The Different Types of 4WD Systems

This article attempts to cut through the murky marketing hype and explain the various types of 4WD systems.

In the good old days four wheel drive technology trickled down from the trucking and agricultural industries. The military, recognising the potential for smaller vehicles capable of negotiating difficult terrain, commissioned the 4WD style that we know today.

There are no fixed definitions or specifications for the different types of four wheel drives. Today's Full-time, Part-time and All Wheel Drive vehicles encompass a wide range of technologies designed to get power to the ground through all four wheels and classifying a particular car is left largely to the manufacturer.

Things begin to get a little confusing when a buyer is confronted with the multitude of marketing jargon which promises real off-road capabilities in a passenger style car.

If you're looking for a four wheel drive, capable of tackling real off-road situations, then a basic understanding of the different 4WD systems is essential to make sure you buy a car that is capable of doing what you require.

The definitions listed below are our interpretations and understandings of the technologies used in various motor vehicles. In no way are we attempting to categorically define the multitude of configurations available on the market. These are simply the benchmarks we use when assessing a vehicle's suitability for a given task - notably: to operate as a 4WD in an off-road situation.

Part Time 4WD
Different Types of 4WD Systems

Virtually all of the 4x4's sold in the early days were based around a part time system. The rear wheels were driven by a conventional transmission and differential and operated in exactly the same fashion as a two-wheel drive car. The driver selected four wheel drive by engaging a transfer case with a second gear lever and turning locking hubs on the front wheels.

The act of engaging the transfer case delivered power through a front differential to the front axles. The locking hubs coupled the front axles to the front wheels and the vehicle was transformed into a four wheel drive.

The transfer case also had the ability to change to a lower gear ratio to provide tractor like capability when tackling severe terrain. Changing from High to Low Range was done with the same second gear lever. This offered 3 distinct configurations - High Range 2WD, High Range 4WD and Low Range 4WD.

Because this part-time system evolved from truck and tractor technology, components tended to be very robust and part-time four wheel drives developed a reputation for ruggedness and durability.

Today there are still large numbers of 4WD's sold that are based on this system. One of the principles of the part-time concept is to reduce moving parts in the front of the vehicle and help conserve fuel. Constantly powering four wheels uses more fuel than powering just two wheels at the rear or front.

Technological advances mean modern part-time 4WD's come equipped with anti-lock braking, traction control, descent control and a host of other features to help the driver hump it over the rough stuff. Four wheel drive can be selected via buttons rather than a second gear stick and locking hubs can couple automatically. The principle remains the same - the vehicle operates in 2WD (part-time) until a transfer case engages the front wheels and delivers identical power to both ends of the car.

All 4 wheels need to rotate at different speeds when turning - inside wheels faster than outside and front wheels faster than rear. A part time system (in 4WD) does not allow front and rear wheels to act independently - the front and rear sections are driven at the same rate. Part time four wheel drives can only be driven on the road in two wheel drive. Driving with 4WD engaged, on hard surfaces like bitumen, generates 'transmission wind-up' and can cause severe drive train damage.

Full Time 4WD

Buyer demand has created an increasing market for safer and more well appointed cars. Many part-time systems require alighting from the vehicle in order to lock the front hubs in order to engage 4WD. This is hardly in tune with a vehicle that may come with heated, leather, electronic seats and full entertainment systems. More importantly the buying public likes the idea of four-wheel-drive on the highways - with it's promise of better traction, cornering and handling.

To overcome the problem associated with transmission wind-up a differential is installed between the front and rear differentials allowing each wheel 25% of the available torque. As with any differential, allowances have to be made for cornering and situations that offer different levels of traction to different wheels. The centre differential allows 'slippage' between front and rear drives while the diffs at either end cater for slippage between their respective wheels.

When the going gets tough the driver has the option of locking the centre differential (either by button or lever) and delivering equal torque to both ends of the vehicle. The 'locking' action enables the car to behave as a true 4WD and tractor over rough terrain.

A full time 4WD system is also equipped with a Low-Range gear ratio for added off-road ability. This means the full time system can operate as 4WD High, 4WD 'Locked' High and 4WD 'Locked' Low.

The driver has no way of selecting 2WD - the vehicle operates full time in 4x4. Just like the part time system, a Full-Time 4WD with the centre diff 'locked' cannot be driven on hard surfaces.

All Wheel Drive or AWD

All Wheel Drive vehicles are sometimes called 'soft 4WD' and are a completely different proposition. Manufacturers would subtly have us believe that their AWD's are really true 4WD's and can potentially overcome any obstacle with dexterity and panache. Not so. The most modern of AWD's couldn't compete with the most humble 1960 Land Rover in a testing off-road environment.

Ground clearance is the most obvious sign of a vehicles ability to tackle tough terrain. All the high-tech gadgetry in the world cannot force a car with 200mm of ground clearance over 500mm boulders. Most all-wheel-drive vehicles have poor ground clearance compared to their 4WD brothers and sisters.

AWD vehicles still utilise a centre differential that delivers power to all four wheels. They can operate in a variety of ways and deliver power to all four wheels just like a full-time 4WD system.

What restricts an AWD from behaving like a 4WD is the ability to lock the centre differential to drive the front and rear diffs at the same rate. As a result the front and rear ends operate independently and will not 'tractor' over difficult terrain like conventional four wheel drives.

Because these vehicles aren't really designed for serious off-road work there is no need to have a Low-Range capability built in their design.

The class of 'Automatic AWD' vehicles consist of a variety of different technologies that deliver torque to different wheels according to need.

Some of these 'Automatic AWD's' effectively operate in 2WD much of the time and deliver torque to the opposite end of the car as required. Sensing-systems determine a loss of traction in one wheel and apply torque to those wheels with the most traction.

The All Wheel Drive system is really developed for bitumen use. While some of the AWD systems can perform adequately in less taxing off-road situations the concept was really developed for safer and better on-road handling. Dressing an AWD up as a small 4WD is a clever marketing strategy.

Summary

Part Time 4WD

  • The vehicle operates by default as a two wheel drive.
  • The vehicle does not have a centre differential.
  • The driver engages 4WD via a transfer case with a second gear lever or a switch.
  • The front hubs engage the front axles by manual or automatic actuation.
  • The vehicle cannot be driven on bitumen or hard surfaces in 4WD.
  • The vehicle has the option to select Low-Range 4WD.

Full Time 4WD

  • The vehicle operates by default as a four wheel drive.
  • The vehicle is equipped with a centre differential.
  • The driver can 'Lock' the centre differential to enable true 4WD.
  • The vehicle cannot be operated in 2WD.
  • The vehicle cannot be driven on bitumen or hard surfaces in 'Locked' 4WD.
  • The vehicle has the option to select Low-Range 4WD.

AWD

  • The vehicle operates by default as a four wheel drive.
  • The vehicle is equipped with a centre differential.
  • The driver has no option to 'Lock' the centre differential.
  • The vehicle cannot be operated in 2WD.
  • The vehicle has no option to select Low-Range 4WD.

Other articles that describe the functions of a 4WD functions are - The Spin on 4WD Differentials and Transmission Wind-Up

The Used 4WD Review, in the 'Four Wheel Driving' section above, details some classic 4x4 vehicles.

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