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A while back I decided to start looking into how the suspension on our bikes worked as it was the area I knew least about (read nothing!).
All of this, unless otherwise stated, applies to both the forks and the shock as the theory behind them is exactly the same with only a few differences in implementation. By adjusting the amount of damping the damper actually provides we can adjust how slowly or quickly the suspension compresses or rebounds. Right so from Part I you should have some idea of how suspension breaks down; the spring and the damper. Now we have to make some sort of suspension from the spring and damper, and this is it shown in the picture above. On the left is a suspension unit as it would be just sitting there, on the right is a compressed suspension unit, like if say somebody sat on it.
So you now know how the main components go together, but how does this a€?black boxa€™, the damper, work?
As I said before you have a large empty cylinder at the bottom with a long rod (the damper rod) going down into.
The piston is basically a flat disk of metal, about 0.5a€? thick attached to the end of the damper rod as shown above.
So the idea is when you push down on the damper rod, this will push the damper rod down into the oil filled cartridge.
So we have a problem here, we need to be able to have different rebound damping to compression damping and the damping needs to be progressive (ie. I think Ia€™ll leave it there for now and let you think this through and try to figure out how to deal with the above problems?
So just to recap and clarify; the speed the suspension compresses, or the shaft speed, is primarily a factor of the terrain you are going over, not so much the speed the bike is going.
At slow shaft speeds you want reasonably soft damping so the suspension feels plush, but the suspension also needs to be able to damp high speed shaft movements as well.
One thin shim on its own would not be stiff enough for out purposes, but if you made a thick one it would not bend enough with small pressures applied.
Explaining and drawing this will get a bit confusing so for now Ia€™m going to ignore rebound. So as I said we want to be able to vary the size of the ports on the piston depending on the shaft speed.
This shim stack is on top of the piston so when oil flows from below the piston through the piston ports it will have to bend these shims out of the way before it can pass. So these two pictures are just side views of the above piston and shims, and show the piston and shims when the shaft is not moving and when it is moving respectively. The shims used deflect (bend) small amounts relatively easily but get progressively more difficult to deflect (bend) the more they are bent. Just for interesta€™s sake here is a graph showing how much force it takes to bend a particular shim (Centre hole diameter - 12mm, shim outer diameter - 40mm, thickness - 0.20mm. You may be getting a bit confused about the way in which shims and shim stacks bend at this stage and that is understandable as it is extremely complex.
Just to mention it was too complicated for me to draw all the ports through the piston, but generally there will be six ports through the piston, three for each direction (compression and rebound). The last three parts of this article have pretty much covered all the basic theory of how suspension in general works. Ia€™m going to start with describing the shock because it is closest to the suspension unit invented in the previous parts of the article. The spring is fairly self explanatory so from now on Ia€™ll leave it out of the diagrams, youa€™ll just have to remember it is always there. So this is what our simple shock looks like (this is looking at the shock from the rear of the bike). When the shock shaft (damper rod) is driven into the damper the shaft inevitably takes up volume in there. This is one of the more common solutions used; a separate reservoir containing a rubber bladder filled with Nitrogen at a pressure of greater then 150psi.
What about compression damping though, we really want to be able to adjust that externally as well right? I know that picture looks quite complicated, I probably should have built the adjuster up from the basics but Ia€™m hoping having read and understood the previous parts of this article you should have a fair idea how it works if I run through the basics. This is a good time to talk about a€?passivea€™ VS a€?activea€™ damping; the a€?passivea€™ damping, in the case of the shock, is the damping provided by the compression adjuster, it is passive because the piston is fixed in place while the oil flows through it, while the a€?activea€™ damping is provided by the main piston on the end of the damping rod, it is active because the piston actually moves, forcing itself through the oil. Thata€™s pretty much it for the shock, if you took apart your shock now youa€™d have a very good understanding of what everything is, assuming it isna€™t a WP PDS shock of course as they are a bit different.
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The first problem with this is even with a very stiff spring there would still be some bounce on big hits. Ia€™ll go into whata€™s inside later but for now just rest assured there is something in side that is providing the damping effect I talked about in Part I. So the damper sits inside the spring and is effectively connected to the top and bottom of the spring, so if the spring compresses, the damper will also be compressed, if the spring rebounds, it will pull the damper rod with it. If this person who is sitting on the right suspension was to jump off the spring would cause the suspension to extend (rebound) and pull the damper rod up with it, which would be providing rebound damping. It is providing compression damping so wona€™t compress too fast, like a spring on its own might. Assume for now that the cylinder is sealed at the bottom and not quite sealed at the top(suppose there is a gap around where the damper rod goes in or whatever. What you see on the end is what is providing all the damping in our simple suspension unit. The outer diameter of the piston is slightly smaller then the diameter of the cartridge so it fits in snugly.
