Preload

Preload applies to coil spring type suspension not air suspension. The weight rating of coil springs can vary depending on they're intended use. Preloading a coil spring simply implies that the spring is being pre-compressed which alters it's stored/potential energy.
For example: A progressive spring with a weight rating of 300lbs when compressed 1/2" will increase it's weight rating to perhaps 350lbs.
In order to adjust suspension for various weight riders, more or less preload can be applied to alter the characteristics of a spring which will result in a smoother or harder ride.

BYKAID: 
SP.500.5.12

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