【正文】 1 The Effect of a Viscous Coupling Used as a FrontWheel Drive LimitedSlip Differential on Vehicle Traction and Handling 1 ABCTRACT The viscous coupling is known mainly as a driveline ponent in four wheel drive vehicles. Developments in recent years, however, point toward the probability that this device will bee a major player in mainstream frontwheel drive application. Production application in European and Japanese frontwheel drive cars have demonstrated that viscous couplings provide substantial improvements not only in traction on slippery surfaces but also in handing and stability even under normal driving conditions. This paper presents a serious of proving ground tests which investigate the effects of a viscous coupling in a frontwheel drive vehicle on traction and handing. Testing demonstrates substantial traction improvements while only slightly influencing steering torque. Factors affecting this steering torque in frontwheel drive vehicles during straight line driving are described. Key vehicle design parameters are identified which greatly influence the patibility of limitedslip differentials in frontwheel drive vehicles. Cornering tests show the influence of the viscous coupling on the self steering behavior of a frontwheel drive vehicle. Further testing demonstrates that a vehicle with a viscous limitedslip differential exhibits an improved stability under acceleration and throttleoff maneuvers during cornering. 2 THE VISCOUS COUPLING The viscous coupling is a well known ponent in drivetrains. In this paper only a short summary of its basic function and principle shall be given. The viscous coupling operates according to the principle of fluid friction, and is thus dependent on speed difference. As shown in Figure 1 the viscous coupling has slip controlling properties in contrast to torque sensing systems. This means that the drive torque which is transmitted to the front wheels is automatically controlled in the sense of an optimized torque distribution. In a frontwheel drive vehicle the viscous coupling can be installed inside the differential or externally on an intermediate shaft. The external solution is shown in Figure 2. This layout has some significant advantages over the internal solution. First, there is usually enough space available in the area of the intermediate shaft to provide the required viscous characteristic. This is in contrast to the limited space left in today’s frontaxle differentials. Further, only minimal modification to the differential carrier and transmission case is required. Inhouse production of differentials is thus only slightly affected. Introduction as an option can be made easily especially when the shaft and the viscous unit is supplied as a plete unit. Finally, the intermediate shaft makes it possible to provide for sideshafts of equal length with transversely installed engines which is important to reduce torque steer (shown later in section 4). This special design also gives a good possibility for significant weight and cost reductions of the viscous unit. GKN Viscodrive is developing a low weight and cost viscous coupling. By using only two standardized outer diameters, standardized plates, plastic hubs and extruded material for the housing which can easily be cut to different lengths, it is possible to utilize a wide range of viscous characteristics. An example of this development is shown in Figure 3. 3 TRACTION EFFECTS As a torque balancing device, an open differential provides equal tractive effort to both driving wheels. It allows each wheel to rotate at different speeds during cornering without torsional windup. These characteristics, however, can be disadvantageous when adhesion variations between the left and right sides of the road surface (splitμ ) limits the torque transmitted for both wheels to that which can be supported by the lowμ wheel. With a viscous limitedslip differential, it is possible to utilize the higher adhesion potential of the wheel on the highμ surface. This is schematically shown in Figure 4. When for example, the maximum transmittable torque for one wheel is exceeded on a splitμ surface or during cornering with high lateral acceleration, a speed difference between the two driving wheels occurs. The resulting selflocking torque in the viscous coupling resists any further increase in speed difference and transmits the appropriate torque to the wheel with the better traction potential. It can be seen in Figure 4 that the difference in the tractive forces results in a yawing moment which tries to turn the vehicle in to the lowμ side, To keep the vehicle in a straight line the driver has to pensate this with opposite steering input. Though the fluidfriction principle of the viscous coupling 2 and the resulting soft transition from open to locking action, this is easily possible, The appropriate results obtained from vehicle tests are shown in Figure 5. Reported are the average steeringwheel torque Ts and the average corrective opposite steering input required to maintain a straight course during acceleration on a splitμ track with an open and a viscous differential. The differences between the values with the open differential and those with the viscous coupling are relatively large in parison to each other. However, they are small in absolute terms. Subjectively, the steering influence is nearly unnoticeable. The torque steer is also influenced by several kinematic parameters which will be explained in the next section of this paper. 4 FACTORS AFFECTING STEERING TORQUE As shown in Figure 6 the tractive forces lead to an increase in the toein response per wheel. For differing tractive forces, Which appear when accelerating on splitμ with limitedslip differentials, the toein response changes per wheel are also different. Unfortunately,