Oil cannot be compressed like say air, so the only way the damper rod can actually move down through the cartridge and hence oil is if the oil goes through the ports (holes) in the piston.
There will be very little damping while the suspension is moving slowly, the damping will then build up as the suspension is moved faster, but only to a certain point which is governed by the size of the ports in the piston. Firstly this will not be very tuneable because you can only set one point when the damping really kicks in hard. Landing from a jump or going over long smooth bumps on the track will cause slow shaft speeds, while hitting sharp edged bumps etc.
So this would call for variable damping, which clearly isna€™t going to be possible with the simple piston with ports through it from earlier. So what is done is to stack a load of thin shims of different sizes on top of each other, this is known as a shim stack. Oil will flow from below the piston when the suspension is being compressed so this shim stack is going to vary compression damping and so is known as the a€?compression stacka€™.
When the suspension is compressed oil has to flow from below the piston, up through the piston ports and out the top. Note how much stiffer the tapered stack is then the single shim is, yet it still has a nice smooth progressive curve.
The picture above shows our usual piston with a shim stack on top of the piston (the compression stack). The first part shows the second tapered shim stack mounted underneath the piston, on the so called a€?rebound sidea€™. Now we can go on to see how the theory is put to use in the actual suspension on a dirtbike. As before Ia€™m going to build up the shock from simple to more complicated to make it a bit easier to understand. The top of this will be mounted to the top of the bikea€™s frame, the bottom will be mounted to the linkage or the swing arm in the case of a WP PDS shock.

The a€?upper shock mounta€™ is what connects to the bikea€™s frame, while the a€?lower shock mounta€™ (aka. Oil cannot be compressed so if the oil cannot get out of the damper the shock shaft simply wona€™t be able to go into the damper because there is nowhere for the oil to go (hydraulic lock again!), ie.
So oil can flow freely (in fact it will always be filled with oil as there is never any air in the damper with the oil) from the damper into this a€?reservoira€™. The shock shaft is pushed into the damper and attempts to displace some of the oil in the damper. Well the main problem is there is no way of adjusting the damping without taking the shock apart and changing the shim stacks, which isna€™t too practical at the side of a track! If you look closely at it youa€™ll notice it is very much like a suspension unit in itself, except it is a€?passivea€™ (fixed in one position, ie.
We added a reservoir beside the main cartridge with a compressible Nitrogen filled bladder in it to give this oil somewhere to go. We have a two way piston just like the one on the end of the damper rod, except it is fixed in place.
This is quite an important concept in suspension and youa€™ll see it more clearly when I move onto the fork. If the item comes direct from a manufacturer, it may be delivered in non-retail packaging, such as a plain or unprinted box or plastic bag. This item may be a floor model or an item that has been returned to the seller after a period of use.
The Traveller series, incorporate high quality hickory timber frames, a standard 2 piece configuration, elevating head piece and a selection of standard heights. If you reside in an EU member state besides UK, import VAT on this purchase is not recoverable. So I figured Ia€™d try to write a short article or two explaining the basics of how and why your suspension works the way it does. A spring is always trying to return to its original length; so when you press down on the bike the springs constantly want to push you back up.
The main problem is that if you have a spring stiff enough to absorb say landing from 20ft up comfortably, it will be WAY too stiff to absorb any of the smaller bumps around the track so it would be like riding a plank of wood around, very uncomfortable! So in this part I want to try and elaborate on the first part, with an emphasis on the damper and how it works.
So all the damper is, from the outside anyway, is a large cylinder on the bottom (assume it is sealed at the bottom and partially at the top for now for simplicity) with a long rod going into the top of it.
So the suspension is not going to shoot up in the air like a spring on its own would, it will extend relatively slowly.
And it has rebound damping, so when the weight is removed from the suspension it will extend in a controlled fashion, not shoot upwards!
There would also be a simple piston ring (same idea as in the engine) on the piston to seal it to the inside of the cartridge, I just havena€™t put it in this picture because it would only make it more complicated to look at. At this point, because no more oil can flow through the ports, the piston simply CANa€™T go any faster!
