Merci 
Ce VTM-4 est-il équipé d'un différentiel central, ou la fonction lock est-elle seulement utilisable à basse vitesse, sur terrain glissant ?
Uniquement à basse vitesse (jusqu'à 30 km/h). Voici le détail de ce système (Honda Pilot):
Variable Torque Management 4-wheel-drive (VTM-4)
Ensuring a high level of all-weather stability, traction and control is the Pilot's available VTM-4 (Variable Torque Management 4-Wheel Drive) system. Unlike conventional on-demand systems that work only when the wheels are slipping, VTM-4 proactively delivers torque to all four wheels during acceleration for excellent dry-road vehicle dynamics as well as outstanding control in wet, icy and snow conditions. A unique "lock" feature is provided to maximize traction for extremely low traction or "stuck" conditions. A compact transfer case is bolted directly to Pilot's front-mounted transaxle. A two-piece propeller shaft delivers torque from the transfer case to a rear axle drive unit. Two computer-controlled, electromagnetically-powered clutches engage as needed to provide torque to the rear wheels.
The Pilot's innovative VTM-4 four-wheel drive system was designed to deliver outstanding traction, stability and control in all weather conditions as well as good medium-duty off-road performance. It was also designed to minimize the weight and packaging penalties associated with conventional four-wheel drive systems.
The VTM-4 system is unique in its operation. Unlike some approaches that use an engagement strategy triggered by wheel slippage, VTM-4 anticipates the need for all-wheel drive and engages the rear wheels whenever the vehicle is accelerating. Additional torque is applied to the rear wheels when wheel slip is detected, up to an approximate maximum of 70 percent depending on conditions.
Another unique feature of the system is the VTM-4 Lock function. Activated by a button on the instrument panel, the VTM-4 Lock mode delivers maximum torque transfer to the rear wheels to aid extraction from extremely low traction or "stuck" conditions. The feature works only when the vehicle is in first, second or reverse gears, and automatically disengages at speeds above 18 miles per hour.
When cruising under normal conditions, the Pilot provides front-wheel drive power for improved efficiency. Torque is proactively distributed to the rear wheels when the vehicle is accelerating or wheel slip is detected. The level of torque delivery, front to rear, is determined by the amount of acceleration (rate of change in velocity) and wheel slip (difference in rotational speed) and is controlled by a dedicated CPU with sensors in the braking, engine and transmission systems.
To avoid the weight and bulk of a conventional transfer case, VTM-4's torque transfer unit is a compact cast-aluminum housing bolted directly to the transaxle. The transfer case is a single-speed, permanently engaged device without a low-range, reducing weight and space penalties while maintaining excellent on- and off-road capabilities.
Attached to the front wheel differential's ring gear is a helical gear that provides input torque to the transfer unit. A short horizontal shaft and a hypoid gear set within the case turn the drive ninety degrees, move it to the vehicle center line and lower its axis by approximately 3.75-inches.
VTM-4 Engagement Modes
There are three distinct modes of VTM-4 engagement:
(1)The first mode, called Acceleration Torque Control (ATC), works whenever the vehicle's throttle is depressed, even on dry pavement - a feature unique to the VTM-4 system. Sensors in the engine and transmission monitor vehicle speed and acceleration. The amount of torque applied, as directed by the system's ECU, is determined according to vehicle speed, the amount of acceleration and transmission status (gear setting). This benefits not only the Pilot's ability to gain traction from a standing start, before wheel slip occurs, but also its overall dynamic stability on both dry and low traction roads. Reducing the propulsive force carried by the front tires under acceleration reduced torque steer and cornering adhesion. Rear wheel torque rises smoothly from zero to a preset maximum in proportion to vehicle acceleration (both forward and reverse). During constant-speed driving, all power is driven to the front wheels for improved fuel efficiency.
(2) The second engagement mode occurs when wheel slip is detected. Differences in rotational speed between front and rear wheels are measured by sensors in the ABS system and monitored by the ECU. In response, the ECU commands an increase in torque delivery to the rear wheels. Torque application is adjusted according to the amount and the rate of change in wheel slip. As slip increases, more power is delivered to the rear wheels for improved traction.
(3) The third mode of engagement is VTM-4 Lock. Lock mode occurs when the driver shifts into first, second or reverse gears and depresses the VTM-Lock button on the instrument panel. When lock mode is selected at vehicle speeds below 18-mph, the system ECU commands a preset maximum amount of rear-drive torque to be delivered to the rear wheels for improved traction in very low-speed, low-traction, conditions. As control is regained and vehicle speed increases, the system gradually reduces rear axle torque until it is completely disengaged.
The maximum torque delivered to the rear wheels is sufficient to climb the steepest grade observed on any public road in America - 31-degrees (60 percent slope) - with a two-passenger load on board. The Pilot will also move from rest up a 28-degree (53 percent slope) dirt grade. On a split-friction grade (different amounts of traction at each wheel), VTM-4 automatically provides sufficient rear-wheel torque to help the vehicle climb something like a steep, low traction driveway.
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