OK the main difference with progressive style springs vs constant rate springs is the more you compress variable rate (progressive) springs the change in resistance is non linear so they act stiffer and stiffer as they compress. The main advantage of this is to prevent bottoming out with stronger forces such as hard braking, hitting a pothole, etc. A constant rate spring will have the same increase in stiffness for 1 inch compression, 2 in compression, etc.whereas a progressive rate will have1 unit of change in stiffness for the first inch, perhaps twice as much change for the 2nd inch, etc.
That being said, I believe the stock RSV is a progressive type, but I havn't taken mine apart yet. Youm can tell a progressive style spring by the spacing between coils. The spacing will not be linear but rather some coils will be closer thanothers. Constant rate springs have the same spacing between coils. I'm still in the dark as to which springs to put into the trike but I want overall increase in stiffness to help minimize front end oscillations. The Sonics are constant rate but supposedly are stiffer than stock.
There seems to be a debate whether or not Progressive makes a better than stock spring for the RSV...