Variable Torque Management VTM-4 Explanation

By Gary Flint - American Honda

Variable Torque Management 4-Wheel Drive (VTM-4)
After studying various all-wheel- and four-wheel-drive systems offered by the wide variety of pickups on the market today, Ridgeline engineers concluded that virtually every one had functional shortcomings and was undesirably bulky and heavy.  The direct result of that research was to use Honda’s innovative fully automatic VTM-4 system. This fully automatic 4WD system is the best match for the Ridgeline owner and proactively distributes torque to all four wheels as needed.  This system provides front-wheel drive for dry-pavement cruising conditions and engages all-wheel drive when needed to improve stability or maneuverability.  Unlike many competitive systems that use an engagement strategy triggered by wheel slippage, the Ridgeline's VTM-4 system anticipates the need for all-wheel drive and engages the rear wheels before slippage begins.  Additional torque is redistributed to the rear for improved performance during acceleration, especially on low friction surfaces and during towing.  In addition, the VSA system provides a limited-slip differential effect by applying braking force to a slipping front wheel thereby directing driving force to the wheel with more grip.

Another special feature is a “VTM-4” lock button, located on the instrument panel, which temporarily holds engagement of both rear wheels to aid traction

VTM-4 Modes of Operation
1. Acceleration Mode
There are three distinct modes of VTM-4 engagement.  The first – called the acceleration torque control (ATC) mode – is unique to this system.  It works even on dry pavement to proactively distribute driving torque to all four wheels as the Ridgeline accelerates from a stop to cruising speed.  One notable benefit of this mode is that traction is immediately available to move the vehicle from rest through a slippery intersection before slippage occurs.  (Once a wheel slips, the traction available for forward propulsion and lateral restraint is significantly diminished.)

A second advantage is that apportioning drive torque among all four wheels greatly diminishes the likelihood of torque steer.  Handling dynamics are also improved.  Reducing the propulsive force carried by the front tires leaves more adhesion for steering the vehicle into a tight bend or for holding cornering arc in the middle of a turn.  In other words, the Ridgeline's dynamic balance is greatly enhanced by ATC logic.

Rear wheel torque rises smoothly from zero to the optimum setting in proportion to vehicle acceleration (both forward and reverse).  At higher speeds, the front wheels are capable of providing the desired thrust with excellent handling so torque delivered to the rear wheels automatically diminishes with speed.  While cruising, all driving torque is delivered by the front wheels in the interests of smoothness, quietness, and fuel efficiency.

2. Slip Control Logic
The second engagement mode uses wheel slippage control logic.  If the difference in rotational speed between front and rear wheels rises because of a slippery surface or poor traction at the front of the vehicle, that condition is detected by wheel-speed sensors which are monitored by VTM-4's ECU.  In response, the ECU commands an increasing amount of torque for the rear wheels.  Torque is proportional to both slip rate and the rate at which the slip rate is increasing. This operation is similar to conventional slip-based all-wheel-drive systems already on the market.

3. VTM-4 Lock Mode
The third mode of all-wheel-drive engagement activates when the driver presses the lock button mounted on the instrument panel.  The maximum amount of rear-drive torque is locked in until the vehicle gets moving and exceeds six mph, at which time rear drive torque is gradually diminished.  By 18 mph, the lock mode is fully disengaged.  When vehicle speed drops below 18 mph, the lock mode automatically reengages.  The shift lever must be in the first, second, or reverse-gear position to use the lock mode.

The maximum torque delivered to the rear wheels allows the Ridgeline to claw up a 28-degree (53-percent slope) dirt grade.  On a split-mu (split-friction) grade (different amounts of traction at each wheel), VTM-4 automatically provides sufficient rear-wheel torque to help the vehicle climb steep and slippery terrain such as a steep driveway with patches of snow and ice.

Honda Ridgeline Rear Differential StructureRear Differential Structure
The Ridgeline's rear axle unit does not use a conventional differential.  Instead, a hypoid ring-and-pinion gear set supported by a cast-aluminum housing switches torque from the propeller shaft's longitudinal orientation to the lateral orientation necessary to drive the rear wheels. 

A connection from the ring gear to each wheel's half-shaft is made by left- and right-side clutches.  Each drive clutch consists of three elements: an electromagnetic coil, a ball-cam device, and a set of 19 wet clutch plates which are similar in design to clutches used in an automatic transmission.  Ten of the plates are splined (mechanically connected) to the ring gear while nine of the plates are splined to a half shaft.  Left and right clutches are identical.

The VTM-4 system's electronic control unit (ECU) determines torque which is to be distributed to the rear wheels, then electric current is sent to the two electromagnetic coils.  The resulting magnetic field moves a rotating steel plate toward each fixed coil.  Friction between that steel plate and an adjoining cam plate causes the cam plate to begin turning.  As it does, three balls per clutch roll up curved ramps, creating an axial thrust against a clutch-engagement plate.  This thrust force compresses the wet clutch plates, thereby engaging drive to the corresponding rear wheel. 

