive
Four-wheel drive
Subaru permanent four-wheel drive - Symmetrical AWD
Advantages
- High stability: the torque is distributed to all four wheels, so that a safe behavior is maintained even on uneven surfaces.
- High flotation: excellent traction in all conditions is ensured by the supply of torque to all four wheels.
- Ease of handling: the tendency to understeer or oversteer is overcome even in extreme conditions.
- Good acceleration dynamics: torque is supplied to all four wheels, making this scheme perfectly combined with high-power engines.
Disadvantages of traditional all-wheel drive that Subaru's symmetrical all-wheel drive eliminates
- High weight, high fuel consumption... All-wheel drive components can be kept simple and light thanks to the longitudinal arrangement of the engine and gearbox.
- Mediocre handling... Thanks to the design advantages, all-wheel drive does not prevent Subaru models from demonstrating refined handling.
Front wheel drive FWD
Advantages
- The opportunity to get a more spacious interior, since there is no cardan shaft under the bottom. (But it is necessary to provide sufficient rigidity of the body, so many front-wheel drive models have a floor tunnel).
- High driving stability: Since the front wheels pull the vehicle, the constantly acting front wheel traction forces increase its stability when driving at high speeds.
- Ease of driving: A front-wheel drive car tends to understeer in extreme conditions. When the accelerator pedal is released and the traction force is reduced, control sensitivity is restored with a return to a given trajectory.
- Excellent Fuel Efficiency: The front-wheel drive layout provides a short torque transmission path and high efficiency.
Flaws
- Worse steering response: Since both traction and steering are carried out only by the front wheels, in extreme driving conditions there is a less clear response to steering and a tendency to understeer.
- With intensive acceleration of a car with a powerful engine, the load is redistributed to the rear wheels, which is why the front tires cannot fully realize their potential. Front wheel drive does not justify itself on cars with a powerful engine.
Understeer
- Centrifugal force
- Side reaction of the tire
- Maximum adhesion force
- Traction force
- Target trajectory
Rear wheel drive RWD
Advantages
- Sharp handling: the front wheels perform only the steering function. The front engine and rear-wheel drive provide the car with good weight distribution over the wheels.
- Smaller turning radius: The lack of front wheel drive allows for greater turning angle.
- Good acceleration on dry roads: during acceleration, the mass is redistributed to the rear wheels, contributing to the realization of more traction.
Flaws
- Less passenger compartment and trunk capacity: a bulky rear wheel drive (cardan shaft, main gear) is located under the bottom of the body.
- More curb weight: Rear-wheel drive vehicles have more components and assemblies compared to front-wheel drive vehicles.
- In extreme conditions, these cars show a tendency to oversteer, which makes them harder to drive front-wheel drive.
For sports models, this is more of an advantage than a disadvantage, as it adds thrills.
Oversteer
- Centrifugal force
- Side reaction of the tire
- Maximum adhesion force
- Traction force
- Target trajectory
All wheel drive 4WD
Advantages
- High stability: torque is supplied to all four wheels, so that a safe behavior is maintained even on uneven surfaces.
- High cross-country ability: the possibilities for implementing traction are much wider than with a monodrive scheme.
- Ease of handling: 4WD vehicles turn closer to neutral.
- Good acceleration dynamics: torque is supplied to all four wheels, so four-wheel drive is very well combined with high-power engines.
Flaws
- Less passenger compartment and trunk capacity: bulky front and rear wheel drive (cardan shaft, final drive located under the bottom of the body).
- Large curb weight due to a larger number of parts, assemblies and assemblies.
- Increased fuel consumption associated with greater mass and the presence of additional rotating parts.
- Worse response to control due to power circulation, and also due to the fact that the steered wheels are loaded with torque as driving ones.
Steering close to neutral
- Centrifugal force
- Side reaction of the tire
- Maximum adhesion force
- Traction force
- Target trajectory
Safety
Reliable grip
The main difference of the symmetrical drive is the same length of the right and left axle shafts, which makes it easy to provide sufficient suspension travel with a clear tracking of the road profile. As a result, the car reliably "holds" the road, the wheels seem to stick to the surface.
High stability
As already mentioned, the combination of Subaru's boxer engine and symmetrical drive results in excellent stability and handling. All-wheel drive guarantees additional advantages over competitors when driving off-road.
Driving pleasure
Economy
As a rule, all-wheel drive vehicles are characterized by greater mass and worse handling, which ultimately leads to increased fuel consumption. The symmetrical all-wheel drive, due to its design advantages, does not require unnecessary components. For some Subaru models, fuel consumption is comparable to that of mono-drive models of the same class from other manufacturers.
Refined handling
Thanks to the longitudinally mounted boxer engine and symmetrical drive, Subaru cars have refined handling. They are endowed with the cross-country ability of all-wheel drive models, and in terms of reaction speed they surpass conventional mono-drive models.
