Ive heard of people modifying that stack but it isnt done much, i wonder if you modify that stack will it just change the range the comp adjuster works over?Another idea is that the main stack has low/mid/high speed shims so why bother with ones that are probably hard to get hold of.
This sounds like a good topic of discussion. If you break the shock down into similar components to the forks, this stack would be the base valve and the stack on the shaft would be the mid-valve stack. The volume of oil passing thru the piston is in the range of 12X that of the compression adjuster (compared to 4X in the forks). The bladder pressure provides the required resistance to cavitation (like the spring does in the showa twin chamber forks). It would seem to me that changes in the compression adjuster stack would mostly effect the amount of change that each click on the needle valve would have. That is, if you make the stack stiffer more oil would be forced to go thru the needle than for a softer stack.
The reason for my question is that I've had three different bikes revalved for hare scramble/woods use. In all three cases, done by two different tuners, they moved the standard adjustments back to the middle of the adjustment range (about 10 clicks out) but the high speed is always 1 3/4 - 2 turns open. That's wide open so the end user (me) doesn't get the choice of trying a very soft high speed setting without a complete revalve, so I was wondering why it too isn't modified.
I have actually heard of modifying this area for woods riding use, but it is apparently rarely done for MX. I think Marcus is closest here as, as I remember it, this shim stack is supported on a spring, similar to fork midvalve. The adjuster preloads this spring...If I remember correctly. I don't think it even effect the LSC clicker at all, as this oil is flowing through the shaft, not this valving stack, but would effec the HSC clicker.
The high and low speed clickers are just variable orfice devices. To gewt it correct the compression adjuster has nothing to do with a midvalve actualy more acuratley the basevalve. The relationships should end there though.
Think of a few cases: What if you had a shock that had just a series of orfices what would happen? The shock shaft fluid displacement would have no restiance until it got to a certain speed and then wham! You'd get way to much compression. Early shocks worked off this system and i beilve George Capidocci (SP) of works perfomance was the first to offer a really good fix as he put a (blow of valve).. This allowed the shock to be tuned for lower speeds where it works best but as speeds increaed you did not get hammered as the shaft tried to compress fluid.
Case two: Clicker and then a shim stack that works in diferential to the clicker. A valve stack is nothing more than a more than a varibale orfice. It allows more or less fluid based on speed. It has potentially about 20 to 30 times more area than a small needle and seat of the clicker. The area that it has compared to the needle depends alot on the valve set-up. At low speeds the fluids path of least restiance is through the clicker, as speeds increase the clicker canot sufficently flow enough fluid and hydraulic pressure opens the valve. At this point the shock moves with less restistance and now the area of clicker is in a ratio of area to the effective orfice of the shim stack. If the shock were to acclerate in speed then the shim stack opens more and then the arear of clicker becomes even smaller in proportion to that of the shim stack. As this continues the clicker becomes less and less effective. Modern high and low speed adjusters work with this concept by allowing the shim stack to be tensioned with a spring and cup washer that applies the force of the spring at a point on the diameter of the stack.
Modern high and low speed adjusters on KYB's even have another stack that works against the needle. So whatever goes throught the orfice has to untimately go throw this stack. It's a way of increasing the damping coieffcient at lower speeds so that thte shock produces more compression sooner without being harsher later. (This actually ties into an earlier theread about rebound effecting compression.)
KTM SXS suspension and Showas don't run the secondary shim stack that apposes the clicker.
Now getting to whay we donm't revalve... We actually have and techically do however most aplications have been very well designed. The speed range is very narow as the shaft has a relatively small area to that of the active compression. Combine low fluid displacement, realtively slow speeds and you have something that works at a very narrow range and is IMO not a big tuning issue. Also it is very sensetive to major changes. You can modify it but many times the price you pay else where is not worth the cost.
IMO as Rob pointed out most guys don't remove it and I feel this is a crime to your shocks long term health.