The upshot of what I was saying is, you need to turn off the traction control (VDC) to let your wheels spin on purpose and rock the vehicle.
I should have said one set of wheels at a time for the forester but our tribeca (and my legacy) has VTD and VDC with an LSD so it has a different system. This is what Subaru has to say about the off button in red, I left the whole text because I thought it was interesting :
Symmetrical All-Wheel Drive is the cornerstone of Active Driving/Active Safety – vehicles are designed to give drivers the control, maneuverability and power to help avoid hazardous situations. First offering a four-wheel drive passenger car over 30 years ago and full-time All-Wheel Drive in 1987, Subaru is a world leader in developing all-wheel drive systems for the passenger car market.
Subaru Symmetrical All-Wheel Drive works transparently, at all times, on any road under any conditions. Symmetrical All-Wheel Drive is robust enough for recreational off-highway trail driving – and suspension changes for the 2005 Outback give it more ground clearance than before for even greater off-highway capability.
Symmetrical All-Wheel Drive Powers All Four Wheels All The Time
It is no secret that some popular four-wheel drive systems function more as part-time traction aids for what are really front- or rear-wheel drive vehicles. With such systems, power is transferred away from the main drive wheels only when they slip. When there is no slippage, these vehicles essentially operate in two-wheel drive.
While such automatic “part-time” or “on-demand” systems can help prevent a vehicle from getting stuck in snow, they may not provide the all-road handling benefits of a true all-wheel drive system. In most cases the Subaru All-Wheel Drive can distributes power to all four wheels, reducing the load on each wheel and reducing tire slip, especially on slippery or loose surfaces. This improves overall responsiveness, safety and performance.
All-Wheel Drive Built In, Not Added On
Every Subaru vehicle is developed specifically around Symmetrical All-Wheel Drive. It’s a crucial element of the brand’s vehicle architecture, which also includes:
Boxer engine for low centre of gravity and better steering response
Transmission located within the vehicle wheelbase, for outstanding balance
Long travel suspension
These elements all work together to provide outstanding traction, handling and overall balance.
Boxer Engine: Compact and Light
Subaru introduced its first horizontally opposed engine more than 35 years ago and today remains committed to this engine layout. The Subaru boxer is ideal for the All-Wheel Drive application because it is inherently compact and lower than “V” or in-line engines. The layout concentrates the engine’s mass in a small area and permits a lower centre of gravity for exceptional handling and steering response.
The horizontally opposed layout results in an inherently smooth-running engine as well, because the motion of the pistons from one cylinder bank (and the vibration the motion creates) cancels the vibrations of the opposing bank. Lower vibration increases durability, a Subaru hallmark. Some inline and V-type engines require a balance shaft to reduce reciprocating vibration, and this feature adds both weight and complexity. Subaru’s aluminum-alloy engine and transmission case also keep drivetrain weight low.
Mounting the boxer longitudinally (front to back) allows the transmission to be mounted directly behind it and within the vehicle’s wheelbase. Power travels in a straight, near-horizontal line to the front and rear differential, minimizing frictional loss. This symmetrical, uniform layout also provides excellent left-right balance. By comparison, in a vehicle with a transverse-mounted engine, an all-wheel drive system requires additional transfer gearing to reroute the power to longitudinal orientation. This adds friction and extra weight on one side of the vehicle.
Four Types of Symmetrical All-Wheel Drive in 2005 Legacy and Outback
Under the Symmetrical All-Wheel Drive brand umbrella, Subaru offers five types of All-Wheel Drive in North America; four of them are featured in the new-for-2005 Outback line and three are used in the 2005 Legacy lineup. A fifth system, Driver Controlled Centre Differential (DCCD), is used only in the Impreza WRX STi.
Viscous Coupling Continuous All-Wheel Drive
In all Subaru models with the 5-speed manual transmission, including the turbocharged Legacy 2.5 GT and Outback 2.5 XT models, the All-Wheel Drive system uses a viscous-coupling in a bevel-gear centre differential mounted inside the transaxle case. This is a simple, compact system that enhances traction on slippery or unpaved roads as well as enhancing dry-road handling.
A viscous coupling contains a series of opposing discs attached to the front and rear drive shafts, surrounded by a type of silicone fluid. In normal operation, the differential distributes power equally between the front and rear wheels (50/50 power split). Slippage at the front or rear wheels causes a rotational difference between the front and rear discs in the viscous unit, which in turn shears the fluid. The shearing action heats the fluid, causing it to thicken. As the fluid thickens, power transfers from the plates rotating faster (the slipping wheels) to those rotating more slowly (the wheels with the best traction). When the slippage stops, all the discs turn at the same speed, restoring the 50/50 power split.
The power transfer process takes place very quickly, invisible to driver and passengers.