VTM-4 Rear Differential Structure

Unlike mechanically actuated four-wheel drive systems, the VTM-4 system is infinitely variable.  The amount of torque provided to the rear wheels is directly proportional to the electric current sent from the ECU and can be adjusted from zero to a preset maximum.  This current constantly changes to deliver the optimum rear torque calculated by the ECU.  An internal gear pump circulates VTM-4 fluid to cool and lubricate the clutches, bearings, and gears within the rear drive unit.  Use of high-strength, low-weight materials – such as die-cast aluminum for the housing – minimizes the bulk and weight of this hardware, helping to keep the weight of the entire all-wheel-drive system to about 212-pounds.

Automatic Mode
The VTM-4® electronic control unit, or ECU, constantly monitors vehicle speed and road conditions. When cruising or braking, the Ridgeline operates in front-wheel-drive only for maximum fuel efficiency. When accelerating, however, the VTM-4® ECU detects wheelspin before it happens, through its sensor data. The ECU then instantly calculates the best power split between the front and rear axles, and engages 4-wheel drive by electronically locking the rear clutches.

Lock Mode
The VTM-4 Lock mode can be activated only at speeds less than 18 mph in 1st, 2nd, or Reverse Gear positions. Note, when the transmission shift lever is placed in 2nd gear, it forces the vehicle into a 2nd gear start condition. This is only desirable on slippery surfaces such as ice or packed snow. To apply the maximum torque to the wheels for climbing a grade or to become un-stuck, you should always use the 1st gear position. Lugging the engine with the transmission in 2nd gear from a stop, may overheat the transmission and limit your ability to climb a steep hill or overcome a difficult traction condition.

VTM-4 Operating Modes
These operating modes are illustrated in the following graphics:

 Honda Ridgeline VTM-4 Operating Modes

Off-Road Recommendations
It is not necessary to operate the vehicle in VTM-4 lock mode to activate your 4-wd system. The vehicle is continuously sensing wheel slip and will transfer torque to the rear axle in proportion to the accelerator position and amount of wheel slip. You may wish to engage VTM-4 Lock as a preventative measure when you are climbing a very difficult steep hill on a non-paved surface, or traversing an extremely muddy section of an off-road trail, or overcoming a condition where the automatic mode is unable to react quickly enough to the circumstances.

If you experience a condition where you feel the vehicle begins to bog down or lack power, you may also wish to disengage the Vehicle Stability Assist (VSA) system which incorporates traction control function which may begin to cut engine power due to wheel slippage or a slight yaw condition caused by the vehicle hitting slush in the road or detecting excessive wheel spin. In conditions, such as slushy snow, sand, or mud may result in undesirable throttle intervention. When driving on public roads in normal snow or ice conditions is always best to leave the VSA engaged to provide the ultimate active safety device to keep you on your intended course. Remember, the VSA system can not overcome the laws of physics. Entering a corner at too high a speed on a slippery surface can still result in loss of control or a crash. The system only overcome situations where there is sufficient traction to return the vehicle to the path you intended to travel. 

Vehicle Stability Assist (VSA)
For enhanced control during acceleration, cornering, and sudden collision-avoidance maneuvers, the Ridgeline is equipped with a standard 4-channel Vehicle Stability Assist (VSA) system that works in conjunction with the VTM-4, drive-by-wire throttle and ABS systems.  This system uses sensor logic to provide active yaw control to help provide even more secure footing.  Active yaw control helps the Ridgeline follow the cornering line intended (by the driver) more faithfully and naturally, greatly enhancing its stability. Unlike some competitor systems, the VSA system is full functional in 4-wheel drive modes.

The VSA system enhances vehicle stability by applying brake force to each of the Ridgeline's four disc brakes independently while also managing the throttle system.  An additional benefit is the limited-slip differential effect the system provides for the front and rear wheels by applying braking force to a slipping wheel thereby redirecting driving force to the wheel with more traction.

Four-Channel VSAAnalyzing data that is constantly received from eight vehicle sensors monitoring speed, steering input and lateral G forces, the VSA system compares the driver’s control inputs with the vehicle’s actual response.  If the actual response is outside a predicted response range — as can happen when cornering forces exceed the tires’ performance capabilities — VSA automatically intervenes with an appropriate corrective action.  In the case of oversteer, VSA applies braking to the outside front and rear wheels to counter the unintended yawing effect.  If understeer is detected, VSA applies braking to the inside front and rear wheels and reduces engine power to help bring the car back onto the driver’s intended course.

The VSA system in the Ridgeline has been carefully calibrated to add handling stability and predictability without intruding on the process of spirited driving.  Whenever the system is actively enhancing the vehicle’s stability, an indicator light flashes on the instrument panel.  In addition, VSA’s stability enhancement and traction control can be turned off with a cockpit switch while still leaving the Anti-lock Braking System fully functional

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