In a linked rear suspension, the dogbone provides increasing resistance/leverage as the swingarm moves through its arc. In this application we use a linear spring. In a linkless suspension there is a one-for-one relationship between the swingarm arc and the movement of the shock. Using a linear rate spring in this application results in a very stiff ride during small movements, and a shock that will blow through its travel during sharp edge transitions. A progressive rate spring provides a compliant ride for the small movements, yet a higher rate for the sharp edge transitions.
For the forks it’s just the opposite. A progressive spring in the fork produces significant brake dive. This, combined with a damping rod provides a fork that is too soft for the initial stroke, and too firm (usually hydrolocked) in the mid-to-late stroke. RaceTech provides a good write up on the limitations of a damping rod fork:
“Damping rods while inexpensive to manufacture have major limitations. To create compression damping, oil is shoved through a hole or holes. Shoving oil through holes creates very little resistance to flow at low vertical wheel velocities as when hitting a dip or gully or applying the front brakes. This allows the forks to shoot through the travel fairly easily, diving or bottoming in these situations. On the other hand when the wheel hits something square edge, especially at speed, it needs a lot of oil to pass through the damping holes very quickly. Unfortunately the nature of shoving oil through holes is that as the wheel velocity increases the damping force increases with the square of the velocity. In other words if you double the velocity you get four times the force. This means the hole basically "hydraulic locks" resulting in a harsh spike. Damping rods give the worst of both worlds; they are both too mushy and too harsh at the same time.
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