Active All-Wheel Drive
In Legacy 2.5i and Outback 2.5i models equipped with the 4-speed direct control electronic automatic transmission, torque distribution is actively controlled by an electronically managed continuously variable transfer clutch housed in the transaxle tailshaft. Power transfer is governed by varying hydraulic pressure applied to clutch plates which transfer power to the rear wheels. The Transmission Control Module (TCM) controls the All-Wheel Drive multi-plate transfer clutch.
Active All-Wheel Drive can adjust the power split in an instant, depending on many input factors. If the front wheels begin to slip, the TCM increases hydraulic pressure on the transfer clutch, reducing slippage and transferring power to the rear wheels. As the front wheels regain traction, the TCM reduces pressure on the clutch, increasing slippage of the plates and transferring power to the front.
The TCM monitors input from speed sensors on the front and rear drive shafts and also takes input from the throttle position and the transmission. All of these factors are taken into consideration by the TCM in conjunction with a software map that determines how aggressively to adjusts the power split. Subaru Active All-Wheel Drive varies the power split according to driving conditions; in most cases power split will be 60 per cent front and 40 per cent rear to match the vehicle’s normal weight distribution and cause the vehicle to slightly understeer.
This makes handling more predictable for the majority of drivers. When throttle input indicates acceleration, the system responds by increasing hydraulic pressure to the transfer clutches and transferring more power to the rear wheels to account for rearward weight transfer. (In a front-wheel drive car, weight transfer during acceleration can cause wheelspin, compromising traction.) When releasing the throttle indicates deceleration, power transfers to the front wheels to enhance braking performance.
The system enhances cornering performance as well. When the driver lifts off the throttle to enter a turn, engine braking is applied to all four wheels – slightly more to the front wheels – making the vehicle more stable and offering greater steering control. As the car exits the turn and accelerates, power transfers to the rear wheels for added traction. Wheel speed differences occur in normal driving, not just in slippery conditions. A front/rear wheel speed difference of up to 20 per cent tells the TCM that the vehicle is simply cornering. Any difference greater than 20 per cent will signal front wheel slippage, and the TCM will then transfer power to the rear.
Variable Torque Distribution All-Wheel Drive
In 2005 Legacy 2.5 GT and Outback 2.5 XT and 3.0 R models with the 5-speed automatic transmission, Variable Torque Distribution (VTD) All-Wheel Drive integrates an electronically controlled hydraulic transfer clutch and a planetary gear-type centre differential to control power distribution between the front and rear wheels. This system is meant for people who want sporty handling as well as the safety of AWD. It achieves this by sending more torque to the rear wheels than the front.
In normal mode the system sends 55 per cent of the torque to the rear wheels and 45 per cent to the front, giving the vehicle more a rear-wheel drive feel and allowing the vehicle to slightly over-steer, if the driver desires, when cornering. This torque split is achieved by using a system of planetary gears that split the torque according to their gear ratios. To ensure straight line acceleration and stability, the transmission control unit locks a set of clutches that split the torque 50 per cent front and 50 per cent rear when full throttle is applied, or if a loss of traction is felt at the front or rear sets of wheels.
By splitting torque 50/50 under hard acceleration, the system ensures that all four wheels are pulling and pushing the vehicle at the same speed, helping eliminate any tendency to fish-tail. Again, if one or many wheels slip when accelerating normally, the system will lock up the centere differential to 50/50 and in this manner, control wheel slip.
Since this centre differential is normally in an open condition (not locked) it will achieve good fuel economy because the gears that distribute torque 45/55 turn freely and are not bound by any clutches or viscous couplings unless, as indicated above, they are activated by the Transmission Control Unit.
Vehicle Dynamics Control
In the Outback 3.0 R VDC Limited, a sophisticated Vehicle Dynamics Control (VDC) stability system incorporates Variable Torque Distribution (VTD), All-Wheel Drive and an all-wheel, all-speed traction control system (TCS) to provide outstanding handling and control in all driving conditions. Controlled by VDC, the VTD centre differential – not the traction control system – determines power distribution.
VDC helps keep the car going where the driver steers it, using individual wheel braking, throttle control as well as centre differential clutch engagement to correct understeer (front-wheel drift) or oversteer (rear-wheel drift). TCS adds an extra margin of driving safety without impeding VTD All-Wheel Drive operation. The VTD system varies the power distribution to respond to traction conditions and vehicle dynamics.
Using special friction-estimation technology, VDC governs power distribution according to available surface friction. The system calculates traction based on sensor inputs and tire characteristics. This information provides finer control through more precise mapping of vehicle control parameters. In addition, the transfer torque ratio is controlled over a broader range.
VDC monitors vehicle stability by continually measuring inputs including steering angle, lateral-g, yaw rate, and individual wheel speed. Using this data, VDC can compare the driver’s intended path to the car’s actual path to detect understeer and oversteer conditions. Working with Variable Torque Distribution (VTD) All-Wheel Drive, VDC can correct understeer, oversteer and wheel slip.
