Have I got this wrong all along ? I thought that tightening preload was supposed to do just that and stiffen up travel if not reducing it.
Spring rates are measured in force per distance. i.e. pounds per inch. This is a linear relationship. i.e. for a 1lb/in spring will compress 1 inch for every 1 pound of force applied. This means, 1 pound force compresses the spring 1 inch, 2 pounds force compresses the spring 2 inches, 3 pounds force compresses the spring 3 inches, etc.
Applying preload is adding force to the spring before adding the force caused by your mass and gravity. So, if you add 1lb of preload to a 1lb/in spring it will now take 2 pounds to compress the spring 1 inch and 3 pounds to compress the spring 2 inches.
All that adding preload does is compensate for your weight. It does not stiffen the suspension or reduce travel (the length of the damper usually limits travel). In extreme cases, with an improperly designed suspension a spring could bind, coils touch, therefore limiting travel. This should not be the case.
All of the above is for linear rate springs. Progressive rate springs function similarly. Progressive rate springs will be composed of two or more spring rates. Some springs are varied over their entire length.
A two rate progressive spring will have one rate for part of the spring and another rate for the rest of the spring. Adding enough preload to a progressive rate spring can eliminate the softer rate effectively giving a liner rate spring of the stiffer rate with some initial preload. This would be a dual rate spring. i.e. two linear rates, 1 lb/in for the first 2 inches then 2 lb/in for the remaining length.
A progressive rate spring with a rate varied over its entire length would increase the spring rate with any added preload making the spring harder and harder to compress as preload is added. This would not be a linear rate spring. An example of this would be 1lb for the first inch, 3lb for the second inch, 6lb for the third and so on.
Plenty of information on the internet if you want to know more.