Double wishbone 1
The above model shows the rather rare, unguided axle with double wishbones, in complete form, shown in figure 6. An axle with four guides in the rear is, in principle no different. In figure 4, e.g., both parts of the lower A-arm guide are arranged very differently, and are adapted to cope more effectively with the forces working on the wheel. Should a type of tie rod be added (also figure 6), allowing the wheel more or less deflection, we then speak of a 5- or multiple guides axle.
Two ball-joints at the points of the wishbones allow for the use as a front axle. As figure 7 shows, the lower guides are often a little longer which causes a negative camber both when the springs are compressed or when relaxed (rebound). In a formula-one racing car we see them made partly from Carbon and very aerodynamically formed (figure 3). Here the centre rod takes up the forces during spring compression and passes them on to horizontal spring elements (pushrods). Sometimes both wishbones, from their axis of rotation, are also slightly bridged (figure 2). This somewhat counteracts the forward dipping action when braking.
In practice double wishbones are installed more in heavier passenger cars and transporters. In lighter vehicles McPherson struts are preferred. These are simpler, cheaper and often more space-saving. although we still find rigid front axles in trucks, they are not found in coaches. Figure 7 shows the wishbones on the guided axle of a modern, articulated city-bus. One special characteristic of this axle construction can be seen in figure 5. The upper ball-joint is placed above the tyre which allows a steering-swivel axle almost concentric to the wheel, which slightly reduces the disturbance force to the steering. 10/08
cartecc.com Top of page Index