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Tech: Truck Traction

Electronic solutions to an age-old problem

Dan Carney
Sep 12, 2005
Knobbies, lifted suspension, winches, tow straps, and locking differentials are familiar tools of the off-road trade, but they're being significantly supplemented--and in some cases replaced--by a new generation of computer-controlled technology that can give highway-friendly trucks and SUVs serious trail capability.
Nowadays, computers can apply the brakes to restrain spinning wheels, directing power to the wheel that has grip. Until recently, such technology was only found on high-end luxury SUVs, but now affordable pickups like the Nissan Frontier and Toyota Tacoma offer traction-control systems. Myriad other tricks also are increasingly available on mainstream trucks and SUVs, including hill-descent control, hillstart control, remapped electronic throttle sensitivity, and electronically lockable differentials.
Photo 2/5   |   Nissan's solution to more traction is to include an electronically activated rear locking differential, only lockable when the vehicle is in low range. In addition, to prevent the Vehicle Dynamic Control system from cutting engine power during high or low-speed wheel slip, Nissan has also installed a "VDC Off" switch.
Purists may scoff at such electronic trickery as gadgets that don't measure up to the traditional hardware, but the electronic devices not only cost less than their mechanical counterparts, but often work better and add capability that's simply not possible with mechanical traction devices.
"I can only see the value of some of these things when I've been able to experience them off-road," concedes Paul Williamsen, an avid off-roader and curriculum development manager for the University of Toyota.
While enthusiasts like to rely on their own skill in sticky situations, "there are two things these systems can do that a driver can't," Williamsen adds. "They can act much more quickly, and, something that even the best driver can't do, these systems can act differently on all four wheels. Even the best among us can't do differential braking of the left-front and right-rear."
Such systems employ wheel-speed sensors used for antilock brakes to detect wheelspin, clamping the brakes to stop the offending wheel in a fraction of a rotation. "Of course, when you're driving off-road, the amount of wheelslip is one of the things you notice the most," says Mike Gabriel, manager of Jeep vehicle development.
Electronic systems have a lighter touch for more precise control than mechanical limited-slip differentials, according to Robert Beaver, manager of the system integration group at technology supplier Continental Teves. "Some of those have fairly low breakaway torques in situations like climbing a hill," when it's too easy for them to spin the wheel with less grip, he explains. Another benefit is that electronic systems can be cheaper and lighter than mechanical limited-slip differentials.
Photo 3/5   |   Many of today's most sophisticated transfer cases use an electronically controlled center differential (taking inputs from several different sensors from around the SUV) to control how much traction is sent to the front driveshaft. This center diff clutch pack is from a Grand Cherokee.
"The electronic limited-slip in the Grand Cherokee is a good example of that," says Gabriel. "Quadra-Trac II was a good system, but the differentials were controlled by wheelspin to drive the pump. Now [with electronic sensors, and not hydraulic pressure] we can detect lower levels of wheelspin and use electronics to drive the pump to transfer the power to the other wheels."
Ford trucks are gaining a smart transfer case to split power 50/50 between the front and rear axles, as well as send all of the available power to either end of the truck if needed, according to Alan Dona, driveline engineering manager for SUV body-on-frame at Ford. The computer-controlled transfer case, which was developed in partnership with Borg-Warner, was introduced on the Expedition and made its first pickup-truck appearance on the new Harley-Davidson F-150. It should move into other versions of the truck over time.
Alternately, the HUMMER H2 SUV and SUT use an open center differential, using electronic controls at each wheel to divvy up power when only one wheel has traction, says Bill Knapp, program engineering manager for Hummer H2. The rear differential also can be electronically locked.
Hill-descent control gives off-roaders a new tool for tackling steep, slippery hills. The system combines the low gearing traditionally used in such situations with automatic braking by the computer to keep the vehicle moving slowly while making sure it's pointed straight down the hill. Getting sideways while sliding down a steep grade is often a one-way ticket to a rollover, so keeping the wheels going the right way is critical. It's even easier to get crossed up when backing down a hill after an unsuccessful hill climb, and the same system works in reverse.
