advice on springing a python trike ? long post !!!

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   Spring preload: The damper will have something to limit its extension (or sometimes on cars there is a separate droop stop). Preload compresses the spring against this stop. The purpose of this is to limit the amount of damper travel required, as the static deflection (the amount the spring compresses when the weight of rider and trike is placed on it) will be reduced.
Imagine a spring/damper requires 1" of travel to absorb a 1g bump. This will mean you will need a spring with the capacity to absorb at least twice that (2"), because the static deflection will be equal to the travel required for a 1g hit. This assumes the spring starts at 'free length', in other words, its uncompressed length. If we were to apply 1/2" of preload to this spring then the static deflection will only be 1/2" and the total damper travel required will only be 1 1/2", not 2".

Preload becomes more necessary the softer the suspension. A saloon car will have a suspension frequency of about 1.5 Hz, so will need about 4 1/2" of travel at the wheel to absorb a 1g bump, so accommodating an equal amount of droop travel becomes impractical. Most cars require spring compressors to fit or remove the springs for this reason.

In the case of your first picture, you can give the spring preload simply by tightening the nut and bolt.

Edited by GeoffBird - 23 October 2017 at 2:09pm