As long as rigid axles are guided on leaf-springs, apart from the spring-suspension, they also take over the task of guiding the wheels. A lot more favourable, also for the responsive properties of the supension, is to separate these two duties. Therefore, one installs coil-springs, the axle is then guided by trailing-arms. With the transverse controlling arm, things become more difficult. One can join the axle to the car-body, through using a wishbone, which changes the wheelbase when the springs are compressed, or with a Panhard-rod , which causes a lateral displacement.
Actually, the best solution to the problem is presented by the Watt-linkage. James Watt, the inventor of the steam-engine, really did solve the problem with his construction of how a piston, always remaining vertical on a rod, could transfer its force to a lever which rotated around a central point. In our case, there are two rods which are connected at different heights to the car body by a pivot. These do not have direct access to the axle, but work through a rocker-arm. This rocker-arm compensates for any side movements when the springs are compressed. 04/10
