subaru technology in detail

(Google-translated from German)

In September 1972, Subaru became the world's first manufacturer to introduce all-wheel drive into industrial passenger car production. With its standard, switchable all-wheel drive, the Subaru Leone Station Wagon AWD ushered in a new era of drive technology. Since then, Subaru has built over ten million all-wheel drive passenger vehicles worldwide, making it the undisputed market leader in this segment.

Until the early 1960s, the term "all-wheel drive" was synonymous with "commercial vehicle." Only military or construction vehicles and pure off-road vehicles had all-wheel drive. While this was the ideal solution for optimal propulsion, the vehicles were slow, reluctant to corner, and uncompromisingly tuned for difficult terrain. In the 1970s, as engines became increasingly powerful and vehicles ever faster, the principle of four driven wheels provided the technical potential for a drive system that could transfer high engine power to the road effectively and in a controlled manner and convert it into propulsion.

Despite all its limitations, the structural advantages of all-wheel drive were evident: the drive system delivers maximum traction, the drive forces do not influence the steering response, and AWD ensures clear and predictable handling. Therefore, it only took a small push to transfer all-wheel drive technology to passenger cars. This push came from the Tohoku Electric Supply Company, which supplied electricity to Tohoku Prefecture in the north of Japan's main island of Honshu. This major customer required a vehicle that would allow employees to reach their worksites easily and safely, even in winter. Subaru developed the Leone Station Wagon AWD and delivered the world's first all-wheel drive passenger car in September 1972. The all-wheel drive Leone triggered a boom in demand among companies that needed its performance in the snowy and mountainous regions of Japan. But it also established itself in export markets: The Subaru Leone Station Wagon AWD became the world's best-selling all-wheel-drive car and the nucleus of Subaru's all-wheel-drive technology. It wasn't until eight years after the Subaru Leone Station Wagon AWD that a southern German automaker launched its "original Quattro" in 1980.

Inside and out, the Subaru Leone Station Wagon AWD was no different from the front-wheel-drive models. However, it featured a selectable all-wheel drive (commonly called four-wheel drive), which also transferred part of the engine power to the rear axle. In the Subaru Leone, 4WD consisted of the mechanical creation of a rigid drive connection using a dog clutch from the originally driven front axle to the rear axle.

Technical specifications Subaru Leone Station Wagon AWD:

• Length / width / height: 3995mm x 1500mm x 1385mm
• Weight: 855 kg
• Engine: Water-cooled four-cylinder boxer engine EA63S
• Bore x stroke: 85 mm x 60 mm
• Displacement: 1361 cubic centimeters
• Compression ratio: 8.5: 1
• Max. power: 53 kW / 72 hp at 6,400 rpm
• Max. torque: 10.2 kg-m/3,000 rpm = 100.06 Nm at 3,000 rpm
• Gearbox: Four-speed manual transmission
• Drive: Switchable all-wheel drive; Mechanical creation of a rigid through-drive by means of a dog clutch from the originally driven front axle to the rear axle
• Chassis: Front independent suspension with McPherson struts, rear semi-trailing arm axle

All-wheel drive has three advantages for driving dynamics: 1. Traction, 2. Handling and 3. Stability. Basically, with all-wheel drive, the entire torque is available at the output side of the transmission. The drive forces do not influence the steering behavior, and all Subaru vehicles impress with their clear, docile and predictable handling. To achieve this driving behavior (which is desired by all car manufacturers), all-wheel drive is ideally suited because the drive forces are distributed across all four wheels.

The power transmission system is of crucial importance because the type of drive and power distribution play a key role in determining when the maximum coefficient of friction between the tires and the road surface is exceeded. This is particularly evident in curves, where three forces come into play: the engine's power, which propels the vehicle forward; the frictional force that presses the wheel against the road surface; and the lateral control (centripetal force), which is the resultant of the other two. This force counteracts the centrifugal force, and the stronger it is, the more confident cornering becomes. Lateral control increases with decreasing power: for two vehicles with the same engine power, a four-wheel drive car (which divides the power by four) will achieve greater lateral control than a two-wheel drive car (which divides the power by only two).

Example: An engine produces 100 hp; under the assumed conditions, each drive wheel can convert a maximum of 30 hp into propulsion on the road. With two-wheel drive, each drive wheel receives 50 hp (100:2) – that's 20 hp more than it can convert into propulsion. Result: The wheels spin. All-wheel drive, on the other hand, distributes the 100 hp evenly across all four wheels. Each wheel receives 25 hp (100:4), so the load remains below the load limit of 30 hp per wheel: the wheels do not spin. This difference applies under all conditions, but becomes more significant on more difficult surfaces with lower friction values. The more accident-prone the situation, the more obvious the safety advantage of all-wheel drive over two-wheel drive becomes.

