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All for four and four for all - AWD explained
By Phil Hanson • 05/12/2015
OUR EXPERT EXPLAINS THE DIFFERENCES BETWEEN AWD AND 4WD, AND OFFERS TIPS ON CHOOSING THE RIGHT VEHICLE
So you’ve got a nice new four-wheel-drive, eh? Sorry, it’s actually an all-wheel-drive.
But isn’t that the same thing, just like chips are really French fries?
Well, yes and no, and there are important differences that might mean the difference between getting home or seriously stuck off the road.
These days, the term 4WD generally describes a vehicle with low-range gearing. When low-range is chosen, drive goes to the wheels via another set of gears that multiplies the “pulling power” of each normal ratio.
AWD Subaru XV,
What that all means is: in low range, the vehicle can crawl along at a walking pace and has the grunt to, say, get a loaded trailer moving or drag heavy objects across the ground. It also offers the driver much better control on off-road tracks and trails.
However, not everyone wants to do any of that, but they may want the advantages of having all wheels driven when launching a boat at a ramp, driving to skifields, or on wet, slippery suburban roads in spring.
So manufacturers responded by offering 4WDs without low-range gearing. These became known as AWDs, pioneered in the 1980s by the Audi Quattro and, in North America, the excellent but poorly assembled AMC Eagle.
AWDs are generally more car-like, lacking the ground clearance (typically in the 175-190mm range) and other specialised features found on 4WDs. An AWD will have some sort of electronic traction control that stops a wheel or wheels from spinning, so maintaining forward progress. It may have electronic hill descent control (HDC) that maintains a safe, controlled speed on steep hills.
If only that’s all there was to it.
Most of today’s AWDs are also known as SUVs — Sports Utility Vehicles, which BMW complicates by calling them Sports Activity Vehicles. However, “proper” 4WDs are also usually described as SUVs. Aargh!
More confusion: an AWD SUV might also be called a crossover, adding another layer of confusion. Sometimes, “crossover” is code for a vehicle that’s only barely an AWD SUV — a soft off-roader.
Soft-roader is a common term, coined to also describe an AWD SUV. However, the term is at odds with some vehicles’ actual abilities. We’re still not sorted.
A 4WD may not even be a 4WD in practice, unless the driver pushes a lever or turns a knob to engage the front wheels. That’s because there are full-time and part-time drive systems.
AWD Hyundai SantaFe.
Full-time 4WD means that drive is going to all wheels all the time, as long as they all have traction. Part-time 4WDs are usually rear-drives on which torque can be sent to the front wheels whenever the driver engages them, via switch or lever. On a few vehicles, such as the Toyota Land Cruiser 70 Series, the driver has to get out and lock the front hubs to complete the drive train.
Part-time 4WD is cheaper to make and has a small fuel economy advantage when only one set of wheels is being driven. Experts continue to argue whether it also lessens maintenance. You can’t drive part-timers in 4WD on high-friction surfaces like dry sealed roads without risking mechanical damage.
Some AWDs, usually the dearer ones, also provide full-time drive to all wheels; others are basically front-wheel-drives that send squirts of torque aft when sensors detect slippage.
Not all AWD systems are born equal. Some don’t have the ability to lock-in drive to all wheels. The computer might then get confused about whether to be in two- or four-wheel drive or where to send the drive and, not being able to see the terrain, make wrong “decisions”. It’s stuck and so, probably, is the vehicle.
A word of caution for shoppers: some AWDs and even 4WDs (such as the Jeep Grand Cherokee) come in 2WD as well as 4WD versions. These became popular because buyers reckoned there was no point hauling around extra weight and complication — and paying more — if they thought they wouldn’t need 4WD. They can make a lot of sense for owners who plan to keep to the road.
After all those confusing and conflicting acronyms, it’s a good job there’s something out there that makes sense.
Weak link: the driver
The best way to make an AWD SUV go farther off-road is to drive it well.
The best way to do that is to take a specialised course.
But for now, adopt the off-road driving adage: as slow as possible, as fast as necessary.
In other words, pointing your crossover at a track obstacle and giving it heaps is not likely to end well. On the other hand, momentum may be your best friend on dry, loose sand.
Know your vehicle. Look underneath and see what bits are vulnerable to damage. Note how long the front and rear overhangs are. Take time to assess an obstacle; look for the easiest way through, have a passenger get out and act as a guide, if you’re fairly sure the vehicle won’t make it, don’t try unless you’re with at least one other vehicle, have proper recovery equipment and know how to use it.
Carry some basic self-recovery equipment, even just a spade and/or shovel and a piece of wood to act as a base for the jack. Know how to use the jack. Start worrying when the cellphone signal fades. If you’re beach driving, allow plenty of time to get back.
What to look for
If you think you might want to do some off-roading, get a proper 4WD with low-range gearing.
But if you must get an AWD SUV and fancy a bit of adventure, look for:
• Good ground clearance.
• Short hangovers between bumpers and wheels.
• A drive system that can be locked in all-wheel-drive.
• A body with the fewest protrusions that can be easily damaged.
• Tyres with a bold tread and high profile – but you’re unlikely to find anything taller than a 65-series.