Honda i-VTM4® All-Wheel Drive

• Torque-vectoring technology enables excellent bad-weather and off-road traction management
• Progressively distributes optimum torque between front and rear axles, and between left and right rear wheels
• Available on Honda Pilot and Passport SUVs, and standard on Ridgeline pickup truck

Honda Intelligent variable torque management (i-VTM4®) all-wheel drive is a sophisticated and technologically advanced AWD system that uses torque vectoring on the rear axle. The benefits of the system are superior all-weather handling, exceptional performance in sand, snow, and mud, and neutral, accurate steering and handling under power that is unmatched by front-drive, rear-drive or conventional all-wheel-drive systems.

Today i-VTM4 is available on Honda Pilot and Passport SUVs, and is standard on Ridgeline pickup, giving all all-wheel drive customers the peace of mind and capability that comes with the sophisticated system.

History of i-VTM4
In 2015, Honda introduced i-VTM4 to the third-generation Pilot as the sole all-wheel drive system available. Mechanically similar to Acura’s third-generation SH-AWD® system, the new i-VTM4 required no intervention from the driver for maximum capability thanks to the torque-vectoring capabilities of the electronically controlled, hydraulically actuated rear differential. The system progressively distributed optimum torque between the front and rear axles, and dynamically distributed engine torque between the left and right rear wheels.

Two earlier technologies used in Honda and Acura vehicles laid the groundwork for
i-VTM4: The Active Torque Transfer System (ATTS) that was introduced on the 1997 Honda Prelude Type SH, and the Variable Torque Management® four-wheel drive system (VTM-4®) used on the on the 2001 Acura MDX and 2003 Honda Pilot.

ATTS used a pair of electronically controlled clutches to actively send up to 80% of engine torque to the outside wheel when cornering, actively overdriving that wheel up to 15%. ATTS not only offered enhanced traction management similar to a limited-slip differential, but the active torque vectoring of overdriving the outside wheel helped significantly reduce understeer, giving the front-wheel drive Prelude Type SH handling characteristics more commonly associated with all-wheel drive cars.

For the 2001 Acura MDX and 2003 Honda Pilot, engineers sought an all-wheel drive system that would automatically distribute torque to all four wheels as needed, but in a compact and lightweight package. The resulting Variable Torque Management® all-wheel drive system (VTM-4®) used a single-speed torque transfer unit bolted directly to the transaxle and an electromagnetic clutch mounted on each side of the rear differential.

During normal operation the clutches were disengaged, allowing the rear axles to freewheel, delivering all engine power to the front wheels. However, if the system’s sophisticated ECU anticipated wheel slippage, the clutches would lock, routing power to the rear wheels to maximize available traction, such as when launching hard from a stop or in slippery conditions. Drivers could also press a VTM-Lock® button on the dash to temporarily engage the rear wheels to aid traction management in situations such as a snowbank or slippery ditch.

These two technologies were combined with the introduction of SH-AWD on the 2005 Acura RL, which applied the benefits of the active torque vectoring used on ATTS to the rear axle of the VTM-4 all-wheel drive system. This world’s-first all-wheel drive system with active torque vectoring was specifically tuned to go beyond improving traction management in poor weather, but also improving handling by introducing torque vectoring to induce a yaw moment to aid in cornering. More information about Super Handling All-Wheel Drive™ is available here.

Mechanics of the i-VTM4 System
To enhance both handling and stability, i-VTM4 can send up to 70% of engine torque to the rear wheels and actively distribute up to 100% of that torque to a single left or right wheel. This is achieved by routing the power generated by the vehicle’s engine to a transaxle, while power is sent to the rear wheels through a torque transfer unit mounted up front rather than a traditional center differential.

Mounted alongside the front transaxle, the torque-transfer unit receives torque from a helical gear that is attached to the front differential's ring gear. From there, a short horizontal shaft and hypoid gear set within the torque-transfer unit's case send power to the rear drive unit though a lightweight driveshaft.

Controlling the torque split between the rear wheels are two hydraulically operated clutch packs, one for each rear wheel. The clutch packs are activated with an electric motor that powers a single hydraulic pump for each pack. The i-VTM4 Electronic Control Unit (ECU) controls a pair of linear solenoids by selectively sending pressure to the packs, which controls the amount of power sent to each rear wheel. Together, the clutch packs control the front-to-rear torque split, and when controlled independently, they can send all the rear-wheel torque to a single rear wheel.