The symmetry of the design is the defining characteristic of Subaru's "Symmetrical AWD" system, which combines a boxer engine with all-wheel drive. Compared to all V- and inline engines, the boxer engine has the advantages of a very low center of gravity and a completely symmetrical design. Its hallmark is its opposed cylinders. Inside, the pistons are arranged in pairs, like two boxers, and the pistons move horizontally. This structure enables a more rigid cylinder block. The mass balance of the boxer engine is perfect; it is flat, short, and compact. The short and rigid crankshaft allows for high engine speeds. The low overall height ensures a low center of gravity and almost ideal weight distribution. A low center of gravity ensures lower body roll in corners, which in turn leads to less body roll and more stable handling. The fascination of the boxer engine has much to do with the aesthetics of its synchronized operation: Even without balance shafts, the boxer engine revs smoothly up to its rated speed.

These advantages contribute significantly to the balance and performance of the Subaru all-wheel drive system: From the engine to the transmission, propshaft, and rear axle differential, the entire drivetrain runs in a straight line with a horizontally symmetrical layout. All key components and assemblies—from the longitudinally mounted boxer engine to the transmission, final drive, propshaft, and rear axle differential—are laid out in a straight line with horizontal symmetry. This results in neutral vehicle balance. The combination of optimal traction and perfect balance results in maximum driving stability. All heavy components such as the transmission, final drive, etc., are located between the two axles. This avoids unnecessary weight from these components at the front and rear and reduces the vehicle's yaw moment. A low yaw moment optimizes the vehicle's steering behavior by reducing the moment of inertia in the steering and improving the vehicle's overall handling. The combination of optimal traction and perfect balance results in driving stability and "accident avoidance capability": This is the true key to safety, because it is better to avoid an accident than "just" to survive one.

The symmetry of the basic design not only ensures excellent handling but also contributes significantly to passive safety, as it leaves plenty of space on both sides of the engine compartment. This allows the use of frame members that lead from the passenger cell directly into the bumper and play a key role in absorbing energy in a frontal impact. Thanks to its installation depth, the boxer engine disappears beneath the floor pan in a frontal impact and does not penetrate the passenger compartment.

Subaru is currently working on optimizing the electronic control of all aspects of the all-wheel drive system. The goal is a complex vehicle concept in which the Symmetrical AWD all-wheel drive, Vehicle Dynamics Control, Yaw Moment Control, and Tire Force Control are interconnected. The design optimization of the center differential, the development of the electronically controlled central limited-slip differential, and the design optimization of the front and rear LSDs almost automatically lead to improved dynamic performance of AWD vehicles.

Subaru has consistently continued the development that first launched the Leone Station Wagon AWD in September 1972. The equation Subaru = all-wheel drive will continue to hold true for the future. The horizontally symmetrical Subaru all-wheel drive system "Symmetrical AWD" is and remains the core technology of the all-wheel drive pioneer, who ushered in the all-wheel drive era in industrial passenger car manufacturing 33 years ago. The potential of Symmetrical AWD is far from exhausted. Subaru is continuously working on optimizing this core technology.

The design differences between the respective all-wheel drive systems arise from the necessity that manual transmissions require different solutions than automatic transmissions.

1972: Mechanically engageable all-wheel drive.

The simplest form of all-wheel drive is the engageable all-wheel drive, commonly referred to as four-wheel drive. In the Subaru Leone AWD, 4WD consisted of a mechanically created rigid drive connection via a dog clutch from the originally driven front axle to the rear axle.

1980: Mechanically engageable all-wheel drive and "dual-range"

Starting with the original engageable all-wheel drive system, Subaru has continuously developed all-wheel drive. The Subaru 1800 (1980) features a manual transmission with engageable all-wheel drive and "dual-range" gear reduction. By pulling the lever, a sliding sleeve in the transmission connects a gear pair and transfers the drive to the rear axle. The second lever position activates the reduction gear and the "dual-range" status.

1983: Pneumatically engageable all-wheel drive.

Subaru also used manual transmissions with engageable all-wheel drive in the Libero (1983) and Justy (1984), but engagement was now electro-pneumatic, at the push of a button via a switch in the shift lever. A diaphragm capsule on the transmission is connected to the engine's vacuum via a solenoid valve on one side, while atmospheric pressure acts on the diaphragm on the opposite side. This pressure difference activates a selector shaft connected to the diaphragm, which in turn actuates a sliding sleeve. This sliding sleeve ensured the frictional connection to the transfer case.

