My own understanding is that rebound springs were historically shortened as a bodge (as per CBY post) to reduce static pre-load on rebound springs which in turn resulted in the fact based massively high spring rate of having both springs in play. This in turn causing uncomfortably hard suspension but then sagging into corners due to inside main spring compression taking rebound spring out of play and effective spring rate reducing to that of main spring alone. Several owners and at least one magazine test of Plus 8 brought this fact to light many decades ago. Peter Ballard explained it and mentioned that ironically as main springs weakened with age the issue could dissapear. Suggesting the rebound pre-load test is rubbish for an entirely different and less important reason is not a fair comment. Its as relative today for trads as ever.
Exactly correct, a pre-loaded main spring due to a compressed rebound spring gives a massively high initial rate followed by a sudden drop in rate as the rebound spring unloads.
All documented and explained years ago by Peter Ballard, I remember the huge consternation it caused among people who didn't understand the physics behind springs in parallel vs. in series. It was hugely entertaining at the time to read all the eMog posts telling Peter he didn't understand how suspension systems worked!
https://www.lambdapi.co.uk/morganatica/Front_Suspension_Spring_Characteristics_-_Trad_Models