Determining how much torque to send to the rear wheels, and how much to distribute to the left and right, falls to the input of multiple sensors. These include the i-VTM4 ECU, and the engine and Vehicle Stability Assist™ (VSA®) ECUs. The i-VTM4 ECU monitors steering angle, lateral g-forces, vehicle yaw rate and electromagnetic clutch engagement for the right and left rear axle shafts, while the engine ECU provides information on engine rpm, airflow and transmission gear-ratio selection, and the VSA ECU provides wheel-speed data. The i-VTM4 system also works in conjunction with VSA and Agile Handling Assist to optimize torque distribution for superior handling and traction utilization.

Improves Foul Weather and Off-Road Performance
Optimized for foul-weather and off-road situations, i-VTM4 uses its torque vectoring to provide unique advantages as it distributes power to all four wheels. In wet or slippery conditions, especially winter conditions such as snow, the system can help stabilize the vehicle when accelerating from a stop by actively routing power to the rear wheel with the most traction.

As traction conditions change once the vehicle begins moving, the system’s quick reactions can help stabilize the vehicle. When driving on a curving road in wet weather, i-VTM4 continues to help maintain maximum traction and stability by constantly adjusting how torque is routed.

In off-road situations, i-VTM4 will continue to route torque where it can do the most good, helping maintain traction and stability in mud and deep sand. It even continues to work in extreme conditions, such as when one rear wheel is off the ground, by sending power to the wheel in contact with the ground to pull the vehicle through and ensure momentum.

Furthermore, i-VTM4 uses input from the push-button-operated Intelligent Traction Management system to further refine the operation through four different modes: Normal, Snow, Mud and Sand. This system was developed, tuned and tested at various locations around the world that include Imperial Dunes "Glamis", California; Moscow, Russia; and Dubai, United Arab Emirates. Intelligent Traction Management allows the driver to select the operating mode that best suits the driving conditions, altering i-VTM4, electronic throttle, transmission shift behavior and VSA using the following parameters:

i-VTM4 Improves Handling
In addition to its off-road capabilities, i-VTM4 overdrives the vehicle’s rear axle by 2.7%, giving the rear wheels the capability to spin faster than the front wheels to create a yaw moment at the rear of the vehicle. This is key to the system’s torque-vectoring function.

The left and right turning motion of a vehicle is known as “yaw.” In conventional cars, this motion is created almost entirely by steering input, applying rotational torque to the vehicle body. However, in some dynamic situations, the front wheels can become overwhelmed with the multiple tasks of steering the car, supporting a large part of the vehicle’s weight, and in the case of front-wheel drive vehicles, delivering power. This can cause the front tires to lose traction, a condition known as “understeer.”

By contrast, a rear-wheel drive vehicle can use the power at the rear wheels to help induce a yaw moment at the rear. Known as “oversteer,” this can make a rear-wheel drive vehicle feel more nimble, but too much oversteer can make a vehicle unstable and cause a spin.

Conventional all-wheel drive systems seek a compromise between front and rear-wheel drive by routing power to the front and rear at the same time to increase stability. The downside to this is that the front wheels are still primarily responsible for inducing the yaw moment, making the car feel less nimble as a result. The fact that the steering is primarily responsible for the yaw moment are why traditional AWD systems typically lack the agile feel of the best two-wheel drive systems.

Using torque vectoring, i-VTM4 combines the nimbleness of a rear-wheel drive vehicle, while maintaining all-wheel drive stability. This agility is due to the overdriven rear-drive unit, and how the system independently controls power delivery to both the left and right rear wheels.

When cornering, i-VTM4 turns the outside rear wheel faster than the average speed of the two front wheels. This helps create a new yaw moment at the rear of the vehicle, relieving the front tires of some of the work of turning the car, reducing understeer to keep the vehicle balanced and controllable. In addition, with the cornering load more evenly distributed between the front and rear tires, the total cornering grip is more fully realized.

This active use of drive torque to help turn the car makes the vehicle more responsive, nimble, and predictable, while maintaining the stability inherent in all all-wheel drive systems. The system has benefits in straight-line acceleration too, sending more power to the rear wheels to help balance power distribution, eliminating wheelspin for fast and repeatable launches.

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