Symmetrical AWD

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Demonstrator at the 2005 Geneva Motor Show

Symmetrical AWD ( Symmetrical All Wheel Drive ) refers to the all-wheel drive system from the Japanese car manufacturer Subaru .

Almost all vehicles from this manufacturer are equipped with all-wheel drive. Unlike other car manufacturers, the type of drive is not available as an option, but an integral part of the vehicle concept. Conversely, the few models without all-wheel drive, such as the rear-wheel drive Subaru BRZ, cannot be equipped with it.

general description

From a technical point of view, Subaru relies on very different all-wheel drive concepts for its vehicles, but (since 2003) they have all been sold under the brand name Symmetrical AWD . The name given to the Symmetrical AWD is the symmetrical arrangement of the drive train: the crankshaft of the engine, the gearbox , the cardan shaft and all differentials are arranged on the longitudinal axis of the vehicle.

The following applies across the board: All engines are boxer engines , with four or six cylinders , some with a turbocharger . The heavy parts of the engine - crankcase, cylinders and cylinder heads - do not protrude above the top edge of the front tires, the overhang to the front is also small. The gearbox is located behind the front axle, the front axle and center differential and an electronically controlled multi-plate clutch for connecting the rear wheels are integrated in the gearbox. Some models are equipped with a rear-axle locking differential . The sports model Impreza WRX STI has mechanical locking differentials (depending on the model year based on Torsen or lamellar design) on the front and rear axles. The arrangement of the engine and transmission results in a lower vehicle center of gravity and a favorable weight distribution. This improves driving behavior without affecting comfort.

Manual models

The manual Subaru Leone was the first Subaru vehicle to be equipped with all-wheel drive in 1972. The car, which is primarily driven on the front axle, allows manual engagement of the rear wheels via a (rigid) transfer case. The next development step was the permanent all-wheel drive of the 1987 Subaru XT , which had an open center differential that could be fully electro-pneumatically locked (100%). The last significant development experienced the manual transmission models in 1988 by introducing a viscous limited slip differential, which in the first Legacy I was used. This center differential was revised again for the 1998 model year and has since been installed identically in all Subaru hand controls (except for the STI models). The drive torque coming from the gearbox is fed into the central differential (bevel gear design), which is combined with a viscous lock . With traction on all wheels, the torque is permanently distributed 50:50 to both axles. In the event of slip , and also to a minor extent when cornering, there is a speed difference between the front and rear axles. In this case, the viscous lock that is now engaging directs additional torque to the slower rotating (or in the extreme case, stationary) axis. The friction used for this purpose of the lamellas rotating in a highly viscous liquid can only transmit part of the maximum possible torque, so that the performance of a full lock is far from being achieved. In almost all models, open, i.e. non-locking differentials are installed on the axles, which ultimately distribute the drive power evenly between the individual wheels. More powerful vehicles such as the Impreza WRX or Legacy GT models also have a viscous locking differential on the rear axle. Many hand-shifted models have an off-road reduction called dual range . In the case of the 4th generation Legacy / Outback , for example, the gear ratio to slow speed is 1.447 or 1.196 (according to the workshop manual).

Models with automatic steps

In the case of the 4-speed automatic transmission with the gearbox designation "E-4AT" (electronically-controlled 4-speed automatic transmission) or "ACT-4", the drive force is generated by a multi-plate multi-plate clutch controlled automatically by oil pressure ("multiplate transfer - MPT ") divided between the front and rear axles. A control unit monitors the speeds of the front and rear axle drive shafts and interprets a difference of up to 20% as cornering; the standard torque distribution of 90:10 (v: h) is only adapted to the situation when it is exceeded (values ​​apply to the previous one US market). The 5-stage automatic system introduced later was based on the VTD design (see below).

Models with Lineartronic

The continuously variable CVT automatic transmission (brand name Lineartronic ) introduced in 2009 uses the so-called Active Torque Split . Similar to the earlier 4-stage machines, this system is based on a multi-plate clutch to distribute the torque to the front and rear axles. The standard distribution is 60:40 (v: h) and is adapted to requirements by evaluating a large number of sensor data. A center differential is no longer installed in this system. There are open differentials on the axles. The only exception at the moment is the Subaru WRX model from the 4th generation of the WRX / WRX STI series (since 2014). In addition to manual transmission, the Lineartronic is also available here, which is then combined with VTD. In Germany, only the WRX STI model is offered from this series , but this will also expire in 2019. In models with Lineartronic , the "X-Mode" is also available (see below).

