VALVING BASICS
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with several tests i made and beeing into suspension for a long time,
i came to the following conclusion:
1.why shims?
IMO we could have the same results with springs instead of shim stacks. why do we use shims?
- springs are larger than shim stacks. we don't have much space in our dampers.
- shim stacks are easier to adjust. you can get a stiffer valving by adding some
shims, but you have to exchange the entire spring to get stiffer valving. you
will not get the same result by preloading the spring!
2. why stacks instead of one (thicker) shim?
while it's good to have a progressive travel (linkage, "airspring" in forks),
it's not a good idea to have progressive damping (rod speed dependent).
if you have sharp edged obstackles (very high rod speeds) you will get a hard
knock, so we search for linear or degressive damping curves.
a single shim offers a progressive damping curve, while a shim stack with
more but thinner shims has only little progression.
3. why taperes stacks?
they are used for the same reasons like the above point. it takes out even more
of the progressive characteristic of the damping curve.
though they act not completely independent, the shims near the piston surface (all
the same OD) are called LS (low speed) or face shims, the tapered shims are called
HS (high speed) shims.
4. single/dual stage stacks
while the two points above try to eleminate the progressive characteristic, a dual stage
stack almost does the opposite. at low speed, only the LS shims bend. when they hit the
HS shims, the entire stack bends. the shim(s) that divides both stages is called crossover shim.
you will see both single and dual stage stacks on stock bikes, on the
compression stack of shocks dual stacks are very common.
5. clamp
this is the last part of a shim stack and is like a fulcrum for all the other shims to bend around.
a smaller clamp provides softer damping
6. float
usually the shim stacks are clamped to the piston surface. on fork midvalves it's common
to let the entire stack lift off of the piston. compared to the bleed, the check valve
function of a shim stack remains by using a float
of course there are different values used, but 0.2mm - 0.3mm is a common value,
at least at modern KYB midvalves.
7. bleed
is a bypass from one side of the piston to the other. we have fixed bleeds, some times
you will find an orifice on some mid valve pistons. variable bleeds, you all know
them as "clickers" you can adjust with a screwdriver. other than the float, the bleed
acts in both directions (compression and rebound). nevertheless they work just
in one direction at a base valve (compression only, because there's no rebound stack),
or much more in one direction (rebound on the midvalve) because the compression
stack is much softer than the rebound stack
the effect of the bypass is mainly at low rod speeds, at higher speeds the resistance
of the bypass comes into play and the damping is controlled mainly by the shim stack
the example in the diagramm shows a damping force reduction of 25% at low rod speeds
due to the bypass, but the effect is only 3% at high rod speed.
8. base valve/mid valve
it's common on both forks and shocks, to control the compression damping with 2 valves.
if you see the cartridge body as fix and the rod as moving, then the base valve is fix
and the mid valve is the moving piston. the first cartridge forks were open bath systems,
which had the base valve located at the bottom of the fork. maybe that's why it's
called base valve.
the mid valve controls the oil flow from one side of the piston to the other, the base valve
controls the oil that is displaced by the rod when it enters the cartridge.
it seems the combination of base valve and mid valve with some float is a good compromise
for the compression damping, while the rebound is controlled by the mid valve only. at least
this is the combination we see on most stock bikes today.
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