Clicked, there seem to be some discrepancies between your two statements? In the latter, you said: These two stacks are the same. They have the same clamp, the same taper and the same crossover. The face shims were set by the thickness cubed rule to be the same. With everything same, same, same you don't need shim factors to figure out the stiffness of those two stacks should have been the same. However, in the post I was quoting from, you said (emphasis added at bottom): From shim factory theory the difference in face shim stiffness is: SF14: Baseline stack of 14x40.2 face shims with a shim factor of sf=112 SF4: Replacement stack of 4x40.3 face shims with sf=108 The replacement face shims are about 4% softer To figure out the change in damping force you need some way to estimate of the stiffness of the overall stack. Using tapered stack shim factors:SF14: The baseline stack with 14x40.2 face shims has a tapered stack shim factor of tsf= 262 SF4: Shim factor scaled stack with 4x40.3 face shims and a tapered stack shim factor of tsf= 258 The replacement stack is about 2% softer in overall stack stiffness ...The replacement 4x40.3 stack was expected to be softer and the dyno shows that. To get the same damping force the 4x40.3 stack needs a shaft velocity that is about 8% higher. That 8% shaft velocity difference implies the stack was about 8% softer, not the 2% expected from shim factor theory. Shim factors missed damping force change by a factor of 4.. I agree with you that the stacks should be comparable without needing the shim factors, by using pure thickness cubed factors instead. However, in your post, your comment of the 4X factor of error on damping force change was specifically referring back to the shim factor numbers (see above). So, I think that if the shim factor equations are +/- 30%, that smaller error (8% difference vs 2% difference) is completely in the noise, and the results actually show good predictive power of the shim factor equations. If we look at the purely thickness-based comparison, I think that does beg some questions, as you indicated. In that case, the results are quite close though. The thickness-cubed calculations suggest that the stack should be 4% softer, and the dyno indicates about 8% softer, meaning that the total system error is only 4%. That is a really good match! I think that a 4% error for this entire system (different shims, different build, re-assembly of the damper, possibly new fluid and nitrogen, dyno calibration error, dyno measurement error, repeatability, etc) is a really really good result! In any case, I still really like your question. We like to simplify shim stacks down to the "beam stiffness theory" behind them, but in reality, in the application, we know that things are a bit more complicated than that. There are fluid flow effects, there are friction effects, there may be preload between the shims, and the deformed shapes of thick shims and thin shims may be slightly different too. IF we can believe the small error between these two tests (I am still on the fence on that), I think it would show that these "other effects" can have an impact on the order of 4% on overall damping, outside of the stack stiffness effects. However, I struggle to separate to what extent this 4% is due to other effects (friction, fluid dynamics, etc) and to what extent it's just misc. error. I think it would be very valuable to do the same exercise with a large difference between stacks (say, 20% or more) and see the predictive power in that case. Large differences between the stacks allow us to more easily separate the small errors inherent in any test, and the true differences between predicted and actual damping that we are trying to investigate.