Spring Rate Explained

The usual approach is to start from the corner mass and a target ride frequency, then work back to the spring rate through the motion ratio. The wheel rate you need is roughly 4π² times the sprung corner mass times the frequency squared. Divide that by the square of the motion ratio to get the spring rate. You should weigh the vehicle at each corner in its working condition, with the extra load fitted, since an estimated corner mass is a common source of error. A lowered car also has less travel before the bump stop, so it usually needs a higher rate to avoid bottoming, and the dampers should be revalved or replaced to suit.

You measure it on a spring load tester, recording the load at two heights within the working range and dividing the change in load by the change in height. It is best to avoid the first and last parts of travel, since the end coils are still seating at low load and coils start to touch near solid height. EN 13906-1 (a European standard) covers the calculation and design of helical compression springs, and the drawing should state the test heights and the permitted tolerance.

Its rate hardly changes. What does fall, as the steel relaxes, is its free length and the load it carries at a given height, which is why a tired spring shows up as lost ride height rather than a softer feel. It is worth checking ride height against the specification and replacing springs in axle pairs. Corrosion pits on the wire are a more serious concern, because fatigue cracks tend to start there.