How To: Understanding 4WD Systems

On one hand we have 'genuine' four-wheel drives - usually identified by the addition of a transfer case delivering dual high and low range modes - then there are all-wheel drives, part time four-wheel drives and wannabe four-wheel drives that may resemble a 4x4 but deliver few of the benefits.

Four-wheel drive systems
Distinguishing a 4x4 from the traditional two-wheel drive layout, the four-wheel drive system transmits power to four wheels instead of the usual two. The obvious benefit of the 4x4 system is that four driven wheels are better able to grip rough terrain and maintain forward momentum.

How 4x4 is engaged
Today there are a number of automatic four-wheel drive vehicles, which require no intervention by the driver to engage 4x4 when leaving a main road. These vehicles are permanent or constant four-wheel drives, yet still provide the driver with the option of selecting low range.

In more traditional four-wheel drives, it is necessary for the driver to engage four-wheel drive. These vehicles are part time 4x4s, with the vehicle usually run in 2WD on main roads and 4x4 selected when necessary. Usually 4WD needs to be manually selected, as does low range.

Transfer case
This is effectively the vehicle's second gearbox and allows drive from the engine through the main gearbox, dividing it along both the rear and the front propshafts to their respective differentials and axles. Both the front and rear wheels turn under power from the engine.

High and low ratio
The transfer box itself has two sets of ratios, high and low, which work in series with the main gearbox. A vehicle with a standard five speed gearbox therefore has five in high ratio (for normal road driving) and five in low ratio (for difficult off road terrain).

Permanent 4x4
The latest 4x4s such as the Toyota 200 Series LandCruiser and Land Rover Discovery 4 are constant or permanent four drives. In these vehicles the four wheels are driven regardless of the road surface. To enable them to operate without 'differential windup', they are fitted with a third differential (located centrally) which allows for variations in the distances travelled by the front and rear wheels.

All-wheel drive
To be an all-wheel drive (AWD), a vehicle need not have a transfer case and low range. Most of the current crop of compact SUVs are either front or rear driven AWDs, with systems that detect wheel slip and can redistribute torque to drive other wheels temporarily. This is done by a traction control device, whereby when one wheel loses grip the brake is automatically applied and the torque is sent to another wheel, thus providing traction. This system is great for improving safety on wet roads, where wheels can momentarily lose grip. While providing all-wheel drive (some of the time), they are not 'true' four-wheel drives.