The Vauxhall 4 Wheel 
Drive System

 

Transfer Box:

The Transfer Box (or T/X box) is possibly the biggest part of the 4WD system in the Cavalier Turbo and is definitely the most important player when it comes to making the 4WD system actually work.

The T/X box is where it all happens, this is where the torque differential is measured, it is also the place where the torque bias between front and rear axles is determined and controlled and its the place where the drive is cut to the rear wheels when needed.

There are contains 3 main components within the T/X box casing which are, from front to back - 

• Gear System

  • This is the part that transfers drive from the Multi Disc Clutch to the Viscous coupling

• Multi Disc Clutch

  • The clutch is the part of the transfer box that dis-engages drive to the rear axle under braking

• Viscous Coupling

  • This part of the transfer box determines the amount of drive distributed between the front and rear axles

Gear System:

The section right at the front of the transfer box is the Gear Section and consists of 4 main gears  - 

• Hypoid Gear

• Planetary Gears

• Ring Gear

• Sun Gear

 

The function of the large Hypoid Gear is simple. It diverts the drive from the gearbox through 90^o and into the transfer box so it can then be used for making the Transfer Box work.

Large Hypoid Gear and smaller Planetary Gears

Attached around (and part of) the Hypoid Gear assembly are 3 smaller gears called the Planetary Gears ( 2 of which can be seen just to the right of the large hypoid gear in the photo above). It is these smaller gears that have the most important part to play within this front gear section of the box. It is these 3 small gears that transmit the power to the Viscous Coupling or, in essence, take the power away from the Viscous coupling when the Hydraulic Clutch is released.

Ring Gear

Around the outside edges of the 3 Planetary Gears sits the Ring Gear. The Ring Gear is connected to the Planetary Gears through the Ring Gear's inside, geared edge.

Ring Gear fitted around Planetary Gears

The Ring Gear is also connected to the front Clutch Rings via the geared outer edge of the Ring Gear and the teeth of the front Clutch Rings.

Clutch Rings fitted around Ring Gear - only front clutch rings are physically
 connected to the Ring Gear

 

The final gear is the Sun Gear.

   
3 views of the Sun Gear unit 

 This is also connected to the Planetary Gears through the angled gear section at the bottom of these 3 pictures, but this time with the inner section where the Planetary Gears meet as shown -  

The straight cut geared section of the Sun Gear is the part of the driveshaft that transmits the power to the Viscous Coupling.

To make it a bit clearer, here is a photo showing the various connections between the different gears within the transfer box - 

Inner Section - This is where the Planetary Gears connect to the Sun Gear. When drive is being applied to the rear wheels (normal driving), these 2 units (Planetary Gears and Sun Gear) are locked together.

Outer Section - This is where the Planetary Gears connect to the Ring Gear. When the drive has been removed from the back wheels (during braking or if there is a fault), these 2 units (Planetary Gears and Ring Gear) are locked together.

Clutch Ring Section - This is where the Ring Gear and front Clutch Rings are connected.

 

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Multi-Disc Hydraulic Clutch:

The hydraulic clutch section of the transfer box is the middle part and its job is to disconnect the 4WD system by removing the drive to the rear wheels.

There are a few reasons why GM and Steyr-Daimler-Puch (Austrian based co-developers) decided it would be best to disable the 4WD system at specific times - 

• System Fault

• During braking maneuvers

By far the most important reason is the bottom one - During Braking.

When braking the front wheels take over the largest part of the braking force as the weight is shifted forwards. If the 4WD system were still active then this would lead to transfer box overheating, and the viscous clutch would transmit the maximum driving power to the rear. 

Also, when braking on variable surfaces the 4WD can interfere too much and the car now becomes directionally un-stable - 

A: - Brakes first applied
B: - Path taken of car with 4WD disconnection enabled
C: - Path taken of car with 4WD permanently enabled

As was explained above the Viscous Coupling reacts very quickly to differences in wheel slip. If 4WD is engaged during braking then the power can suddenly and un-expectantly transfer the drive to the rear wheels causing sudden over steer to which the driver cannot react to in time (Point C) If the 4WD is disengaged then the car will react identically to a front wheel drive car and under steer predictably and safely (Point B)

The disconnection of the rear wheels is not necessary at low speeds and so the 4WD is NOT disconnected if the brakes are applied when the car is moving at speeds of 16MPH or less

The main final reason for disconnection of the 4WD system during braking is the ability to allow the inclusion of an ABS system. If the car remained locked in 4WD then both the 4WD system and the ABS system would be fighting over where the power is sent to or taken from.

 

How the Hydraulic Clutch Works:

Now that we understand WHY the 4WD is disengaged during braking, lets find out HOW its done in relation to the Multi-Disc Clutch assembly - 

There are 2 main sections to the clutch assembly. These are the - 

• Clutch Rings

• Pressure Plate

CLUTCH RINGS

The Clutch Ring assembly acts in much the same way as the disc packages of the Viscous Coupling.

As you can see here, there are 2 distinct and separate types of Clutch Rings in the transfer box - Rings with teeth on the inside and rings with teeth on the outside. The ring at the top of the picture has no teeth as it is the outside ring which is used to be squeezed onto by the pressure plate.

