To The Point - Shim Stacks

Jun 25, 2013
by Mike Levy  
RockShox's Eric Neely is putting his Master's degree in vehicle dynamics to good use, having played a large role in the company's shorter travel offerings over the last few years. His handiwork includes the new 'Dig' valve damper used in the Sid and Revelation platforms, although Neely's projects have moved up in stroke over the last six months, with him transitioning to working on the dampers used in their downhill forks.



First off, what exactly is a shim stack and what is its job?

Shims are thin metal discs that come in a wide variety of diameters and thicknesses, and
they are typically stamped out of spring steel for good fatigue life. We use very high
quality steel that allows them to be tumbled to remove the burrs or sharp edges left
from the stamping process. A shim stack is exactly what it sounds like, a stack of these
thin metal discs stacked together in various combinations of diameters and thicknesses
against the face of the damping piston. Shim stacks can be configured in almost infinite
variations: straight (same diameter shims stacked), pyramid (shim diameter decreases
as you get further away from the piston
), split (two different shim stacks directly on top
of each other separated by a small spacer
), etc. The piston has ports that are configured
in a pattern to allow oil to flow through as the damper moves. Generally, there are ports
for compression flow and another set of ports for rebound flow, with dedicated shim stacks
for each set of ports. During compression, the rebound shims completely block oil flow
through the rebound ports while the compression shims bend open to allow oil flow. In
rebound, the opposite happens.

RockShox Vivid Air shim stack

The piston can be seen at the top left; the shim stack is laid out below it. Oil flows through the openings in the piston during high shaft speeds, forcing the shims to open and bend, slowing down the passage of the oil and creating damping. The piston bolt that holds it all together sits at the far right.



How does a shim stack provide damping?

Damping is created by generating fluid pressure inside the shock. The shims on the
face of the piston resist opening, creating fluid pressure through this restriction. In order
for oil to flow through the piston, it has to bend the shims up to create an opening. If the
shims are thin the oil can easily bend them open very far, allowing a lot of oil to move
through the piston to creating very little damping effect. If the shims are thicker and harder to
bend, then it takes more force for the oil to open the shims and flow through the piston,
increasing the damping effect.



What part of the fork or shock's stroke does a shim stack control?

A shim stack doesn’t necessarily control a particular part of the damper stroke, but rather a range
of speeds. In a basic configuration, the orifice will control rider inputs such as pedalling,
pumping, or braking, and the shim stack will control bumps. In this setup, the shims will
control the middle part of the stroke where the damper is moving fastest. In more complex
shim stacks the range of speeds depends on the configuration. For instance, the shims
can be preloaded so that they don’t open until a particular damper speed to improve low speed
control (i.e. a pedalling platform), or they could be configured as a split stack where the first
stack controls low speed until the shims bend far enough to contact the second set of shims
so that the combination of the first and second shim stacks control high speed. Having all
of these options easily achievable by changing a few shims is one of the reasons shim valves
are so common in high performance dampers.

Rockshox suspension testing at Whistler with Mike Levy and Brad Walton

A Vivid Air's internals (left) spread out for inspection, and a fully assembled piston (right). The white ring encircling the piston is the 'glide ring' that prevents oil from bypassing its intended path through the bleed holes and the piston.



How can a shim stack be used to tune how the suspension performs?

Because the shims have to bend to open the valve, the damper is tuned by it making more or less
difficult for the shims to bend open. There are a number of ways this can be achieved: adding
shims, changing to thicker shims, or changing the pivot shim diameter to name a few. The
end result of each of these changes is that the damping force at different speeds can be changed
to achieve a desired result on the bike. Generally, increasing the damping force in compression
or rebound will result in better control of the chassis, which makes the bike more responsive to
rider inputs. There is a limit to this, though, and excessive damping force will limit the wheel's ability to
react to bumps, making for a harsh ride. For instance, a damper with too much compression
damping usually translates to a bike that tends to kick and deflect off of bumps, making it difficult
to hold a line.

Steve Smith on a tear... Steve was another rider that just looked liquid on the track all day where other riders struggled Smith was all flow grace and smashing power.

The speeds that Devinci's Steve Smith rides at means he requires a heavier tune used within his fork and shock, but his setup would very likely create a bike that rides harshly under a more average rider.



