Cooley On Cars
AWD systems explainedFrom 4-wheel drive to all-wheel drive, CNET's Brian Cooley explains how they have evolved and the differences between them.
-In the beginning, there was four-wheel drive in 4 x 4-- tough off-roaders like early jeeps or dodge power wagons. In their guts was a second gear box called the transfer case that split the power coming out of the transmission, sending it for an [unk]. And it was controlled by a second gear lever inside. And then there were geared hubs, you had to get out and lock by hand for max off-road traction. Driving all-wheels then was not for the faint of heart. But today, it's a highly sought after feature in cars that's different from a jeep as this Jag. Why? Well, two main reasons neither of which will necessarily ever take you off the road. Weather, in the snow all-wheel drive is much better than rear-wheel drive or even front-wheel drive to get power through the snow to the road. Performance, all-wheel drive is much better taking all the engine's power and getting it to the road through the maximum number of wheels to get around the corner fastest or just off the line. This Jaguar XJ is a good example of both modes. Its all-wheel drive system biases the power mostly to the rear wheels by default. But when wheel slip is detected, up to 50 percent of the cars power is sent to the front wheels usually before you even know it's needed there. Rear-wheel drive is the classic layout for great handling, except in snow where it tends to spin and slip and leave you stranded. Most cars today are front-wheel drive mostly for efficiency reasons, but that also does tend to land better snow traction. But both the two-wheel drive systems leave half the wheels largely unused and both suffer from cornering issues, either over steer when the rear of the car wants to come around in a corner, or under steer, when the front end sort of plows to the outside of a corner. Enter all-wheel drive. You know Subaru for their obsession with all-wheel drive today, but you may not know they started all this-- 1971, the Leone-- The first regular car that had all-wheel drive stuck under it. For the first time, you could drive something that wasn't a truck and get out of almost any kind of trouble on the road. Then in 1980, all-wheel drive got cool, with the arrival of the Audi Quattro and its eponymous all-wheel drive system. It's rally car DNA completely repositioned all-wheel drive has something you didn't just use a few days in the winter. It opened consumer's eyes that all-wheel drive is the performance thing. Then in 2004, Honda brings out super handling all-wheel drive, the first that could bias power not just between the front and rear wheel sets, but also differently across the left and right rear wheels using power for cornering in a completely new way that is still cutting edge. -Simultaneous and continuous control ensures that the optimum amount of torque is always distributed to each of the 4 wheels. -Now this 13 Pathfinder's a good example of how cars have technology that allows all-wheel drive to become much more gentrified, to be honest. You don't need to understand anything about low range or locking hubs to get the most out of the system. Check it out. Here's what it's come to, you got a knob now that handles all-wheel drive. No more big old levers and transfer case handles. This thing defaults to auto which is very telling. You can also roll it two-wheel drive or put it in lock if you even know what that's for. Typically, you leave it in auto, it'll figure out the rest. I first saw this on Land Rovers. They have-- what they call terrain response control. You rotate this knob to rock or mud or snow. Jeeps do something similar but eventually, I think this highly simplified control is the real harbinger of the future and after this, the computer will figure out what you're stuck in and it will do that knob's job for you. Okay, now a reality check, all-wheel drive is not just milk and honey, and traction and cornering, it's got some downsides. First, cost. All-wheel drive adds a fair amount of serious hardware over front or rear-wheel drive, and that can add a couple thousand bucks or more to the MSRP. Complexity, all-wheel drive does create more to go wrong and more than has to be shoe-horned underneath the belly of a car. Efficiency, all-wheel drive cars may give up an MPG or 3 on the highway or on city fuel economy due to both added weight as well as more hardware to be turning all the time, depending how the system is designed. But rear-wheel drive will still have its place in particularly performance automobiles, front-wheel drive will remain the mainstream drive system for most cars. But all-wheel is getting closer to being there-- partly because the cost is now fairly modest to add it on, and partly because it's become so much easier to use. These seamless, digitized, transparent sort of think-for-you systems have made all-wheel drive a lot more approachable for a lot more drivers. If it weren't for the cost penalty and some MPG penalty, it would be the pinnacle of getting power to the road.