All Wheel Drive Systems
A simple concept:
From this simple concept we can derive an cardinal rule:
Subaru's All Wheel Drive systems are engineered to insure this rule is always followed regardless of traction conditions.
Two principles
Two principle all wheel drive systems exist, permanently engaged and automatically engaged.
All Subaru all wheel drive systems are considered permanently engaged, while most of the others fall into the automatic variety.
Automatic systems detect slip and direct power to the other axle. But wheel slippage has already occurred!! It is always better to avoid wheel slippage rather than react to it. In the time that it takes for the system to react, loss of control is possible. The delay also means that the section of road that caused the front wheel to slip is now under the rear wheel when it receives power, so it too may slip.
Read about Non-Subaru All Wheel Drive Systems
Subaru's all wheel drive system is designed to always drive the front and rear axles at the same speed, regardless of wheel slippage conditions. If the front axle encounters ice and the system is designed to drive that axle at the same rate as the rear, wheel slip will NOT occur!
This is the Subaru advantage.
Subaru All Wheel Drive Systems
Subaru uses five different All Wheel Drive (AWD) systems.
System 1: Continuous All Wheel Drive
Used on manual transmission (5MT) vehicles
Uses a viscous coupled centre differential to balance the engine power 50/50 between the front and rear axles. Allows slight axle speed differences for easy turning. If wheel slippage occurs on either the front or rear axle, the viscous coupling reacts to match the speeds of both axles and eliminate the slippage.
This system has an excellent capability of dealing with wheel slippage, but because the default condition is to split the power 50/50 front/rear, slippage rarely occurs.
Some models are equipped with a viscous coupled limited slip rear differential (LSD) which prevents a single rear wheel from slipping. If the right tire begins to slip, the LSD reacts and increases power at the left wheel and reduces it on the right and vice versa.
System 2: Continuous All Wheel Drive with Driver Controlled Centre Differential (DCCD)
Used on manual transmission (6MT) WRX-STI models only.
Operates in an identical manner to the standard viscous coupled centre differential system, but has a dial control that allows the driver to adjust to default front/rear power distribution from 50/50 to 35/65. By directing more than 50% of the power to the rear axle, the car behaves more like a rear wheel drive vehicle which is advantageous for sporty driving.
Front and rear viscous coupled limited slip differentials (LSD) are used with this system. This configuration can cause understeer and rob engine power, however the 300hp WRX-STI has more than enough power to counteract these effects.
System 3: Active All Wheel Drive
Basic system used on automatic transmission (4EAT) equipped Subaru's.
Uses an electronically controlled multi plate clutch pack to adjust the amount of power directed to the rear axle. Normally splits engine power 90/10 front/rear and can adjust distribution up to a to 50/50 ratio.
The Transmission Control Unit (TCU) determines the vehicles centre of gravity and traction requirements and continuously varies the power sent to the rear axle. During acceleration, the vehicle centre of gravity moves to the rear, and power is increased to the rear axle. During braking, the vehicle centre of gravity moves to the front, and power is reduced to the rear axle. If front wheel slippage occurs, more power is sent to the rear wheels to compensate. In most cases this system can predict situation where slippage is likely to occur (ex. acceleration) and make the required adjustments BEFORE slippage can occur.
This system uses axle speed sensors within the transmission unit itself, not the Antilock Brake System (ABS) sensors, the ABS and AWD are two completely independent systems.
Some models are equipped with a viscous coupled limited slip rear differential (LSD) which prevents a single rear wheel from slipping. If the right tire begins to slip, the LSD reacts and increases power at the left wheel and reduces it on the right and vice versa.
System 4: Variable Torque Distribution (VTD)
More advanced system used on some automatic transmission (4EAT) equipped Subaru's.
Unlike the standard Active All Wheel Drive system, this system allows more than 50% of the engine power to be directed to the rear axle. Normally engine power is split 45/55 front/rear, and during acceleration even more power is directed to the rear wheels. By sending more power to the rear wheels, a sporty feel is obtained. Aside from being able to send more than 50% of engine power to the rear axle, this system functions in an identical manner as the Active All Wheel Drive System.
This system is always equipped with a viscous coupled limited slip rear differential (LSD) which prevents a single rear wheel from slipping. If the right tire begins to slip, the LSD reacts and increases power at the left wheel and reduces it on the right and vice versa.
Some VTD equipped vehicles are equipped with a SPORTSHIFT feature which allows for manual shifting.
System 5: Variable Torque Distribution (VTD) with Vehicle Dynamics Control (VDC)
The standard Variable Torque Distribution (VTD) system with one of the most advanced traction control, anti lock brake system (ABS) and skid control systems available.
This system uses the antilock brake system's (ABS) wheel speed sensors to detect wheel slippage. If wheel slippage is detected, the VDC system can activate the brake on the slipping wheel to control its speed. This system can control slippage on 1 to 3 wheels simultaneously.
If wheel slippage is severe (more than one wheel), the VDC system will instruct the engine management system to reduce engine power output, which is accomplished by turning off one or more fuel injectors and retarding ignition timing.
In addition to traction control this system can take corrective action when understeer, oversteer or vehicle drift is detected. Understeer is a condition where the vehicle turns less than what is desired, while oversteer is a condition where the vehicle turns more than what is desired. Drift is the initial moment of a slide or loss of control.
The VDC system uses the ABS wheel speed sensors, steering wheel position sensor and yaw sensors to determine if the vehicle is responding to driver inputs, no more, and no less. If the VDC system detects that the car is not responding correctly to driver inputs, the system takes corrective action to restore control. Corrective action includes braking one or more wheels, adjusting engine power and front/rear torque split to restore directional control of the vehicle.
This system is NOT equipped with a viscous coupled limited slip differential, since the traction control system is more effective at controlling individual wheel slippage.
This system can correct many loss of control conditions at the instant that they occur, however it can not accommodate for driver stupidity.
Future advancements:
Subaru is continuously improving their all wheel drive systems. The following systems will be available in the next few years:
5 speed automatic transmission (5EAT)
Semi Automatic Manual Transmission
This system has many advantages: