A 400lb spring requires 400lbs to compress the first inch. And it takes 800lbs to compress two inches. etc. Coil springs are not produced to “exact” standards. If a coil is offered in 50lb increments (example: 350, 400, 450) a 400lb spring may actually be anywhere from 376 to 425lbs. Most quality spring manufactures have a slightly tighter tolerance, but that’s the concept.
Springs define total length as well as stroke. Running a coil that is too long or short in either measurement can create problems, most often premature coil bind, which limits travel and creates an infinite spring rate when bottomed, which means all your “suspension” is in the tyres. I grew up building custom cars and we had to be really aware of this. Now, some auto race teams deliberately bind corners out which is great, except there’s no forgiveness if the tyre misbehaves. Anyhow another thing for another day.
A coil spring’s support is based on its diameter (OD and ID), the actual coil’s wire diameter, how much elasticity the material has, how many active coils it has, as well as the angle of the active coils. Understanding coil springs is really an understanding of how the active coils behave, less active coils in a spring increases the resistance to compression. Most MTB springs have closed and ground ends for performance and ease of mount. These wrapped ends do not count as active coils. When a spring is compressed the outer coils contact the wrapped ends. Technically all the coils are compressed equally, but the outer active coils have a lesser distance to the wrapped end coils which is why they make contact and,
Preload applies additional tension to the spring. It decreases the number of active coils. Tighten the upper spring perch to increase Preload, loosen it to decrease Preload.
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