Life used to be so simple. Pickups looked like pickups. SUVs looked like SUVs. Everything else was a car. And when it came to 4WD and AWD, one was serious, and the other was a gimmick. All that has changed. Today, there are so many fractured segments, subsegments, different styles, and looks that some people might consider a certain vehicle a car, while others call it a four-wheel drive, and still others think it's a crossover. And in one way or another, they could all be right. Here, we take a look at the basic assets and strengths of 4WD and AWD systems, discuss the major players in each category, and suggest what to keep in mind when using these systems to the best of their capabilities. In fact, the possibility exists that in the future our current naming choices won't be adequate to distinguish the different types of powertrain systems. Maybe simply calling something 4WD or AWD (or even 4x4) isn't good enough. Jeep's Grand Cherokee uses a full-time 4WD system with a dedicated transfer case that offers several terrain settings. Selec-Terrain has a setup similar to Ford (with Snow, Sand & Mud, Rock, and Normal) with an added Sport setting. This allows the Grand Cherokee to be a better, more spirited street driver and back-country explorer.
4WD
The 4WD vehicles were simpler. The output shaft from the transmission went into a transfer case where the engine power was split between front and rear driveshafts. In 2WD, the rear driveshaft turned a pinion inside the rear differential, spinning a ring gear that turned the right and left axle shafts, eventually turning each wheel. With the transfer case lever engaged in 4High or 4Low, a fork would engage a chain drive, turning the front driveshaft and pinion (at the same speed as the rear driveshaft) in the front axle's differential, also turning the ring gear and right and left front axle shafts, then the wheels. This is also referred to as a "part-time" 4WD system because the 4WD had to be manually engaged, and without a center differential it could be used only on low-grip terrain or driving surfaces. Four-wheel-drive systems are more complicated now, but the principles are the same. As you might expect, there are more
computers
and advanced electrical sensors in most of today's 4WD systems, but there are still a few brand-new vehicles that aren't that different from the originals (such as the Jeep Wrangler and Ram HD Power Wagon). Still, as we define it, a basic 4WD system will have some kind of dedicated transfer case with a separate low-range gear that allows the driver to multiply the transmission and ring-and-pinion gearing to provide for more (and slower) control of the vehicle. The slower speeds this crawl ratio (first gear x axle gear x low range) allows for more useable tractive force (or torque) to each tire, whether on rocks, snow, gravel, or a steep incline. The low-range gearing effectively acts as a torque multiplier, giving the vehicle more grunt at the wheels to climb over nastier terrain or obstacles. Of course, it doesn't hurt that many vehicles equipped with a dedicated transfer case also have a good amount of ground clearance and larger wheels and tires. Many who enjoy this type of driving for recreation often remove some air pressure from their tires at the trailhead to give them a larger contact patch and more sidewall flex on the terrain they want to navigate over. The new 4Runner offers two different four-wheel-drive systems, with this being full-time 4WD model. The dial is set to all-wheel-drive mode, designated at "H4F" for high range power to all wheels, all the time. Other settings include high range with engine power equally split to front and rear axles (center differential locked), and low range with engine power equally split to front and rear axle.
AWD
According to the Society of Automotive Engineers, an all-wheel-drive vehicle is one with an on-demand feature that occasionally or intermittently sends power to the non-primary powered wheels. The most basic systems often begin with a front-drive platform with some kind of power-splitting differential housed inside the transmission in or in a separate box. AWD is able to send rotational power through a prop shaft to a rear differential where power is directed to each of the rear wheels. In some cases, a certain percentage of engine power is sent to the rear wheels, until the computer sensor detects front wheelslip and redirects more power. Many crossovers or small SUVs were originally developed from passenger car platforms, so the AWD systems were adapted to work for the platform, rather than being designed specifically for that vehicle. Although this type of system doesn't offer an extra low-range gear, it does provide a good amount of traction in snow, rain, or icy conditions when the
computer system
can detect small and large amounts of wheelslip between front and rear wheels. Just as 4WD systems have benefitted from the advancement of added computer power, so have AWD systems, especially in the last decade. A typical AWD or 4WD drivetrain in a pickup truck or large SUV has the engine sending power through the transmission and into the transfer case, where the torque is split between the front and rear driveshafts, with transfer to the front usually handled by a chain.
