Tech Tip: Basic suspension theory (parts 1-4!)

[Zenith]

Basic Suspension Theory Intro I

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!). I soon discovered just how little information, even at the most basic level, there is on the Internet. So I figured I’d try to write a short article or two explaining the basics of how and why your suspension works the way it does. This is the first of those articles.
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. So I’ll refer to suspension, meaning either a fork or shock.

Essentially suspension breaks down into two components; the spring and damping. We’ll isolate the two components here to make it easier to follow.
The spring is what supports your weight and lifts the bike back up when you compress (ie. Press down on the bike) the suspension by say hitting a bump in the track. 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 problem with a spring is if there is weight put on it it tends to want to bounce up and down after being compressed. 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 ‘rebounding’ in the suspension world), but your weight is still on it, so it will get to a certain point and start going down again. This will continue a number of times depending on the spring and amount of weight put on it, ie. it will be bouncing. So you can see the problem? 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. Get fully compressed) very easily. 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. You would then land again and the same thing would happen, over and over, very like being on a pogo stick. Far from ideal obviously! The bike would be almost unrideable; every little bump you hit would send the bike hopping up into the air. 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.
First instinct might say why not just use a heavy spring? The first problem with this is even with a very stiff spring there would still be some bounce on big hits. 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! The solution is to use what is known as damping! It is a bit difficult to explain damping if you have never seen a damper from your suspension. 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 won’t increase the speed the pedal goes down at. Or another analogy Germany suggested is damping is similar to walking through water; if you don’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 won’t really go much faster. That make sense? Right so suppose we add something like our foot pump to the spring only suspension from before? 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 won’t allow the spring compress too quickly. 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. Perfect! The next problem is the spring rebounding (returning after being compressed) and launching the bike back off the ground. The solution? Have the damping work in both directions! So when the spring is rebounding, the damper is slowing and controlling it just like when it compresses. This is where the terms ‘compression damping’ and ‘rebound damping’ come from; ‘compression damping’ is the damping involved in slowing the compression of the suspension spring while ‘rebound damping’ is the damping that controls the spring as it rebounds/returns. Just for completeness I’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 weren’t floating ), you would just slowing sink to the bottom and stay there.
By adjusting the amount of damping the damper actually provides we can adjust how slowly or quickly the suspension compresses or rebounds. (The ‘damper’ is the unit in the suspension that provides damping, I’ll go into the how, where and why of this in the next article, hopefully with some diagrams to help.)
- For instance if the suspension rebounds too quickly it will launch you back up into the air when you land from a jump. In this case we need to slow down how quickly the spring rebounds, so we make the rebound damping stronger/Harder.
- If it rebounds too slowly it won’t be putting your wheels back down onto the ground quickly enough meaning for instance you won’t get good traction over acceleration bumps because the rear wheel will be off the ground most of the time! This time the damper is doing too much damping and slowing the suspension too much, so we need to make the rebound damping weaker/Softer.
- Suppose the suspension compresses too slowly; this will mean when your wheel hits a bump the damper won’t allow the suspension compress quickly enough so the whole fork/shock will be forced up into the air, ie. You will feel the bumps in your arms/legs. We need to speed up how quickly the suspension is allowed to compress, ie. We need to weaken/Soften the compression damping.
- Finally what about if the damping is too weak/soft on the compression stroke? This is great for small bumps because the suspension will compress very quickly so you won’t feel anything in your arms. However the problem is if you land off a jump or hit say a whoop the suspension will compress very quickly and easily and will rapidly use up all its travel and bottom. So we need to strengthen/Harden the compression damping and find a balance between small bump absorption and larger bump absorption. <There are more factors involved to aid this balance in modern suspension, such as air gap and different strengths of damping depending on how fast the suspension compresses, but to keep this article simple I’ll avoid these for now. I hope to add them in the next part or two of these article though>

I think that’s all I wanted to say for now. If there is anything apparently unexplained PLEASE ask, I didn’t write this for the good of my health, I want to help people understand this fascinating (IMHO anyway) subject.
So if there is any interest in this I’ll write another part explaining damping in some more detail (with diagrams), the how’s and why’s of the damper and possibly how the clickers work if it is not too long and boring already .


Philip

[Zenith]

Basic Suspension Intro II

Right so from Part I you should have some idea of how suspension breaks down; the spring and the damper. I didn’t really explain how the damper actually works, only the effect of it. So in this part I want to try and elaborate on the first part, with an emphasis on the damper and how it works. I’m going to use pictures as much as possible as I think it makes it much easier to understand.

So what you have is a spring and this “”black box”” we’ve called the damper.



