【文章內(nèi)容簡介】 specially at high speeds are large enough for a viscous coupling front differential to bring improvements in straightline running.High powered frontwheel drive vehicles fitted with open differentials often spin their inside wheels when accelerating out of tight corners in low gear. In vehicles fitted with limitedslip viscous differentials, this spinning is limited and the torque generated by the speed difference between the wheels provides additional tractive effort for the outside driving wheel. this is shown in figure 12Figure 12: tractive forces for a frontwheel drive vehicle with viscous limitedslip differential during acceleration in a bend The acceleration capacity is thus improved, particularly when turning or accelerating out of a Tjunction maneuver ( . accelerating from a stopped position at a “T” intersectionright or left turn ).Figures 13 and 14 show the results of acceleration tests during steady state cornering with an open differential and with viscous limitedslip differential .Figure 13: acceleration characteristics for a frontwheel drive vehicle with an open differential on wet asphalt at a radius of 40m (fixed steering wheel angle throughout test).Figure 14: Acceleration Characteristics for a FrontWheel Drive Vehicle with Viscous Coupling on Wet Asphalt at a Radius of 40m (Fixed steering wheel angle throughout test)The vehicle with an open differential achieves an average acceleration of while the2/smvehicle with the viscous coupling reaches an average of (limited by enginepower). In these 2/stests, the maximum speed difference, caused by spinning of the inside driven wheel was reduced from 240 rpm with open differential to 100 rpm with the viscous coupling.During acceleration in a bend, frontwheel drive vehicles in general tend to understeer more than when running at a steady speed. The reason for this is the reduction of the potential to transmit lateral forces at the fronttires due to weight transfer to the rear wheels and increased longitudinal forces at the driving wheels. In an open loop controlcircletest this can be seen in the drop of the yawing speed (yaw rate) after starting to accelerate (Time 0 in Figure 13 and 14). It can also be taken from Figure 13 and Figure 14 that the yaw rate of the vehicle with the open differential fallsoff more rapidly than for the vehicle with the viscous coupling starting to accelerate. Approximately 2 seconds after starting to accelerate, however, the yaw rate falloff gradient of the viscouscoupled vehicle increases more than at the vehicle with open differential.The vehicle with the limited slip front differential thus has a more stable initial reaction under accelerating during cornering than the vehicle with the open differential, reducing its understeer. This is due to the higher slip at the inside driving wheel causing an increase in driving force through the viscous coupling to the outside wheel, which is illustrated in Figure 12. the imbalance in the front wheel tractive forces results in a yaw moment acting in direction of the turn, countering the When the adhesion limits of the driving wheels are exceed, the vehicle with the viscous coupling understeers more noticeably than the vehicle with the open differential (here, 2 seconds after starting to accelerate). On very low friction surfaces, such as snow or ice, stronger understeer is to be expected when accelerating in a curve with a limited slip differential because the driving wheelsconnected through the viscous couplingcan be made to spin more easily (powerundersteering). This characteristic can, however, be easily controlied by the driver or by an automatic throttle modulating traction control system. Under these conditions a much easier to control than a rearwheel drive car. Which can exhibit poweroversteering when accelerating during cornering. All things, considered, the advantage through the stabilized acceleration behavior of a viscous coupling equipped vehicle during acceleration the small disadvantage on slippery surfaces.5 / 13Throttleoff reactions during cornering, caused by releasing the accelerator suddenly, usually result in a frontwheel drive vehicle turning into the turn (throttleoff oversteering ). Highpowered modeles which can reach high lateral accelerations show the heaviest reactions. This throttleoff reaction has several causes such as kinematic influence, or as the vehicle attempting to travel on a smaller cornering radius with reducing speed. The essential reason, however, is the dynamic weight transfer from the rear to the front axle, which results in reduced slipangles on the front and increased slipangles on the rear wheels. Because the rear wheels are not transmitting driving torque, the influence on the rear axle in this case is greater than that of the front axle. The driving forces on the front wheels before throttleoff (see Figure 10) bee over running or braking forces afterwards, which is illustrated for the viscous equipped vehicle in Figure 15.Figure 15:Baraking Forces for a FrontWheel Drive Vehicle with Viscous LimitedSlip Differential Immediately after a Throttleoff Maneuver While CorneringAs the inner wheel continued to turn more slowly than the outer wheel, the viscous coupling provides the outer wheel with the larger braking force . The force difference between the frontwheels applied fBaround the center of gravity of the vehicle causes a yaw moment that counteracts the normal turnin GCM0reaction.When cornering behavior during a throttleoff maneuver is pared for vehicles with open differentials and viscous couplings, as shown in Figure 16 and 17, the speed difference between the two driving wheels is reduced with a viscous differential.Figure 16: Throttleoff Characteristics for a FrontWheel Drive Vehicle with an open Differential on Wet Asphalt at a Radius of 40m