The Tuesday Tune Ep 17 - Moar Shimz (and what they do)
Alright, here it is; shim stacks. Shimmed valves are the most common form of high-performance valve in suspension dampers, and the raison d'être for many forums and indeed entire websites to exist. Some say they control the flow of oil, others say they are responsible for controlling the flow of karma throughout the cosmos. Being light, simple, compact, highly tunable and relatively cheap to manufacture, shimmed valves very often turn out to be the right tool for this particular job.
So what we've put together here is a very brief overview of the simplest form of a shimmed valve. Within this video are many technical omissions for the sake of simplicity so that we can focus on the very basics of a single-stage (non-crossover), unpreloaded, zero-float shim stack, and garner some insight into the way that the actual shims function.
We feel that the simplest way to understand a shim stack is as an unpreloaded spring holding a valve closed, and it is predominantly the stiffness of that spring that determines how much pressure it takes to open the valve a certain distance—this dictates the relationship between pressure drop (damping force) over the valve and volumetric flow rate (determined by shaft speed) through the valve. A shim stack of this configuration will deliver quite a linear force vs velocity characteristic over the majority of its operating range, up until the point at which it cannot open any further to increase the available flow area. Other variations on this type of valve can deliver substantially digressive or progressive curves, but we aren't covering those here.
This video certainly isn't comprehensive, it provides exactly nothing in the way of precise methods of calculation, and it makes many generalizations, simplifications, and omissions for the sake of explaining the stack's function and demands within a short video. However, hopefully, you'll find it interesting and informative one way or another.