Cool Cars & Tech
Both all-wheel drive (AWD) and four-wheel drive (4WD) are meant to help vehicle handling through inclement road conditions or otherwise unpaved terrain, but there has been a notable difference between the two. Lately, however, the disparity is narrowing as technology brings the systems together. Of course, that just adds to the confusion among consumers. What the heck is the difference in the first place?
Four-wheel drive (4WD) is a drive system with a transfer mechanism that delivers power in the form of “torque” to all four wheels at the same time. Power to the front axle and power to the rear axle is split again through a gear mechanism called a differential that allows wheels on the left and right to spin at different speeds. This allows a car to take a corner without grinding tires (the wheel on the inside of a turn travels less than the one on the outside, so needs to rotate less).
With all-wheel drive (AWD), on the other hand, power is available to all four wheels at a time as well, but only “on demand” thanks to a handy-dandy differential mechanism. Thus, AWD is actually two-wheel drive (2WD) – usually front-wheel drive on smaller vehicles, rear-wheel drive on crossovers, trucks and SUVs – unless the vehicle senses a loss of traction on a tire or two, whence it throws some torque to the other pair of wheels to keep the lot of them rolling. And it does all of this automatically without any input from the driver.
AWD can be used on any road surface due to the control provided by that centre differential. As wheels only get power when others are slipping, problems such as “driveline bending,” wheel hop and other issues typical of Ye Olde 4WD are avoided. On 4WD vehicles, you actually only want 4WD activated when driving off road in mud or loose dirt and gravel, or in inclement conditions as with snow and ice.
On dry pavement with 4WD active, the wheels tend to fight each other, with slower wheels front or back getting dragged or pushed by the faster ones. Not a problem when a wheel is being pushed or dragged out of some slippery mud or snow to fall into step with the others, but a heck of strain when all tires have a solid grip on the road. Something has to give, and that something is either a driveline bent or wheels hopping and chirping.
Neither system is perfect. While 4WD is only appropriate off-road or in inclement weather conditions, AWD is wimpier when the going gets tough in that it does not have a low gear option. 4WD, on the other hand, has two gear modes, low and high, with low being bullishly powerful but slow – good for mountain goat trails and pulling tree stumps.
A vehicle is better served with AWD on roads with only typical traction issues – the issues most drivers face day to day: a snow day (season) here, some icy patches there, the odd slick hither and yon, but a non-bumpy road underneath all the time. While newer AWD systems are starting to accommodate harsher off-road conditions such as mud, sand and rocks, those are still typically best left to 4WD vehicles.
Still, while AWD and 4WD are distinct, they share enough similarities to cause confusion for consumers.
Both 4WD and AWD provide a certain peace of mind when driving in weather. The 4WD system just requires a conscious decision to use it. “All-Wheel-Drive is typically a system that doesn’t require the customer to do anything,” said Jeff Williamson, an engineer in GM’s Powertrain division. “A Four-Wheel-Drive system typically will require a [driver] to pull a lever or button during a transfer case to select a mode.”
While 4WD offers a little more flexibility in terms of road surfaces, manufacturers are moving toward AWD or a hybrid 2/4-wheel drive where changes in drive settings are made by the car, not the driver. “Drivers want the confidence that comes with traction and stability but don’t want to do anything,” Williamson claimed.
In something of a middle ground, newer vehicles such as the Ford Explorer give the driver scenarios to choose from instead of a single mechanical decision between 4WD high and 4WD low. The Explorer has a knob with icons describing clear road, sand, mud and ruts, and snow. The driver can easily select the best mode based on the observed conditions and then let the vehicle figure out how to dole out the torque. (If a car could see or feel the slippery section ahead, then drivers wouldn’t be needed at all…)
In the case of GM’s Cadillac line, the sedans and crossovers use a “full-time active drive” system that falls under the AWD category. For its Escalade hybrid, the manufacturer uses a transfer case that can switch to 2-wheel drive mode to manage fuel economy.
Across the board, things are moving toward an AWD direction in the sense that operations are becoming more and more automated, less driver-operated. That puts the control in computer chips and software systems.
It also means that 4WD capabilities aren’t going anywhere, but they are becoming “active” drive systems; aware when they’re not needed even if the driver doesn’t. “Active systems are pretty much the wave of the future,” explained Williamson.
Among manufacturers, the differences in drive systems are typically in the software. “The magic’s in the software and the calibration that separates these things,” explained Hampden Tener, Cadillac product director with GM. Tener asserted that it’s now software, not hardware that plays the key role.
To the typical buyer, just know that when counting “all wheels” on a vehicle, you’ll find 4. If they’re driven by an AWD system, that’s great; better traction and handling all around, through thick and thin. If it has a 4WD system, all the better, but only if that 4WD is “active” or “on demand,” which makes it an AWD in disguise with the added ability to get low and bullish.
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