Why is it that less expensive suspension forks or shocks sometimes don't use a shim stack?

The main reason is cost. Shim stacks use a lot of shims and usually require more precision parts to
function properly. This adds up quickly and it’s easy to overshoot the target cost of a less
expensive product. The other aspect is the development cost. It takes a lot of testing, in the
field and the lab, to fine tune a shim stack to work well across a really wide range of riders
and conditions.


How does a suspension company decide how to configure a shim stack?

This really starts with the intended application of the damper and what the performance priorities are for
that application. For instance, the shim stack in a small air sprung cross-country race shock is
drastically different than the shim stack in a downhill shock. Once the priorities are understood,
a baseline shim stack can be developed on the dyno, and from there a combination of field and
lab testing is used until we come up with a shim stack that provides the best possible performance
for that platform.



www.sram.com/rockshox


73 Comments

  • + 76
 Manitou uses shimmed rebound and compression dampers in every fork and shock they make. Even the 250$MSRP Manitou Match. Saying it cost to much is a joke, The real reason Rock Shox doesnt do it is because if a Recon had the same damper as a Revelation, Most people would buy the lower end Recon and save the money. Rock Shox doesnt want that to happen.
  • + 10
 That's a pretty good observation. Keep in mind, though, that TPC has been around since the 90s so their development cost is practically zero. They're saving cost on using steel. The fact that Manitou has slipped pretty deep from being a leading fork manufacturer prevents them from charging top dollar like Fox or Rock Shox. We pay a lot for perceived performance.
  • + 2
 You can just drop a rev damper into a recon crown and bingo your in. A lot of the cheaper RS stuff does actually use shims just they're not easily accessible to swap out. £200 monarch does, £200 sektor does in the rebound piston and the RLT damper also uses shims from what i can see of mine. Im sure you probably could change them but getting into it might not be the easiest thing and not only that what shims do you use ? theres no real guides. main difference between a recon and a rev is the CSU, the rebound assembly (dual flow in the rev) and the RCT3, XX and RLT dampers but these parts can be fitted to a recon/sektor if you wanted to. i have a sektor kitted out with an RLT damper and it works great.
  • + 5
 Thats true, but the ABS+ damper is different from the TPC damper of the past because it allows for the shim stack to be preloaded. All of the shim stack data from the TPC damper would be a moot point with the current ABS+ because it wouldnt cross over unless you used a linear(non preloaded) stack which is not used in a stock fork.

I agree with them not being a leading fork manufacture, and not being able to charge more, but they still make money and have shimmed dampers, while selling less forks. You would think Rock Shox would be able to make money with a shimmed damper based on the volume of forks they sell. If Manitou can do it while selling just a fraction of what Rock Shox does, there is no excuse not to have shims in at least the mid level forks like the Sektor.
  • + 4
 Sektor does not have a shimmed rebound damper. RLT dampers are not shimmed either. Only forks in rock shocks line up with shims are forks with Mission control (both compression and rebound) the new RCT3 (both compression and rebound) and the RLT Black box dampers(rebound Only)

Motion control IS has a speed stack on the compression damper, but its not the same thing as a normal shim stack.
  • + 0
 Well i wonder what those metal shim like deelies are below the piston head on my sektor then?
  • + 5
 Rebound check valve is probably what you saw. The RL damper(top of the line for a stock Sektor) uses the standard rock shocks port orifice rebound damper. A rebound shim stack is not on the shaft side of the damper.
  • + 7
 My manitou 6 elite from over a decade ago has a shim stack, maybe i'll go tune it now I'm an expert
  • + 1
 Looks identical to this. I know this is from a reba, but looks like a scaled down version of the same assembly from my lyrik. ive only replaced the TK damper with the rev RLT.

joyride-cycles.com/Images/ProductImages/BTI/RS3976.jpg
  • + 2
 But sektor is not a dual flow rebound assembly so how can the rev and sektor share the same rebound damper ?

