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.
How does a shim stack provide damping?
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.
Damping is created by generating fluid pressure inside the shock. The shims on the What part of the fork or shock's stroke does a shim stack control?
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.
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.
How can a shim stack be used to tune how the suspension performs?
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.
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.
Why is it that less expensive suspension forks or shocks sometimes don't use a shim stack?
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.
The main reason is cost. Shim stacks use a lot of shims and usually require more precision parts to How does a suspension company decide how to configure a shim stack?
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
This really starts with the intended application of the damper and what the performance priorities are for www.sram.com/rockshox
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.