How suspension works, the in & out

[Jeff Gilbert]

I piddled with suspension for a couple of years now on my own bikes and even changed a few fork seals, what I'd really like to know is how it all works.
I do understand how valves work by limiting oil flow and the fact that springs are there to keep the travel extended. What I don't know are what all these shims, stacks, etc are and what portion of the stroke is affected by which component. Can someone direct me to information that can school me in how it all works and the best way to understand what I need to do to effect a desirable setup? I know experience is probably the key here but I have to start somewhere, maybe this time I'll start at the beginning. :confused:

 

[JTT]

Here is a simplistic description of what is going on, with damping anyways:

OK, let me give this a try. I'm not even close to an expert (but am learning). First the shock and fork are slightly different, but the principals are similar.
Picture a piston on the end of a shaft, now that shaft is in cylinder filled with fluid. The piston has holes in it that pass oil from above the piston to below and below to above as the piston moves through the fluid (if it didn't there would be a hydraulic lock and the shaft could not move).
Now the damping effect is created by the resistance of the fluid moving through these holes in the piston. The smaller the holes, the greater the resistance. Similar to passing a cupped hand through water or an open hand. In a crude system this is what you've got to control damping.
For our purposes though, we need to better control that fluid flow to give greater control of movement. Shims (thin little washers) are stacked on the top and bottom of the piston, covering the holes, or ports.
Now you must imagine this piston having 8 holes. The shims on the top cover 4 of these holes completely, the remaining 4 holes are covered by the shims on the bottom of the piston, but exposing the other 4. So, on each side there are 4 holes covered and 4 holes exposed.
Now move the piston upwards (as if the shock is compressing). The oil is displaced from above the piston and forced to flow to below. The shims are restricting this movement, but because they are thin, they deflect allowing the fluid to pass by.
On rebound, you will see the same effect takes place only in reverse, wit fluid travelling through the other set of 4 holes, past the shims and into the upper side.
This piston and shim arrangement is the "active" valving that you will hear referred to in this forum. Active in that it is moving through the oil, as opposed to being stationary and having oil flow through it.
...starting to make any sense?
Now, I going to throw another piece of the puzzle your way. Go back to when we compressed the shock. As the piston is moving upwards through the oil, the shaft that is pushing it is also filling the lower chamber of the cylinder. If the cylinder is sealed, then you again have a hydraulic lock situation as the internal area that the fluid occupies has now been diminished by the additional area of the shaft. (like putting an ice cube in your drink, that level in the glass rises).
So we have to do something with this displaced fluid. A second valve is provided, in a similar fashion to the one I just described above, only this one is stationary (ie: passive). This displaced fluid is passed through this valve arrangement and controlled in a similar manner (with stacked shims).
I hope I haven't made this any more complex than it has to be, but here's another curve...
In addition to the fluid that is passing through these valve assemblies, builders have given the oil an additional "out". Bypass holes or bleed are usually provided to allow a percentage of the fluid flow to bypass this valving arrangement. For simplification (if that word can even be used at this time ) I will not get into different types of bleed too in depth.
Remember our piston on the shaft? Make the shaft hollow and provide holes in shaft, above and below the piston so that some oil can flow past valve freely. Now we install a restriction between those two holes and into that restriction we install a tapered needle that can be threaded into and out of the restriction hole, thereby allowing us to control how much fluid travels through our "bypass". Voila! this is your clicker! (rebound that is anyway).
Do a similar bypass in your "passive" valve. You now have your compression clicker.
I am sorry for the length of this post, but I hope you understand my ramblings. If anyone has any additional items to add, or corrections to make, please do.
If you ever get the opportunity to see inside a shock or forks, take a close look and try to trace the flow of the fluid. It makes much more sense when you can see it in your hands.
Hope this helps!

 

[JTT]

My previous post was the very basics...which, Jeff, you probably already understand. Have you had a fork or shock apart to actually see the components? The shims are arranged in a "stack", generally in decending ODs from the piston....forming a sort of leaf spring arrangement.

Typically, the shims closest to the piston were considered to be LSC, middle ones MSC, and farthest from the piston to be HSC...in actual fact, it does not appear to be that simple, as every change, no matter where in the stack, effects the remainder of the stack to some degree. This is where is become complicated...and why guys like Jer "make the big bucks" ;)

This place is the best I have found for information on this subject, but it is an ongoing education, at least for me. Just when I think I have it figured out...I find out I'm way off base.

I have learned to not be afraid to make radical changes to my valving, for the sake of experimentation, as the results are often very surprising (but also be prepared for failure). What you may think will be "unridable" may just be "closing on perfection" in some cases.

I wish I could be more specific on this stuff. I remember asking and wondering the same things a couple of years back, and being disappointed by vague responses, like I just gave you...only to find later that there was reason for this....the goal posts keep moving :think: That is to say the lines between "what portion of the stroke is effected" blur as the shape and design of the stack changes.

Hopefully someone smarter than me can give you better insight...there's lots of them around here :thumb:

 

[marcusgunby]

Joe your post was very good but i feel it was a little too indepth-what we need is a picture of a fork/shock internals and labels with all the parts-on one of the suspension tuners web sites is a working diagram-i will look for it and post back.

like Joe says its alot easier with all the parts in your hands-next time you change your fork seals(which should be very soon knowing KTM's;) take the time to move the parts, as they do when in use, and see how they work.

 

[marcusgunby]

www.tootechracing.com has some good info as does jers mx-tech.com
not found the working diagram as yet.

 

[JTT]

There are a few static diagrams HERE and HERE

 

[JTT]

I think I know the one you are looking for Marcus...the animated cutaway going through the stroke, right? ARRGGHH...I can't find now...

 

[Lonewolf]

JTT thanks for that info that you posted, i made a post similar to jeff's earlier in the week and go no responces to it. i would like to learn suspension systems in depth, i wish there were books or something that i could learn a little more on modifications. were do the people who work at places like pro-action and mx-tech learn this stuff?

 

[JTT]

Sorry Lonewolf, I did not see your earlier post I guess. I don't know of any books or such available. I know Race Tech puts on seminars from time to time (just need to sift the facts from the marketing ;) )

I think most guys get into it simply through experience or working with others. Education in engineering and fluid dynamics wouldn't hurt either :laugh:

 

[bclapham]

i am thinking of getting the race tech videos

there is another website that could be good with diagrams and such, but they are having trouble getting started- they also have a forum

www.suspension-network.com

http://suspension.proboards6.com

 

[KTM-Lew]

Bruce....I've pretty much given up on that web-site.....it's down more than it's up. :(

This one is related to it but even more interesting for me. www.shimprogram.com
As allways, these sites are trying to drum up business....so read, think,read some more, ride even more.....etc,etc..... :yeehaw:

 

[DEANSFASTWAY]

Yeah I looked at it and trieds to joins up but got zoided out somehow . Somethimes Is can Reeds its Thoughs