"4x4" in a vehicle means there are 4
wheels total and 4
wheels that are driven.
There are two types of 4WD systems found in the 4x4 vehicles in today’s market – either Part-Time 4WD or Full-Time/Constant 4WD. Both are ideal and meant for off-roading.
Virtually all of the 4x4's sold in the early days were based around a Part-Time system with some still in use today, although the Full-Time /Constant 4WD has become prolific (but more about that later).
PART-TIME 4WD
The rear
wheels of the 4x4 are driven by a conventional transmission and differential and operated in exactly the same fashion as a two-wheel drive car. The driver selects four wheel drive only when required by engaging a transfer case. In older models of 4x4 this was done with a second gear lever and turning locking hubs on the front
wheels (ie. stopping and getting out of the vehicle). However, now this is usually done by pressing a button on the dash and the hubs will lock automatically. Whilst modern part-time 4WD’s come equipped with many extras the principle remains the same - the vehicle operates in 2WD (part-time) until a transfer case engages the front
wheels and delivers power 50/50 between the front and rear diffs. 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.
The transfer case will also have the option to switch into a lower gear ratio to provide extra torque when tackling extreme terrain. Changing from High to Low Range is done with the second gear lever (in older models), or by pressing a button (with newer models). There are 3 modes of operation - High Range 2WD, High Range 4WD and Low Range 4WD.
Having less moving parts in the front of the vehicle is an advantage of the part-time system and aids in conserving fuel usage.
Part time four wheel drives can only be driven on a bitumen surface in conventional mode (two wheel drive). Driving with 4WD engaged, on hard, smooth surfaces generates 'transmission wind-up' and can cause severe drive train damage.
FULL-TIME (CONSTANT) 4WD
The Full-Time 4WD has a centre slip diff allowing each wheel 25% of the available torque. The centre differential allows 'slippage' between front and rear drives while the diffs at either end cater for slippage between their respective
wheels. The driver of the vehicle can lock the centre diff (by button or lever) to ensure torque is delivered equally to both ends of the vehicle. When the centre diff is locked, you must be conscious not to get transmission wind-up, so just as with the part-time 4WD, the vehicle cannot be driven on hard, smooth surfaces such as the highway in this mode.
The Full-Time 4WD system is equipped with low-range gearing for added off-road ability. There are 3 modes of operation: High Range 4WD, 4WD “Locked” High, and 4WD Locked Low. There is no 2WD mode.
With this type of system the vehicle and driver will benefit from the AWD feature on the road and the 4WD feature when off-roading, so can be considered the best of both worlds.
4x4 vehicles (whether constant 4WD or part-time 4WD) are deliberately constructed for the purpose of driving in off-road terrain so they are also equipped with large profile tyres and rims, have low range gearing, have high ground clearance with superior approach, departure and break over angles. They would also typically be large vehicles with a large load and towing capacity.
These vehicles are therefore ideal for those drivers who need to tow a large caravan, boat, or trailer, and for drivers you want to negotiate difficult off-road terrain when more torque and/or slower speed is needed.
Importantly for Australian touring, if you wish to have a vehicle that you can take over
sand dunes (either beach or dessert) or through natural bush terrain such as hilly unmaintained tracks where erosion causes washouts and track rutting, and to negotiate rocky sections where wheel travel and angles become critical factors, then you will require a 4x4 (and it doesn’t much matter if it’s full-time or part-time).
HIGH/LOW RANGE
To enable a 4WD to travel at lower speeds while travelling on rough terrain it needs lower gear ratios. Not all 4WD’s have low range gearing and this restricts their ability to tackle rough terrain. However 4WD’s that lack low range gearing are generally not built for severe off-road conditions.