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Old 06-25-2012, 09:51 AM
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Default General shock question

Hi All,
Hoping someone can explain something to me related to how a remote reservoir shock functions. I am curious why, when the shock is pressurized to 200psi (or any psi for that matter) that the shock wants to return to fully extended length?

I keep thinking that the shaft should not want to move to the extended position because:
--The total volume of space for the oil and gas remains the same.
--The bleed holes in the piston should allow for the pressure to equalize on the either side of the piston.
--That the nitrogen was present in the system to pressurize the oil and keep the cavitation down in the oil.


Does the bleed valve only work in one direction? Is the bleed valve covered by one of the shim stacks such that you have to have a minimum amount of force to use the bleed valve?

Thanks, just can't seem to reason this one out in my head as to why the shocks act the way they do.
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Old 06-25-2012, 10:39 AM
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the remote res contains a moving piston that divides the res into two halves, on one half is an air chamber (nitrogen) and the other half is the oil res connected to the main shock body.

as the shock travels, the shock shaft displaces some oil volume from the main shock body, connected to the resi this causes the floating piston to compress against the air chamber in the resi.

so in essence, even a regular shock is sort of a "mini air shock" and by varying the air chamber pressure you can modify the overall spring rate of your damping system. changing this pressure can also have a minor effect on damping and rebound rates as it contributes to preloading the damping shim stacks.

a remote resi shock shouldn't suffer cavitation, as the piston divides the oil and air chambers.
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Old 06-25-2012, 10:45 AM
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Here is a cutaway showing how it works. This is actually a Fox DHX RC4 for a DH mountain bike, but its all the same. well sorta, i think this actually has a lot more adjustment than your regular 4x4 shock does.

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1950 cj3a: FJ60 Axles, brakes, and steering, Willy's
sheetmetal and a Buick engine, held together with
metal from the scrap yard glued together in a garage in
Louisville and dragged around behind a F350 Diesel.
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Old 06-25-2012, 10:47 AM
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Quote:
Originally Posted by euroford View Post
the remote res contains a moving piston that divides the res into two halves, on one half is an air chamber (nitrogen) and the other half is the oil res connected to the main shock body.

as the shock travels, the shock shaft displaces some oil volume from the main shock body, connected to the resi this causes the floating piston to compress against the air chamber in the resi.
Thanks for the response. But wouldn't the pressure equalize over time through the bleed valve in the piston? Also, if there is some displacement of oil from the main shock body into the resi, this is what can cause cavitation. Its a sealed system so the rapid change of pressure via the piston can create a pressure differential.

But I still come back to the same point in my mind, that unless the bleed valve is covered by the shims (not sure if this is true?) that the pressure should equalize to both sides of the piston???
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'74 project pig (sold)
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Old 06-25-2012, 11:02 AM
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Ok, think I answered my own question. The volume of gas+oil doesn't remain constant. At full extension the shaft takes up none of that volume. At full compression the shaft takes up volume within the shock body. So, the pressure is higher at full compression than at extension.
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'74 project pig (sold)
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Old 06-25-2012, 11:22 AM
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^^ yup!

and there are of course holes/shims on the -shaft- piston, and yes the pressure remains constant on both sides, but there are no holes on the -resi- piston which serves as a 100% barrier between the air and the oil.

(if that piston leaks, its broke!)

fyi: the only reason why nitrogen is used on these is because its a readily available source of gas without any moisture in it, if you want to mess around with and tune this pressure, you can use a hand operated shock pump like this:

http://www.polyperformance.com/snow/...I-p-25357.html

and when you get it to where you like it you can release the pressure and have a shop fill it with nitrogen, no harm done.
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1950 cj3a: FJ60 Axles, brakes, and steering, Willy's
sheetmetal and a Buick engine, held together with
metal from the scrap yard glued together in a garage in
Louisville and dragged around behind a F350 Diesel.
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