Terrain Control Explained

New SUV systems make off-road driving easier.

The joke that's followed Ford's terrifically popular Explorer (six million have been sold) since day-one twenty years ago has been that owners never actually drive the off-road-equipped SUVs off the pavement. Marketers have been playing to a consumer's desire to have their friends think that they might, one day, be the type of person who would rock-crawl a mountain pass.

The truth is that the popularity of the off-road capable SUV in general has been about size, riding height and, for many, all-wheel or 4-wheel drive for getting through snow, or through unpaved roads.

But each generation Explorer has been equipped with real off-road equipment such as a two-speed transfer case as part of its four-wheel-drive system. The transfer case is additional to the transmission and provides ultra low gearing for steep, slow climbs over rocky terrain.

But many owners, says Amy Marentic, Ford's marketing manager for the Explorer, didn't know when or how to use the low range function of the transfer case. "I didn't want to damage the car," one Ford Explorer driver told Marentic. "I never knew what that switch was for." And so the capability went unused.

Several SUV makers have introduced systems that are easier on customers' brains this year, the newest are included in Ford's all-new 2011 Explorer, Jeep's all-new 2011 Grand Cherokee, and Land Rover's revised 2011 LR4 and Range Rover.

An All-Weather SUV For Soccer Moms

"Our customers do not rock climb," continues Marentic. "Customers are intimidated by the [2010 and earlier] four-wheel-drive system." That doesn't mean that Ford has turned the new Explorer into a pavement-only machine, even though it looks like a four-wheel-drive off-roader. In fact, the butch-looking Explorer shares drivelines and other hardware with the decidedly non-off-road Flex station wagon/ crossover and the Lincoln MKT luxury wagon.

Four-wheel-drive systems confuse SUV newbies, mostly because every system works differently. And there are dozens on the market. SUVs with four-wheel-drive (as well as so-called cross-overs and tall wagons) are generalized into categories like "Full-Time," "Part-Time," "All-Wheel-Drive," or "On-Demand." None of these labels are particularly descriptive of how they work. Some systems the driver has to select and some work automatically, changing between mostly front-wheel-drive and four-wheel-drive in a way that is invisible to the driver.

The types of terrain where SUVs can be driven are even more varied. "When our customers say 'off-road'," explains Jim Holland, chief engineer for the Explorer, "they mean they want to drive on a dirt road to the cabin." They don't mean they want to bushwhack over two-foot boulders or cross the Serengeti desert, he explains. For this type of off-roading, the Land Rover LR4 or Jeep Grand Cherokee are much better choices.

Electronic sensor technology and drive-line computers have made all of these SUVs easier for customers to choose where to drive off the pavement. Ford's is called Terrain Management. Land Rover's brand Terrain Response. And Jeep's system is dubbed Selec-Terrain. But these systems are not fully automatic--they still require the driver to make some selections.

The Explorer's makers have settled on four driving conditions for their Terrain Management system, which must be operated by the driver via a console-mounted switch to four conditions: Normal, Snow, Sand, and Mud/Ruts. The Explorer's four-wheel-drive system has no low-range for rock-crawling or bushwhacking. And instead of mechanical differential locks, it relies on individual wheel braking to control wheelspin. The Snow setting is also the best choice for ice, water, wet grass, and gravel. In this position, the shifts are slowed to prevent abrupt torque changes that could spin a tire, and the automatic transmission upshifts at lower engine speeds. The Sand setting is for, not surprisingly, driving on the beach, but also is the best choice for loose and deep gravel. At lower speeds, the transmission remains in as low a gear as possible, so the Explorer doesn't bog down while the wheels churn through the loose surface.

The Explorer's stability control system is also relaxed, allowing the SUV to slide to prevent getting stuck. However, if the sand is wet, then the Mud/Ruts setting is recommended for best traction, relaxing the shifting to reduce wheelspin. All settings utilize a Hill Descent Control system that holds low gear, and automatically applies the brakes without locking any wheels.

Jeep Goes To A Place Called "Hell's Revenge"

Jeep and Land Rover both identified five conditions for their systems, which also make use of an extra low set of gears, which allow their SUVs to creep over big boulders and up and down steeper hills than the lower-riding fixed-height Explorer. Jeep's marketing director Jim Morrison says that the Grand Cherokee was tested extensively in an area near Moab, Utah, called "Hell's Revenge." The Jeep's hill descent control will work automatically on 20-percent grades, which may not sound steep, but when you're inside an off-roader headed downhill at this angle, it feels like the Jeep should topple over its own hood-yet it doesn't.

Jeep's five conditions are Auto, Snow, Sand/Mud, Rock, and Sport. For on-road driving, the Auto position will allow the wheels to roll freely on pavement but will limit slip on any wheel on a slick surface. In the Snow setting, the Jeep will start from a stop in second gear, and wheelspin is heavily limited, similar to the Explorer's Snow setting. For Sand/Mud, wheelspin is allowed somewhat, and stability control is relaxed so the Jeep can slide to keep up momentum. In addition, the optional air springs raise the height of the Jeep, and lower gears keep engine torque at a higher output. In the Rock setting, the suspension raises the Jeep more than four inches above its normal ride height, and the differentials combine with the traction control to limit wheelspin, especially when one or more wheels are airborne, while the accelerator becomes more sensitive to small inputs from the driver. Opposite of these settings is Sport, which increases damping and lowers the Jeep for tight body control at higher speeds on pavement.

The Brit Boulder Crawler

The Land Rover LR4's Terrain Response system (also on the Range Rover), has General Driving, Grass/Gravel/Snow, Sand, Mud/Ruts, and Rock Crawl settings. Like the Jeep, the Land Rover also has air springs that vary the ride height of the LR4 to clear obstacles, but it adds a special setting that will lift its body higher to free the vehicle if it gets stuck on top of a boulder or mound of dirt. Like the Jeep's Auto setting, the Land Rover's General Driving selection is for pavement and occasional slick spots, although Land Rover recommends its more aggressive "Ruts" setting for mud, while Jeep says its "Sand" setting will do mud just fine.

Finally, Land Rover has a special Rock Crawl setting that increases pedal travel, raises ride height, limits differential slippage, and engages special settings in traction control, hill descent and stability that keeps riders comfy on steep, cratered billy-goat trails. Special systems keep the Land Rover from rolling backwards on very steep hills, and control forward speed going down steep hills when the brakes are first released.

Back when the original Explorer was introduced, it took skills such as delicate throttle movements, selecting the right gearing and differential locking, and a phobia for the brake pedal to drive off-road without damage. Today, however, not only can anyone drive a terrain-controlled SUV off-road with little risk, but it can be done with a level of passenger comfort that keeps rising.

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