After this point it will be impossible to compress the suspension any faster; the problem here is if you were to hit a very bit square edges bump in the track you would feel most of the shock in your body because the suspension could not compress fast enough to absorb the bump. Secondly you cana€™t differentiate between compression and rebound damping, they will effectively be the same because the same ports are used for both directions! Putting these shims on top of the piston and effectively varying the amount of oil that can flow through is like a valve, so this system of shims etc. But now the shims are blocking the ports so the oil will have to bend them out of the way to get through, this is shown in the second part of the picture. At slow shaft speeds only a small amount of oil wants to flow through the ports so the shims only need to be deflected a small amount, which is relatively easily done. Youa€™ll notice in the above example the final stiffness has only been increased about three times, even though you have gone from one shim to 10 shims! Now is probably a good time to mention that different manufacturers and models use different mechanisms to achieve certain things, so things will be slightly different if you open up your shock or fork, especially if it is say a few years old or a WP PDS shock or Showa Twin Chamber technology etc. This time though the pressure created in the oil by the entering shaft begins to squash the Nitrogen filled bladder. So a hole is drilled down the centre of the shock shaft to below the piston and shim stack on the bottom of the piston, then a few holes are drilled into the side of the shock shaft to meet the bottom of this first hole. Firstly we have a similar problem to the one we had before where if you make the size of the bypass too small it will have an effect at slow shaft speeds but when there is a lot of oil flowing though the piston and shim stacks the tiny amount going through the bypass will have an insignificant effect on damping. So there will be a passage from the main cartridge to the reservoir where this displaced oil flows through; we are going to stick a damping unit in there to make use of this oil flow!
When the shock compresses the damper rod is going to displace oil which is going to want to flow from the main cartridge into the bladder reservoir, ie.
There are two ways oil gets pushed through the various damping circuits of a suspension unit. The problem with a spring is if there is weight put on it it tends to want to bounce up and down after being compressed. Ia€™m going to use pictures as much as possible as I think it makes it much easier to understand. To compress the damper you would push the long rod (known as the a€?damper roda€™) into the larger cylinder body(often known as the damper(even though a damper is really the damper rod as well) or cartridge).
Note the spring is compressed and the damper rod has been pushed down into the damper body; the cartridge.
When you push the damper rod down, the piston and damper rod are trying to move down in the cartridge, so oil has to get from below the piston to above; it flows through the ports in the direction of the red arrows. This is your damping; you can compress the damper or suspension unit up to a certain speed, after that speed you wona€™t be able to make it accelerate any faster. But in real world applications compression damping and rebound damping need to be quite different, typically by a factor of three! Actually varying the size of the ports in the piston would be more or less impossible, the solution is to use shims!
Note the stack in your dirtbikea€™s suspension will probably have a few more shims then this one.
The top of the piston where we have mounted the compression stack is known as the a€?compression sidea€™, likewise the bottom of this piston would be known as the rebound side, where the rebound stack would be mounted, but we arena€™t dealing with that just yet. However at higher shaft speeds more and more oil wants to flow through so the shims need to be deflected more and more, the deflection of the shims gets increasingly more difficult as they are bent more and more.
The centre hole diameter is ignored, then you have the a€?shim diametera€™.a€™shim thicknessa€™) a certain amount. It functions exactly the same as the piston ring in your engine.) Now you may have noticed a big problem with our piston from above. Ia€™ve put the spring seat and preload nuts onto this picture just to show where the spring is and how it a€?connectsa€™ to the damper.
It is basically a very thick balloon which is filled with Nitrogen (Nitrogen is used because it doesna€™t expand or compress with changes in temperature like air would).
Whereas if the bypass is made large it will have an effect at high shaft speeds and allow adjustability but at slower speeds it will come to a point where oil only flows through the bypass and will not need to go through the piston and shim stacks, bypassing them entirely.
You can see this damping unit in the above picture, it is known as the compression adjuster. The oil obviously cana€™t flow through the red ports as these are blocked on the right hand side by the shim stack so it must flow through the yellow ports.
The first which I described earlier is where the piston on the end of the damping rod is forced through the oil, meaning the oil MUST pass through the ports in the piston if the rod is to move. The reason for this is when you press down on a spring it will compress, then return back up again (this returning is known as a€?reboundinga€™ in the suspension world), but your weight is still on it, so it will get to a certain point and start going down again. It is a bit difficult to explain damping if you have never seen a damper from your suspension.
So when you push the rod down into the damper some oil will be displaced because of the rod taking up the area it would have before, so some will squirt out the top of the cartridge. Obviously this works in the other direction; if you are pulling the damper rod up (or the spring is rebounding and pulling it up) the oil will have to flow through the ports in the direction of the yellow arrows, ie.
So to try and clarify this a bit; if you were to go over a small bump the damper would compress slowly and hence with no resistance, only the force of the spring would be slowing the compression. So what you could do to try and tune at what speed the damping cuts in is to change the size of the ports in the piston; bigger will flow more oil and allow greater shaft speeds (shaft speeds refer to the speed the damper rod and piston are moving at).
