Are progressive springs really progressive?Some force curve measurements of progressive springs by GreenPylons.
Pylon, who designed and built an open-source force curve meter (featured in KBD #21), measured the 63.5g SPRiT complex and progressive springs from a gBoards.ca SPRiT spring sampler, and the Thic Thock 68g MP springs. The SPRiT springs were placed in a Gateron Yellow (one test with the leaf, and another test with the leaf removed), and the Thic Thock spring was placed in a JWK Alpaca "V2" / "B".
Obviously graphs don't look very nonlinear. The Thick Thock 68g MP pretty much looks linear and very similar to the Durock 67g spring when installed in an Alpaca switch. The SPRiT springs deviate about 1-2gf from linear, and also don't seem to behave nonlinearly by very much compared to a regular Gateron Yellow spring. The SPRiT springs are also much lighter than the 63.5g claimed.
It's unclear if this is an implementation issue with these particular "progressive" springs (the tighter wound sections on the SPRiT and TT springs are relatively short compared to the regularly-wound portion of the spring, and seemingly too short to have much of an effect), or if keyswitches just inherently don't provide enough travel and compression for progressive springs to really enter their nonlinear regime.
Pylon also hasn't had a chance to test other progressive springs, and it is possible that some of them in fact behave nonlinearly as expected. But in any case, you should probably be skeptical about purportedly nonlinear "progressive" springs for the time being, until you see measurements that validate their claims.