When buying a new SUV, crossover or bakkie, you’ll have to choose between four-wheel drive, part-time four-wheel drive and all-wheel drive. What are the differences, and which should you go for?
As an amateur adventurer, you’ll likely know the golden rules of off-roading. There are quite a few of them, but for the purposes of this article, we’d like to focus on the rule stipulating that you should know the limitations of your car.
You wouldn’t, for example, use a Duster to negotiate Divorce Pass in Namibia. It may offer an off-road experience beyond that of the average soft-roader, but it’s worth keeping in mind that it is, and will always be, a soft-roader.
A proper 4×4, as they’re called in Leisure Wheels lingo, has four-wheel drive, or part-time four-wheel drive, and not all-wheel drive, as is the case with the Duster.
We used to think that the differences between these three set-ups were common knowledge in our little world of mud and adventure, but a few recent phone calls and some online correspondence proved to us that this was not the case.
As such, we offer this missive, which hopes to explain the difference between all-wheel drive and four-wheel drive.
Down to basics
A car has an engine, which sends its power via the gearbox to the driven wheels. The most basic example of this is a front-wheel drive SUV, but since we’re focusing on 4x4s in this article, we’ll leave the fronties out of it.
By dismissing the fronties and focusing on the 4x4s in SA, a pattern soon emerges. There are three basic systems in play: all-wheel drive, part-time four-wheel drive and four-wheel drive.
Part-time four-wheel drive
This is the four-wheel drive system most commonly found in SA and it offers three driving modes – two high (2H), four high (4H) and four low (4L). A part-time four-wheel drive system relies on the driver to select which of the three should be used on any given surface.
Two high is normally used in normal day-to-day driving conditions. Only the rear wheels are driven and the engine power is split between the two rear wheels.
The secret to this system’s four-wheel drive mode is a transfer case. This transfer case links the front and rear drive shafts, transferring the torque to all four wheels.
When things get really tricky, it’s advised to switch to four-low, which basically means more torque at slower speeds compared to four-high.
We’ll explain low-range in more detail in an upcoming article…
The upside of this set-up is that it’s cheap, which is why it’s used in so many off-roaders. There’s also an improvement in fuel economy in two-wheel drive and less wear on the components.
A perceived negative is the fact that, although the centre differential locks the drive between the front and rear axles in a 50/50 split, the split between the wheels remains unchecked. So if you encounter an axle twister in your part-time 4×4 with the centre differential locked, individual wheels with the least resistance will still spin, resulting in loss of traction and forward momentum.
A mechanical, manually activated differential lock (front and/or rear axles) ensures that both wheels on the same axle rotate at the same speed, hence offering much improved traction in cross-axle conditions.
Part-time 4x4s should never be used on tarmac with four-wheel drive engaged. Because of the simplicity of the system (50/50 split between front and rear axles), the front wheels are unable to move faster than the rear wheels, which can lead to transmission problems.
The other downside is that part-time four-wheel drive doesn’t necessarily mean more grip in day-to-day situations, like the rainy season in Gauteng. With all-wheel drive, the additional grip is an added safety measure, but since a part-time four-wheel drive system can’t be used on tarmac, it means zilch when the roads are wet.
Examples include Toyota Hilux, Nissan Navara and most other double cab bakkies.
In layman’s terms, a four-wheel drive system permanently sends a percentage of the engine’s power to all four wheels at all times. A big difference between it and a part-time four-wheel drive car is that there’s no transfer case.
Instead, a four-wheel drive car has a centre differential, and unlike a part-time set-up, it can regulate the amount of torque deployed between the front and rear axle, which means it can be driven on any surface. This centre differential can, in some cases, be locked to distribute torque evenly between the front and rear axle.
These kinds of vehicles usually come as standard with a rear differential lock, in case one or more of the wheels have little grip.
The upside of this system is that the power is always sent to all four wheels, with no driver involvement necessary. The downside is increased weight, fuel consumption and higher cost compared to all-wheel drive and part-time four-wheel drive.
Examples include Subaru Forester, Audi’s quattros and the Toyota Land Cruiser 200.
The most important concept to grasp with all-wheel drive is the idea that there is a primary and secondary axle.
Under normal driving conditions, such as tarmac driving, the power is sent to the primary axle, which, in most vehicles, is the one up front. Once the electronic sensors pick up a loss of grip at the front axle, it diverts power to the rear axle, and activates it. This is usually done via a hydraulic clutch or viscous coupling.
As soon as the front and rear axles are spinning at the same speed again, the rear axle is unlocked and it’s back to front-wheel drive business only.
Some all-wheel drive vehicles have a locking function, which allows the driver to override the electronic all-wheel drive system and split the torque equally between the front and rear axle, but as soon as the driver exceeds a certain speed (usually 80km/h) the ECU overrides the lock and reverts back to the standard set-up.
It sounds like a perfect solution, but there are a few reasons all-wheel drive will never be as good as permanent four-wheel drive or part-time four-wheel drive.
First, there is no transfer case involved, which means that you don’t have the added advantage of low range. The second problem is that it can’t transfer power to the wheel with the most grip and will most likely send power via the path of least resistance. In an axle twister, where both a front and rear wheel will likely be in the air, an all-wheel drive car has no hope of wiggling itself loose, except if it comes with a good traction control system, which is becoming the norm these days.
For the most part, these vehicles were designed with light off-roading in mind, but the main reason they exist is to offer an enhanced driving experience in slippery conditions like snow, ice and wet surfaces.
The upshot is the additional grip they provide without any driver interference. The computer does all the work for you.
It’s much cheaper as well and due to the compact nature of the set-up, it can be installed into most vehicles with relative ease. There are numerous saloons that make use of this system, and we’re seeing quite a few performance manufacturers moving over to all-wheel drive as their cars get more powerful.
Examples include Honda’s CR-V, Nissan’s X-Trail and Kia’s Sorento.
The next step
These days we’re seeing more manufacturers bragging about ‘intelligent’ all-wheel drive, which is exactly the same as normal all-wheel drive, but the driver is completely removed from the equation.
There’s no centre differential lock, which means the driver has to rely on the drivetrain to decide where the power should go. This makes for a safer and more thrilling driving experience on road and in wet conditions, but the same can’t be said of off-roading.
An intelligent all-wheel drive will only get you so far before the ECU is completely flummoxed by the data it is receiving from the wheels. It can easily cope with snow and rain, but show it a dune and it will be utterly defeated.
You’ll note that we gave the positive and negative attributes of each system, with good reason. There’s no right or wrong answer here and there’s no clear winner.
The best system depends on what you’re going to be using the car for.
If you’re a hardcore off-roader, you’ll want to go the four-wheel drive, or part-time four-wheel drive route.
If, however, you just want to drive the occasional dirt road, or want to tour through Namibia, Botswana or Mozambique, a decent all-wheel drive soft-roader is up to the task. It’s also worth looking at if you just want the added benefit of all-wheel drive for the rainy season.
Just don’t go and try driving up the highest dune in Namibia, or the rockiest mountain pass in SA. Leave that to the proper off-roaders.
Text: Gerhard Horn