Subaru designed VDC to increase stability on slippery surfaces and to increase cornering performance on dry roads. The system improves high-speed stability and helps prevent oversteer if the throttle is quickly lifted while cornering. Using various sensor inputs, VDC detects potential instability at either the front or rear of the vehicle. If VDC detects an unstable condition, it will produce a counteracting force to help restore stability by adjusting power distribution between the front and rear wheels and momentarily applying (or releasing) brake force on one or more wheels.
The VDC system also incorporates an all-wheel, all-speed traction control (TCS) function, which can apply a braking force to slow a slipping wheel or wheels. The system operates while driving in a straight line or cornering and helps maintain traction even if up to three wheels are slipping. If the driver is applying more throttle than available traction will allow, VDC will reduce engine power. It is important to point out that the Subaru VDC system uses traction control as a second line of defense, activated only when VTD alone cannot maintain sufficient traction.
All-Wheel Drive Operational At All Times
The Outback 3.0 R VDC Limited includes a “VDC Off ” switch to turn off the stability and traction control for driving in or extricating the vehicle from deep snow or mud. Under such conditions, the selective braking action that a stability or traction control system provides is not desirable. It is important to point out, however, that even with VDC off, the VTD All-Wheel Drive system operates at all times, and the standard viscous limited-slip rear differential is helpful for extricating the vehicle from deep snow.
How VDC Works
The VDC control module monitors various sensors (wheel speed, yaw rate, lateral-g) and evaluates the data to determine the vehicle’s direction. At the same time, the VDC control module receives driver inputs indicating steering angle and brake pressure to determine the driver’s intended direction. If there is a discrepancy between driver input and vehicle direction, VDC applies a corrective measure.
To correct understeer (front wheels don’t follow intended path), VDC applies braking force to the inside rear wheel. This counter-action pivots the car around the braked wheel and helps put it back on the driver’s intended course. At the same time, VDC optimizes All-Wheel Drive control by decreasing transfer clutch engagement to reduce power at the front wheels.
To correct oversteer (rear end slides out), VDC applies braking force to the outside front wheel. This counter-action pivots the car around the braked wheel to help bring the rear of the vehicle back in line. At the same time, VDC optimizes All-Wheel Drive control by increasing transfer clutch engagement to transfer more power to the front wheels. During an emergency lane change, for example, the quick change in direction could induce both understeer and oversteer. VDC responds by making continual adjustments to optimize stability and control.
VDC Advanced Logic
The Subaru VDC system responds to the varied conditions a driver might encounter on both dry and slippery surfaces. During straight-line acceleration, VDC controls engine output and braking of individual wheels (through TCS) and VTD All-Wheel Drive transfer clutch operation. During straight-line braking, the ABS function controls the braking action and the All-Wheel Drive transfer clutch operation.
Responding to oversteer with throttle depressed, VDC will:
Apply a strong brake force to the outer front wheel
Apply a slight brake force to the outer rear wheel
Increase transfer clutch engagement to transfer power to the front
Reduce engine output
Responding to oversteer with the brakes applied, VDC will:
Reduce brake force on the inner front wheel
Reduce brake force on the inner rear wheel
Increase brake pressure on the outer front wheel if the brake force applied by the driver is insufficient (in addition to the previous two steps)
Responding to understeer with the throttle depressed, VDC will:
Apply a slight brake force to the inner front wheel
Apply a strong brake force to the inner rear wheel
Reduce transfer clutch engagement to reduce power at the front wheels
Reduce engine output
Responding to understeer with the brakes applied, VDC will:
Reduce brake force on the outer front wheel
Reduce brake force to the outer rear wheel
Increase the brake force on the inner front wheel if the brake force applied by the driver is insufficient (in addition to the first two steps)
Increasing straight-line stability, VDC will:
Activate the centre differential clutches and lock the centre differential to stabilise the vehicle once it is driving in a straight line after a sharp steering manoeuvre. This takes advantage of the tendency for a vehicle with a locked centre differential to go straight.
Limited-Slip Rear Differential
For even greater traction capability, Subaru equips the 2005 Legacy 2.5 GT and all 2005 Outbacks with a viscous limited-slip rear differential. If one rear wheel starts to lose traction, the differential automatically diverts power to the other wheel. A limited-slip differential not only enhances traction on slippery road surfaces, but it is also a handling aid. For example, as the car enters a curve and weight transfers to the outside wheel, the inside wheel can lose traction. The limited-slip differential would, in this case, maintain the inside wheel’s speed making the vehicle more stable.
2016 BRZ, 2010 Leg 3.6 R, 2013 OB, 2001 Forester S and 1999 Miata