Suspension - Rear Shock Damper Design

DAMPER DESIGN   SHIM STACK   PLATFORM   PROPEDAL   MANITOU SPV   ROCK SHOX   OIL FLOW                 DIAGRAMS   CURNUTT   FOX   MANITOU   ROCK SHOX               Damper design is most often a cylindrical tube, sealed at one or each end depending on design purpose, a penetrating shaft is attached to a Teflon lined aluminium piston which plunges through suspension fluid pressurized by a separate internal floating piston ( IFP ) contained in the damper body or reservoir.   Rear shock dampers are constructed using several designs, one being a mono tube that houses damper internals in an in line arrangement. With the internal reservoir and IFP at one end of the damper body, the damping piston and shock shaft are situated almost directly behind the IFP. Most often with an external rebound adjustment, nitrogen charged or reservoir air pressure adjust via a schrader valve. Although the design is reliable it is often limited in shock travel and oil volume, and an increase in both damper size and stroke length to avoid this can only be achieved where frame chassis allows.   Other damper designs include mono tubes with attached ( external ) reservoirs, or remote reservoirs attached by a flexible hose, these being the more popular design due to larger volume, increased shock stroke length although compact over all, and the added advantage of oil routes ( between piston and reservoir ) to which seperate oil ports with external adjusters can be located. Both coil over and air shocks share the attached external reservoir design and due to the larger capacity, oil volume, shock shock length etc has encouraged the use of air shocks over a wider range of applications like free ride and down hill.   Designs like twin tube dampers incorporate one tube inside another, creating a separate oil transfer route, that often includes an external reservoir, or a twin tube damper with an internal reservoir, this created in the cavity between the inner and outer tube. Designs like pull shocks and through shaft dampers are also used, although not so common thier application is still used on some bicycle chassis.   Rear damper bodies normally act as the main shock chassis where coil springs are installed, or if air, the spring chamber surrounds the damper. Constructed with mounting to a suspension chassis/swingarm, the shock incorporates eyelet ends, normally situated at one end of the body and another on the end of the shock shaft and often with a sacrificial DU bushing and aluminium reducers the width relevant to the shock mount area on the chassis.   Damper Piston/Shim Stack The damper body houses the primary damping piston, this a ported valve, and the ports are designed relevant to oil flow. The entrance of the ports are strategically placed on the outside edges of the piston, and shims of varying size cover the exits of these ports. A standard shimmed damper piston has shims on both sides of the piston, and the resistants of the stacked shims regulate oil flow as the piston cycles in and out of the damper on the compression and rebound stroke. Note, that SPV damper pistons use only rebound shims as the stable platform valve is situated in place of compression shims.       Shim Stack/Oil Flow The flat red lines in figure 1 indicates the oil travelling through the piston ports and ( high speed ) oil flow is regulated by the shim stack, the oils initial transfer into the piston is slowed by a deliberately choked entrance and then met with considerable resistance before exit, the oil flow determined by shim configuration......altering the shims, alters the damping characteristics.   The piston is attached to the hollow shock shaft ( 3 ) which also houses the rebound needle, oil flow through the hollow shaft is metered by the tapered end of the needle situated in the fix orifice, this a separate port from the piston and the oil flow is restricted as the needle is adjusted in, ( by means of an external adjustment control ) or the opposite effect when adjusted out. The thin red line indicates oil flow ( low speed compression ) through the hollow shock shaft.   The direction of flow indicated by the red arrows in figure 1 is the compression stroke, which makes it obvious that rebound and compression are affected by the same adjustment by the needle, although increased oil flow ( high speed ) is directly effected by the shim configuration on either side of the damper piston. The over all design of the needle orafice, piston ports and stacked shims all contribute to damper operation at various shock shaft speeds, and high and low speed compression and rebound forces are effected by this configuration.   Internal Floating Piston To operate a high pressure hydraulic system and allow displacement of oil, an internal floating piston separates oil and gas in an internal or external reservoir. As the shock compresses displacement of oil is necessary to allow room for the entering shock shaft, and the IFP adjusts to accommodate oil volume change. Often the oil route to and from the reservoir is used for various adjustments to damping characteristics by the addition of separate oil ports with external adjusters.   The more common and reliable dampers are referred to as the De Carbon design, the de Carbon concept means that the damping oil is placed under pressure by gas and separated from the gas by a floating piston. This concept has many advantages. It prevents the chance of cavitation, which happens when the oil can not move fast enough and becomes hard as a rock (compare with an unsuccessful dive into water). It offers better cooling, especially if the shock absorber has an external reservoir (the external reservoir is in fact an extension of the shock absorber and more oil, larger cooling areas improve performance and durability). Allows consistent damping, regardless of the shock absorber's working temperature, and increases durability and shock life.   