Spring Rate is the amount of weight needed to compress a spring a certain . Springs are rated in LB/in (in metric system kg/mm), or specifically, how many pounds of weight are required to depress the spring by one inch. To convert LB/in to kg/mm international, you must divide divide figure by 56.
Consider you have 2 springs having different spring rates: One with 345 g/mm and the other with 480 g/mm. So what does it mean?
It means the 1st spring will compress 1 mm if you put a load of 345 grams, while the 2nd one will not. The 2nd one will need a 480 g. load to compress 1 mm. According to this, we can say that the 2nd spring is harder than the 1st one, or we can state that:
Springs that have a low Spring Rate are soft, while springs that have a high Spring Rate are stiffer.
If there are 2 different values listed, it means that the spring starts at one rate, and ends at another rate under full compression.
For example: a 10lb to 25lb progressive spring will need 10lb to compress it the first inch, then 13lb the next inch, and so on, until the end of the travel; it will take 25lb to compress it the last inch. The benefit of this is that the spring can be soft enough at the start of the travel to offer a soft ride yet be stiff enough at the end of the travel to performance well during hard braking and turning.
What Affects the Spring Rate?
There are 3 things that affect the spring rate:
1. Diameter of the wire: Diameter of the wire itself affects the spring rate because when diameter of the wire increases it gets stronger, meaning a wire which is harder to compress. So, if we know that a wire becomes harder when its diameter increases, we can say that: When wire diameter increases, spring rate increases.
2. Diameter of the spring: That is in fact 'the mean diameter of the spring', achieved by subtracting the diameter of the wire from diameter of the spring:
The overall outside diameter of the spring (mm) - diameter of the wire (mm)
When diameter of the suspension spring increases, the spring rate decreases.
3. Number of Active Coils (length / height of the coil spring): Determination of the active coil number varies according to spring design. Total coil number -2 for springs with both ends closed. Count the total coils -1 for springs with one end closed and one end open.
As the number of active coil decrease, the spring rate increases.
Normal Springs has a fixed spring rate.
Step Linear Springs are springs which have 2 different spring rates.
Progressive Springs have a variable spring rate.