The Tuesday Tune Ep 17: Moar Shimz
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.
MENTIONS: @VorsprungSuspension
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@steve: I have been trying to tune a old rock shox lyrik rcdh midvalve in order to get a faster rebound. But it nearly seems to be impossible, because it only came with two 0.1 shims and one 0.4 clamp shim. So it seems hard to decrease the thickness of the stack. And oil viscosity is already at a very low level. After watching this my last hope is to swap the clamp shim to fix the issue.
The lyriks rebound is way slower when compared to my marzocchi 44 rc3 which only has a ls rebound bleed port. This made me scratch my had more than one time.
Another thing - so far you've covered the more-or-less standard suspension devices used in MTBs today (shimmed dampers, poppet valves, boost valves, fixed-size orifices). Can you say a few words about the hydropneumatic dampers that we've been hearing about as of late? From what I've read thus far, hydropneumatic suspension systems are unique in the ways that they interconnect damping circuits/springs for multiple wheels, to an extent allowing them to react to different inputs (acceleration, braking, roll, bump) (the creuat system/hydragas/etc patents looked quite funky the first time I saw them
Semi-dependent suspension systems have been around for a long time. Most if not all of them provide some potential advantage in some manner (usually with other disadvantages as well), but often the complexity and introduction of more tuning variables means the net result is not as good as conventional suspension.
If it helps the decision wether to continue the TT next winter I for one would happily pay to watch your videos, e.g. with a "support the TT" purchase (donation) on your website.
A topic I'm really curious about is the relative effect that different viscosity damping fluid has on suspension behaviour, as compared with the effects of the external adjustments and the changes possible with a revalve. I've been experimenting with ghetto damper tuning recently with different fluids and haven't found as much difference in feel as I thought I might. Very unscientific though, I realise there are many many variables to consider...
Anyway, please keep up the good work, you clearly have a large and faithful following!
Typically, this means, unlike a standard check shim which would be clamped, it's backed by a spring, allowing it to open progressively, rather than purely on flow direction.
Incidentally, there's a reasonable conclusion that the midvalve tune on the Pike damper is too stiff, leading to the spiking that many people complain about. I know of at least 2 aftermarket tunes that make it lighter, & in the case of the FAST damper upgrade, they actually use a stiffer compression shim stack.
FWIW, my charger currently has a lighter than stock compression tune, & it still spikes. Unfortunately, finding replacement springs for this usage is much harder than finding shims.
Unless @VorsprungSuspension wants to sell me one?
What you could try instead is replacing the midvalve shim with a thinner one. Those things reach their full float position very quickly, any damping after that is to do with the combination of float distance, port size and shim thickness, but the port volumes on those pistons are huge and realistically this probably is not your problem.
It's also possible that the sharp harshness you're experiencing is not actually spiking of the compression damper. Excessively stiff or excessively soft setups can create the same sensation, as can overly high tyre pressures.
My preference for a stiffer spring, & less damping, is probably somewhat at the root of the issue as well: The pike seems to be a designed to be very damped, with a lot of sag, & I hate the way that makes bikes feel.
:-)