Time was that all 4X4s just drove the rear wheels on road, but now there’s a choice of all wheel drive. So should you buy a part-time 4X4 vehicle?

THE TWO BASIC drivetrains for 4X4 vehicles are full-time 4X4 and part-time 4X4.  Full-time 4X4 is also known as constant 4WD, all wheel drive (AWD), or full-time 4WD, and part-time is also known as selectable 4WD.

What are we talking about here?

The difference is simple; the full-time 4X4 system drives all four wheels all the time, even on high-traction bitumen, and it uses some sort of centre differential system to allow the front axle to be driven yet rotate quicker around a corner which avoids transmission windup.

The part-time 4X4 system has no trick differential in the centre so it cannot be driven onroad in 4X4 mode, otherwise it’d get transmission windup. Instead, it drives on high-traction surfaces in 2WD, which for 4WD vehicles usually means it is rear-drive.

Examples of part-time 4×4 wagons are the Toyota Fortuner, Isuzu MU-X, Toyota FJ Cruiser and Nissan Patrol GU. Pretty much all utes are part-time too, with notable exceptions being some Amaroks and all Defenders. Mitsubishi have an unusual system called Super Select for the Triton which offers both part-time and full-time modes.

So the difference between part-time 4X4 and full-time 4X4 is that part-timers drive just the rear wheels onroad, and the AWDs drive all four wheels, all the time. Now the question is whether you care or not, and that’s up to you – I’ll just describe the differences in real life.

Driving differences: onroad

Most of the time the driver wouldn’t know the difference between part-time and full-time. The differences start to come in when traction starts to drop, and typically at slower speeds. For example, roundabouts, particularly wet ones,  especially slower corners you power out of.

Here you can find the part-time vehicle wanting for traction, with perhaps a little bit of rear wheel spin, and especially now with today’s powerful engines. Yet thanks to modern tech it’s not a huge drama as the vehicles typically have traction and stability control which quickly quells wheelspin, and any chance of the rear end getting loose. But you’d never have that traction loss with an AWD, so the electronics would never need to work their magic.

You may well argue that we all got around very well thank you before electronics existed to curb traction loss. And we did…or did we? Plenty of accidents happened due to traction loss, and stability control is recognised as a lifesaver. And yesteryear’s 4X4s weren’t very powerful and had four-speed autos or five-speed manuals. Today’s vehicles are far more powerful with more gear ratios, so much more able to kick the rear out….particularly once you run offroad tyres which have less onroad grip than road tyres.

It only takes the one moment gone wrong over your lifetime to change your life forever.

Now there is an argument that the part-time 4X4 uses less fuel. And that is true. But the difference is barely measureable, so it’s not really a valid argument. The best example is driving Tritons or Pajeros in 4H (full time 4X4 mode) then in 2H (part-time 4X4, 2WD mode) and you’ll see barely any difference.

Driving differences: dirt roads

On dirt roads the full-time 4X4 is markedly superior to the part-time 4X4. It has better traction, handling and even braking on those occasions you engine brake.

There’s a reason rally cars are all wheel drive, and it’s traction…all else being equal, a 2WD rally car cannot keep up with a all wheel drive version.

However, those advantages of the full-time 4X4 vehicle can be equalised by selecting 4X4 on your part-timer. But then you need to remember to select and de-select 4X4 when you drive onto high-traction surfaces like bitumen, or even some high-traction dirt roads.

Driving differences: offroad

Whether there’s a difference offroad depends on whether the centre diff on your full-time 4×4 can be locked. If it can be locked, then there’s no difference between full-time and part-time when in the rough.  Examples of vehicles with lockable centre diffs are Prado, LC200 and Pajero Sport.

If the centre diff can’t be manually locked and is computer-controlled, as per Everest, Discovery and H9, then you are likely to find yourself at a disadvantage because no car manufacturer has yet delivered an “intelligent” centre diff or clutch which is as good as a lockable one, and in most cases they are worse. For example, a big failing is that when you apply the parkbrake only the rear wheels lock, leaving the fronts unlocked and therefore making it hard to secure the vehicle on a hill. There are many claims that such systems improve traction, but the last decade of offroading testing such vehicles has yet to convince me, and no manufacturer has provided proof of their claims either.

Also, if you break a CV joint then you will find that the computer-controlled centre diff is not very keen on letting you drive only the rear wheels, whereas if you could lock it then you’d have rear drive at least.

There is one advantage of a computer-controlled centre diff offroad and that is the turning circle, which is smaller with such vehicles as the computer allows the front and rear axles to turn at different speeds during a turn.

There is one useful, but rarely used feature of a part-time 4X4 system, and that is you can put it in 2WD and spin the rear tyres. I have done this a few times to slew the back end around. It’s not often you need it, but when you do it’s handy. Even did it once with a trailer attached to slide the back end sideways by 30cm into a rut.

Towing

Driving all four wheels is better for towing that just driving two. You won’t have any drama with wheelspin on takeoff, more traction everywhere, and it’s even better at slow speeds. Ever tried back a trailer up a slope, or slow-speed drive around a slippy campsite? Often your part-time 4X4 will spin a wheel, whereas the full-time 4X4 never misses a beat.

Which to buy and when?

All types of 4X4 transmission can work onroad and offroad when designed correctly, so it is a matter of selecting the best one for your needs. This shouldn’t be the primary criterion for buying a vehicle – consider size, general capability, cost and all the other attributes first.

That said, here’s some buying advice:

• Full-time 4X4 is better onroad and for towing – this is due to the greater traction. However, a part-time can work perfectly well if driven sensibly, especially with modern traction aids like stability control.
• Preference lockable centre diffs – if your full-time 4X4 has a computer controlled centre differential or clutch then it is best if there is a lock option, as per Prado, Grand Vitara, LC200 and Pajero Sport. However, some computer controlled systems are effective, such as Land Rovers. For the ones which can’t be locked read the reviews on this site to determine how effective they are and whether that is likely to cause you any trouble.