There are several different 4WD drive systems in the marketplace and many 4WD buyers are confused about the relative merits of each type.
All non-hybrid 4WDs have a main transmission driving through ‘transfer’ gearing that can send drive, mechanically, to the front and rear axles. Most hybrids are different and we’ll come to that later.
In the case of vehicles with low-range gearing the transfer case contains reduction gears as well, to provide better off-road tractive effort.
That’s where the similarity between different 4WD systems ends.
With some Land Rover and Toyota exceptions, most drivelines behave similarly when operating in low range, where mechanical drive to the front and rear axles is ‘locked’, but the major differences between competing systems are noticed in high range.
Full-time 4WD and on-demand 4WD are the most fool-proof systems for owners who know nothing of the mechanicals underfoot. These systems mean that the driver needs to intervene only when engaging low range or when activating the centre differential lock and, where fitted, rear differential differential lock.
Full-time 4WD spreads the driving torque between all four wheels, which gives good traction when running on loose or slippery surfaces. On-demand systems sense a loss of tyre grip and do the same, until better grip returns and they revert to two-wheel-drive.
Selectable-full-time 4WD as used by Jeep and Mitsubishi is intended to reduce high-grip-surface fuel consumption, by letting the vehicle operate in rear-wheel-drive, but in our on-road testing we’ve been unable to measure any of the claimed fuel consumption difference between 2WD and 4WD operation.
The difficulty in settling on a preferred driveline type is that the decision is bound up with other purchasing factors. For instance, most utes are part-time 4WDs and most large wagons have full-time-4WD.
We’d love to list the different 4WD systems by make and model, but this is a constantly changing scene. It’s essential you find out what driveline is fitted to the vehicle you want to buy, so you can determine if it suits your needs.
Don’t rely on the sales person. Check the engineering specification sheet or the vehicle handbook.
Part time 4WD
Part-time 4WD is where the whole business began and is still the preferred choice of most working-vehicle makers. Nearly all utes and cab/chassis and many wagons with ute-based powertrains are part-time 4WD vehicles.
The traditional part-time 4WD driveline has served working-vehicle owners well, but is easily abused by those who don’t understand what transmission ‘wind-up’ is and how much damage it can do to a poorly-driven 4WD vehicle.
When operating a part-time 4WD, the machine must be driven on hard or high-grip surfaces in rear wheel drive and put into 4WD mode only when conditions warrant it.
Which brings us to the subject of ‘free-wheeling’ front hubs that are fitted to some working 4WD utes.
There is great confusion about the function of free-wheeling hubs. Some owners fear they’ll ‘wind up’ their driveline and increase front tyre wear if they drive on high-friction surfaces with the hubs locked. They won’t, as long as the vehicle is driven in 2WD. Locking the front hubs does not select 4WD; it merely makes that selection possible.
All part-time 4WD vehicles had free-wheeling front hubs years ago, but very few do these days. Manual types are engaged and disengaged by turning a knob on the outer face of each hub. With the hubs in the ‘free’ position, there is no connection between the front hubs and the front axle shafts, so the front wheels can turn without causing the front axles, differential and propellor shaft to turn.
Once in ‘lock’ mode and with the vehicle running straight ahead, four-wheel-drive high range can be selected via the transfer case lever or dashboard switch at any speed, without stopping the vehicle, because front and rear propellor shafts are turning at synchronous speed, or very close to it.
Automatic free-wheeling hubs were a popular way for 4WD makers to get around the inconvenience and misunderstanding of manual hubs, but they’re seen only on used 4WDs these days. Auto hubs functioned in much the same way as manual hubs, but were locked by stopping the vehicle and selecting 4WD.
Modern part-time-4WD systems use electronic 4WD selection and part of that system is pre-engagement of the front differential, to get it up to speed, before 4WD engagement. Some part-timers don’t even have a disconnecting front diff, so that pre-engagement feature isn’t necessary.
Full-time 4WD is the most popular choice of top-shelf 4WD wagon makers. The original 1970 Range Rover introduced full-time 4WD, by means of a differential in the transfer case. The transfer case lever was needed only to select low range. An additional control was a mechanical lock on the central diff, to force both propellor shafts to turn together, just as in the case of a part-time 4WD. However, most full-timers have automatic centre-diff locking in low-range-4WD.
Full-time 4WD improves traction and handling on high-friction surfaces, and evens out tyre wear, while providing an easy transition onto loose or slippery surfaces.
Jeep and Mitsubishi offer selectable-full-time drive systems. Selectable-full-time 4WD has the central coupling design necessary to allow full-time 4WD operation, but the system can be operated in rear-wheel-drive only if the driver selects that mode.
In between the extremes of part-time 4WD and full-time 4WD are on-demand 4WD drivelines that transfer automatically between 2WD and 4WD as conditions demand it, without driver intervention. Nearly all ‘softroaders’ (SUVs) have on-demand 4WD.
Various electronically controlled mechanical couplings are used on these predominantly front-wheel-drive vehicles, to activate rear-wheel-drive when required. Some systems have the means to ‘lock’ the vehicle into what essentially becomes part-time 4WD and that mode shouldn’t be used on high-friction surfaces.
Traction control influences
The 4WD driveline subject is complicated by electronic traction control and, while an examination of different traction control systems is a separate subject, our testing shows that TC has altered the basic behaviour of different 4WD driveline designs.
Traction control shifts the responsibility for reducing wheel spin from the driveline components to the ABS and EBS systems, so a powerful TC system can improve the tractive ability of any 4WD.
Electrification is coming at full speed and the concept is a boon for 4WDs that use on-demand 4WD.
Hybrid powertrains interpose an electric motor between an internal combustion engine and the driveline. Extending that package to 4WD is simply a matter of putting an electrically powered axle at the other end of the vehicle. There’s no need for any mechanical connection between front and rear axles.
The fact that an electric motor delivers peak torque at zero revolutions means that there’s also no need for low-range gearing. (Gearing is there to multiply torque, remember.)
Also, it’s very easy to have torque proportioning related to tyre grip, so there’s no need for a diff lock on an electric axle.
Some hybrid makers are testing motor-in-wheel electric drive, which puts an individual electric motor in each wheel hub, as some battery-electric cars do.
Our testing of hybrid on-demand SUVs indicates that the principle works very effectively off-road and the all-terrain ability of these vehicles is limited only by their SUV design, with its low ground clearance, considerable front and rear overhangs and inadequate wheels and tyres.