further looking into this seems to show that the picture i posted is also the rev dual flow damper from what i can see. you you are right in that the standard sektor is not a shimmed rebound, im just confused as to why i have a fork with a dual flow rebound assembly and TK comp damper from stock lol Big Grin
  • + 3
 There is a big difference between the recon and revelation besides the damper. The lower end recons have steel stanchions, among other things. My RS domain 318 is a tapered, alloy steerer tubed fork (that for the price I really love) that has steel stanchions. Compared to my friends totem, it is noticeably flexy. The price differential across product lines really does reflect manufacturing costs. If you don't want a differentiated product, buy Fox Shocks, they have essentially one line per bike type. Sram, Fox, and Marzocchi aren't pumping out millionaires from their companies. If they were, there would be more players in the mountain bike suspension game.
  • + 2
 Obviously there are bigger differences than just dampers between a Revelation and a Recon. That wasnt my point. Im pretty sure the flex you feel in your domain would have a lot to do with its 35mm stanchions compared to a totems 40mm stanchions. jaydmf, the sektor and revelation RL forks share the same damper. The other revelations have the dual flow.
  • - 2
 Rev is a super light fork using shitty damping for loads of cash. Light weight per travel ratio is it's only advantage. A huge argument for majority of buyers thus great marketing story. If you look deeper though - It is flexy, and compression damper is crap, and you can'tdo anything about it, You can gave double flow or tripple action rebound in it, with diamond coating and it will still either dive excessively under braking or stutter on harder compression setting making you loose vision. And you can't tune it.

Recons in the past (until silver gold series came along) used to have Motion Control damper.

Lyriks did not perform that well in air spring models either, comparing to much more stable coil versions. Excessive diving or hard to use full travel.

New RCT3 damper for 2014 PIKE is the first high-performance, air-spring specific damper from RS.
  • + 2
 2 pairs of lyriks, a pair of pikes, 2 pairs of recons and a pair of sektors. All of them have been good forks, the pike with original moco dived under braking when full open but add a bit of compression and that doesnt seem to be much of an issue. RLT damper seems to be a tweaked version of the original pike moco damper with proper detents and i think it does a pretty good job.

Marzo were better back in the day i will admit but they were tanks and quality went out the window in the late 00's. Fox 32/36 float and vans have been great, talas has generally been shit IMO (that may just been my experience however) but theyre way over priced and require far too much messing to service fully either that or you need a phat wallet. Manitou are the only forks ive never really ridden much due to reliability issues from friends experiences so generally i think RS are doing ok as of around 2004.

But hey each to their own. i think the most expensive fork ive bought was my original lyrik @ £580 most have been around £200-£300 and on that basis id say price/performance ratio is pretty damn good tbh
  • + 1
 @jaydmf

never liked any of my 36 Talas (excess friction, inconsistent spring rate between travel adjustment, quick degradation of travel adjuster)

converted them all to Float (and for many customers too...)and a much better fork after what was a quick and relatively affordable job

36 Float RC2 = awesome fork
  • + 1
 Cost cutting = death of RC2...which is too bad because that so far has been the best Fox has produced. I've got a '07 36 RC2 Talas that I rode with no maint other than squeezing some oil past the wipers for 5 yrs, 3-4x a week and never had any of the typically described ish's....I was trying to see how long it would last without re-building! Upgraded to a '12 Fit Talas (freebee as I was a Fox employee at the time), and other than the weight, and slippery Kashima, I wouldn't mind putting the old one back on my rig!...sorry to digress from the shim stack tutorial.
  • + 38
 Why show us the vivid? None of us are good enough.
  • + 8
 zing!
  • + 18
 Shimmy shimmy yall shimmy yeah shimmy yay give me the mic so I can take it away---ODB
  • + 18
 I bet for a normal rider, Steve Smith's Wilson would feel like riding a full rigid bike, or close to it!
  • + 7
 Wow, my fat cousin must ride like a pro!
  • + 10
 What would really make you laugh is that Fox managed to brand "pro-pedal" from a simple ring shim in the top of the shim stack.
In essance pre-loading the shims behind it causing the platform feel.