So this is the spring, pretty straight forward!



This is what the damper looks like from the outside. I’ll go into what’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 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. To compress the damper you would push the long rod (known as the ‘damper rod’) into the larger cylinder body(often known as the damper(even though a damper is really the damper rod as well) or cartridge). Pushing down on the rod you will feel the damping effect I explained before. This is usually done when the spring compresses so is known as the compression damping. 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. This usually happens when the spring is rebounding (returning to its full length) so is known as the rebound damping. So that’s what the damper is like from the outside.



Now we have to make some sort of suspension from the spring and damper, and this is it shown in the picture above. 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. Just assume the top of the spring and the top of the damper rod have been welded to say a plate of metal, as have the bottom of the spring and the bottom of the damper/cartridge.
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. 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. So the suspension is not going to shoot up in the air like a spring on its own would, it will extend relatively slowly. Note the spring is compressed and the damper rod has been pushed down into the damper body; the cartridge.
So we effectively have a functioning suspension unit here. It is providing compression damping so won’t compress too fast, like a spring on its own might. And it has rebound damping, so when the weight is removed from the suspension it will extend in a controlled fashion, not shoot upwards!


So you now know how the main components go together, but how does this ‘black box’, 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. 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. This is not the case but is the best way to explain it for now). The large cylinder (the cartridge) is filled with oil. 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 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 isn’t quite true but it can be ignored for now).



This is the damper rod on its own, having been removed from the cartridge. What you see on the end is what is providing all the damping in our simple suspension unit. For now it is just a piston.



The piston is basically a flat disk of metal, about 0.5” thick attached to the end of the damper rod as shown above. The outer diameter of the piston is slightly smaller then the diameter of the cartridge so it fits in snugly. 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 haven’t put it in this picture because it would only make it more complicated to look at. Basically there are holes (known as ports) drilled right through the piston so oil can flow (shown with the red/yellow arrows) from above the piston to below it or visa versa. These ports are shown in green on the pictures.
So the idea is when you push down on the damper rod, this will push the damper rod down into the oil filled cartridge. 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. Ie. 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. 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. from above to below. 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.
So how does this piston with ports through it actually provide damping? Well if the ports/holes are small enough and you move the damper rod and hence piston fast enough it will get to the point where no more oil can actually be forced through the ports. At this point, because no more oil can flow through the ports, the piston simply CAN’T go any faster! This is your damping; you can compress the damper or suspension unit up to a certain speed, after that speed you won’t be able to make it accelerate any faster. 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. 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 obviously works for rebounding as well. There is a maximum speed the damper can rebound/compress.
Of course you might have spotted a problem here? 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. 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. You’d probably end up on the ground. 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). 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.
Again there are a couple of problems here. Firstly this will not be very tuneable because you can only set one point when the damping really kicks in hard. Secondly you can’t differentiate between compression and rebound damping, they will effectively be the same because the same ports are used for both directions! But in real world applications compression damping and rebound damping need to be quite different, typically by a factor of three!
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. the damping needs to start off quite soft so the bike is nice and plush (smooth/soft) over small bumps, but must build up gradually to deal with larger and larger bumps). Remember larger bumps will compress the suspension faster, ie. cause higher shaft speeds, so need stiffer damping to stop the suspension bottoming.

I think I’ll leave it there for now and let you think this through and try to figure out how to deal with the above problems? I’ll give you a hint; this is where shims come into play!

I hope that was remotely interesting. As I said before PLEASE post any questions or corrections you have, no matter how small or trivial they might seem to you.

Philip

[Zenith]

Basic Suspension Intro III

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. Landing from a jump or going over long smooth bumps on the track will cause slow shaft speeds, while hitting sharp edged bumps etc. will cause high shaft speeds. At slow shaft speeds there is a small amount of oil flowing through the piston ports, then as the speed builds up more and more oil flows through, up to the point where no more oil can flow through (known as hydraulic lock) and the shaft/piston can not accelerate (note it cannot ‘accelerate’ anymore, it can however continue moving at the same speed though, it’s not like it stops dead) anymore.

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. So this would call for variable damping, which clearly isn’t going to be possible with the simple piston with ports through it from earlier. We need some way of effectively varying the size of the ports depending on how fast the piston/shaft is moving. Actually varying the size of the ports in the piston would be more or less impossible, the solution is to use shims! A shim is basically a thin flexible washer that can be bent and will spring back to its original shape.


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. 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. A shim stack is shown below, this is known as a tapered stack. Note the stack in your dirtbike’s suspension will probably have a few more shims then this one.


