A Confusing Mess
While SUVs have fallen out of favor of late, added traction has not. There is simply no substitute for the added capability to reach remote campsites, navigate unimproved roads or provide confidence when the weather turns nasty.
But buyers can be forgiven for not knowing what the systems do, or which system best fits their lifestyle. Symmetrical All-Wheel Drive? Selec-Trac II Full-Time Four-Wheel Drive? How is a consumer to choose between the myriad systems?
Much of the blame can be placed at the feet of auto manufacturers and their endless need to differentiate their vehicles in the marketplace, despite the fact that there are only a few basic system designs.
The first step in unraveling this ball of confusion is to separate four-wheel drive from all-wheel drive.
AWD vs. 4WD
Differentiating between these two types of systems used to be easy: all-wheel drive allowed no driver interaction and was on cars; four-wheel drive allowed the driver to shift in and out and was primarily on trucks. It’s just not that simple anymore. Today, many SUVs have what is essentially an all-wheel-drive system, but labeled as four-wheel drive system.
Forget what the manufacturer calls the system. Quite a few experts agree on this definition for an all-wheel drive: a system that requires no driver interaction to send power to all four wheels, has no four-wheel-drive “low” setting, and is meant primarily for on-road use. A four-wheel-drive system, on the other hand, typically features a four-wheel-drive “low” setting that aids some kind of off-road use. What’s a four-wheel-drive “low” – often called “4LO” or “4LOW” – setting? It’s simply another set of gears that multiply the engine torque even further to help in severe low-traction situations. A four-wheel-drive “low” setting is essential for true off-roading.
I think you’ll see by this definition that many of the so-called four-wheel-drive systems on the market are actually all-wheel drive instead. That’s ok, since few of these vehicles really need the extra torque the “low” setting provides.
Even with the differences between all-wheel-drive and four-wheel-drive systems, the actual operation can be similar when you get to the most sophisticated systems. But we need to get the least sophisticated system out of the way first.
This less sophisticated system is only on some four-wheel-drive vehicles. Since it needs to be shifted in and out of four-wheel drive, it is by definition not an all-wheel-drive system. It’s called “part time” because you can only use it part of the time; namely when the road or trail you are driving on is slippery or loose – on snow, ice, gravel or off-road trails, for instance. You cannot use this system in everyday driving on dry pavement.
The part-time system makes a solid connection between the wheels on the front and rear axles. Unfortunately, the wheels need to turn at different speeds when going around corners – the outside wheels actually travel farther than the inside wheels. There is no allowance in this system for that to happen, so the wheels need to “slip” to prevent eventual damage to the driveline and tires. Unless you plan on driving only in straight lines, don’t engage part-time four-wheel drive on dry pavement.
That limitation means you can’t be ready for constantly changing road conditions. Most buyers want a system that does the thinking for them. This is not that system.
So who would want this kind of system? Buyers on a budget who don’t mind shifting in and out of four-wheel-drive, and serious off-roading types. Off-roaders appreciate the proven, simple design and ability of the system to “lock” the front and rear axle speeds together. That’s why Jeep Wrangler still has a part-time system to this day.
Automatic Systems: Full-Time and On-Demand
The automatic systems – whether all-wheel drive or four-wheel drive – are what most buyers want today. They provide “set-it-and-forget-it” convenience in all road conditions. There are some differences between the two systems on how they get to that point.
Whether the system is full-time four-wheel drive or full-time all-wheel drive, it generally means that all four wheels have power directed to them even during everyday driving.
This type of system operates primarily in a two-wheel-drive mode – either front-wheel drive of rear-wheel drive – until more traction is needed.
Modern full-time and on-demand systems can transfer the power between the front and rear wheels as traction needs dictate. How they do that shouldn’t matter too much to buyers. Whether they use a viscous coupling or an electro-magnetic clutch, it’s mostly a matter of nuance.
Which automatic system is better? I would test drive both systems under slippery conditions, if possible, and judge for yourself. Most systems today transfer power so quickly it might be hard to choose between these two. Some experts like the full-time systems better because they are a bit quicker to respond to changing conditions.
Determining A Vehicle’s System
Make sure you know what kind of system you are getting beforehand, so there are no regrets after the purchase. The marketing names make it devilishly difficult to pin down the operational details of system. You simply have to go to the manufacturer’s Web site and look for the key words that describe the system operation. Don’t get sidetracked by marketing-speak, focus on the operational details.
For instance, after doing a Google search and clicking on the area entitled “What is Subaru Symmetrical AWD?” from the company’s Web site, you would think the answer to that question would be readily found, right? Sort of. After digging through the info you learn that Symmetrical AWD has nothing to do with system operation; rather, it is the balance of the system components. I could find nothing about the operation of the system. Finally, I came across a Subaru press release from July 2008 that said, “Subaru employs four versions of Symmetrical AWD system across its model line. The type used in the WRX utilizes a viscous coupling locking center differential that distributes torque 50:50 front/rear. Should slippage occur, the system transfers more power to the wheels with the best traction.” Bingo. It’s a full-time system. It’s just a shame that the information is so deeply hidden.
As a further caution, I’ve seen systems that are referred to as “full-time, on-demand.” What? I know they are implying that it’s a full-time system that can change how much power goes where “on-demand,” but could they be any more confusing? Again, ignore the names and focus on how the system operates.
Which System for You?
Only you can decide which system is right for you. If don’t want to worry about whether you have the correct amount of traction for the given road conditions, then an automatic system – whether four-wheel drive or all-wheel drive – is right for you. If you don’t mind shifting in and out of four-wheel drive, and you do some off-roading, then a part-time system might save you some money.
As for the difference between four-wheel drive and all-wheel drive, you might not have a choice with the vehicles you are looking at. Still, you might want a four-wheel-drive system with a four “low” setting if you do serious off-roading or otherwise need the added pulling power on slippery surfaces – when maneuvering a boat trailer on a slippery boat launching ramp, for instance.
Whatever you decide, do some research on the Internet before visiting the showroom. If you can’t find the system operation details on the manufacturer’s site you, find a review of the vehicle from a reputable enthusiast magazine like Road & Track, Car and Driver, Motor Trend or Petersen’s 4WD and Off-Road. Doing that will add a level of confidence to your buying decision and ensure that you’ve chosen the right system for your needs.