When you know how minor adjustments in these shimstacks can cause massive changes in the characteristics of a damper and use that knowledge to get your setup exactly how you want. then there is no need for all these fancy dials on the outside of the shock.
The simplest shock is one that is setup for the exact user and preferred usage.
  • + 1
 Which works great if you have the time to adjust your bike for the exact terrain you'll be riding. For me, I love having the ability to play around with rebound/compression damping to tune my 6" travel trail bike for the type of ride I'm going on. Being able to tweak my suspension for a more XC ride one day followed by moderate lift-served DH the next is a HUGE time saver. Of course, the bike is generally best at the trail style riding I do most often, but making those small tweaks certainly makes it more capable if I stray to either side, without requiring a few hours of fork and shock rebuilding.
  • + 9
 Great write-up! But how about offering different shims that riders can buy and slap on, like high-end moto and off-road shocks instead of fine-tuning "a shim stack to work well across a really wide range of riders and conditions?"
  • + 17
 That's exactly what dvo is promising for it's products aswell as the new Marzocchi stuff. You can already mess with this on existing products like the 888 or a vivid-shock but these require a complete rebuild. On the Emerald and the 320/new Marzo-Shock you will be able to do this on the fly which is really cool because finding the best individual shimstack-configuration requires testing.
  • + 3
 now if they can only make the emerald or the new marz 320 affordable, hmmm........
  • + 1
 He was talking about "less expensive suspension forks or shocks". Vivid has a few different stacks depending on the frame.
  • + 0
 Aren't these open-bath style dampers? Much easier to swap out, but if you have a closed system that requires bleeding, there really is no easy way to make it user-friendly
  • + 3
 You get what you pay for.
  • + 1
 The 88 is open-bath but it sucks to drain all the oil everytime you wanna reconfigure the shimstack. If dvo and MZ keep their promises you can do this in the parking lot, swapping out shims or the whole stack without all the oil-mess. To see what the shims will do for you there is a program called shimstackreactor.
  • + 5
 Unless they sell you complete shim stacks, with good description of intended use, and results you can expect by using them, selling loose shims to peoplefortuning is aperfect way to f*ck up a well working default setting. You think you want it, but no you don't. Just as this article says: it takes lots of time and knowledge to set it up. A company "shimming" a shock for certain bike model, can take 10 shocks, pack each of them with different shims and test them first on duno, then on trail within 2-3 days, then change stuff in each of them. In float shocks you need to recharge nitrogen everytime you want to mess with the stack. You at home have no chance to match such experimenting power and tools, neither knowledge and experience to manage it.
  • + 1
 I adjusted the shim stack in my Domain to make it plush enough for trail use. Shims should totally be sold, just like springs.
  • + 1
 Yes, in the fork you might get a good result after 5 take-aparts, but in the shock you can forget it, especially in some virtual pivot frame, that has a frame rate like my mental stability Big Grin
  • + 1
 Agreed that shocks are much harder to tune, and without a dyno, or at least a few dyno charts to reference, its a shot in the dark on getting a proper stack and it will take multiple tries.Same goes for forks. Rock shox does a good job of making the stock tunes for their rear shocks available, and they can be used as guides if you know what you are looking to change and understand the principles behind shim tuning.

Also,shims can be bought individually from motocross suspension companies like MXTech(in america anyway). They are not hard to get, but you will pay more for shipping then you do for the shims.
  • + 1
 just buy a ccdb and forget about shims. As for the forks like the 888 there are forum-thread out there with description of what a certain shim does. So you know in which direction you have to go to get the desired result.

Also: be sure to get the exact springrate and external adjustments right before you mess with the shimstack. You can also use different damping-oils before you change shims.
  • + 4
 $12 car shock has shimstacks. The article started well and ended in corporate desinformation. Shimstacks are microcents...

Most rearshocks and rearends are engineered fubar. Elka, Curnutt and amazingly Fox DHX with Coil are not.

There is not enough space in oil-forks to mess with physics.So most coilforks work surprisingly well. Even a RS Domain.
  • - 3
 I have a coil Sektor and it works like crap comparing to my coil Lyrik. PIKE was 0,4kg heavier but at leadt it was stiff. Motion Control damper is useless in anything with more than 100mm stroke. But yes, add air i to it and you can sing holy diver Smile Motion control is not damping, it is a mere suspension action prevention.

I will be soon trying out 32 Float Fit RLT 140 on Blur TRc - we'll see what's that worth. 2014 Pike or BOS Deville in long sighted plans.
  • + 4
 Yep - this stuff is nogo. One eye on the competion, one eye on not hurting own productlines and no eye on engineering... Have a Pike and it sucks. Best thing is lockout and the mattblack finish. Happiest with a Boxxer RC. Silverspring, 2 Clicks on LSC and LRS only - more and it stops working.... Even came with the spring the wrong way in from the factory... The more you pay for perceived niche the more esoteric stuff gets.
  • + 4
 This article is great. Thanks for breaking it down for us, Eric.

