Four wheel drive, all wheel drive, 4WD, AWD, full-time, part-time,
4Hi, 4Lo, 4×4. There are many names and just as many ways of motivating
every wheel a vehicle has on the ground. What’s the difference between
four-wheel-drive and all-wheel-drive? In one word: Marketing. Want to
know more? Click past the jump as we dive in the most controversial
topic since “Dodge vs Chevy.”
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 1970 Land Rover Range Rover (above), the
self-proclaimed “first mass-produced vehicle with full-time AWD.” (Note
they didn’t call it 4WD until later.) The system used a lockable center
differential that allowed the front and rear axles to spin at different
rates on pavement allowing the system to be engaged at all times. The
system was designed with off-roading in mind, so the transfer case had a
low range like like the rugged truck based systems at the time in
addition to the full-lock feature.
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.
Then Audi released the Quattro in 1980, but despite featuring a
manually lockable center differential, Audi chose to call it
“all-wheel-drive” or AWD. (Later Quattros were automatic.) The AWD vs
4WD differentiation was born. Soon everyone was getting into the
four-wheel-motivation game but nobody agreed what to call the systems.
In 1982 Fiat introduced the world to the first four-wheel-motivated
vehicle with a transverse engine layout and a transaxle (the Fiat Panda
4×4). It was the start of a revolution. Some car companies followed
Audi’s suit and referred to car systems as AWD while the Toyota Tercel,
Dodge Colt and others sported 4WD or 4×4 labels. This was the start of
the “that’s not four-wheel-drive, that’s all-wheel-drive” argument.
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.
The 21st Century
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?
The Current State of Affairs
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.
Part time locking systems with a longitudinal layout
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.
Full time systems with a longitudinal layout
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.
Subaru and Audi you ask? Yes indeed. Audi’s longitudinal systems and
Subaru’s AWD systems claim to be different or superior to the
competition, but in reality
the only difference
is that they merge the center and front differentials into the
transmission housing resulting in a space savings, but not necessarily a
weight savings. (Mercedes claims 4Matic will take a scant 150lb toll in
2014, 50lbs lighter than Quattro.) This also means that the Subaru
systems share design elements with traditional rugged body-on-frame
SUVs, something that Subaru owners seem to rarely know but might want to
brag about.
Transverse engine based systems
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.)
While there are exceptions to this rule, 99% of transverse FWD
systems have a fundamental difference from longitudinal systems because
of the integrated front differential. Instead of creating a purpose
built AWD transaxle, what car makers do is just extend the power output
of the transmission (before the differential) out of the transmission
case and into the angle gear that sends power to the back. (See the
diagram above.) This means that the input to the front and rear
differentials are tied, just like a part-time locking system that we
discussed above. To keep the system from binding and improve fuel
economy a clutch pack or a viscus coupling is placed between the angle
gear and the rear differential. This allows the rear wheels to be
uncoupled, but does nothing about the front wheels. Systems like this
are incapable of sending more than 50% of the power to the rear unless
the front wheels have zero traction. Acura’s SH-AWD system takes things
one step further and uses an “acceleration device” aft of the clutch
pack to make the rear wheels spin
faster than the front wheels thereby giving the vehicle a slight rear “bias” even when the front wheels have traction.
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.
