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In-Depth: GMC Sierra All Terrain Heavy Duty Concept

A halo truck for a halo brand?

by on Jan.13, 2011

Will GMC put the Sierra All Terrain HD Concept into production? Inside sources say that's likely.

Could GMC add a halo off-road truck to its full-size pickup lineup? The diesel-powered GMC Sierra All Terrain Heavy Duty Concept — which made its debut, this week, at the 2011 Detroit auto show — points to how the “professional grade” brand thinks it might deliver a credible competitor to the Ford F-150 SVT Raptor and the Ram 2500 Power Wagon in the current or next-generation Sierra HD.

The light-duty Raptor and heavy-duty Power Wagon are two of the most capable off-road pickup trucks ever to jump or climb off the showroom floor. We love both because these purpose-built rigs can go places and do things off pavement that would kill lesser trucks within yards of the trailhead. The Raptor flies across the desert at high speed, like a Baja trophy truck, while the Power Wagon can rock crawl over the toughest terrain you’ll find in Moab.

But these traits make an apples-to-apples comparison of the trucks difficult because their off-road strengths lie at opposite ends of the wheeling spectrum. This is where GMC smells opportunity.

The short-box crew-cab Sierra All Terrain HD threads the needle between the half-ton Raptor and three-quarter-ton Ram 2500 Power Wagon by carving a new niche that blends the off-road prowess and features of the Ford and Ram trucks with a few new innovations.

The GMC Sierra All Terrain HD concept would go up against the likes of the Ford Raptor or even heavier models, such as the F-250.

As with all off-road pickups, the heart of the All Terrain HD is its suspension. The concept’s running gear makes good use of the overhauled box-frame ladder, shocks, springs and axles that were introduced for the 2011 model year.

“The capability-enhancing attributes of the Sierra All Terrain HD build on the already outstanding capabilities offered in the all-new production Sierra HD trucks,” said Lisa Hutchinson, GMC product marketing director. “Although it’s strictly a concept, it’s a pretty realistic one.”

But some heavy-duty off-road users have criticized GM’s HD pickups for their torsion bar independent front suspension instead of a coil spring solid front axle, like Ford’s and Ram’s heavy-duty pickups use.

An independent front suspension generally provides better ride comfort on- and off-road, which the light-duty Raptor uses for high-speed desert running. A solid front axle allows higher ground clearance and superior articulation — for low-speed rock crawling — which the Power Wagon excels at.

The All Terrain uses independent front suspension to its advantage. For improved off-road stability, the standard A-arms have been replaced with custom double-wishbones and offset wheels that give the Sierra a 73-inch track up front — 4.2 inches wider than the current Sierra HD and nearly identical to the Raptor’s 73.6-inch track.

The All Terrain’s rear track is also 73 inches from a stock rear axle and offset wheels. Ram Power wagon’s track is 68.3 inches in front and 68.2 inches in back.

The GMC Sierra All Terrain HD concept combines greater heavy-duty truck capability with greater off-road versatility.

An independent front suspension isn’t the only thing the Raptor and All Terrain HD have in common. They also share Fox Racing internal bypass shocks, though instead of piggyback reservoirs in back, the Sierra has remote reservoirs (for improved capacity and cooling) integrated into the truck’s wheel wells at all four corners.

If GMC wanted to get Ford’s attention, this should do it. Fox’s shocks are the key components that give the Raptor such awesome wheel travel at high speeds off-road.

Fox Racing’s internal bypass technology is slick and maintenance-free. Instead of placing the oil routers outside the shocks, Fox sealed them inside the main tube so that they can’t be adjusted. The valves have been replaced with very small gates, precisely placed for optimal damping in all conditions.

Hardcore off-roaders might not like this setup, but it solves several potential issues both for GMC and for less-obsessive desert-running enthusiasts. There are no external bypass tubes to be damaged by offroad debris striking them; there are no worries about check-valve durability; and the shocks can be tuned specifically to the Sierra All Terrain’s off-road character.

In the Sierra, Fox’s long-travel dampers add 2 inches more travel up front (11 inches total) and 3 inches in the rear (11.75 inches total) over the Sierra’s stock monotube shocks.

While Ram doesn’t offer Fox shocks for the Power Wagon, Chrysler Mopar performance parts division will also feature Fox Shocks as part of its aggressive Ram Runner dealer-installed off-road package for the light-duty Ram 1500.

We wouldn’t be surprised if this latest Fox development effort causes a major rift with Ford, which brought prominence and high-volume production expertise to Fox in OEM off-road applications.

The All Terrain HD also borrows a cool trick for its front suspension from the straight axle Power Wagon. The Ram features a front sway bar disconnect system that increases wheel travel and articulation to climb over tall obstacles. The Sierra All Terrain also has an electronic front stabilizer bar disconnect.

While GMC officials continue to say the Sierra All Terrain HD is nothing but a concept, inside word seems to be heavily leaning towards putting the truck into production, probably sooner than later.  Factors that would abort such a move include a huge rise in fuel prices, or the simple fact that General Motors’ engineering resources, post-bankruptcy, have been overwhelmed catching up on product program delays caused by the run through the courts.

Mike Levine is publisher of PickupTrucks.com.