Are there good references for proper suspension set up? Something with info like you stated "if you have too much high speed compression, your front tire will deflect, and hard to keep a line".. I think something like that would be helpful for a lot of people, myself included. I understand how the compression and rebound settings work, but I don't know the 'on trail indicators' I should be looking for, like you described above. "If you notice X on the trail then turn knob Y +/-. While it's generally better to be on the firmer side, you'll know it's too firm when X happens.. or you'll notice it's too soft when Y happens..

I think generally, people start off with their settings too wide open, then progressively firm them up. It would be great to know what boundaries you're looking for to get your settings into their 'sweet spot' for their type of riding. Thanks in advance. Cheers
  • + 2
 There are some good motorbike things on this sort of thing that might suit you. The thing with these setting guides is that they can only be very general. I know from my EXP with road motorcycles that a setting that suits one rider can be WAY out for another rider to feel comfortable. I can imagine it will be the same for MTB!
  • + 8
 love me some shimzz!!
  • + 12
 moar shimzz
  • + 2
 Would be nice to know how most RS forks are shimmed, by that I mean for what size rider. I am a light weight rider and even the low dampened stuff feels a little harsh.

I am currently looking for some shims (near impossible to find in UK) so i can play about with the shim stack in my Boxxer Race to get some relevent damping for my size (148LB).

Speaking of the Boxxer Race; Motion control. I am certain this compression damper has been purposefully restricted in its design, with some light modification I am certain I can get a low speed controlled by orifice and high speed controlled by shim stack (similar to the team/WC damping). Stock all compression goes through orifice then shim stack!!?? Basically you only need to open up 3 holes that are already there but closed at bottom cap that will go through shim stack (HSC) and then bypass the shim stack with the stock orifice holes (LSC)...

As someone has said, this is probably to make their more expensive versions more appealing...
  • + 2
 That was a great read, thanks!
This article is a good match with a href="http://forums.mtbr.com/shocks-suspension/tune-your-mission-control-high-speed-shim-stack-how-guide-pics-541403.html">this online post/a> I followed to mod my Totem/MiCo damper.
  • + 1
 Not a bad intro to what shim stacks are, but no information on how they work. If you actually want to learn the basics of how shim stacks work, have a look at the DVO website under their tech guides www.dvosuspension.com/2012/06/shims-what-do-they-do
  • + 3
 Manitou has had a custom shim kit for the ABS+ for a couple of years. issuu.com/hayes_components/docs/decline_article
  • + 1
 By far the best technology exist now (and was also used in fox dhx 5.0 too in that time...) you get wider range of damper especially in the rebound side - with too high rebound you can easily can get the knock sound and feel, it is so wider with the shim stacks, not to mention it is 100% tunable for your liking - because if you need a specific damper feel you just need to change the shims and you are there already (well you need someone who knows his job, but you have the ability to change at least). The mountain biking industry slowly upgrades - i just wonder why was it so slow, when the motorcyle industry it was common for a period long of time already... Anyway great article, keep them coming!
  • + 4
 Not so sure about this. The poppet valve technology found in the Cane Creek Double Barrel is vastly superior in terms of user tune-ability and broad range of adjustment.

On the flip side, some will argue that the shim stack offers better performance when tuned correctly, but, an individual rider will be heavily restricted by available adjustment dials and ability to fully customize the shim stack is probably best left to companies like PUSH, since they have all tools required to do so and then bleed out any air; which comes at a heavy premium.
  • + 1
 I think shims make sense but home tun-ability is as you say "troublesome". Just finding spare shims to buy for the suspension is hard, also having a rough idea what sort of stack you should be shooting for is a thing of experience and knowledge of the suspension unit/bike/weight/riding style... Finally, especially with shocks (forks are much more accessible) the shim stack can be a nightmare to get to!
  • + 2
 Shim shimenee, shim shimenee, shim shim Sheree.
Your fork is as damped, as damped as can be..