1987: Permanent all-wheel drive.

The era of permanent all-wheel drive at Subaru began with the XT coupe at the 1987 Frankfurt Motor Show. For the first time, Subaru combined all-wheel drive in a model with either a five-speed manual or a four-speed automatic transmission. In the manual version, the center differential distributes drive power equally between the front and rear axles; if speed differences occur between the axles, it can be locked electromechanically: An electric switch activates a mechanical pawl at 100 percent.

1988: Permanent all-wheel drive with viscous lock.

In the first-generation Legacy, Subaru used a center differential with a self-locking viscous coupling for the first time. The transmission design is essentially identical to the XT, but the viscous lock in the center differential locks automatically and continuously, depending on the size of the speed difference between the front and rear axles, until the differential is fully locked. This design—a center differential with a viscous lock—is still used in current models with manual transmissions.

2005: STi-AWD with planetary gear center differential, helical LSD, DCCD.

The Impreza WRX STi, the base vehicle for the Impreza WRC, varies the basic design according to sporting requirements: The six-speed transmission is equipped with a center differential designed as a planetary gear set. When the clutch is in the disengaged position, the torque distribution between the front and rear axles is 36:64 percent. The driver can vary this torque distribution manually. Using a rotary switch, the driver activates an electromagnetic clutch that locks the planetary gear set according to his specifications (DCCD - Driver Controlled Center Differential). With the planetary gear set fully locked, the power distribution between the front and rear axles is 50:50 percent. The integration of a Helical LSD into the front differential optimizes the traction of the front wheels when cornering

1981: Automatic transmission with multi-disc transmission 4WD

The groundbreaking "multiplate transfer 4WD" (MP-T) system utilizes hydraulic pressure from the automatic transmission and combines the automatic transmission with an all-wheel-drive system that can be engaged while driving at the simple push of a button. The system consists of a seven-disk clutch on the output axle, which is connected to the rear driveshaft. When all-wheel drive is engaged, hydraulic pressure from the transmission oil pump is automatically transferred to the multi-disk clutch, which engages and transfers torque to the rear driveshaft. MP-T thus compensates for speed differences between the front and rear wheels. In "Auto-4WD" mode, the vehicle enters an "intelligent" all-wheel-drive mode, in which hydraulic pressure for the multi-disk clutch is activated by applying the brakes and activating the windshield wipers (sic!).

1987: Permanent all-wheel drive.

In the Subaru XT with the four-speed ACT-4 (ACT = Active Control of Torque) automatic transmission, torque distribution is differentiated: mechanically to the front axle via a pair of equal-sized gears, and hydraulically to the rear axle via a multi-plate clutch operating in an oil bath.

1989: ECVT transmission and selectable all-wheel drive.

In the Justy with ECVT transmission (1989), all-wheel drive is engaged at the push of a button in the gearshift lever. The switch hydraulically actuates a selector shaft, which connects the transfer case drive via a sliding sleeve.

1991: Permanent all-wheel drive with VTD.

With the Gran Turismo SVX (1991), Subaru introduced the further developed automatic transmission with VTD (Variable Torque Distribution): A center differential consisting of a planetary gear set and a rear axle differential with viscous coupling normally directs 36 percent of the engine power to the front wheels and 64 percent to the rear wheels. If traction changes, the electronics redistribute the torque to the wheels that provide the best traction under the respective conditions. The hydraulic multi-disk lock continuously locks the planetary gear set when speed differences occur between the front and rear axles. The viscous coupling on the rear axle ensures that no power is wasted on the spinning wheel.

1998: Permanent all-wheel drive with VTD and Vehicle Dynamics Control.

In 1998, Subaru integrated VTD and Vehicle Dynamics Control into the top models of the Legacy series. The "hardware" (the design of the VTD) remains the same, but electronic control is significantly improved by the optimized sensor technology of the Vehicle Dynamics Control unit via CAN communication.

2004: Permanent all-wheel drive with optimized VTD and Vehicle Dynamics Control.

In the newly developed five-speed automatic transmission with VTD via a planetary gear set, the multi-plate lock can vary the drive torque for the front and rear axles over a relatively wide range. This makes the automatic transmission with VTD particularly suitable for vehicles with Vehicle Dynamics Control, as its control unit communicates with the transmission control unit. If the Vehicle Dynamics Control unit detects understeer or oversteer behavior in the vehicle, it sends corresponding information to the transmission control unit. The transmission control unit reduces the drive torque of the unstable axle by controlling the multi-plate lock.

All Legacy 3.0s from model year 2004 onwards have this system.