1、 1 The Effect of a Viscous Coupling Used as a Front-Wheel Drive Limited-Slip Differential on Vehicle Traction and Handling 1 ABCTRACT The viscous coupling is known mainly as a driveline component in four wheel drive vehicles. Developments in recent years, however, point toward the probability that t
2、his device will become a major player in mainstream front-wheel drive application. Production application in European and Japanese front-wheel drive cars have demonstrated that viscous couplings provide substantial improvements not only in traction on slippery surfaces but also in handing and stabil
3、ity even under normal driving conditions. This paper presents a serious of proving ground tests which investigate the effects of a viscous coupling in a front-wheel drive vehicle on traction and handing. Testing demonstrates substantial traction improvements while only slightly influencing steering
4、torque. Factors affecting this steering torque in front-wheel drive vehicles during straight line driving are described. Key vehicle design parameters are identified which greatly influence the compatibility of limited-slip differentials in front-wheel drive vehicles. Cornering tests show the influe
5、nce of the viscous coupling on the self steering behavior of a front-wheel drive vehicle. Further testing demonstrates that a vehicle with a viscous limited-slip differential exhibits an improved stability under acceleration and throttle-off maneuvers during cornering. 2 THE VISCOUS COUPLING The vis
6、cous coupling is a well known component 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 coupl
7、ing 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 front-wheel drive vehicle the viscous coupling can be installed insi
8、de 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 charact
9、eristic. This is in contrast to the limited space left in todays front-axle differentials. Further, only minimal modification to the differential carrier and transmission case is required. In-house production of differentials is thus only slightly affected. Introduction as an option can be made easi
10、ly especially when the shaft and the viscous unit is supplied as a complete 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
11、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
12、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 ro
13、tate at different speeds during cornering without torsional wind-up. These characteristics, however, can be disadvantageous when adhesion variations between the left and right sides of the road surface (split- ) limits the torque transmitted for both wheels to that which can be supported by the low-
14、 wheel. With a viscous limited-slip differential, it is possible to utilize the higher adhesion potential of the wheel on the high- surface. This is schematically shown in Figure 4. When for example, the maximum transmittable torque for one wheel is exceeded on a split- surface or during cornering w
15、ith high lateral acceleration, a speed difference between the two driving wheels occurs. The resulting self-locking 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
16、Figure 4 that the difference in the tractive forces results in a yawing moment which tries to turn the vehicle in to the low- side, To keep the vehicle in a straight line the driver has to compensate this with opposite steering input. Though the fluid-friction principle of the viscous coupling 2 and
17、 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 steering-wheel torque Ts and the average corrective opposite steering input required to maintain a straight course
18、during acceleration on a split- 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 comparison to each other. However, they are small in absolute terms. Subjectively, the steering inf
19、luence 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 toe-in response per wheel. For differi
20、ng tractive forces, Which appear when accelerating on split- with limited-slip differentials, the toe-in response changes per wheel are also different. Unfortunately, this effect leads to an undesirable turn-in response to the low- side, i.e. the same yaw direction as caused by the difference in the
21、 tractive forces. Reduced toe-in elasticity is thus an essential requirement for the successful front-axle application of a viscous limited-slip differential as well as any other type of limited-slip differential. Generally the following equations apply to the driving forces on a wheel VT FF With TF
22、 Tractive Force VF Vertical Wheel Load Utilized Adhesion Coefficient These driving forces result in steering torque at each wheel via the wheel disturbance level arm “e” and a steering torque difference between the wheels given by the equation: eT = loHhiH FFe c o s Where eT Steering Torque Differen
23、ce e=Wheel Disturbance Level Arm King Pin Angle hi=high- side subscript lo=low- side subscript In the case of front-wheel drive vehicles with open differentials, Ts is almost unnoticeable, since the torque bias ( loHhiT FF / ) is no more than 1.35. For applications with limited-slip differentials, h
24、owever, the influence is significant. Thus the wheel disturbance lever arm e should be as small as possible. Differing wheel loads also lead to an increase in Te so the difference should also be as small as possible. When torque is transmitted by an articulated CV-Joint, on the drive side (subscript
25、 1) and the driven side (subscript 2),differing secondary moments are produced that must have a reaction in a vertical plane relative to the plane of articulation. The magnitude and direction of the secondary moments (M) are calculated as follows (see Figure 8): Drive side M1 =vv TT ta n/)2/ta n (2
26、Driven side M2 =vv TT ta n/)2/ta n (2 With T2 =dynT rF T= systemJoTf in t,2 Where v Vertical Articulation Angle Resulting Articulation Angle dynr Dynamic Wheel Radius T Average Torque Loss 3 The component cos2M acts around the king-pin axis (see figure 7) as a steering torque per wheel and as a stee
27、ring torque difference between the wheels as follows: )t a n/2/t a n()s i n/2/t a n(c o s 22 liwhiw TTTTT where TSteering Torque Difference W Wheel side subscript It is therefore apparent that not only differing driving torque but also differing articulations caused by various driveshaft lengths are
28、 also a factor. Referring to the moment-polygon in Figure 7, the rotational direction of M2 or T respectively change, depending on the position of the wheel-center to the gearbox output. For the normal position of the halfshaft shown in Figure 7(wheel-center below the gearbox output joint) the secon
29、dary moments work in the same rotational direction as the driving forces. For a modified suspension layout (wheel-center above gearbox output joint, i.e. v negative) the secondary moments counteract the moments caused by the driving forces. Thus for good compatibility of the front axle with a limite
30、d-slip differential, the design requires: 1) vertical bending angles which are centered around 0v or negative ( 0v ) with same values of v on both left and right sides； and 2) sideshafts of equal length. The influence of the secondary moments on the steering is not only limited to the direct reactio
31、ns described above. Indirect reactions from the connection shaft between the wheel-side and the gearbox-side joint can also arise, as shown below: Figure 9: Indirect Reactions Generated by Halfshaft Articulation in the Vertical Plane For transmission of torque without loss and vdvw both of the secon
32、dary moments acting on the connection shaft compensate each other. In reality (with torque loss), however, a secondary moment difference appears: WDDW MMM 12 With TTT WD 22 The secondary moment difference is: DWM VWWVWWVDVDW TTDTwTT t a n/2/t a ns i n/t a n 22/2 For reasons of simplification it appl
33、y that VVWVD and TTT WD to give VVVDW TM t a n/1s in/12/t a n DWM requires opposing reaction forces on both joints where LMF DWDW / . Due to the joint disturbance lever arm f, a further steering torque also acts around the king-pin axis: LfMT DWf /c o s loloDWhihiDWf LMLMfT /c o s Where fT Steering
34、Torque per Wheel fT Steering Torque Difference f Joint Disturbance Lever L Connection shaft (halfshaft) Length For small values of f, which should be ideally zero, fT is of minor influence. 5 EFFECT ON CORNERING Viscous couplings also provide a self-locking torque when cornering, due to speed differences between the driving wheels. During steady state cornering, as shown in figure 10, the slower inside wheel tends to be additionally driven through the viscous coupling by the outside wheel.
