Shock Trivia
By Randy Norian
Date: Thu, 3 Sep 1998 01:36:12 EDT

Thanks to some great emails from helpful listers, brain twisting conversations
with loquatious Brits, and a fair bit of introspection lately, I have some new
insights on what is going on inside these little shock absorber thingies. 

Some of this may be old news, some of it is just theory, some of it is fact-
read on!

here is a tidbit to remember:  when I was hanging out all day at computrack,
they mentioned an odd thing.  Turning up the compression damping knob will
increase your rebound damping, at some point.  They showed me on MY bike, and
it was a very noticeable effect.  I have a Fox shock.

I will come back to that later!  But remember that.

I just rebuilt the shock on my ninja.  That poor bike is the catalyst for half
of my RG-related shenanigans.

The ninja shock has a 40mm shock body, and is a very ordinary modern unit
with remote reservoir w/ gas bladder and compression damping on the reservoir
body.  The piston is 14mm diameter.  The volume of the inflated gas bladder is
approximately 100cc.  The gas bladder is pressurized to 200 psi.

basics we all know-  on compression, oil is valved through the shock
'cartridge' and flow is metered by the valving stack.   On rebound, the
opposite occurs with the rebound damping stack.  

So what's up with the compression adjuster on the reservoir body?

Richard K. astutely pointed out that one reason the reservoir is there to
provide someplace for the oil to go when it is displaced by the *shaft* of the
shock, as it extends into the shock body.  With no reservoir, and a shock
topped off with oil, it  would be hydraulically locked, as the shock shaft
could not move into the body.

How much oil flows into the reservoir?  what happens with gas pressure and the
compression adjuster?

for starters, I will assume the shock is moving *slowly*.

My shock shaft is 14mm diameter.  1.54 sq cm sectional area-    with 3" of
shock travel, it will pump 11.7cc of oil into the reservoir at full bump.
Nitrogen pressure will increase to about 228 psi under those conditions. 

At rest, the gas pressure contributes 18.8 lbs of force to extend the shock.
at full bump, this increases to 21.4 lbs.  with 300 psi of gas pressure, this
would be 28.2 lbs and 32.1 lbs.  At full bump the spring is exerting about
900+ lbs of force to extend the shock, so the effect of gas pressure as an aid
to spring pressure is almost nil.

I only considered the effect of gas pressure against the shaft, not the piston
(piston/shim assy) as the shim stack has pressure exerted against it from both
sides. 

But what about  compression damping?   That shock shaft displaces almost 12cc
of oil, and it HAS to flow into the remote reservoir.  This 12cc of oil passes
through a small orifice, which is adjusted by the compression knob.   Compare
that to 96 cm3 which is swept by the shim stack assy.  This oil will pass
through the compression shim stack, and ends up on the other side of the
shims.

So on the surface, lets say that the compression damping is 90% controlled by
the shim stack, and 10% controlled by the adjuster knob.  However, it can be
more complicated than that.  I will refer to "swept oil" as swept by the
piston/washer assy, and 'displaced oil'  (or "rod oil") as displaced by the
shock shaft.

The compression adjuster is widely regarded to be a low speed adjustment.  I
could not understand this--

The  compression adjuster usually selects one of several orifice sizes for the
oil to pass through before it can enter (or leave) the reservoir, the smallest
of which can be pretty damn tiny.  This varies from shock to shock, so I'll
stick with the 'pretty damn tiny' unit.

Suppose the adjuster went all the way down to zero (no orifice).  The shock
would become hydraulically locked.  Still think it has no effect on high speed
compression?

Your shock is partially 'cartridge' shock, and partially a conventional fixed-
orifice shock.  The adjuster selects a  fixed orifice, thru which a small
amount of oil MUST flow.  At higher shock speeds, this fixed orifice can
display all the bad behaviour of an 'old style' shock-  sharply increasing
resistance to motion with increased oil velocity.  In fact, the higher the
shock speed, the compression knob has more and more effect on damping.

I am not able to do fluid dynamics, so I can not guess at the forces
involved.. but imagine hitting a 3" rounded bump at 100 mph.  The bump is 4"
long (to the top).  at 150 fps forward velocity, your shock will  compress
about 1.5 inches in 2.2 milliseconds.  you will pump 5.8 cc of oil thru a
(2mm?) hole in 2.2 mS.  we're looking at oil moving at avg 2700+ fps thru that
hole.   hydraulic lock?  too extreme?  use a 5mm orifice and 50 mph.  oil is
moving at 440 fps thru that hole.  does it matter?  hey, I have no idea.

enough of that.

when bleeding the shock, with 40 psi in the bladder (depressing the shock
suddenly to force air out of the washer stack) and the shock seal not yet
installed, you can force the shock shaft into the body, and the oil level will
drop, then rise again a moment later.  This means that some of the 'swept' oil
is blowing into the reservoir, and  compressing the nitrogen bladder, then is
forced out again a moment later by the gas pressure. (rather than ALL the
swept oil flowing thru the shim stack) This at 40psi.  At 200 psi, it the gas
pressure will offer more resistance to 'swept' oil blowing in there
temporarily, however, some swept oil will still flow thru the hose and into
the reservoir, momentarily.  So even more oil may be metered by the fixed
orifice of the compression knob...  Although at higher fluid velocities, the
increasing restriction of that small hole may just mean that more and more of
the oil is forced to go thru the shim stack. 

That is the case, as I  found out.  At low speeds, the compression valving
flexes little or none.  The shock moves displaced oil (rod oil) into the
reservoir, but also PUMPS swept oil into the reservoir as well. so you may
have 80% (just a random number) of the swept + displaced oil passing thru the
compression adjuster orifice at low speeds.  Therefore it is the main source
of low speed compression resistance. 

At high speeds, the valving opens and swept oil passes through the compression
shim stack.  Little more than the displaced oil moves into the reservoir thru
the compression orifice.   So now we have 10+% of the total oil moved passing
thru that orifice.  There is proportionally less of the compression damping
coming from the adjuster hole.   However,  the adjuster still provides a
portion of the total compression damping, and the smaller the setting, the
greater the proportion.

Now I want to get back to what the computrack techs said about the compression
adjuster altering rebound damping.  The orifice on the adjuster controls not
only how easily oil can flow INTO the reservoir, it also controls oil LEAVING
the reservoir - and that is how it increases rebound damping.  Turn  your
rebound and compression damping down as low as they go, and bounce  your bike
in back.  Notice how it returns from the bump.  Now increase compression at
the knob, and continue bouncing.  Notice how it rebounds.  Keep increasing
compression and at some point you will probably notice a sizable increase in
rebound damping- the bike  comes up more slowly-  oil is having  trouble
flowing out of the reservoir, thru the small , adjustable 'compression'
orifice.

The compression adjuster orifice *does* create unwanted rebound damping.  The
number I got was approximately 20% of your rebound damping comes from the
compression adjuster, unless there is extra valving/fluid path to allow
rebound oil flow to bypass the compression adjuster's port.   Different shocks
will vary.  It is generally a higher end feature, and the best shocks do not
exhibit this "crossover" effect. 

If I were more clever I  would modify my shock to do this- maybe a separate
return oil path thru a one-way popoff valve.   

I guess  the thing to do, with what little I know at the present, is to valve
the shock so that you get the desired damping action while leaving the
compression adjuster on the largest possible orifice size (lowest compression
setting).  And just be aware that everything interacts! 

Randy Norian

Rob Koopman ( Rob.Koopman@inter.NL.net ) ^{RG500 Index}