The Tuesday Tune Ep 17: Moar Shimz

Mar 14, 2017
by Vorsprung Suspension  
Views: 2,912    Faves: 17    Comments: 4


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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35 Comments

  • + 11
 Moar shimz - a life changing recipe that every primate understands. But, can they fix a missing link? Asking for a friend.
  • + 5
 So many in-jokes there, my head hurts!
  • + 2
 @VorsprungSuspension: hi,great series of talks. Could you do one explaining how the millyard rear shock/strut works and whether it would be as good as it's been described by the people who've ridden it. I.E. Steve jones from dirt mag. Thanks
  • + 2
 Only if your riding a Rux.
  • + 6
 Best video series on mtb suspension made so far. I hope you will continue next winter.
@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.
  • + 1
 Random thought, have you tried how the fork rebounds without the rebound shims installed at all (well, just one for checkvalving), or even without oil in the damper (though this might throw your feeling way off)? Could be that you can't get faster due to a lack of stored energy (spring) in the first place.
  • + 1
 If you have two 0.10mm shims, and it's still too slow (which seems very odd, they should be lightning fast at full open), check that nothing is obstructing the damping circuits (especially the LS bleed) and if necessary remove one of the shims or exchange it for a smaller diameter.
  • + 1
 @Lwerewolf: yes, I disassembled the fork quite a view times now and also tried the fork just with the spring installed. And there it rebounded like pogo stick. The oil I am using is a red line 2,5 which has a quite low viscosity.
  • + 1
 @VorsprungSuspension: that's a good hint maybe the low speed rebound does not open wide enough.
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.
  • + 9
 You look pretty tired man...Great vid though!!
  • + 6
 +1, the video's preview pic says it all.. Get some sleep xD
  • + 2
 Apparently he was sick last week which is why there wasn't a video - probably still on the back end of that.
  • + 7
 That's my default look haha
  • + 2
 Thanks heaps Steve, the Tuesday Tune is the highlight of my pink bike week!
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!
  • + 2
 Since the fox fit2 cart was mentioned again, can you explain why the 2010 fit was considered problematic? My (wild) guess is the bottom seals, but I can't find more specific info.

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 Big Grin ), but in the case of a system controlling a single wheel (i.e. an MTB rear shock)... ummm... the only thing I can think of that I haven't seen on an MTB yet is the variable size metering pin. Am I missing something? Smile
  • + 2
 The pre-inverted Fit cartridges had some issues with oil ingestion, because they used a compression bladder (rather than an expansion bladder) that meant once the damper sucked some oil in through the shaft seal (and it does happen over time) the bladder was no longer able to sufficiently compensate for the shaft volume entering the cartridge body. When you bottomed it out in that state it could blow the bottom out of the cartridge and dump all the oil into the fork lowers.

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.
  • + 2
 With a specific thickness, a shorter straight spring is stiffer than a longer spring. Does this not apply to shims as well? You state that a wider shim is stiffer than a narrower shim. Even bending them by hand, I find this to the contrary.
  • + 2
 The reason smaller diameter shims feel stiffer to bend by hand is because you have less leverage to bend them with. However, in the case of a shimmed valve, the effective leverage that you have to bend them with is a constant, since it is the larger diameter shims in front of it that are actually dealing directly with the resolved force from the oil pressure in the port. The distance from port to clamp diameter doesn't change, so the effective leverage that the oil pressure has does not change either.
  • + 3
 Good stuff, man. I love to tinker with the sometimes daunting and tedious. If you know what you're doing with suspension, it pays off BIG time.
  • + 1
 @VorsprungSuspension i guess i'm still a little confused about the purpose of a midvalve as an additional compression damper. i know that it acts as the rebound damper's check valve, but what does a midvalve do that the shim stack on the compression damper can't do?
  • + 1
 @VorsprungSuspension why usually dont we see dh forks with midvalves that actually have shims instead of a single checkvalve. It isnt better to control the damping with a midvalve due to larger flow it sees?
  • + 1
 Because if you generate much pressure drop over the midvalve (which isn't adjustable externally) with a very open compression adjuster setting, you can end up causing cavitation as most DH fork cartridges are not (substantially) pressurised.
  • + 2
 Got overwhelemd. Can you explain what exactly the base valve and mid valve is? I think the midvalve is the piston with the compression and rebound stack, bit not sure.
  • + 1
 You're right Wink
  • + 1
 That was the moment where I threw in the towel haha.
  • + 2
 A midvalve is essentially a way of tuning the check shim on the rebound piston to damp compression (contrasted with a standard check shim, whose only function on a compression event is to get out of the way to allow oil flow to the compression damper.)

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? Wink
  • + 3
 @groghunter: if you're having issues with your Pike's performance it's almost certainly not the midvalve spring stiffness, they really aren't overly stiff in spite of what some people have been saying. You can prove this for yourself by removing your compression shim stack entirely and cycling the fork. FYI - bit.ly/2mJPmP1
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.
  • + 2
 @carym the base valve is the compression circuit whose location in the fork or shock is fixed (ie where your compression adjuster is), the mid valve is the compression circuit on the moving piston attached to the damper shaft. The moving piston is not always referred to as a mid valve in a rear shock, I just use the terms interchangeably because its function in a rear shock is conceptually/potentially the same as that in a fork.
  • + 1
 @VorsprungSuspension: Thanks for the advice. I think I'm good on tire pressure & spring rate (sagging somewhere around 20% on a 160mm with no tokens) so i'll think about using a thinner shim. This also confirms what my instincts told me, which was that damper spiking is a coarse tuning issue, & the mid-valve spring is a fine tuning adjustment. Hmmm, I don't see any reason why I couldn't leave the compression valve out for some controlled tuning of the mid-valve, provided I'm only testing it for spiking (what I wouldn't give a dyno for that, but I'll figure something out.)

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.
  • + 2
 My favourite program anywhere. This and Jeopardy.
  • + 2
 Don't tell me I don't need moar shimz, I don't need such negativity.
  • + 2
 Out of interest @VorsprungSuspension what do you run on your bike?
  • + 1
 My personal bikes are rolling test beds. The forks/shocks on there vary from time to time.
  • + 1
 @VorsprungSuspension: All though it's undoubtedly the truth, nice dodge of the question! Thanks for the series buddy!
:-)
  • + 2
 BTW - my last comment is supposed to be a bit tongue in cheek so please don't read it like I'm being an arse! When I re-read it, it didn't sound so good!

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