All Wheel Drive Systems
A simple concept:
-When driving in a
straight line, all four wheels are turning at the same speed.
-When turning in forward and reverse the rear axle follows a shorter
turning radius than the front axle and rotates at a slower rate.
From this simple concept
we can derive an cardinal rule:
-The front axle will
always rotate at the same or slightly faster rate than the rear axle.
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.
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)
-An extra gear for
improved performance and economy.
-Available on 2005 models?
Semi Automatic Manual
Transmission
-Uses a conventional
manual transmission clutch mechanism, but instead of a clutch pedal to
release the clutch an electronically controlled hydraulic system is
used.
-Rather than a traditional H pattern shifter, a shift lever similar to
an automatic transmission lever is used. The shift lever is used to
select direction (park, forward and reverse), transmission mode
(automatic or manual shifting) and to manually change gears (+ or -,
usable in both automatic and manual shifting modes). Steering wheel
mounted paddle controls will also control gear changes.
-A conventional manual transmission is used, but rather than
controlling the shifting forks which engage and disengage the required
gear by the manual H pattern shift lever, electronically controlled
hydraulic motors are used to perform this function.
-This system is used in BMW and Ferrari vehicles and is becoming more
popular. This system has been used in both the Formula One (F1) and
World Rally Championship (WRC) circuits for many years.
This system has many
advantages:
-Typically lighter in
weight and smaller in size than automatic transmissions
-More reliable than automatic transmissions
-Typically more gears can be fitted to these transmission than with
automatic transmissions
-Clutch abuse is eliminated
-Gear grinding is eliminated
-Shifts faster than any driver could, and almost as fast as automatic
transmissions
-Integrated hill holder mechanism (detects incline, and if required
holds brake engaged until clutch take up).
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