Are you new to the off-roading scene? Does your head spin when you hear terms like low range and axle wind-up thrown around? We’re here to help.
Regular family sedans are generally driven by just two of its four wheels. Sometimes it’s the front pair and sometimes the rear. As you’d expect, four-wheel drive vehicles are driven in all four wheels. Why? For the same reason that it is easier to climb up a rock using both hands and feet rather than just your feet. The first mechanical fourwheel-drive system was invented in 1902 by two Dutch brothers named Hendrik-Jan and Jacobus Spyker. Back then, race cars used unpaved dirt tracks, and the Spyker brothers developed fourwheel drive to improve their car’s handling. It was a simple system but it enabled them to win several races. The competition took notice, and manufacturers like Mercedes-Benz immediately embraced the technology. Nowadays there are three types of four-wheel-drive systems – actually four, but we’ll get to the fourth kind later. The three systems are: permanent, parttime, and all-wheel drive.
PERMANENT FOUR-WHEEL DRIVE
The Spyker brothers’ race car had permanent four-wheel drive (4WD), but how did it work? Since the basics remain unchanged in over a century, we’ll use a Land Rover Defender as an example: A Defender’s engine is connected to a gearbox and this gearbox is in turn connected to a centre differential. From here a short driveshaft extends forward to a second differential on the front axle. A second, longer driveshaft leads aft to a third differential on the rear axle. Occasionally it may happen that none of the four wheels rotate at the same speed, and these three differentials prevent differences in torque to cause a phenomenon called axle windup, which can damage moving parts. The centre differential also allows more engine power to be transferred to the axle that requires it most. The two differentials on the axles send power to the wheels with the least resistance. That’s all well and good but if you need to exercise more control over where the power is sent, the centre differential can be locked to force the system to divide power equally between both axles. This is particularly useful when you’re negotiating an obstacle and you need the front axle to pull just as hard as the rear axle is pushing. If you need even more control over power distribution, you can fit locking differentials on the axles, which will send an equal amount of power to every wheel – in other words, each wheel receives 25% of the engine’s output. The drawback of a vehicle equipped with permanent 4WD is that they are heavier on fuel than a car driven by just two wheels. The benefits are that your vehicle is always ready for 4x4 action, and because the system is relatively uncomplicated, the odds of the system failing is low.
PART-TIME FOUR-WHEEL DRIVE
In 1911, US truck manufacturer The Four Wheel Drive Auto Co. became the first to develop a system by which the front axle could be disengaged from the gearbox when 4WD wasn’t required. As a more recent example, we’ll use the Nissan Patrol. A Patrol’s engine is connected to a gearbox and the gearbox is connected to a transfer case. From this transfer case a driveshaft extends forward to a differential on the front axle. Another driveshaft leads to a differential on the rear axle. When on paved roads, only your vehicle’s rear axle receives power from the engine, and the rear differential determines which of the two wheels will receive the lion’s share of this power. If road conditions deteriorate, you only need to tug on a short lever next to the gear lever (or press a button) and a strong chain belt will grab hold of the front axle and turn it along with the rear axle – almost like the centre differential of a permanent 4WD system. The major advantage of a part-time system is that you can disengage one of the axles and thereby improve fuel economy. The drawbacks are that the increased complexity raises the odds of one or more parts failing. For instance, if you engage 4WD on hard surfaces, the aforementioned axle wind-up caused by >
unequal torque between the axles can cripple your ride. Mitsubishi found a way around this by employing both a transfer case and a centre differential. It’s called Super Select and it allows you to disengage the front axle and save fuel, or you can leave it in 4WD all of the time and drive it like you would an old-school Land Rover on any surface.
ALL-WHEEL DRIVE (AWD)
Technically all 4WD vehicles are also all-wheel drive, as are six-wheel-drive vehicles. But the term is generally reserved for sedan cars that are driven in all four wheels but are not specifically designed to go offroad. Think Subaru and Audi. Certain AWD systems are similar to permanent 4WD in that all the wheels are powered all of the time, as is the case with a Subaru. Other vehicles such as the Q5 has a part-time system that favours the front wheels and only engages the rear axle when wheel spin is detected by electronic sensors. But why would you want a regular car with all-wheel drive? As the Spyker brothers showed, four powered wheels grip better than just two. Another important consideration is convenience. If it suddenly starts to rain or snow you don’t have to pull over to the side of the road, engage 4WD, and continue – the car does it automatically.
go! Drive and Camp says
4WD works great in the soft sand of the Kgalagadi, and AWD is perfect for slippery mountain passes.
FRONT ON DEMAND ENGINE DIFF GEARBOX TRANSFER CASE PART-TIME 4WD ALWAYS DRIVEN DIFF REAR
AWD REAR ENGINE GEARBOX DIFF FRONT ON DEMAND ON DEMAND