Explaining and drawing this will get a bit confusing so for now I’m going to ignore rebound. I’ll explain it at the end but for now just try to ignore rebound altogether and assume this suspension unit just deals with compression...

So as I said we want to be able to vary the size of the ports on the piston depending on the shaft speed. What we do is mount one of these shim stacks on top of the piston as shown below. 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. is known as valving in the suspension world. This is where the term getting your suspension ‘revalved’ comes from; among other things the suspension tuner changes the shim stack, adding/removing shims of different sizes and thicknesses.

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. 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 ‘compression stack’. It is also known as the compression valving. The top of the piston where we have mounted the compression stack is known as the ‘compression side’, likewise the bottom of this piston would be known as the rebound side, where the rebound stack would be mounted, but we aren’t dealing with that just yet.


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. When the suspension is compressed oil has to flow from below the piston, up through the piston ports and out the top. 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. This bending of the shims is known as ‘deflecting’ the shims in the suspension world.

The shims used deflect (bend) small amounts relatively easily but get progressively more difficult to deflect (bend) the more they are bent. This is how the variable damping is provided. 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. 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 effect being that at slow shaft speeds you get a small amount of damping, then as the shaft speed increases so does the damping!

Just for interest’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. This would generally be written 40.20. I.e. The centre hole diameter is ignored, then you have the ‘shim diameter’.’shim thickness’) a certain amount. (The graph is from http://www.shimprogram.com, hope they don’t mind . By the way after you’ve read all these articles if you’d like some more detailed info on the workings of shims specifically I’d strongly recommend checking out the articles on http://www.shimprogram.com!). Note the way the force required to deflect the shim increases exponentially with the amount the shim is deflected.


Below is another graph from http://www.shimprogram.com, it shows the forces required for different amounts of deflection of the single shim from before (in red) and a tapered stack (in green) like the one from the pictures above. Note how much stiffer the tapered stack is then the single shim is, yet it still has a nice smooth progressive curve. Again for your interest the tapered stack used to create the green curve is this (Note the largest shim sits up against the piston) –
40.20 (ie. Shim outer diameter is 40mm, thickness is 0.20mm)
38.20
36.20
34.20
32.20
30.20
28.20
26.20
24.20
22.20



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. It can take years before it starts to make sense so be patient. You’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!


Back to the piston again. The picture above shows our usual piston with a shim stack on top of the piston (the compression stack). (I’ve also added the piston ring around the piston which seals the piston to the walls of the cartridge so oil can’t blow around the piston and avoid the valving (the shim stack and piston ports etc.). 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. When the suspension rebounds (ie. the shaft and piston want to move up in the damper) there is no way the oil can flow back though from the top of the piston to the bottom because the shims are completely blocking the top openings of the piston ports. So something different needs to be done or our suspension will compress but never rebound again! Also when we get oil flowing from the top to bottom we are going to want to add some form of rebound damping. 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 the picture above shows the solution. The first part shows the second tapered shim stack mounted underneath the piston, on the so called ‘rebound side’. This is known as the rebound stack or rebound valving. The shape of the piston has also changed quite a bit. It now has one way ports through it. 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. We don’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. 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 you’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 can’t be bent that direction as they are up against 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! And visa-versa when the suspension is rebounding. So compression damping and rebound damping have now been isolated; you can change the stiffness of the compression shim stack (ie. add shims) but this will not effect how stiff the rebound damping is, and visa-versa for changing the rebound stack! This was one of our original aims.
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). Also, generally the two shim stacks (compression and rebound) will not be identical as it appears in the above pictures...


Right I think that’s enough said for this part. Again if you have any questions or I’ve explained something badly or incompletely PLEASE reply and say so, it’s the only way I’ll know. You should now have a very good idea of how your suspension is working at a detailed level. There are still a few things left uncovered, like how this applies to a fork or shock which I hope to get to in the next part or two.

[Zenith]

Basic Suspension Intro IV

The last three parts of this article have pretty much covered all the basic theory of how suspension in general works. Now we can go on to see how the theory is put to use in the actual suspension on a dirtbike. 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.

I’m going to start with describing the shock because it is closest to the suspension unit invented in the previous parts of the article. As before I’m going to build up the shock from simple to more complicated to make it a bit easier to understand.


