How the Honda RealTime 4 Wheel Drive Works

The CRV and the Element both use the RealTime 4WD system, different from the Pilot and the Ridgeline's VTM-4 Lock (which I explained in an earlier article). Here is how it works (technical explanation);

The CR-V�s 4-wheel drive system is designed to best match the majority of driving situations that SUV�s realistically encounter. The fully automatic Real Time 4WD system enhances the CR-V�s all-weather and light duty off-road capabilities when driving in rain, snow, dirt roads and sandy conditions � without the significant weight, fuel economy and handling performance drawbacks of a conventional four-wheel-drive system. Enhancements have been made to the system for improved performance by optimizing materials and construction, resulting in a more rigid assembly with the capacity for up to 20 percent more torque transfer to the rear wheels.

The CR-V's Real Time 4WD system sends power only to the rear wheels when there is insufficient traction for the front-wheel-drive system. The system consists of the conventional front-wheel-drive system, a compact transfer case that distributes torque to a propeller shaft running the length of the vehicle, a dual-pump system with a multi-plate clutch, a cam unit mechanism, the rear differential, and left and right rear-wheel driveshafts.

The core of the system is the dual-pump unit. It consists of two hydraulic pumps, one driven by the front wheels via the propeller shaft and one driven by the rear wheels via the rear differential. A hydraulically actuated, multi-plate clutch, similar to the clutches used in Honda automatic transmissions, connects the propeller shaft to the rear differential.

When the CR-V is operating with the front and rear wheels turning at the same speed, for example on dry pavement, the front and rear hydraulic pumps operate at the same rate. Hydraulic fluid circulates between the two pumps; however, no pressure is generated. In effect, the fluid pressure created by the front pump is equalized by the rear pump.

If the front wheels begin to turn faster than the rear wheels, as would be the case if they were spinning on snow or ice, the two hydraulic pumps would turn at a different rate and hydraulic pressure proportional to the difference in their speeds of rotation would be generated. The resulting hydraulic pressure opens a valve body and feeds pressure to the multi-plate clutch, which engages the front propeller shaft to the rear differential. The rear differential then feeds the drive torque to the right and left rear wheels.

To provide a quick torque transfer response time for transparent 4-wheel drive operation to the driver, the multi-plate clutch is supplemented by a one-way ball cam unit mechanism. The cam unit mechanism consists of two plates separated by six ball cams (large bearings) that move within ramped grooves. When a slight difference in rotation speed exists between the rear wheels and front wheels when driving forward, the ball cams move within the ramped grooves and create pressure that instantly begins the engagement of the main clutch (prior to the build-up of sufficient hydraulic pressure). By engaging the main clutch at a sufficient force even before the hydraulic pressure is generated by the two hydraulic pumps, the driving torque is transferred without a hint of time lag.

Overall system operation is completely automatic; no electronics or driver action is involved. The greater the degree of front-wheel slippage, the greater the amount of torque fed to the rear wheels. Real Time� 4WD is also low maintenance. The Maintenance Minder system will indicate that the dual pump fluid should be changed at about 100,000 kilometers.

Source: 2008 Honda Product Guide for the 2008 Honda CRV.