Hi, Phil.
A complex question that you ask.
First, I believe that although heavier, coil over shocks are far superior to the air shocks Ernie provides. Better damping and no worry of air leaks.
As to preload, think of it as changing the length of the shock, eye-to-eye, when the gyro is sitting at rest on the ground. If the shock is too short, the keel drags, if too long, you are way up high and the wheels gain a positive camber which is bad. This can be demonstrated during a hang test, BTW. A longer spring would add preload but are difficult to find and near impossible to adjust. Adjusting the spring's upper clamp to mast location provides a small amount of what I call 'ride height' adjustment.
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