Freelander 4 Wheel Drive
The benefits of 4WD
In slippery conditions, on or off-road, four driven wheels are always better than two.
With power being spread to all four wheels, 4WD effectively provides double the traction and half the risk of wheel slip or spin, on or off-road
Freelander's 4WD System
Four Wheel Drive
- Car-like driving qualities on road, with a bias to the front wheels in normal tarmac driving conditions.
- An intelligent and automatic transfer of torque to the rear wheels to provide maximum traction when required..
Car-like feel, 4WD traction when required
In order to give car-like manoeuvrability and driving feel, most of the drive is channelled to the front wheels in a normal tarmac straight-line mode.
If the vehicle is operating in slippy conditions and one of the front wheels starts to lose traction, the difference between the front and rear propshaft speeds is sensed by the Viscous Coupling. The instant this happens, drive is progressively split as required between front and rear axles to maintain forward motion.
- Car-like driving qualities on-road.
- Good high speed stability.
- 4WD traction and handling off-road
- Reduced noise from transmission.
How it works.
The Freelander 1.8 petrol is fitted with the conventional 5-speed PG1 gearbox. The Freelander does not have a separate transfer box splitting the power from front to rear.
Instead, it has an Intermediate Reduction Drive unit bolted onto the gearbox where the Final Drive unit is located. This reduces the ratios by a factor of 1.458 to give relatively lower gearing, and also rotates the drive through 90 degrees to the rear axle, via the Viscous Coupling. This gives the benefits of lighter weight and reduced transmission noise.
The Intermediate Reduction Drive
The IRD incorporates a differential assembly, a primary and secondary helical spur gear train, and a hypoid gear set. It replaces the traditional transfer unit and performs 3 main tasks:
I R D
- Reduces the gear ratios of the manual gearbox by 1.458 to the front wheels via the helical spur gear train.
- Rotates the drive through 90 degrees to drive the propshaft and rear wheels via the hypoid gear set.
- In conjunction with the Viscous Coupling, gives the vehicle an automatically activated four wheel drive system without any extra levers.
The IRD is driven from the manual gearbox Final Drive carrier. Depending on the slip at the front wheels, power is transmitted to the rear axle with torque split front/rear to suit the prevailing driving conditions.
The Viscous Coupling
The concept of the Viscous Coupling Unit (VCU), introduced on Classic Range Rover, was a world first for Land Rover. The VCU has been colloquially described as a "tin of treacle with discs in it". It is a canister between the front and rear propshafts, filled with a silicone fluid sandwiched between steel plates. The fluid has the characteristic of increasing its viscosity when stirred and heated.
When the wheels on the front axle start to lose traction, they cause the front and rear propshafts to rotate at different speeds to each other. This makes the discs in the VCU rotate at different speeds, which in turn causes a rise in temperature, progressively locking the discs together. This effectively starts to couple the front and rear propshafts, eventually dividing torque equally between front and rear axles to maintain traction and control. The response is almost instantaneous, and undetectable by the driver.
As the front wheels regain traction, the temperature in the VCU drops, the viscosity of the fluid returns to its original level, and the propshafts are unlocked, reverting to normal drive mode.
Through this ingenious system, the Freelander is able to give car like handling in normal conditions, and extremely good performance in adverse or off-road conditions, without the driver having to use additional controls.