To The Point - Shim Stacks
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?
![RockShox Vivid Air shim stack]()
How does a shim stack provide damping?
What part of the fork or shock's stroke does a shim stack control?
![Rockshox suspension testing at Whistler with Mike Levy and Brad Walton]()
How can a shim stack be used to tune how the suspension performs?
![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.]()
Why is it that less expensive suspension forks or shocks sometimes don't use a shim stack?
How does a suspension company decide how to configure a shim stack?
www.sram.com/rockshox
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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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.
Motion control IS has a speed stack on the compression damper, but its not the same thing as a normal shim stack.
joyride-cycles.com/Images/ProductImages/BTI/RS3976.jpg
This is a Sektor/Revaltion RL/RLT damper
www.google.com/shopping/product/2565189403161763234?q=rock+shox+sektor+rebound+damper&safe=off&bav=on.2,or.r_cp.r_qf.&bvm=bv.48293060,d.aWc&biw=1517&bih=714&tch=1&ech=1&psi=UOjJUY2jEMbeyAHinICADQ.1372186706490.5&sa=X&ei=X-jJUfiuJKnkygHO34H4DA&ved=0CFUQ8gIwAg
joyride-cycles.com/Components/forks-and-rear-shocks/suspension-fork-service-parts/rock-shox-rebound-damper-2010-revelation-150mm
If you replaced it and got what you pictured, got the black box version which is shimmed
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
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.
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
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
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.
docs.google.com/file/d/0B-s3OzrP-sd0ZWI4MjdkZDEtNTcyYS00MDhmLThhMmUtMDM2ZGFlNGUwZGQw/edit?pli=1
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.
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.
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
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.
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.
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.
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!..
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.
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...
react to bumps, making for a harsh ride."
-Hello Bos Deville
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.
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