Repost from http://www.thetruthaboutcars.com
Motivating four wheels in a car isn’t new, we’ve been doing it for over 110 years. If you thought this was a recent affectation, you’re not crazy. Over the last 30 years there has been an explosion in the number of vehicles powering a quartet of tires. There has also been a similar explosion in the number of ways we power four wheels. At the same time the way systems are designed, marketed and used have converged and with them the terms AWD and 4WD have have practically merged. Of course, the SAE does have a definition “an all-wheel-drive vehicle is one that has an on-demand feature that occasionally sends power to the non-primary powered wheels.” But what that means has changed a great deal over time.
The Good Old Days
Let’s set the way-back-machine to 1970. Trucks and “Jeeps” had 4WD aka 4×4 systems. The system had to be engaged manually once you were on a loose surface because they “locked” the inputs of the front and rear differentials together making turning difficult on high traction surfaces. Engaging AWD on pavement could result in damage to the systems, or at the very least strange road manners. These systems were found on vehicles that would otherwise be RWD like trucks and truck-based “things.” Frequently the transfer case featured a reduction gear for more severe situations.
Then came the AMC Eagle. AMC jammed a new NP119 transfer case made by New Process Gear behind a Chrysler transmission. The unit featured a viscus coupling to the front axle that would allow power to flow to both axles simultaneously while still allowing them to turn at different rates. But this AMC wasn’t a truck, didn’t have a low-range and had an independent front suspension. Not knowing what to call it, AMC called it 4×4. So much for standards.
By the ’90s SUVs started to roam the land. The box-on-frame creatures borrowed their drivetrains from truck parts bins and brought with them 4×4 and 4WD monikers. (And a bevy of full-time and part-time systems.) Meanwhile, the proliferation of AWD systems exploded and we soon started seeing them in everything from Chrysler minivans to the Porsche 993. Despite the proliferation, the industry had more-or-less settled on calling longitudinal “truck” systems 4WD/4×4 and “car” systems (especially transverse systems) AWD.
Crossovers happened. In truth the crossover was born in the 20th century, but the era of the “modern crossover” dawned within the last 15 years. In 1995, crossovers were a microscopic segment composed of jacked-up station wagons. By 2005 the non-truck utility vehicles accounted for more than 50% of the segment. At the dawn of the 2014 model year there are few “traditional” SUVs left, especially in the volume mid-sized segment. Those that remain account for a minority of sales.
Back to the marketing. Now, more than ever, the lines between truck and car are being blurred by marketing speak. Ford calls their Explorer AWD while Nissan is claiming the Pathfinder had 4WD and Chrysler says the Jeep Patriot is a 4×4. The truth is all three drivetrains operate on the same general design as that 1982 Fiat Panda: the transverse AWD system. The system Fiat called “4×4″ in the 1980s is now thought of as “AWD” by Fiat in this decade. What gives?
This brings us to the present. Now that we know the AWD vs 4WD vs 4×4 battle is a war of marketing speak, and we have a bit of history under our belts. Let’s talk about how AWD systems work. Why? Because it’s more important to know how the systems work than what they are called. Let’s go over them one by one. Since I’m not a graphic artist I’ll toss in a rough power-flow diagram to show how each system works.
In the picture above we have a traditional “truck” system, the one that some people will call a “real 4×4.” There is no center differential so the system shouldn’t be used on-pavement because the front and rear axles cannot spin at different speeds. The system has to be engaged by the user in some manner, either with a lever or a button. Most systems use a chain drive to connect the front and rear axles so power flow is (in theory) locked 50/50 front/rear. If one rear wheel is freely spinning, the front wheels will still have grip. If one front wheel and one rear wheel freely spin, the vehicle won’t move. To solve that problem the systems usually include some form of locking or limited slip differential in the rear or both rear and front axles. The systems are typically very rugged and if the system employs fully-locking axles on the front and rear power is exactly 25/25/25/25 percent wheel to wheel and if three wheels lost traction the remaining wheel can consume all 100% of available power. Some systems integrate a low-ratio reduction gear into the transfer case.
