I see you've latched on to a marketing buzz word. Yes, the stock springs are progressive.
Evenly spaced spring coils store increasing amounts of energy as they are compressed, until they are fully compressed (coils touching each other) at which time storage accumulation stops abruptly as well as motion. The amount of energy to compress increases at a more or less linear rate.
How to Determine Rate For Compression Springs
Rate which is the change in load per unit deflection, may be determined by the following procedure:
1. Deflect spring to approximately 20 percent of available deflection and measure load (P1) and spring length (L1).
2. Deflect spring to approximately 80 percent of available deflection and measure load (P2) and spring length (L2). Be certain that no coils (other than closed ends) are touching L2.
3. Calculate rate (R) lb./in. (N/mm)
R = (P2 - P1) / (L1 - L2)
Unevenly spaced coils (assuming total wire rod length is equal), store the same energy until the closer spaced coils begin to make contact with one another, this shortens the wire rod length and begins to require an increased rate of energy to cause further compression of the coil spring. This is the progressive portion of the non-uniformly wound coil spring. And, the spring rate changes during this compression extreme.
The stock spring uses these progressive properties to resit hitting hard stops during bottoming out operations and to ease the damage or loads on the rubber snubbers that are part of the shock absorber.
It is possible to design springs (also progressive) to require higher energy storage while in high G load turns. Meaning they are designed to begin "stacking the coils" sooner in the load energy accumulation graph. This would be beneficial to keep the swing arm from bottoming and causing tire contact deformation. The stock spring does do this to some degree but is not very compliant to uneven road surfaces while at this compression point.
A progressive spring should have more than one spring rate spec. One applied to "normal" travel, and another for progressive rate increase, as well as, at what total compression height it becomes progressive.
But, no one in marketing and sales wants to explain all that (and few wish to give it that much thought), hence "progressive" becomes the end all explanation with all the ancillary connotations it conjures up in the uneducated consumer. It is such a good sounding word, isn't it?
Cheers,
