Delivering the Goods - May - 2018

All Wheel Drive Systems: A New Twist

When we mention the terms all wheel drive (AWD) and four-wheel drive (4WD), you might immediately think of a truck or SUV that’s capable of sure-footed traction in poor driving conditions. When we mention cars that have this capability, we think of Subaru, Audi, or maybe Mercedes.

In all cases, we envision a vehicle that’s more expensive, utilitarian, or comfortable, rather than nimble and exciting. There’s also an expectation of compromised fuel economy due to added components. But thanks to technology, the image of 4WD and AWD has changed.

Light duty AWD systems have introduced the advantages of traction and control to another class of vehicles. Small SUVs, compact cars, and sports cars are taking advantage of these new, lighter drivetrain components to give consumers more choices and new driving experiences.

All major US auto manufacturers have light-duty AWD vehicles in their lineup now, and the number of models is on the rise. Armed with this information, we need to know more about this technology and what makes it work, as well as service techniques, projected issues, and diagnostic procedures.

First, it’s fair to suggest that this technology has been with us, lurking in the background under the skin of the most expensive super cars and luxury vehicles. Though state of the art, they all lacked practical application that could be replicated in a more affordable package until now.

Fast forward to the present, inexpensive high-speed computers and CAN bus networking can seamlessly micro-manage all-wheel drive systems to provide a unique driving experience at a reasonable price. Let’s take a closer look at what’s available now.

A company called GKN is taking the automotive world by storm with an integral AWD system, managed by its own control module that can easily be incorporated into a powertrain control system. Ford, General Motors, and Range Rover are a few companies that presently have models that use this system.

The package offers different configurations that allow manufacturers to tailor their systems to meet their specifications for fuel management, traction, performance, and cost.

The computer control system was designed to be CAN friendly, so pairing with existing stability, traction control, and ABS systems takes just a meeting of the minds on the software level. The rest is fitting the drivetrain hardware beneath the vehicle and plumbing the wiring looms.

The GKN system offers two levels of driveline coupling control. One level offers a direct drive PTU transfer case with a full-time, live, rear drive axle. The rear differential coupling clutch-controlled system acts directly on each axle, eliminating the conventional spider gear setup (figure 1). This allows for minimal rolling resistance and a fuel savings while in 2WD mode by completely uncoupling the rear wheels.

The next level offers a coupling clutch-controlled system at the PTU. Adding this feature optimizes driveline control to maximize fuel economy by eliminating torque to the rear driveline during 2WD operation. With choices like these, the engineers are limited only by their imaginations. The Ford application is a noteworthy example.

The Ford engineers decided to use the Focus as a technological playground for this system. Under the badge of Focus RS, the GKN system couples to a 350 horsepower, 350 pound-foot torque engine, and a manual shift gearbox.

It uses the basic PTU with the Twinster rear end. Ford chose to use an overdriven rear end. In other words, the rear differential (full lock) ratio is higher than the transaxle final drive ratio. It is 1.8% higher! Sounds weird, huh? Well they did this to produce a driving sensation called torque vectoring.

What is torque vectoring? Simply put, it’s the process of counteracting the adverse steering conditions called understeer and oversteer. They achieve this by directing torque to the wheels that have the greatest traction and limiting torque to the ones with less traction, using the ABS brake system or an overdriven rear differential.

The concept is simple: Suppose you’re preparing to make a right turn. The natural tendency of the vehicle is to keep traveling in a straight line. The inertia of the vehicle shifts the weight of the vehicle to the left (figure 2), causing the wheels on the left to have greater traction than the wheels on the right. This creates an oversteer condition.

Applying the brakes to the right wheels applies more traction to the left wheels, which counteracts the oversteer. If the system applies greater brake pressure to the right rear wheel than the right front, then the left rear wheel can provide a “pushing” sensation to neutralize this feeling of oversteer.

With the addition of the Twinster coupling system, when the vehicle goes into a turn, the system can apply torque to the outer rear wheel to “push” the car into the turn. Since the rear ratio is higher than the front, the system can actively control the amount of push, based on driving conditions and sensor feedback.

The GNK controller uses data from wheel speed sensors, inertia sensors, the steering angle sensor, and other drive input related sensors to calculate how much torque to distribute to each wheel. It has a sample rate of about 300 times per second. This also decreases the need for using the brake system to create the vectoring effect.

Torque vectoring also comes into play on heavy acceleration. In this state, the weight of the vehicle shifts toward the rear, causing the rear wheels to be less likely to lose traction than the front. By applying as much torque as possible to the rear wheels, the driver gets a sensation of being pushed hard from behind while retaining a feel of full directional control.

The Focus RS system also has some fun features added. The all wheel drive controls are switchable from 4WD (normal), Sport, Track and Drift mode (figure 3). Each stage provides more aggressive handling characteristics.

Drift mode is totally new: Since the rear end ratio is higher than the front, the Twinster coupling system can increase the amount of torque delivered to the rear wheels, causing a controlled wheel spin in a turn, called drifting. The computer recognizes when the driver intends to induce a drift condition. This mode is for off-road driving only.

So this is all wheel drive the likes of which we aren’t used to. This is all wheel drive on steroids! Knowing what good looks and feels like is very important for diagnosing this system properly. A customer complaint would most likely be very vague based on the “feel factor” that’s present with this system.

A fault would disable the system, returning the system to a typical, 2WD manual shift Focus with too much horsepower. If one or both Twinster coupling clutches were to fail (clutches welded on), it would create a bind that would render the vehicle undriveable.

As far as statistics and reality are concerned, this vehicle remains reasonably limber at a curb weight of 3470 pounds; about 500 pounds heavier than the base model. Servicing the rear differential and PTU units will prove challenging due to limited space (figure 4). Any repairs needed on the rear differential require you to drop the entire rear suspension as an assembly.

Technology is constantly changing and adding more conveniences and driveability aids. To accommodate those changes, you need to become familiar with these systems so that you can diagnose and repair them correctly. Knowing what looks and feels good is the beginning. I encourage you to visit your local dealerships to experience this cutting-edge technology for yourselves.