progressive means the spring rate increases as the axle goes up. it is progressive because there are different sized leaf's. what it basically gives you is the ride quality of a soft spring, but wont bottom out. i put a ton of progression in to the springs when i build them (yes there are lots of tricks). on mine, when i was winching it down for those pics earlier,it proved this. there is a total of about 10" uptravel. when it gets to about 5" left, the rate has increased quite a bit. when it gets around 3", the rate is so high, i couldnt pull it down more with a 1ton winch!
How can you change the spring rate change? Being that the force put out by a spring equals the product of the spring rate and the distance the spring is streched from its equilibrium position (F = -kx), any spring will in effect be progressive, even with a constant spring rate. I don't see how the spring rate of a leaf spring could change through out its compression. Am I missing something?
multiple leaves with different radius bends will have different spring rates at the same flexion point. when stacked on top of each other and interplay between them, springrate will lose its linear rate.