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Normal after rebuild?

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I just finished rebuilding the shock on my '99 XR400. I revalved a bit, but I'll post that after results. I followed the Honda manual (new band and o-ring, Maxima 5W fork oil, debubbling, 7 psi air, release, 142 psi Nitrogen).

On the bench, no spring installed, after charging with Nitrogen, and with both compression and rebound set to full soft, I noticed that when I compressed the shock about 1" for 1-2 seconds, it would slowly expand back to fully extended on it's own when I released it. This makes sense to me given that there is some reservoir pressure built up during compression by the spring nature of the compression valves. Is this normal?

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yes this is normal on any shock,you have pressurized the area and when you push the shaft down in that area you are adding mass to the area and it just naturaly wants to displaces it and pushes it back out-------what i do is to see how well the valving and the rebound pin is sealing i will push the shock all the way down and then turn the rebound adjuster all the way in and the shaft will stay there if everthing is sealing good and i can click it out a click or two and see it move back up and then close it again and it stops-----its just a little test

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Kelstr,

I see what you're saying. I didn't know how fast the fluid would seep past the compression shims and equalize the pressures on above and below the stack. Apparently, not very quickly (it is a damper after all). The shock is installed now, so I'll do the rebound test after the weekend. Thanks again for all your suggestions and help :cry:.

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You still have the volume taken up by the shaft, so the pressure will still want to push it back out.

From the shock rebuild guides I've read if the shock doesnt do that something is amiss.

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I think I finally get it. It's the surface area of the end of the rod inside the shock. It's got 142 psi on it while the other end (outside the shock) has only atmospheric pressure on it. Roughly 1/2" rod OD, 142 psi equals roughly 28 lbs of force pushing the rod out. Hence, the rod pushes out. I expect that force increases as the rod is pushed in. Cool.

I may be slow, but .............. :cry:

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XR250 had it with volume being the key - not surface area or atmospheric pressure. As you compress the shock, you are stuffing the chrome shaft inside the shock body. But the shock body is already full of oil and parts. So, to make room for this additional volume of metal now inside the shock, the rubber bladder is squished down and reduced by an equal volume, while increasing its pressure. Once you release the shock, the pressure the bladder exerts on the surrounding oil then pushes the shock shaft back out (like sticking a pencil in a tube of toothpaste and then stepping on it!)

:cry:

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All true, however, the pressure inside the shock is 142 psi or more at all times. It only goes up from there (due to displacing oil into the bladder area) as you push in the shaft. All of the pressure on the sides of the shaft inside the shock is counteracted by the pressure on the opposing sides. The only surface of the shaft whose pressure is not counteracted on by oil is the end of the shaft inside the shock. It's counter acting force is produced by the atmospheric pressure outside the shock on the other end of the shaft. So even when the shaft is fully out, there is some force trying to push it out further because of the 142 psi versus atmospheric pressure difference. At least that's the way it seems from a physics standpoint :cry:.

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I follow you now, yup. Actually I think there was even a factory spec for the initial compression force of the damper unit (no spring) in the service manual somewhere. As you've noted, it will be directly related to the bladder charge pressure. Speaking of which, here is a tip I put in some other post some while back...

When charging the shock with N2, note that a tank/regulator set

to 150PSI may not charge your bladder to 150PSI - it depends on

the style of Schrader filler nozzle. Typical brass garage style

ones have the tip contact the valve core before the rubber seal

ring contacts the brass outer circle. This means that when you

release it, the seal breaks contact 1st, and you lose some of your

fill. I found you could easily loose 40PSI this way. Stacking a

second (or thicker) rubber ring into the filler nozzle helps

remedy this problem. When in doubt, check with your gauge, and

then refill (or purge and refill, N2 is cheap)

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Yes, releasing pressure is a concern. One of the guys at work remembers a Schrader fill valve fitting. It screws onto the outer Schrader shell, and has an o-ring sealed screw-in plunger to depress the center pin separately. It also has a gas fitting. This allows you to set the pressure, release the center pin (without losing pressure), then remove the pressure from the Schrader assembly. I'm looking for one now.

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yes you can get the set up at fox or lockhart phillps----its the only way to get the pressure correct,---i built mine so many years ago and cant remember where i got the fitting from --.

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you know i was so mad when i lost mine and could not find it , wasnt that a kool guage

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Hairy,

We had a chance to fill the shock to 140 psi today and measure the shaft force with no spring. It turns out to be almost exactly as calculated, 28 lbs. Cool.

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