What spring rate tells you

The number needs a unit

A linear spring rated at 30 N/mm needs an extra 30 newtons of force for each additional millimetre of compression. A 60 N/mm spring needs twice the force for the same movement. This describes the spring itself. The car also has suspension leverage, tyres and dampers, so that number cannot predict how two different cars will ride.

Two springs under the same added force

The animation uses two illustrative linear springs and an added force of 300 N. From F = k × x, the 30 N/mm spring compresses 10 mm and the 60 N/mm spring compresses 5 mm. Their drawn movement uses the same scale. These are worked examples, not spring recommendations or the load on a particular corner of a car.

Preload is kept out of this example so the extra compression is easy to compare. In an installed assembly, preload and available travel need checking separately. Progressive springs change rate as they compress, so a single constant-rate calculation does not describe their whole travel.

Keep front, rear and units together

Copy the complete front/rear specification when comparing kits. Listings may use N/mm, lb/in or the kg/mm shorthand commonly used for kgf/mm. One kgf/mm is approximately 9.81 N/mm. A front figure in one unit cannot be compared directly with a rear figure in another.

Keep the spring part code too. Free length, internal diameter and end shape must suit the seat and available travel; equal rate does not make two springs interchangeable.

Check a rate change with us

Send the exact kit code and current spring markings, then describe the change you want. Include the car, tyres and significant weight changes. We can check whether the proposed spring suits the damper and application. A custom spring choice may need a matching damper specification. Keep the original setup record so any later change has a useful reference.