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  1. #1
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    getting acquainted with my rockshox digital fork pump

    I just started using a RockShox digital fork pump for the first time tonight and I'm not sure if I'm using it correctly. I screwed the hose to the fork but for one full stroke I get a very large increase in air pressure, like 20psi. My only other context for pumping air is into my bike tires. When I pump air into my tires using a Joe Blow bike pump, it takes many large strokes to get +20psi. A fork tube has a much smaller capacity than a tire but the RS fork pump is a much smaller pump than the Joe Blow tire pump. Am I seeing the expected behavior? I.e. should I really expect the fork psi to go up by 20psi in a single stroke? Doesn't this make it difficult to achieve a finely-tuned, granular and precise psi within approximately 2psi?

  2. #2
    Sneaker man
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    Yeah shock pumps are high pressure low volume, so 1 pump gives a lot. Tyre pumps are large volume low pressure so takes a bit to go up. OS something like 20psi form 1 stroke may well be about right.
    All the gear and no idea.

  3. #3
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    Quote Originally Posted by mik_git View Post
    Yeah shock pumps are high pressure low volume, so 1 pump gives a lot. Tyre pumps are large volume low pressure so takes a bit to go up. OS something like 20psi form 1 stroke may well be about right.
    What you are describing does not follow the laws of science. Just because something is labeled high pressure, low volume it does not mean you get high pressure without multiple pumps. My mini pump for tires is high pressure, low volume and you have to pump the crap out of it to fill a tire vs a low pressure, high volume floor pump. At the end of the day pressure and volume are inversely proportional. A floor pump would add greater PSI per stroke if you could apply enough force and the pump did not fail because it has more volume that it is trying to push into a confined space.

    For the OP, 20 psi per stroke does seem high. Do you have another gauge that you can use to compare? (you may have to compare at lower pressures than you would run to fit within the range of the other gauge)

  4. #4
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    Sounds like you didn't screw it on all the way and are just pressurizing the hose.

  5. #5
    Flat Pluto Society
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    Sounds like maybe the shock pump isn't depressing the valve on the shock. You might be just filling up the pump hose?

    D'oh. Jeremy beat me by seconds, lol.

  6. #6
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    My shock pump has two screw valves on it. The first screws on to the shock and then you screw the other ring to open the valve on the shock. This allows you to remove the pump without losing any pressure. Otherwise the tiny bit of air that escapes when you remove the pump would significantly change the pressure in the small chamber.

    To remove my pump I unscrew the small ring that is above the main attachment to the shock. If that second valve isn't opened then the air isn't getting into the shock and it will look like you are getting high pressure but it's only in the tube of the pump not the shock.

    That make any sense???

    getting acquainted with my rockshox digital fork pump-shock-pump.jpg

  7. #7
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    Quote Originally Posted by Tribble Me View Post
    My shock pump has two screw valves on it. The first screws on to the shock and then you screw the other ring to open the valve on the shock. This allows you to remove the pump without losing any pressure. Otherwise the tiny bit of air that escapes when you remove the pump would significantly change the pressure in the small chamber.

    To remove my pump I unscrew the small ring that is above the main attachment to the shock. If that second valve isn't opened then the air isn't getting into the shock and it will look like you are getting high pressure but it's only in the tube of the pump not the shock.

    That make any sense???

    Click image for larger version. 

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    Isn't that "tiny bit of air" just what's in the hose and not the shock?

  8. #8
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    ^^^correct, for a properly functioning shock pump and shock valve. The shock valve closes before air escapes the connection. The only air that escapes is what's left in the pump and hose, not what's in the shock.

    Most pumps do this by design when simply unscrewing the hose the from the shock. Some pumps have a lever or second barrel that closes the shock valve and is actuated before detaching the hose.
    Do the math.

  9. #9
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    I had to leave for work but I did a few more pumps before I left. Looks like the pressure is more normal if I don't tighten the connector as much. Otherwise, it seems like it may have a similar effect to when you try to pump a tire when the valve is a bit blocked. Has anyone else here had this experience with the digital fork air pump?

  10. #10
    100% PRIME ALBERTA BEEF
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    Quote Originally Posted by Finch Platte View Post
    Isn't that "tiny bit of air" just what's in the hose and not the shock?
    Agreed. This issue has been beaten to death on here countless times.

    In any event, I have the exact same pump and can confirm that it does not behave as the OP has described.

    OP - make sure it's threaded to the end. Or back it off a bit as you have most recently posted. If you still don't like the way it is performing, return it. As an aside, I had to return the first one I bought because the readout went all screwy on me early on (although it could have been a battery issue)

  11. #11
    Sneaker man
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    Quote Originally Posted by BlueCheesehead View Post
    What you are describing does not follow the laws of science. Just because something is labeled high pressure, low volume it does not mean you get high pressure without multiple pumps. My mini pump for tires is high pressure, low volume and you have to pump the crap out of it to fill a tire vs a low pressure, high volume floor pump. At the end of the day pressure and volume are inversely proportional. A floor pump would add greater PSI per stroke if you could apply enough force and the pump did not fail because it has more volume that it is trying to push into a confined space.

    For the OP, 20 psi per stroke does seem high. Do you have another gauge that you can use to compare? (you may have to compare at lower pressures than you would run to fit within the range of the other gauge)
    Sorry, probably explained very badly, should have said with a shock pump you are pumping INTO low volume at high pressure (the way the system works). I mean it would take forever to pump up a tyre with a shock pump. 20 does seem high, but I figure i get ~15 from one stroke of my pump. But it'd been a while, so I just went and had a play and sure enough, it's like 5psi from a stroke if that. So I'll shut up now.
    All the gear and no idea.

  12. #12
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    Ok I figured out the issue for real this time. I wasn't screwing in the hose hard enough. I had twisted the hose on all the way on with medium pressure but apparently, at least in my particular scenario, I needed to twist the hose on more forcefully.

    When attaching a pump hose to a bike tire, it the hose is not connected to the receptor securely then you will hear air spewing out. It seems to work a bit differently with the RS fork pump. When the hose doesn't have a secure fit to the receptor, instead of spewing air, the air will collect in the pump hose and give an invalid psi reading.

    Now the pump display shows a pressure increase of about 1/2 pound per pump which seems much more realistic.

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