The slide shows a rigid axle guided by a Watt linkage.
The Panhard rod should be as long as possible, for transverse offset of the axle during suspension movement to be kept as low as possible, since this would have a negative effect on straight-running properties.
The position of the roll center agrees with the virtual point of transfer of lateral forces between axle and body. For rigid axles suspended on leaf springs, the roll center is located at the height of the spring-mounting points on the body. In case of the transverse suspension consisting of Panhard rod, the roll center lies at the height of the (horizontal) Panhard rod; while for the Watt linkage, at the height of the middle hinge.