The teeth on the Inner Rings are connected directly to the Ring Gear of the transfer box and the rings with the teeth on the outside are connected to the outer housing of the transfer box and do not move - 

Again, like the Viscous Coupling, every front ring is followed by a rear ring and this is repeated many times over.

The "front" rings only (teeth on inside) have inserts on both sides made of a clutch like material - 

 

 

PRESSURE PLATE:

The Pressure Plate can best be described as a large round metal plate with a notched ring or ram around it whose main job is to squeeze the individual Clutch Rings together.

It does this by receiving a high pressure supply of fluid through its inlet connection (bottom left corner of picture) from the accumulator assembly in the engine bay. This high pressure fluid (5,200 kPa) pushes the ram towards the Clutch Rings with enough force to squeeze the individual Clutch Rings together so that they become one unit. 

When this happens, the front Clutch Rings are no longer able to spin with the Ring Gear as the rear rings are preventing this. This in turn prevents the Ring Gear from being able to spin as it is locked to the front Clutch Rings. The knock on effect is that the 3 smaller Planetary Gears are also locked off by the Ring Gear not moving and this means that, instead of all the energy of the Planetary Gears being used to spin the Ring Gear and then front Clutch Rings, the Planetary Gears spin the previously stationary Sun Gear instead which in turn transmits the power to the Viscous Coupling.

When the pressure is released (3,600 kPa), the ram falls back with the aid of a spring and the Clutch Rings come apart which in turn disconnects the drive to the rear wheels as the energy from the Planetary Gears now turn the Ring Gear and front Clutch Rings instead of the Sun Gear

This is all done in milliseconds with no driver interaction past pressing the brake pedal and as was mentioned above, the rear wheel drive remains engaged if the brakes are applied when the car is moving at speeds of 16MPH or less.

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Viscous Coupling:

The Viscous Coupling is the section of the transfer box that determines how much of the driving force goes to the rear axle and how much is sent to the front wheels i.e. this unit is the sole operator of the power split in the Cavalier or Calibra 4WD system.

 

1. Drive from front wheels	5. Connection for Rear Driveshaft
2. External Disc	A. Drive from Transfer Box gearing
3. Internal Disc	B. Drive to Rear Wheels through 5.
4. Housing of the Viscous Coupling

As can be seen above, the Viscous Coupling (VC) is a completely sealed, drum shaped unit and resides at the "rear" of the transfer box assembly. Contained within this unit are 2 different types of steel plates or discs all of which are immersed in a silicone fluid.

The interior discs are slit along the entire radius of the plates as can be seen in Point 3. of the above diagram. The inner discs are also connected to the front wheels via a driveshaft by the teeth that form the inner edge of these plates.

The exterior discs have holes punched in them and these also cover the full diameter of the plates (Point 2. above). The exterior discs are connected, again by teeth, with the housing of the Viscous Coupling which is bolted to the rear driveshaft that is connected to the rear wheels. These teeth form the outer edge of the exterior plates as can be seen above.

And this is how the information about the numbers of revolutions (speed) of front and rear axle arrives at the Viscous Coupling which is then used to determine wheel slip.

Within the Viscous Coupling, an interior disc (slit plate) immediately follows an exterior disc (punched plate). This repeats itself many times but NONE of these plates physically ever come into contact with each other. The whole unit, as mentioned above, is then submerged in silicone fluid which is how the drive is transferred from plate to plate.

 

How the Viscous Coupling Works:

Under normal conditions, the drive is split so that 75% of the power is driven to the front wheels and 25% is driven to the rear wheels. The secret of the Viscous Coupling is based in the silicone fluid it contains.

When driving normally, the difference in RPM (rotational speed) of the front wheels and the rear wheels is minimal. That is they are travelling at virtually identical speeds. This also means that the internal discs and the external discs are also turning at the same speed as these are connected to the corresponding axles.

If the speed of one set of wheels were to be more than the other, for example the front wheels lose grip so spin faster than the rear, then the difference in speeds of the corresponding disc's in the Viscous Coupling will also increase proportionally. In this case the inner discs would spin faster than the rest as these are connected to the front wheels.

This difference in speed of the discs means that the silicone fluid can no longer flow freely between the plates. This would cause a "cutting" action within the silicone fluid as it is sliced between the slits of the interior discs and the holes of the exterior discs resulting in the silicone fluid heating up instantly - 

1. External Disc
2. Silicone Fluid
3. Internal Disc

The properties of the silicone fluid are such that as the temperature increases so does the viscosity or thickness of the fluid. This results in the previously freely spinning plates to become increasingly locked together allowing more of the power to be transferred to the rear axle via the external discs.

In extreme cases, the power transfer can be as much as 100% to the rear wheels.

This is the basis of the Viscous Coupling system within the Vauxhall Cavalier/Calibra 4WD system

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For Information on the removal, stripping and rebuilding of the Vauxhall Transfer Box then click the link below for Richard King's site - 

Acknowledgements -

Many, many thanks go to the following people for all their help with this section -

• Ciubuca Dragos -  for all his help with this part of the technical section including photographs and translations.

• Steyr-Daimler-Puch (Company that designed this 4 Wheel Drive system for Vauxhall) - for their technical drawings and their confirmation of the accuracy of this section