Road test: GMC Sierra All Terrain HD


At the 2011 North American International Auto Show, GMC showed a pickup truck concept based on the Sierra Heavy Duty (HD).

The GMC Sierra All Terrain HD adds some greater off-road versatility to the standard heavy-duty truck. Aside from bodywork modifications such as reduced overhangs for greater departure angles, this is achieved via some interesting chassis and suspension modifications.

Carl Zipfel is GMC’s chief designer:

“We started with a standard Sierra HD chassis but we worked with [off-road racing shock developer] Fox Racing to develop a set of Fox shocks just for our application and the geometry modifications we designed into the concept vehicle,” he tells VDI. “We didn’t design our truck to be a racing truck, but we went to Fox because they’re probably the most knowledgeable people when it comes to off-road suspension systems.”

The remote fluid reservoirs for the Fox shocks are integrated into the wheelhouse liners.

A custom set of aluminum, front upper and lower control arms was designed by GMC engineers for the Sierra’s independent front suspension. They contribute to the concept vehicle’s wider track, as do the 20in, machined-aluminum all-terrain wheels. The wheels are deep, with six split-spoke elements, and are constructed in a reverse drop-flange method for added strength.

Project engineers also developed a secondary jounce shock system in conjunction with Light Racing. The goal here is for additional compression damping and more controlled rebound, leading to increased control and stability, and better handling. The jounce shocks themselves are mounted low down by the front and rear axles.

“When you get into a full compression the jounce shocks manage the last four inches of travel,” adds Gipfel. “It works as a great addition in HD applications for trailer and off-road work.”

The concept vehicle is based on a 2500-series Sierra chassis, but the rear suspension starts with a shortened, modified leaf spring rate from a 3500-series truck. “The rear jounce shock setup is a little different because of the leaf spring,” he explains. “It has a jounce rocker arm – a triangulated, two-pivot-point rocker that’s connected to the secondary jounce shock. It was custom-designed internally but is based on some existing jounce shock race technology.”

A further bespoke feature of the concept is a button-operated, electronically decoupled front anti-roll bar for improved rock-crawling capability.

“The electronic decoupling isn’t standard but is within our component set,” Gipfel reveals. “We haven’t announced that we’ll offer it [on Sierra]but it is under study internally and we’re exploring with the concept vehicle. The version on the concept was internally developed and modified for this application.”

Tech spec – GMC Sierra All Terrain HD concept

Body style / driveline: five-passenger crew cab, 3/4-ton, four-wheel-drive heavy-duty pickup

Dimensions: 5,864mm (L) x 2,106mm (W) x 2,077mm (H)

Wheelbase: 3,774mm

Track: 1,853mm (F/R)

Construction: body on frame

Engine: Duramax 6.6-liter turbodiesel, 397bhp, 1,037Nm

Transmission: Allison 1000 six-speed automatic

Front suspension: long- and short-arm independent with torsion bars

Rear suspension: semi-elliptic two-stage multi-leaf spring

Steering: integral power-assisted recirculating ball

Brakes: power-assisted, Hydroboost brake-apply system, all-around discs, ABS. 355 x 40mm front discs, 360 x 34mm rears

Wheels: 20 x 9.5in aluminum

Tires: BFGoodrich KM2 325/60 R20

Minimum ground clearance: 536mm (at rocker panels) / 300mm (at skid plates) (76mm more than normal)

Approach angles: 39˚ (F), 31˚ (R)

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About Author


Graham Heeps is a regular contributor, and knows the dynamics industry well, having previously edited the title. Graham also writes regularly on automotive and motorsport subjects for other magazines from Vehicle Dynamics International’s publisher, UKi Media & Events (as well as editing Tire Technology International), and contributes to a range of online and print publications in the UK, USA and Canada.

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