A shim is basically a thin flexible washer that can be bent and will spring back to its original shape. The effect being that at slow shaft speeds you get a small amount of damping, then as the shaft speed increases so does the damping!
The spring would sit on top of the spring seat, then the preload nuts are screwed down on top of it to hold it in place and compress it a small amount (known as a€?preloadinga€™ the spring. Earlier I said there was a gap around where the damper rod went into our damper but didna€™t really explain why; this is the reason, to allow a€?displaceda€™ oil out.

When the shock begins to rebound again the bladder simply expands back to its original shape so the oil is back to the way it was. The main reason this would be a problem is small adjustments of the screw will have the desired small effect on a€?high speed dampinga€™ (ie. This will continue a number of times depending on the spring and amount of weight put on it, ie. A damper is a bit like a foot pump in that if you push down on it slowly it moves down easily with little effort, but if you push harder it gets more and more difficult to push to a point where pushing any harder wona€™t increase the speed the pedal goes down at.
Obviously there must be something in the damper to create the damping because the rod entering a cylinder of oil is not going to do very much (this isna€™t quite true but it can be ignored for now). However if you hit a big sharp bump the suspension and hence the damper would compress very quickly, however the speed with which it can compress is limited by the fact that the piston ports can only flow so much oil so the piston can only move so fast.
This is actually what the Race Tech Gold Valves do, they have larger ports so more oil can flow through the piston, although there is a LOT more to this which I will probably cover later. For example when the suspension is compressing, as in the second part of the picture, oil wants to flow from beneath the piston to above. Of course having a hole in the top (or bottom in the case of a shock) of the damper will be no good because all the oil would simply pour onto the ground the first time the shock was compressed! As I said before ita€™s important to remember there is NO air in the damper where the oil is, it is effectively filled to the brim with oil, including into the reservoir around the outside of the bladder, not inside the bladder of course as it is filled with pressurised Nitrogen.
So the oil cana€™t flow down the yellow ports in the piston as they are covered on the left hand side by a a€?check platea€™ (more on this later) but it can flow through the red coloured ports as they are open on the left hand side; these are the compression damping ports.
It is basically a washer (known as a a€?check platea€™) sitting up against the piston with a light spring holding it there, so it blocks oil flowing into the ports in one direction but the light spring allows the washer move back out of the way for the other direction! Or another analogy Germany suggested is damping is similar to walking through water; if you dona€™t make too much effort and walk slowly it will be easy enough to move, however if you start to make a lot of effort and try to move fast it becomes very difficult and you wona€™t really go much faster. Now if you were to push the rod down a bit then pull it back up again you would again feel the damping effect but in the opposite direction.
Another way of thinking of this, which is a bit more confusing, but more correct, is that when the damper rod and piston are moving DOWN, oil pressure builds up in front of the piston and decreases behind it and because there is a passage between these two different pressure areas they will naturally try to equalise, so oil will flow from below the piston to above it. Note the way the force required to deflect the shim increases exponentially with the amount the shim is deflected.
So something different needs to be done or our suspension will compress but never rebound again!
We dona€™t want the rebound stack coming into play here, only the compression stack (the one on top of the piston), however we do want oil to flow from below to above. Youa€™ll note in the above picture there is a seal at the bottom of the damper body that seals against the damper rod (aka shock shaft) and keeps the oil inside the damper, so if you tried to compress the above damper the damper rod would try to move into the damper and displace oil but the seal prevents the oil escaping so the damper rod simply couldna€™t be moved! We need to be able to adjust the amount of oil that can flow through this bypass so a long screw is put up the centre of the shock shaft from below, this is shown in pink. There is only one practical solution and that is to leave the bypass relatively small so it only really affects low speed damping, hence this becomes a a€?low speed damping adjustera€™ or just a€?low speed adjustera€™.
Oil can also flow through a bypass (just like the one up the centre of the damper rod) through the centre which is controlled by another clicker type screw, shown in pink and blue. So when the oil tries to come from the right hand side it easily pushes the a€?check platea€™ out of the way and flows through unimpeded! The damping rod is entering a sealed cartridge, and seems as oil can not be compressed some of the oil has to make way for the damper rod or it would be impossible for the rod to enter any further (causing hydraulic lock). This usually happens when the spring is rebounding (returning to its full length) so is known as the rebound damping.
Also when we get oil flowing from the top to bottom we are going to want to add some form of rebound damping.
