4WD Systems Are More Complex Than You Think
Watch as Engineering Explained runs through the details of the Ford F-150 Raptorby Mark McNabb, on
The Ford F-150 Raptor is the undisputed king of off-roading pickups – at least in the full-size segment. And now with the Raptor having entered its second generation for 2017, Ford upped the ante with whiz-bang tech that makes this Baja blaster even more capable in nearly every driving condition. Much of that tech centers on the Raptor’s 4WD system. But rather than some dogleg shifter protruding from the floor, the Raptor’s transfer case is multi-talented. It can be both 4WD and AWD, not to mention having modes for RWD and neutral. Hang on to your thinking caps, kids.
In order to make the Raptor a true all-terrain monster – meaning good on-road performance and good off-road performance no matter the type of terrain – Ford developed a special transfer case that intelligently distributes power to where it’s needed. There are four main modes, all of which are electronically controlled via both the dash-mounted knob and the Raptor’s drive modes. While the Raptor is the only vehicle that currently employs this exact system, other vehicles have had similar drivetrain setups with automatic 4WD. General Motors debuted its Autotrac system in the late 1990s. It could operate in RWD, 4WD, or a 4WD Auto mode. Likewise, the Toyota 4Runner offers a full-time 4WD system with a similar operation, minus a RWD mode. Technically speaking, this means the 4Runners has a full-time 4WD system rather than a part-time 4WD system. Land Rovers are perhaps the best-known SUV for having a full-time 4WD system. All come standard with a computer-controlled full-time system that diverts power to whatever wheels have traction. Many models go a step further by adding a 4WD Low range, which uses different gears within the transfer case to multiply torque that’s sent to the wheels. 4WD Low is best for low-speed crawling or pulling a heavy load at low speeds over a short distance, like recovering a stuck vehicle or pulling a tree off a trail.
But how’s the 2017 Ford F-150 Raptor’s 4WD system work? Glad you asked; Keep reading for more – and that informative video from Engineering Explained.
Breaking Down the Raptor’s 4WD System
The 2017 Ford F-150 Raptor has four main 4WD setting. These encompass both electronic setting and gear selections within the computer-controlled transfer case. These settings are 2 High, 4 Auto, 4 High, and 4 Low.
The 2017 Ford F-150 Raptor has four main 4WD setting
Naturally, 2 High sends power only to the rear wheels, regardless of wheel slip. However, the magic starts happening in 4 Auto. This mode engages the locking front hubs so the front axle shafts, differential, and drive shaft are spinning the same speed as the front tires. A wet multi-plate clutch inside the T-case is left as the sole break in connection to the engine’s torque. Should wheel slip occur and the computer determines more traction is needed, this clutch can partially or fully engage to send power to the front wheels. 4 Auto is great for bad weather conditions like rain or snow. Not only does it provide extra traction, but that multi-plate clutch allows for slip between the front and rear axles. Without this slip, the driveline can bind under pressure, which causes wheel scrub on dry pavement. Obviously, this isn’t good for either the driveline components or tires.
4 High completely locks the 4WD system into use. Like an old-school transfer case, this mode equally splits engine power to the front and rear axles – something that’s called a 50/50 torque split. This is great for off-roading, since wheel spin does not affect the power delivery between the axles. This mode can also be engaged, disengaged, and used at any speed.
4 Low is for more serious off-road situations. The transfer case uses different gears to multiply engine torque to provide greater turning force at each wheel. This is great for rock crawling, climbing a steep hill, or pulling something heavy like a boat out of the water at a boat ramp. But because of this torque multiplication and gearing limitations, 4 Low isn’t for going fast. Most vehicles with 4 Low engaged top out around 25 to 30 mph, with some even lower.
While the Ford Raptor isn’t the first to employ a full-time 4WD system with a 2WD mode, it certainly has the most complex and feature-laden. That’s part of why the second-generation Raptor is so good.
The Confusing Terminology
Asking someone to explain the difference between 4WD and AWD – or even full-time and part-time 4WD – is often met with a blank stare. Hopefully, this will clear things up.
Part-time 4WD is the oldest and least advanced type of 4WD system
Part-Time 4WD: This is the oldest and least advanced type of 4WD system in vehicles like trucks and traditional SUVs. It uses a transfer case that only engages the front axle when the driver shifts it into gear, hints the name “part-time.” In regular driving, only two wheels are turning (most commonly, the rear wheels). When 4WD is engaged, the transfer case is locked into a 50/50 torque split, with the engine’s power equally divided between the front and rear axles. Low-range gears allow for torque multiplication, increasing torque to the wheels. Modern vehicles that use this system include the Jeep Wrangler and pickups with manually operated transfer cases like the Ram Power Wagon.
Full-Time 4WD: This 4WD system is similar to all-wheel drive (AWD) in that all four wheels are getting engine power almost all the time. However, unlike AWD, a full-time 4WD system offers low-range gears. The driver makes the choice if/when to engage 4 Low. Vehicles like Land Rover Discovery, Range Rover, and the higher-spec Toyota 4Runners mentioned above use this type of 4WD system.
Yet even with these 4WD and AWD systems neatly categorized, there are some systems that defy these classifications.
All-Wheel Drive: An AWD system powers all four wheels regardless of road or weather conditions. While AWD systems can vary the percentage of torque split between the front and rear axles, the system never disconnects one axle or another. More modern systems like Subaru’s Symmetrical AWD system even actively vary the percentage of torque split between the left and right, sending power to the outside wheel in a turn, helping the vehicle rotate around a curve. This is called torque vectoring.
Part-Time AWD: Many of today’s crossover and AWD cars use a part-time AWD system. This type of system allows all four wheels to receive power, but automatically and seamlessly disconnects one of the axles to boost fuel economy. After all, turning extra components and wheels uses fuel. The 2017 Jeep Compass is offered with two types of AWD/4WD systems beyond its base front-wheel-drive configuration. The available Active Drive system defaults to FWD mode in normal driving conditions, but will send power to all four wheels if traction becomes an issue – all with no driver intervention. Active Drive does offer a “4WD Lock” mode the driver can engage should he desire the rear axle to remain under power.
Yet even with these 4WD and AWD systems neatly categorized, there are some systems that defy these classifications. The 2017 Jeep Compass’ other 4WD system, Active Drive Low, is a perfect example. Not only is it a part-time AWD system, but it also incorporates low-range gearing. This means ADL hovers between a part-time AWD system and a part-time 4WD system. Technically, it could be classified as an automatic part-time 4WD system. Boy, this does get tricky, right?
So, what do you think? Did you learn anything about AWD and 4WD systems? Let us know in the comments below.
Read our full review on the Ford F-150 Raptor