Right so the first thing to do is turn our suspension unit upside down as above. The top of this will be mounted to the top of the bike’s frame, the bottom will be mounted to the linkage or the swing arm in the case of a WP PDS shock.
The spring is fairly self explanatory so from now on I’ll leave it out of the diagrams, you’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). The ‘upper shock mount’ is what connects to the bike’s frame, while the ‘lower shock mount’ (aka. Clevis) is what connects to the bike’s linkage. I’ve put the spring seat and preload nuts onto this picture just to show where the spring is and how it ‘connects’ to the damper. 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 ‘preloading’ the spring. The amount you screw it down my is known as the ‘preload’.). I’ll leave these out of the next few diagrams to save them confusing things...
When the shock shaft (damper rod) is driven into the damper the shaft inevitably takes up volume in there. Oil cannot be compressed so if the oil cannot get out of the damper the shock shaft simply won’t be able to go into the damper because there is nowhere for the oil to go (hydraulic lock again!), ie. the shock would be rock hard. So we need to allow the ‘displaced’ oil out of the damper somehow. Earlier I said there was a gap around where the damper rod went into our damper but didn’t really explain why; this is the reason, to allow ‘displaced’ oil out. 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! You’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 couldn’t be moved! If you pressed hard enough you would eventually blow the seal out altogether or cause the damper to explode under the pressure. 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.


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. 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 ‘reservoir’. In the reservoir there is a rubber ‘bladder’. It is basically a very thick balloon which is filled with Nitrogen (Nitrogen is used because it doesn’t expand or compress with changes in temperature like air would). The Nitrogen in the bladder is kept completely separate from the oil though.
So now what happens when the shock is compressed (as in the second part of the picture)? The shock shaft is pushed into the damper and attempts to displace some of the oil in the damper. This time though the pressure created in the oil by the entering shaft begins to squash the Nitrogen filled bladder. So the shock can now compress properly without all the oil pouring 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. As I said before it’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.
That’s the shock basics covered! If you built the above shock and stuck it on a bike it would work, not too well, but it would function...

So what’s wrong with it? Well the main problem is there is no way of adjusting the damping without taking the shock apart and changing the shim stacks, which isn’t too practical at the side of a track!


This is one part of the solution to external adjustability. It is basically a bypass through the two shim stacks and piston etc.. 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. These are show in red. So this creates a bypass around the valving (piston, shim stacks etc.), ie. when the shock compresses of rebounds as well as oil flowing through the piston the deflecting the relevant shims some will flow through this bypass. 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. Screwing this screw in (clockwise) screws it further up into the shock shaft and simply blocks more of the passage, restricting oil flow. You can adjust this using the blue screw head as shown. 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 I’ve stuck it on the bottom of the clevis for now. 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 ‘clickers’.
So what’s the problem now? Two main things. 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. 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. The main reason this would be a problem is small adjustments of the screw will have the desired small effect on ‘high speed damping’ (ie. the damping provided at high shaft speeds) but will have a huge effect on ‘low speed damping’ destroying the progressive effect we achieved with the shim stacks. 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 ‘low speed damping adjuster’ or just ‘low speed adjuster’. 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. Clearly you will rarely want to adjust the compression and rebound together like this. But remember before I said rebound damping is typically a lot stiffer then compression damping? This is how the rebound adjuster (clicker) manages to have most of its effect on rebound damping! The rebound shim stack on the piston (ie. the one on top of the piston) is much stiffer then the compression stack, therefore the piston mainly provides rebound damping. So adjusting the bypass will primarily affect rebound with a small effect on compression!

What about compression damping though, we really want to be able to adjust that externally as well right? To some degree you could change the compression damping characteristics (ie. how the compression damping functions, how hard/soft it is etc.) externally by changing the pressure of the Nitrogen in the bladder but it would be difficult and very ineffective so we need some way of changing compression damping externally. Something similar to the adjustable bypass we put in the damper rod for rebound?




Right well these two pictures show what is known as the ‘compression adjuster’. If you look closely at it you’ll notice it is very much like a suspension unit in itself, except it is ‘passive’ (fixed in one position, ie. the piston doesn’t move).
I know that picture looks quite complicated, I probably should have built the adjuster up from the basics but I’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. If not please say so and I’ll do a more detailed description, it’s no problem at all…
As I described earlier, oil will be displaced by the damper rod entering the cartridge. We added a reservoir beside the main cartridge with a compressible Nitrogen filled bladder in it to give this oil somewhere to go. 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! You can see this damping unit in the above picture, it is known as the compression adjuster.
So preferably what we want is an adjuster that only affects compression damping. We have a two way piston just like the one on the end of the damper rod, except it is fixed in place. 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. from left to right in the pictures above. This oil flow is represented as usual with RED arrows. So the oil can’t flow down the yellow ports in the piston as they are covered on the left hand side by a ‘check plate’ (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. 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 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 isn’t strong enough to bend the shims yet). So low speed compression damping can be adjusted using the clicker screw. 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. It will encounter a shim stack blocking its way which it needs to deflect (bend) to get by. There is also a spring up against this shim stack to effectively control when it starts to bend and let oil by. Make the spring stiffer and it will be more difficult for the shim stack to bend, hence making it more difficult for the oil to get by, hence making the damping stronger/stiffer! The stiffness of this spring is controlled by compressing it a bit, which is done by an external hex nut on the shock. This hex nut typically surrounds the clicker screw I mentioned earlier. 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 won’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! We now have an external high and low speed compression adjuster!
What about rebound? Well when the shock rebounds it sucks oil from the reservoir to the main cartridge, ie. from right to left in the above pictures. The oil obviously can’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. On the left hand side of the piston there is what is known as a ‘check valve’. It is basically a washer (known as a ‘check plate’) 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! So when the oil tries to come from the right hand side it easily pushes the ‘check plate’ out of the way and flows through unimpeded! 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. 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.

