The gas chamber volume has to be very small to get a large enough pressure change for the pneumatic spring effect to become significant. That said if you increase the shock Hc value by a factor of 10 will produce constant pressure at near atmospheric in the air pocket. That will eliminate any pneumatic spring.

If you haven't already you should check out the shock dyno testing that Icecyc1 did. He tested hole sizes/number, oil viscosity, bladders, different piston geometries..... Very interesting work. https://www.rctech.net/forum/electri...t-results.html.

I have thought about including progressive spring rates but it adds a whole other level of complexity. It makes everything dependent on the displacement of the spring. For example the roll stiffness would have a fixed value at zero displacement with a small factor increasing the rate as the spring is compressed.

I'm not surprised that linear springs are not 100% linear. I plan to investigate this as well in the near future.