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reprinted from Automotive Engineering, September 1996

Honda Direct Yaw Control, variable torque-split system for front- and all-wheel-drive vehicles

Sir Isaac Newton observed in the treatise deMotu that a moving body will continue its uniform motion in a straight line unless acted upon by external forces trying to deter it. In the automobile, such forces are exerted by the driver turning the steering wheel from the straight-ahead position, but the vehicle's reaction entails all kinds of complications, such as understeer, oversteer, or simply losing the tires' grip.

Honda began developing an active yaw control system, called the "Direct Yaw Control System," to help the vehicle follow the cornering line intended (by the driver) more faithfully and naturally, greatly enhancing its stability. Honda R&D completed a DYC applied to the all-wheel-drive system in 1991. A prototype small car demonstrated the DYC-AWD's tremendous cornering power and remarkable stability; however, its application to series production models was judged too costly and complex at the time. Honda has since completed a two-wheel-drive version of the DYC, which will be adopted in the soon-to-be-launched, front-drive Prelude sport coupe replacement.

Honda likens the DYC to a row boat without a rudder. To turn, the outer oar must be rowed harder than the inner one to provide a difference in the right and left drive force. How does it work? Think of a frustum, or more simply a paper or styrofoam cup, and roll it. It moves in a circular line. If a vehicle has a larger wheel on the outer side and a smaller one on the inside, it naturally turns toward the inner side, ultimately making a circle. The DYC does that, not literally, but by splitting and varying driving torque unequally between the left and right wheels when cornering.

The system is an accelerator mechanism, which, when combined with a conventional differential, increases the outer wheel rotation, i.e., increases driving torque to the outer wheel. A reaction force equivalent to the increased amount of torque is directed to the inner wheel as a braking force, thus reducing torque to the inner wheel, and varying the lateral distribution of driving torque.

In the front-drive configuration, the driving torque distribution unit is located inline, outrigger of the transaxle's final drive. The unit comprises twin concentric shafts (outer for the right wheel, inner for the left), triple-pinion planetary geartrain, and left- and right-turn clutches. Two sets of planetary gears enable either wheel to revolve more than the other, providing optimum ratio for stable cornering performance. Torque distribution between the left and right wheels is steplessly variable for smooth transition. The DYC for front-drive attains a maximum increase of 15%, splitting driving torque to 80 (outer wheel)/20 (inner wheel).

The ECU consists of two control functions: feed-forward and feed-back controls. Vehicle speed sensor, steering wheel angle sensor, lateral acceleration (g) sensor, and the ECU (engine rpm and torque) feed into the feed-forward part of the ECU, which determines the driver's intention by such factors as steering input, vehicle speed, lateral acceleration, and engine performance, and controls driving torque distribution between the wheels. The feed-back part determines the vehicle's lateral slip angle, beta, by vehicle model (vehicle speed, steering wheel angle, and lateral acceleration) and yaw-rate sensor input, and feeds reference data to the feed-forward unit for precise distribution control.

Honda lists among the DYC's virtues: improved tractability during cornering providing secure "on-the-rail" feel, improved stability, and reduction in steering effort.

Honda had completed the development of an all-wheel-drive DYC system some years ago (the unit is mounted at the rear, varying torque distribution between the rear wheels), however, its product application will come after the front-drive version.

Jack Yamaguchi