Once only available on high-dollar Land Rovers, hill-descent control now is available on more affordable models. The only problem with such a system is that its use takes an act of faith: Driving down a hill without your foot on the brake (after pressing the button to activate the system) can be unnerving. Touching the brake typically deactivates it, once again giving the driver direct control. Using the system properly means laying off the brakes in situations that appear to demand their use. But first you have to trust the truck won't start racing downhill.
Photo 4/5   |   The LR3 Terrain Response 4WD system uses a series of sensors to change ride height, locking differential settings (front, rear, center), transmission gearing, throttle response, and more, depending on the type of terrain you need to navigate. This is probably the most advanced electronic traction system available on any 4x4.
Calibrations vary from one manufacturer to another, but they typically limit downhill speed to 2-4 mph. Nissan's system works in high and low range, with a higher descent speed of 6.7 mph in high range, according to Jeff Ceccacci, Xterra and Pathfinder product planner.
Another trail-ride circumstance electronics can help with is the dreaded uphill stall. Getting started again without too much backward roll is important (and especially difficult when driving a manual transmission), because there might be something behind you that you could hit, or you may be on a slick surface with less traction. Restarting while rolling backward is a good way to spin wheels and break traction. Of course, the parking brake is always available in such situations, but the Frontier and Tacoma's hillstart systems automatically apply brake pressure when the truck's on a hill to prevent backward rolling. The computer monitors throttle position, vehicle speed, and brakeline pressure to produce smooth starts in troubling situations. The system gives the driver two seconds to get moving, then releases total pressure to avoid overheating the brake-control valves.
It might seem that truckmakers are conspiring to deprive skilled enthusiast drivers of direct control of their machines through the increased use of electronics, but, in fact, electronics can help give the driver even more control. Nissan and Hummer, among others, include a different throttle map when in low range.
Photo 5/5   |   Several manufacturers offer selectable locking rear differentials as part of their off-road option packages. Electronic lockers are designed to allow for 100 percent of the available torque (sent from the engine through the driveshaft) to distribute all available traction between the two tires.
With this map, the gas pedal requires more travel to produce the usual amount of throttle opening, resulting in less jerkiness and jump. That makes it easier for the driver to apply the right amount of throttle in precarious situations. And what enthusiast can complain about technology that makes us better drivers?
Flexible Electronics
by Dan Carney
Not all electronic off-road devices are in the powertrain. In many cases, better traction comes from improved suspension control. Case in point: Dodge's electronically disconnecting front anti-roll bar. Off-roaders have long known that to keep the tires on the ground, you need maximum articulation, which means disconnecting the anti-roll bars. Under normal conditions, it's the job of these bars to encourage the left and right tires to compress and extend at the same time to limit excessive lean through curves. But off-pavement, drivers want each wheel to do its own thing as much as possible, seeking maximum grip no matter how far the suspension might have to stretch to reach the ground. Some off-road enthusiasts have been known to whip out a wrench at the trailhead to disconnect the links and at the trail's end to reattach. But that's all changed--the new Dodge Power Wagon Ram offers a pushbutton anti-roll-bar disconnect.
An electromechanical coupling in the middle of the bar disengages the right and left halves when the button's pushed, and, because the system's electronic, it's easy to retain on-road handling by making the system too smart to disconnect at highway speeds. If you forget to press the button when heading off the trail and onto the highway, the computer will automatically reconnect the bars for you. So forget about "I, Robot" scenarios where machines try to spoil things for humans. These electronics are here to help us. Really.
The Dodge Ram Power Wagon (lead image) uses an electronic front anti-roll bar disconnect coupler to control how much movement, or flex, the front axle will have. The push-button disconnect is partnered with push-button electronic front and rear locking differentials to make the Power Wagon a formidable steep-trail climber.

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