Stability and traction
The efficiency of all-wheel drive depends on the vehicle concept. The more actively the distribution of torque over the wheels, the higher the cross-country ability, however, most often to the detriment of controllability.
For Subaru models, with quick response and high efficiency of all-wheel drive, torque can be actively distributed to the wheels, while maintaining good stability and high cross-country ability on different types of roads without compromising fuel efficiency and handling.
It's easy to see the difference between 4x4 based 2WD vehicles and Subaru's perfect layout built from the ground up.
An all-wheel drive vehicle with a free center differential stops when one of the wheels slips. To avoid this, a blocking mechanism is used.
However, the operation of such a mechanism may adversely affect driving. So, when driving on dry asphalt with a locked differential, power circulation occurs, causing jerks and making it difficult to turn. Therefore, on dry roads, the differential must be unlocked, and in difficult areas with low grip, it must be locked. The permanent all-wheel drive system can automatically lock and unlock the differential depending on driving conditions.
This solution is necessary to prevent jerks when the lock is turned on. In addition, better control is required in the face of a sharp change in road conditions. That's when the experience and technical knowledge in the field of four-wheel drive system management really matter!
center differential
Center differential unlocked
Center differential locked
- Potential traction force transmitted by the wheel
- Traction force spent on internal losses
- Actual traction force transmitted by the wheel
Controllability
Multi-mode active center differential system
The multi-stage manual mode and three automatic control modes of the DCCD system provide a choice of one of two types of center differential lock. This provides the perfect balance of excellent traction and agility in all road conditions. The basic proportion of torque distribution between the front and rear wheels is 41% / 59%. The redistribution of torque is provided by the control of a multi-plate electromagnetic torque transmission clutch and a mechanical self-locking differential.
Multi-mode dynamic stabilization system
Vehicle Dynamics Control System
Standard on all Subaru models, Dynamic Stability Control monitors whether the car's behavior is in line with the driver's intentions through multiple sensors. If the vehicle approaches a buckling state, the torque distribution system, engine, and brake modes of each wheel are adjusted to maintain the vehicle's predetermined trajectory.
Maneuver stability
When cornering or maneuvering around sudden obstacles, Dynamic Stability Control compares the driver's intentions with the actual behavior of the vehicle. This comparison is based on signals from the steering angle sensor, the brake pedal pressure sensor, and the lateral acceleration and yaw rate sensor.
The system then adjusts the engine power output and brake modes of each wheel to keep the vehicle on track.
Subaru symmetrical all-wheel drive systems
All-wheel drive system VTD *1:
A sporty version of the electronically controlled all-wheel drive that improves cornering characteristics. The compact all-wheel drive system includes a planetary center differential and an electronically controlled multi-plate hydraulic lock-up clutch*2. 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, 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.
Subaru WRX with Lineartronic transmission.
Previously installed on cars: Subaru Legacy GT 2010-2013, Forester S-Edition 2011-2013, Outback 3.6 2010-2014, Tribeca, WRX STI with automatic transmission 2011-2012
All-wheel drive system with active torque distribution (ACT):
An electronically controlled all-wheel drive system that provides greater vehicle directional stability on the road compared to mono-wheel drive vehicles and all-wheel drive vehicles with a plug-in drive to another axle.
Subaru's Genuine Multi-Disc Torque Clutch adjusts front-rear torque distribution in real time according to driving conditions. The control algorithm is embedded in the electronic transmission control unit and takes into account the speed of rotation of the front and rear wheels, the current torque on the engine crankshaft, the current gear ratio in the transmission, the steering wheel angle, etc. and with the help of a hydraulic block compresses the clutch disks with the necessary force. Under ideal conditions, the system distributes torque between the front and rear wheels in a ratio of 60:40. Depending on the circumstances, such as slipping, sharp turns, etc., the redistribution of torque between the axles changes. Adaptation of the control algorithm to the current driving conditions provides excellent handling in any traffic situation, regardless of the level of driver training. The multi-plate clutch is located in the body of the power unit, is its integral part and uses the same working fluid as other elements of the automatic transmission, which leads to its better cooling than in a separate location, like most manufacturers, and therefore greater durability.
Current models (Russian specification)
On the Russian market Subaru Outback, Subaru Legacy, Subaru Forester *, Subaru XV.
* For modifications with Lineartronic transmission.
All-wheel drive system with center self-locking differential with viscous coupling (CDG):
Mechanical all-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 WRX and Subaru Forester - with manual transmission.
All-wheel drive system with electronically controlled limited slip active center differential (DCCD *3):
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 VTD: Variable Torque Distribution.
*2 Controlled limited slip differential.
*3 DCCD: Active Center Differential.