This might be hard to visualise without seeing an actual piston but the red and yellow above represent ports through the piston just as the green did before, but now youa€™ll notice that oil can only really flow through them one way; one direction the oil can flow and deflect the shims out of the way, but the other direction the shims block the entrance to the ports and prevent flow as the shims cana€™t be bent that direction as they are up against the piston!
If you pressed hard enough you would eventually blow the seal out altogether or cause the damper to explode under the pressure.
Screwing this screw in (clockwise) screws it further up into the shock shaft and simply blocks more of the passage, restricting oil flow. Now the second problem is that as I said oil will flow through this bypass when the shock both compresses and rebounds, hence changing the adjuster screw is going to affect both compression AND rebound damping together. So when the shock compresses slowly (low speed) a small amount of oil will want to get by, this will simply go through the bypass (the low pressure of the oil isna€™t strong enough to bend the shims yet). So there is basically NO rebound damping provided by this compression adjuster as there is no real restriction to the oil flowing back during the time the shock is rebounding. If you had a bike that just had springs for suspension the first problem is, without having a VERY stiff spring, the suspension would bottom (ie.
Right so suppose we add something like our foot pump to the spring only suspension from before? So again we are going to use a shim stack, but this time it is going to be placed on the bottom of the piston. So taking the suspension compressing, oil will not be able to flow up the yellow port as the rebound (shim) stack is blocking its entrance so instead it will flow up the red port and cause the compression stack to deflect! So we need to allow the displaced oil escape from the damper but not just pour onto the ground, it needs to go back into the damper when it rebounds. Also the bypass that was added to allow low speed compression damping has no effect during rebound because the oil is free to flow as much as it wants through the piston and by the check valve.
We added the bladder and compressible Nitrogen earlier to give this oil somewhere to go and we just added the compression adjuster which the displaced oil will have to flow through before it can get into the reservoir containing the bladder, creating damping. Now we have a way of slowing the compression of the spring so we can run quite a soft spring but not have it bottoming really easily because the damper simply wona€™t allow the spring compress too quickly. For convenience this blue screw will actually be found on the side of the clevis (lower shock mount) on most shocks but to keep the idea simple Ia€™ve stuck it on the bottom of the clevis for now.
But remember before I said rebound damping is typically a lot stiffer then compression damping? Now if the shock is compressed faster the bypass will not be big enough to flow all the oil so it will start to go through the red ports in the piston. The main problem though is that when you landed from a jump the suspension would compress, then the springs would push the bike back up very quickly, bouncing you back up into the air. So you can now have the suspension acting nice and soft (plush) over small bumps on the track while also being able to land from a big jump and not have the spring compress so fast that it bottoms.
So compression damping and rebound damping have now been isolated; you can change the stiffness of the compression shim stack (ie. The screw is usually a special type that clicks as it rotates so you have some idea how much you have rotated it by, hence the reason these adjusters are often called a€?clickersa€™.
This is how the rebound adjuster (clicker) manages to have most of its effect on rebound damping! It will encounter a shim stack blocking its way which it needs to deflect (bend) to get by. Whata€™s important to realise here is the piston on the damper rod with its shim stacks is providing its damping due to it moving through the oil and forcing oil through its ports, while the damping unit we are about to add is providing its damping by the displaced oil, two quite different ways. You would then land again and the same thing would happen, over and over, very like being on a pogo stick. There is also a spring up against this shim stack to effectively control when it starts to bend and let oil by. The next problem is the spring rebounding (returning after being compressed) and launching the bike back off the ground. The bike would be almost unrideable; every little bump you hit would send the bike hopping up into the air. The stiffness of this spring is controlled by compressing it a bit, which is done by an external hex nut on the shock.
So we need some way to control how fast the spring compresses and some way to control how fast the spring rebounds (returns). Slowing how fast the spring compresses will stop it from bottoming easily and slowing how fast it rebounds will stop the bike getting launched back into the air and if you slow it enough the bounce effect will also be stopped.
So when the spring is rebounding, the damper is slowing and controlling it just like when it compresses. So oil will only flow by this shim stack at high speeds because at lower speeds the oil will be flowing through the bypass and wona€™t have the pressure to deflect the shim stack out of its way, hence adjusting the stiffness of the spring is effectively changing the compression damping only at higher shaft speeds, hence it is a high speed compression adjuster! Just for completeness Ia€™ll explain what would happen if you had just damping for your suspension. Well without the spring to support your weight as soon as you would take the bike off its stand the suspension would just compress slowly to fully compressed and stay there.
This is much like if you were to just lie on top of the water(assuming your lungs were empty so you werena€™t floating ), you would just slowing sink to the bottom and stay there.

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