Tech Briefs

May 2002
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GM shows versatility with Versatrak

The rear-differential module contains a twin gerotor, which consists of two gerotor hydraulic fluid pumps, two seven-plate wet clutch packs, and five hydraulic pressure valves.
Click to enlarge

General Motors has developed an all-wheel-drive system that it claims is quiet, efficient, and effective in low-traction situations. Called Versatrak, its design is "so light and compact that it fits under a flat rear load door," said Anna Kretz, Vehicle Line Executive for GM's minivans and crossover vehicles. "This enables GM to make this system available in vehicles such as the Pontiac Aztec and Buick Rendezvous, as well as our trio of minivans." The GM minivans include the Pontiac Montana, Chevrolet Venture, and Oldsmobile Silhouette.

A team of engineers from GM and Austrian driveline specialist Steyr-Daimler-Puch Fahzeugtechnik developed originally for the Aztec the on-demand all-wheel-drive system, which uses front-wheel drive for ordinary road conditions and automatically adds rear-wheel drive as necessary while the vehicle is operated under slippery road conditions. This change in drive is done without perceptible influence on vehicle handling or driving comfort, according to GM, without buttons to press or levers to throw.

"Versatrak is a mechanical system that transfers torque directly when front-wheel slippage occurs and can direct as much as 44% of the engine's power to each rear wheel either individually or simultaneously as needed," said Charles Kingsley, Vehicle Chief Engineer for GM's crossover vehicles.

The short- and long-arm independent rear suspension of the Buick Rendezvous employs a rear crossmember, upper and lower control arms, and knuckle assemblies.

GM describes the Versatrak system as beginning with a power take-off (PTO) unit bolted to the right end of a vehicle's front-mounted transaxle, a propeller shaft from the PTO to a rear-differential module, and halfshafts from the rear-drive module to each rear wheel. An aluminum driveshaft links the PTO unit with the rear-drive module. Inside the rear module, sensors in the twin Geromatic gerotor pumps react to differences in the rotational speed of the front and rear wheels. The gerotor is a "non-conventional assembly," according to GM, of two gerotor hydraulic fluid pumps, two seven-plate wet clutch packs, and five hydraulic pressure valves. The twin gerotor is licensed from Santa Barbara, CA-based ASHA Technologies.

All power is directed to the front wheels as long as no speed difference is sensed. If one of the front wheels begins to slip, the rear-mounted gerotor units pressurize fluid to engage the clutches that redirect torque to one—or both—rear wheels, enabling the rear wheels to provide traction that allows the vehicle to keep moving forward despite muddy, snowy, or slippery road surfaces. Hydraulic pressure forces a disc-shaped piston to engage the seven-plate clutch and apply torque through the halfshaft to each rear wheel. If the rear wheel is also on a slick surface, it will match the speed of the front wheels, reducing hydraulic pressure and disengaging the halfshaft and wheel. Torque from the propeller shaft is then re-routed to the other rear wheel. The propeller shaft uses a rear sliding constant velocity joint, which allows the driveshaft to vary in length to compensate for rocking motions of the transverse engine and transaxle.

The PTO unit has a three-piece housing with a helical geartrain consisting of three helical gears and a 90° hypoid gearset. An input helical gear welded to the right side of the differential carrier transfers torque through the other two helical gears and a hypoid gearset that turns 90° to route the torque to the propeller shaft. Four stamped, steel stiffening braces secure the PTO to the transaxle case, engine block, and oil pan.

Sensors in the twin gerotor pumps react to differences in the rotational speed of the front and rear wheels.
Click to enlarge

According to GM, the system performs the functions of a four-wheel-drive system that has both center and rear differentials—performed by the twin gerotor—with limited slip capabilities. This feature enables performance equivalent to a four-wheel-drive vehicle with torque-sensing center (between the front and rear axles) and rear (between the rear wheels) differentials, which have been available generally on only luxury and high-performance cars with four-wheel drive. Some other on-demand four-wheel-drive systems allow one front and one rear wheel to slip since they have open differentials in both front and rear axles.

The Versatrak system is mechanically and functionally identical on all the vehicles. However, the torque tube bolted between the rear-drive module and the propeller shaft is about 102 mm (4 in) longer on the Rendezvous to accommodate its longer wheelbase. The minivans, with wheelbases of about 200 mm (8 in) more than the Rendezvous and 305 mm (12 in) more than the Aztec, use a longer driveshaft to accommodate the extra length.

- Jean L. Broge