This is a good time to talk about ‘passive’ VS ‘active’ damping; the ‘passive’ 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 ‘active’ 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. This is quite an important concept in suspension and you’ll see it more clearly when I move onto the fork. There are two ways oil gets pushed through the various damping circuits of a suspension unit. 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. This is the ‘active’ part. The other way oil gets pushed around is by displacement. 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). The oil is therefore ‘displaced’ by the damper rod. 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. This will be known as ‘passive’ damping. What’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.

That’s pretty much it for the shock, if you took apart your shock now you’d have a very good understanding of what everything is, assuming it isn’t a WP PDS shock of course as they are a bit different.

There is a great animation of a shock very similar to the way I described it above everybody should take a look at. It was from http://www.ccycle.com but they don’t seem to have it up anymore so I’ve made it available here http://homepage.eircom.net/~thefoundati ... ckdemo.mov .

[AV8R]

WOW! Most excellent!

[Spanky]

WOW! Most excellent!

Zenith's suspension knowledge is impressive.

[Mobil1rally]

Hi to everyone!

I registered just to say that after endless researches, the guide Zenith wrote is the best one I've found on the internet. Simple as that.

But also, you wrote:"....This is quite an important concept in suspension and you’ll see it more clearly when I move onto the fork...."

I'm begging you, please make a fork theory explanation!! It will be great!

Thanks for your great work

PS: If you find weird or offensive something I write, please tell me, because I'm italian so I could write something wrong... Thanks again!

[Mobil1rally]

And why don't you add this explanation on wikipedia? It's really worth reading from everyone interested in this argument...

[JR Nolan]

Thank you. Please, the fork?

JR

[ssmx]

Excellent write up! By far the best on the net, would really like to see the forks aswell if you have time?

Got some questions,any help appreciated.

The compression stack on the shaft,is this just low speed then the high speed adjuster and stack controls high speed?

Also heard about "crossover",what is this?

Face shim,is this the shim closest to the piston face?


Sorry for all the questions,just seems to be the only thing to do with motocross thats kept a secret!

[redhot007]

Very good job
And hope you will improve eeven more this thread

[Zenith]

ssmx - It's been a while so I'm a bit rusty on this stuff.

The main shim stack on the damper rod/shaft controls both high and low speed, the adjuster just allows you tweak high AND low speed depending on whether you use the outer hex bolt (HSC) or inner screw (LSC). If you open one up you'll find the shims on the compression adjuster are tiny in comparison to the main shims. The relatively complex adjuster was just the shock manufactuers way of allowing you adjust high and low speed somewhat independently, where as the more basic mechanism on forks only allows adjustment of pretty much low speed compression. The fork adjuster is very like the rebound adjuster on shocks, a screw partially blocking an orifice.

The "crossover shim" is part of something else I meant to cover in future, two stage shim stacks. In a two stage shimstack you have a tapering stack of shims with a gap somewhere near the middle of the stack, followed by more of the tapering ship stack. This gap is created with a much smaller but quite thick "crossover shim". What happens is when small volumes of oil are passing through the piston only the first few shims in the stack have to bend around the crossover shim, while at higher volumes of oil the whole stack has to bend. I'm a bit hazy on this part now as it has been so long, but I think it may be the case that the first few shims actually have to work harder because they have to bend a tighter angle around the crossover shim before you reach volumes of oil where the whole stack bends together. Again I "think" this allows you setup stiffer low speed damping with softer high speed. It's very common in MX forks, if you open yours up you'll almost certainly find a two or three stage stack and see what I mean.

Yes on the face shim question.

Yeah it's odd that it's so secret. I think it may be more to do with the complexity involved, that VAST majority of people aren't interested or find it to difficult to get to the point where they can do something useful with their suspension by themselves, so they don't look for info.