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seven
08-19-2003, 09:37 AM
Here is some info on how to check your nitrogen pressure on you works shocks with cans.Pressure in Works Performance shocks.
First of all the normal pressure that we put
in most of our shocks is 250lb of nitrogen.
If a shock has our small 1 1/2" diameter body which
is typical of our twin shocks and a reservoir and
a 5/8" shock shaft we often use 175lb of pressure and
also on some BMW applications. It would not actually
hurt anything to run 250lbs in these applications
but would feel stiffer and or a little harsher.
If you have tried to check the pressure in one of our shocks
you have let out at least most of the pressure and will need
to repressurize, if it was an emulsion shock and you lost a
few drops of oil(which can make a pretty good little
mess) that should be no problem. The shock is mostly full
of oil but we leave 1/2" of air space in emulsion shocks.
The pressure in an emulsion shock when filled to 250lb
will normally drop to about 175lbs and that is its normal
running pressure and may vari with temperature (sort of
carbonation in reverse). A reservoir shock, floating piston
or bladder type will pretty much hold the pressure you
put in.
If you are really serious and want to check the pressure,
it is almost impossible to put any kind of pressure gauge
on the valve and not lose the pressure. There is another
way, the area of a 1/2" shock shaft is about 1/5 of a
square inch (14mm 1/4 sq in, 16mm or 5/8" shaft 1/3 sq in).
So with spring removed compress the shock just a little
against a bathroom scale and see how hard it pushes back.
Multiply this by 5 for 1/2" shaft and you have a good
approximation of the pressure (40lb on scale would be
200lb of pressure in the shock). On a 1/2" shaft
emulsion shock 30 - 40lbs on the scale is a more or
less fully charged shock. 1/2" with reservoir should
show 40 - 50lbs on scale. A large body reservoir
shock (5/8" shaft) should show about 80lbs on scale
that would be about 240lb pressure.
Where to get nitrogen, of course Works Performance,
almost anyone who services shocks, most Yamaha
dealers, and some other motorcycle shops. Many welders
have nitrogen and can do this, you may substitute Argon
but avoid Welding Argon if it has carbon dioxide in it.
Last (I have never done this but) people who service
commercial fire extingishers pressurize normally with
nitrogen, use about this pressure, and the ones I have
seen charge through a schrader valve (tire valve). That
means if you can talk the guy into it, he can charge your
shocks (not normally the fire department).
Why don't we just fill the thing with oil and forget
about the air space? Actually we do on some of the Softail
Harley shocks that are pull shocks but that means when you
pull the shaft out of the shock there becomes a blank space
inside the shock (cavitation). On a normal push shock
if you filled the shock all the way with oil and tried to
compress the shock, the shock shaft would become a hydraulic
ram and blow the shock appart in some way. Oil will not
compress (exept on neutron stars and black holes, places I
don't go) and the shock shaft does take up room inside the
shock when you push it in. We must leave an air space
and if we do not pressurize than the air space it is very
soft easy to compress and we have to use stiffer valving
to make up for that. Or we can pressurize and make the
air space stiffer and valving softer which works much better
overall. Even though the air space is small if we put in
a bunch of pressure we are putting a bunch of gas into
the shock and we do not want a bunch of oxygen (to help
burn up the oil) or much moisture (to mess everything up).
Nitrogen is clean, dry, and not expensive, and does not
react with anything in a shock.
What about a reservoir shocks? Proper reservoirs sperate
the gas and oil, they normally are about 2/3 gas space
and 1/3 oil sperated by a bladder or floating piston.
The shock has access only to the oil and is full oil itself.
The shock works in oil only, not an emulsion of gas and oil.
If the reservoir is not pressurized the shock piston
will actually push oil ahead of it into the reservoir
when you hit a bump, no compression damping, and a
big cavitation behind the shock piston so the shock
just slaps back. Basically while an emultion shock
can work kind of OK without pressure, the reservoir
shock can NOT work at all without pressure. Now
with your reservoir normally presserized and working
properly when the shock compresses the amount of shaft
that you push into the shock, that amount of oil has to
go into the reservoir and compresses the gas space by
that amount. The pressure pushes the oil back into
the shock as the shock extends. Air could be substituted
for nitrogen in an emergency but should be let out again
and replaced with nitrogen within a few days. (truck stops
should have better than 100lbs of air pressure available).
People think that different gases have different expension
rates but that is not true. All gases expand the same with
temperature, the exception in a shock is something like
water which can go back and forth between liquid (not a gas)
and steam (which is a gas) and does unmentionable things to the
pressure as it goes between the two states, a very little
bit of moisture can mess up a shock that is being used
hard and therefore subject to a very wide range of operating
temperatures. Nitrogen is a very good gas for a shock but is
not magic in any way it just does not do anything bad.

Juggalo
08-19-2003, 11:22 AM
great info! this belongs in the FAQ forum.