Based on the part-time systems we just talked about, Land Rover was the first company to use an existing idea to improve their new luxury off-roader and added a center differential after the reduction gear. This system became all the rage after AMC brought it to the mainstream in 1979 for the 1980 Eagle. These systems can take a variety of different forms. The “center differential” can be a simple open unit, a limited slip, a Torsen that apportions power unequally (i.e. 75% rear, 25% front unless slip occurs) or a simple viscous coupling which isn’t technically a differential at all. Each type of stem has benefits and drawbacks depending on your application. Open diffs apportion power equally, but if the front or rear wheels loose traction the car can’t send power to the other axle. Limited slip systems (including manual or auto-locking units) can connect the front and rear together, thus operating like a part-time system when the unit is fully engaged. If the system engaged on pavement however you can get a “binding” feel in tight turns. Torsen units are primarily used in performance oriented systems like high-performance variants of SUVs where you want added traction but a decidedly RWD bias.
You’ll find full-time systems of some description in the current Audi Q7, Jeep Grand Cherokee/Wrangler, Mercedes ML/GL/GLK/G, BMW X1/X3/X5/X6, GM’s full-size SUVs, Dodge Durango, Infiniti EX/FX, Land Rover LR4/Range Rover/Range Rover Sport, Lexus GX/LX, Nissan Armada, Porsche Cayenne, Volkswagen Touareg, Subaru Forester/Tribeca/Outback/XV, Toyota FJ/Land Cruiser/4 Runner/Sequoia.
Is that a long list? Yes. However that a complete list (insofar as I know) of SUVs currently sold on our shores with this type of a system. Why did I bother to list them all? Because it shows how few of this type of system there really are in the utility vehicle segment. Just a few years ago this number was higher and the market share of this system was higher still.
British Motor Corporation popularized transverse engine front-wheel-drive systems in 1959 with the launch of the original Mini. The drivetrain layout has been so popular that the same basic design is used by 16 of the top 20 best-selling vehicles in America. (Everything but the full-size pickups on the top-20 list.) This drivetrain layout represented a challenge to AWD development, so it wasn’t until 1982 that Steyr-Daimler-Puch produced a four-wheel motivation system based on a transaxle. (For that Fiat Panda.)
What’s a transaxle? Excellent question. A transaxle is a transmission that integrates a front differential into its casing. That’s an important thing to keep in mind because the transaxle is why FWD layouts are preferred for fuel economy. In a transverse transaxle the power doesn’t have to “turn” 90 degrees to spin the front wheels. HOWEVER, in a transverse transaxle based AWD system, the power has to make two 90 degree turns on its way to the rear wheels. First power leaves the transmission, then heads to an angle gear which sends it to the back. Then power flows to the rear differential which turns power 90 degrees to the wheels. This is part of the reason that transverse full-time systems that always send power to the rear are [in general] just as efficient as longitudinal “RWD based” AWD systems. (This is why most of them disconnect the rear wheels whenever possible.)
Transverse systems come in many different flavors so it’s important to know what you’re buying before you sign on the line. Some systems on the market are “slip-and-grip” systems like the Honda CR-V which won’t lock the center clutch pack unless front wheel slip occurs. Then we have systems like the Ford Explorer which usually sends some power to the rear, locks the coupling during hard acceleration and varies it depending on vehicle dynamics. The Honda Ridgeline allows the center coupling to be locked in first gear while Jeep’s Patriot allows the center coupling to be almost fully locked at all speeds.
I’ll close by posing a question: If my 2001 GMC Envoy (GMT360 SUV) with its two-speed transfer case and locking center differential can be considered a 4WD/4×4 vehicle. What is the Cherokee? AWD or 4WD? With 4-Low range and a locking rear differential it meets all the traditional requirements, but under the hood you’ll find a four-cylinder or V6 engine sitting sideways. This author’s humble opinion is that the name doesn’t matter if the vehicle does what you expect of it. That Cherokee? We’ll have to wait and see but I suspect it will be as capable as a Grand Cherokee mostly thanks to a substantially lighter curb weight.