In every modern mountain bike fork, there are 3 key areas which make everything work. Damping, which covers both compression and rebound, and the spring, be it air or coil. This is a quick, basic insight into how the damping works on a couple of popular brands.
Damping is the means to reduce the size of oscillations in the forks when they hit a bump. Without it, the fork will act like a pogo stick and just continue to extend and compress after a bump. By using oil and slowing it as the fork compresses and rebounds, we can control the rate at which the fork absorbs the bump and returns to its original, static state.
Two different systems are used in both compression and rebound to control the oil flow-Shim stacks and adjustable sized holes or slits, generally referred to as port orifices or needle valves.
This article will compare Rock Shox’s Motion and Mission Control and Magura’s Albert Select.
First we shall look at the compression damping.
Rock Shox’s Standard Motion Control uses a simple adjustable size slot to control the oil flow.
It’s hard to see, but as you turn the blue dial, the Compression Control Valve cover (brass disk) rotates and blocks the slit which the oil flows through.
Magura’s damping system uses a slightly more complex method of controlling the oil.
The Albert Select uses a small shim stack to control the oil flow during general use and then by engaging the Albert Select, it diverts the oil through an adjustable size hole (orifice).
A shim stack is a small pyramid of very thin washers that bend when oil is forced though the small holes under the stack that are otherwise sealed. When the oil is forced through these holes, the amount it resists by depends on the shape of the stack, the number, diameter and thickness of the shims.
Shim stacks vary between forks and can be customised to suit the rider’s personal preferences.
Here are two examples of different shim configurations used in Mission Control DH:
Compression shims/Rebound Shims
As you can see, the number of shims depends on the job they have to achieve.
Back to the damper….
Albert Select ‘Off’:
Albert Select ‘On’:
Red indicates oil flow
With Albert select off, the central sleeve is closed and oil can pass through the three slots in the bottom washer to engage the shim stack. When it is on, oil passes through the central hole, down the inner sleeve and out, bypassing the shim stack. The size of the hole at “A” is adjustable using the external gold platform dial, and can be adjusted from fully open, allowing full use of the travel, to fully locked out.
On the rebound, the oil simply flows behind the shim stack and pushes the large washer aside, which is held in place by the small spring making a check valve.
The Black Box Motion Control is similar to the Albert Select in that it uses an adjustable port orifice to deal with slow hits, and has a shim stack to deal with the high speed impacts.
Flood-gate will be explained later.
When Rock Shox introduced the Lyrik and Totem, they desperately needed a damper more suited to long travel and the higher oil flow needed to retain controlled damping. Their answer to the problem was Mission Control. This will look at Mission Control DH, found in 2010 Boxxers (no floodgate)
Mission Control uses an adjustable needle valve to deal with low speed compression whilst high speed compression is handled by a shim stack with adjustable preload, but what does that all mean….?
At the very end of the Mission Control rod is an adjustable needle valve. As you dial in Low Speed Compression at the top of the fork using the silver knob, you are slowly screwing in a rod which closes up a hole, limiting the oil flow.
This is simple and works in the same way as rebound later on.
The High Speed compression employs a shim stack to dampen the oil flow.
The HSC adjustment uses a spring at the stop of the Mission Control to preload the shim stack at the lower end of the rod. This is accomplished by moving the whole rod in and out of the fork.
To see the change the adjustment makes, compare the two photos below and look at the ring of oil at the top.
The spring has been compressed by ~4mm. This preload is transferred through the black rod which sits directly on the shim stack.
Now that your fork has absorbed the bump and compressed, the spring will cause it to extend again. To control the rate that it returns to its original state, a similar method to compression damping is used, just reversed.
Most forks on the market use a simple adjustable port orifice/needle valve which is used to adjust the oil flow. This includes Motion Control forks. Shown below are Magura, Rockshox and Fox:
Fully Open (fast) Fully Closed (Slow)
Now the 2010 Boxxers feature a dual flow rebound circuit that separates the beginning and end stroke.
Similarly to the compression damper, this uses an adjustable port orifice and shims to control the oil as the fork extends back to its original state.
Some of you might be wondering, if the compression carts are a fixed length and don’t move how is the oil flowing through them?
If you look at this schematic diagram of the fork leg:
And compare the oil levels before and after compression, as the rebound rod moves into the shaft, the volume it takes up forces the oil through the compression damper.
On various Rock Shox forks you will find a flood gate dial. In an ideal world, when the fork is locked out, there should still be a small amount of movement to let it track properly. And this is where the floodgate comes into its own.
But what does it do?
If we look inside the Motion Control cart, you will find a slender aluminium rod with a long notch in the bottom, the bottom cap with the brass disk attached (the Compression Control Valve), and a plastic tube (or titanium on Black Box Motion Control) that looks like Swiss cheese. When locked out, the plastic tube will compress by up to a few millimetres, allowing the aluminium rod to push the compression control valve off the bottom of the cart uncovering the slit for the oil to flow through. When you turn the gold Flood Control knob, you adjust the length of the rod and how much the tube will have to compress to move the brass disk.
Hope that helps to explain how your forks work!
Next week we’ll be going through Servicing Rock Shox Boxxer Teams in a step-by-step guide.
Words and images by Matt Holland