Spring ratio/Damper ratio is a ratio between the displacement of the spring/damper and the vertical displacement of the wheel centre.
As the spring and dampers on a suspension system are mounted inboard of the wheel centre, the spring/damper ratio on production cars is almost always less than 1 (i.e. the damper or spring displacement is less than the wheel centre displacement). On race cars using push or pull rod suspension systems, ratios greater than 1 are possible.
The ratio is governed by the position of the spring/damper relative to the wheel centre and also by the inclination of the spring/damper. It is generally desirable to have a spring or damper ratio greater than ~0.7. From the damper perspective, this ensures that for any wheel centre displacement, there is a reasonable amount of damper displacement to allow the damper to generate force and influence the motion of the suspension. It is less critical from a spring point of view, but a higher ratio allows a softer spring to be used to deliver the target wheel rate. Lower damper and spring ratios do result in less damper or spring travel for a given amount of suspension wheel travel, this may be a consideration in some suspension designs.
Spring and damper ratios are not constant and change with suspension travel. The influence of this ratio change will be examined in a future post.