Special systems

VTD

In vehicles with VTD ( Variable Torque Distribution ), a central differential distributes the drive torque asymmetrically between the front and rear axles. This was introduced in 1991 with the Subaru SVX , by means of a planetary gear (not bevel gear) differential (distribution: 36% front / 64% rear). An additional lamella lock with a high locking capacity ensures a sensible distribution of the force when the wheels slip. In 2004 the VTD was further developed and combined with a 5-speed automatic transmission, which is used in the 3 liter or 2.5 liter turbo version of the Legacy IV as well as the Legacy V and the Tribeca , with the latter having a 45:55 torque distribution .

VDC

The Vehicle Dynamics Control is employed since 1998 electronic stability program of Subaru, which additionally supports wheel drive system by braking of spinning wheels and thus diverts drive torque to grip wheels. This principle is also commonly referred to as "electronic limited slip differential".

DCCD

The Driver's Controlled Center Differential is a fully adjustable center differential specially developed for the Impreza WRX STI or WRX STI sports model. Similar to the VTD , it consists of a planetary gear differential that asymmetrically divides the drive torque from the transmission between the front and rear axles. In addition, an electromagnetically controlled multi-plate clutch is integrated, which increases the locking effect between the front and rear axles by up to 100%, depending on the driver's wishes. Alternatively, the multi-plate clutch can also be controlled automatically. The DCCD has been in use since 2000 at the latest and has been further developed several times. For example, the front-to-rear torque split in the 2005 STI model is 36:64; in the 2018 model 41:59.

X-mode

Models with X-Mode do not have any additional mechanical elements in the drive train, but rather have an assistance system that can be activated as an option to increase off-road capability . This uses and influences the systems already in the vehicle as follows:

  • the electronic throttle valve opens less when the accelerator pedal is pressed lightly, and then reacts more sensitively when the pedal is pressed harder
  • CVT transmissions preferentially translate into high torques instead of low speeds
  • The degree of locking of the central multi-plate clutch is permanently increased
  • VDC control times are becoming shorter; spinning wheels are braked faster
  • Hill descent aid (up to approx. 20 km / h)

Since the 2019 model year, the dual-function X-MODE has been available for certain models, which, in contrast to the previous system, offers setting options. The new system can work with greater wheel slip ("wheels spin") in order to provide better propulsion in deeper mud or snow.

Web links

Individual evidence

  1. ↑ Legacy Outback Press Information. (PDF) FUJI HEAVY INDUSTRIES LTD., 2003, p. 4 , accessed on September 18, 2019 .
  2. ^ A b Subaru History Book. (PDF) (No longer available online.) Fuji Heavy Industries, April 1, 2014, archived from the original ; accessed on August 1, 2019 .
  3. ↑ a b c d e f Symmetrical AWD. (No longer available online.) Subaru Deutschland GmbH, March 18, 2012, archived from the original ; accessed on August 1, 2019 .
  4. ↑ Locking effect Subaru center differential, Legacy Impreza Outback Forester. YouTube , accessed July 5, 2019 .
  5. ↑ Subaru Impreza. Promotional brochure of the 1998 WRX model. Subaru. Retrieved August 2, 2019 .
  6. ↑ SUBARU'S ALL WHEEL DRIVING SYSTEM. (No longer available online.) Auto World / VIS, 2001, archived from the original ; accessed on October 9, 2019 .
  7. ^ Subaru Symmetrical AWD Explained. Quality Subaru, accessed August 20, 2019 .
  8. ↑ Subaru WRX / WRX STI. (PDF) Advertising brochure for the 2014 WRX & WRX STI models. FUJI HEAVY INDUSTRIES LTD., March 2014, p. Print page 34 , accessed on August 23, 2019 .
  9. ↑ 2007 Subaru B9 TRIBECA. (PDF) advertising brochure. Subaru of America, p. 35 , accessed August 20, 2019 .
  10. ↑ "SUBARU Impreza WRX Sport Parts Catalog". Subaru Tecnica International Inc., August 1, 2000, accessed August 2, 2019 (original Impreza WRX & STI accessories catalog from 2000).
  11. ↑ The Subaru WRX STI: Dare more sports. (PDF) - Long version model year 2018 -. SUBARU Deutschland GmbH, August 1, 2017, accessed on August 2, 2019 .
  12. ↑ Subaru X-Mode Explained. (PDF) Sport Subaru, Orlando, Florida, accessed October 4, 2019 .
  13. ↑ Denis Flierl: Subaru Forester's New Dual-Function X-MODE; Why Adventure-Seekers Want It.TorqueNews, January 21, 2019, accessed January 6, 2020 .