Shim shimenee, shim shimenee, shim shim Sheroo.
No knowledge of shim stacks will help me or you!.. Smile
  • + 5
 Orifice is a funny word.
  • + 2
 indeed. although it makes me cringe every time i hear it.. just like the word 'discharge'
  • + 1
 Moist.
  • + 1
 Welcome to the world of simple shim changing and tuning. Welcome to DVO> www.dvosuspension.com/2012/06/shims-what-do-they-do
  • + 1
 "There is a limit to this, though, and excessive damping force will limit the wheel's ability to
react to bumps, making for a harsh ride."
-Hello Bos Deville
  • + 3
 The coolest thing about this article is that photo of Stevie.
  • + 7
 duhh, unless you like how shims look! this article was more about the words, pics are their for just visual aid!
  • + 2
 Thats a pretty ignorant thing to say. The performance of our suspension that makes our mtb's most able to be mtb's often boils down to how well shim stacks (both compression and rebound) are tuned. Learning how to change shim stacks in your suspension can make the differnce between owning an ok feeling fork and one that feels amazing.
  • + 1
 Wish i had these for my forks because they are squelchier than the inside of my mouth
  • + 1
 Perfect timing, iv been looking for information on shim stacks. Too bad this was not more in depth.
  • + 1
 Thanks for this, PB -- always a good thing to understand what my components are doing underneath me. Smile I'm a fan.
  • + 2
 sooo what stack should i use on my argyles?
  • + 1
 I still don't understand shims and shim setup.
  • + 4
 If you imagine a gate with a spring on it. The harder the spring the harder it is to push past the gate. Imagine the spring and gate is the shim(s) and you are the oil. Different spring strength/ number of springs and where they are mounted on the gate can all affect how fast you can open the gate and get through...

Now think of the fork compressing over a bump, this movement pushes the oil, the oil can't go anywhere until it can push past the shim stack (the gate is in its way). The oil pushing past the shim takes energy away (the person pushing the gate gets tired) so the suspension is slowed down.

All damping does basically is slow down a forks movement, shims are a way of doing this as it takes energy to move them.

Hope that helps a little.
  • + 2
 Check out DVO suspensions website. They summarize pretty well how the shims work and what different widths/thicknesses/setups affect.
  • + 5
 +1

Lsc and high speed compression comes down to velocity. A low speed hit moves oil slowly so the oil is forced through a orifice port at a rate it can cope with. When the bike hits a high speed velocity the oil is forced so fast that the orifice isn't big enough so it over flows and is pushed through the holes into the shim stack.

A dual shim stack works the same way but the first stack is shimmed so it opens for LSC and then when enough oil is forced in it will hit the HSC shim stack after it.

You can think of it as a wall with three holes all next to each other. The middle hole is LSC if you slowly (slow velocity) poor oil towards the wall it will all fit in the middle hole. If you than throw the bucket at it (high speed) the middle hole is not big enough and the oil will overflow into the outside holes.

The shim stacks are behind the holes. This than controls the force against the oil. (Controls the force in which your fork compresses)

HSC and LSC doesn't really stiffen the support much at all, it primarily preloads the forks ability to open the shim stack, so by adding HSC you are making the oil reach a harder force before opening the shim stack. That's why you can up compression but never get more support because as soon as the shim stack is opened the oil still flows through it at the same rate.

My understanding anyway Smile
  • + 2
 Indeed Slidways. Sounds about right to me. In the bikes of the Boxxer R2C2/team, flow is controlled by how far the shims are allowed to open, the activation point is altered by the the compression preload, though both can be controlled by shim stack style, and ideally would be.
  • + 1
 To add there is a LSC orific which can be changed internal, I'm still unsure how effective the LSC shim stack is, so far from what I can establish is the initial LSC is controlled by the orifice and the LSC support is controlled by the shim stack. The bigger the orifice the less oil flows theough the shim stack and visa Versa, so in theroy it allows control over the lsc range both intial and support.That's my guess I may be wrong. As you know I'm just learning this stuff.
  • + 1
 The stock race is supposed to have a shim to support all compression. I can tell you that it breaks small bump absorption which is horrible! Orifice is good enough for low speed in my limited experience!
  • + 1
 TBT Racing....enough said
  • - 2
 all i know is that i need MOAR

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