Also the dual spring set up gives a rising rate system. As the main spring reaches maximum compression the lower spring, which has a higher rate, comes into play.
Although I initially thought the same as you it's actually the other way around Peter. Dan at SSL explained as follows:
Both springs compress initially but the shorter spring becomes coil bound first (at around 25mm total compression from static) despite being the higher rate, thus leaving the longer main only in play. Seems counter-intuitive, but when both springs in play, being in series, the combined rate is lower still. Once you've got your head round this it all makes sense
Have a look here at the first graphic with the red line.
https://sites.google.com/site/morganatica/suspension/springs-progressive-rising-rateI am not talking specifically about the Suplex type, but first of all about the normal front axle.
It is definitely counterintuitive
In the arrangement of the Morgan front suspension it is not a series arrangement. It is a parallel arrangement. With that the contra-intuition already starts. Both springs add their forces in the arrangement. It is not a series connection. When the lower spring presses together with the upper spring on the wheel hub, force is very high and the suspension is hard. Unfortunately, this is already the case when you only need little spring power and the car could be driven more comfortably (this is what the properties of the Suplex principle build on to change this disadvantage).
If you drive over a bump with the lower spring already preloaded and the upper spring is compressed but suddenly the lower spring loses contact, then only the upper spring is in play. As a result the car suddenly sags.
In principle the lower spring is responsible for two things. Firstly, it is responsible for preventing the car from tilting in curves (on the unloaded side it prevents the car from climbing up very high) and secondly, it is responsible for the controlled escape from a bump.