The diagram on the right is a 2004 Fox Vanilla RC, Nitrogen charged internal floating piston to 200psi, external rebound and compression adjustment, typical shimmed damper piston, fixed orifice rebound port and blow off valve in the compression circuit. The first mass produced bicycle shock that equalled motor industry standards. The active Fox damper is no longer produced.   The above explains a basic shimmed damper, which consists of a damping piston, specifically placed oil ports and controlled by a stack of shims on each face of the piston, this effects both compression and rebound oil flow. A fixed orifice situated at the base of the shock shaft controls rebound oil flow by adjusting a tapered needle through the fixed orifice, but no matter which way the oil is moving it will always choose the route with the least resistants, and at low speed the oil will flow in both directions through the rebound needle fixed orifice. ( high speed oil flow tends to create large volumes of oil that utilize the larger oil ports in the piston, and regulated through the shim stack ) With equalization occurring in the damper via the fixed rebound orifice ( allowing the damper to be active over small bumps ) a stable platform valve such as Curnutt, Manitou and Fifth Element SPV will not operate correctly, an SPV damper requires differential pressure either side of the piston for threshold to be effective, hence the fixed rebound orifice has a gate to prevent any compression flow ( other than direct oil flow through the SPV ). For some, the greatest advantage of SPV is reduced rider input on the suspension, what we don’t see is the stable platform valve achieving what it was designed for originally, by operating a threshold at high shaft speed the mechanism slows the damper piston at the end of the compression stroke preventing unnecessary travel of the piston and shock shaft. Its interesting to note that Fox have different approach in their damper design concerning this, and the Fox position sensitive damper still retains the fixed rebound orifice.   Stable Platform Valve With the introduction of SPV, the compression shim stack has been replaced with a pair of overlapping aluminium cups. They fit into one another, and the resultant atmospheric pressure trapped in between causes them to naturally push apart, no springs, no shims. They are mounted on the compression side of the damper valve. In static state, at the top of a suspension's travel, the cups are held closed against the valve by air pressure on the oil in the damper, preventing oil flow. Depending on the level of air pressure, it's possible to tune out low amplitude forces, like pedal-induced bob, and create a breakaway threshold from which the shaft begins moving through its travel. More air pressure on the oil will make for a higher breakaway threshold, less air will make for a plusher initial state, but with more resultant pedal bob.     The diagram on the left shows a 2007 Manitou Swinger coil over shock, circled is the one way valve, damping piston and stable platform valve. Curnutt, Manitou and 5th Element all share the same design.         The diagram above shows the SPV in the closed and open position, as mentioned the SPV replaces the compression shim stack, and the larger of the two parts of the SPV covers the compression ports on the damping piston.   Propedal Fox shox have introduced their own platform design, and pedal efficiency is dealt with in two ways, the more cost effective dampers have a modified valve that is situated at the head of the damper piston, basically a simple check valve, integrated into the fixing bolt that attaches the damper piston to the shock shaft. The propedal bolt inhibits low speed oil flow by preventing compression flow back through the fixed orifice ( rebound needle port ) and forces oil to use the compression shim stack, note that all of Fox’s damper pistons still retain shims on both faces of the damper piston, also, Fox dampers that use the propedal bolt only have non adjustable IFP reservoirs and continue to use nitrogen charged reservoirs.   The more complicated version of propedal incorporates a position sensitive valve    ( PSV ), the internal floating piston situated in the reservoir is adjustable, and air pressure is introduced through a Schrader valve at the reservoir. Propedal is integrated with the PSV, and introduced air pressure alters the threshold on the position sensitive valve, the design of the propedal adjustment also alters threshold resistants but only effects low speed compression damping. Note, there is no PP bolt used in this system and the main damper piston still retains shims for both compression and rebound, the fixed orifice of the rebound needle still exists.     The diagram on the left is a 2007 Fox DHX 5.0 Air shock, air charged IFP, adjustable rebound, propedal, bottom out, compression threshold and main air spring. With a position sensitive valve on the compression circuit a threshold is achieved by adjusting IFP air pressure. Like the DHX coil over, the DHX air no longer uses nitrogen in the IFP reservoir and relys on introduced air pressure for correct operation.     The basic but reliable Fox Van R 2008, nitrogen charged IFP reservoir ( non adjustable ) Retains a standard shimmed damper piston with the addition of a propedal bolt situated at the head of the damper piston, this inhibits low speed compression, diverting oil flow directly into the shimmed piston. Rebound and spring preload adjustable only. More Fox rear shock diagrams here <<       Through Shaft Dampers A basic through shaft damper does not require an IFP for correct operation, displacement by the shock shaft entering the oil filled damper is balanced by the retreating shaft at the opposite end. The design below is a rear shock application although suspension fork ( through shaft ) dampers operate on the same principal ( Fork Damper Cartridge ) Sealed through shaft cartridge in suspension forks are not so common as technoligy improves.   Through shaft dampers are still used in some rear suspension designs.   Motor Industry/Penske Formula 3 damper on the right is a two way adjustable in compression & rebound ( high & low speed ) and gas charged for improved performance.

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