If you’ve ever set your air fork or shock’s suspension pressure, checked sag and then put the pump back on to make an adjustment, you’ve probably noticed things weren’t where you left them just seconds ago.

We’ve all heard that psssssst of escaping air as we’ve disconnected the shock pump. And when we put the pump back on the fork or shock and notice it’s a few PSI lower than what we just left it with, it’s easy to assume we’re losing a bit of air by removing the pump. Common sense then dictates we should put a bit more than we need into the chamber, that there’s something wrong with our pump, or something else. Which can be frustrating.

But don’t worry. There’s probably nothing wrong with your equipment, and it’s working the way it should. Here to ease your concerns is Cane Creek’s Andrew Slowey to explain how it all works…

you can lose up to a few psi when attaching the shock pump but not when you remove it
Filling the hose and gauge of a shock pump can take a few PSI out of your fork or shock, but nothing should be lose when you remove the pump. Left to right: Before removing the pump it’s at 80psi, but drops to 78psi after re-attaching the pump.

BIKERUMOR: How much air is actually lost when disconnecting a shock pump?

LUKE: If your air can’s valve is working properly, no PSI should be lost while disconnecting the shock pump.

The air you hear when disconnecting the pump is actually air from the pump’s hose. When you put a pump back on you lose some air pressure in the air can due to it charging the pump again. That’s why recording (i.e. writing down) your setup is important for riders looking to tune their suspension to suit their individual wants and needs.

BIKERUMOR: What’s the best way to remove a shock pump to minimize air pressure leakage when disconnecting the pump?

LUKE: Ensuring that your air valve on your air can and shock pump are clean and working properly. There is no special technique to minimize air leakage as this is a common misconception.

BIKERUMOR: So there’s no need to over inflate our suspension slightly to account for the air lost when disconnecting the pump?

LUKE: No, a properly working shock pump and air can valve will not allow air to escape the air can while disconnecting the pump. The air you hear when disconnecting the pump is actually air from the pump’s hose.

BIKERUMOR: Why don’t air forks and shocks have built in air pressure gauges?

LUKE: Building in a pressure gauge onto a suspension fork or shock would add extreme complexity to the overall design. The pressure gauge would have to be able to handle forces from 100psi to 1000psi due to the ramp of pressure during use and to get the precision as well as reliability needed would be very difficult.

How a Shock Pump Works

how much air really leaks out when I remove a shock pump

So, taking a look inside the valve, here’s why it won’t leak…assuming everything’s in good working order. For simplicity’s sake, we’ll refer to any air suspension as a “shock”, but you could replace it with “fork” and the effect is the same.

do we lose air when removing a shock pump from the valve

When the shock pump is threaded onto the valve, it creates a seal before the plunger is depressed to open the airway. Once the plunger is depressed, it lets air escape from the shock into the pump, filling its hose and gauge. Luke calls this “charging” the pump, and that’s what makes it seem like there’s less air in your shock than what you started with. We’re only talking a few PSI, but it’s enough to register.

 

why does air leak out when I remove my shock pump from my mountain bike air fork or shockWhen you’re done pumping up the shock and start to unthread the hose from the valve, the plunger moves back into the closed position (Fig. C) before the seal between the valve and pump’s head is broken, keeping all of the intended air inside the shock. As you unthread the pump’s hose further to remove it, that’s when the seal is broken and you hear the pressurized air escape from the pump’s hose and gauge.


The fun never ends. Stay tuned for a new post each week that explores one small suspension tech, tuning or product topic. Check out past posts here. Got a question you want answered? Email us. Want your brand or product featured? We can do that too.

24 COMMENTS

  1. So you put a Schrader valve on a shock or fork instead of a tube or car tire and all of a sudden people are confused about how it works?

    • No, but the relative volumes are so much different. When you fill your car tire to 30 psi, then connect a shock pump, it’ll still read 30 psi.
      Fill your shock to 150 psi, disconnect, then re-connect, and it’ll read 130 psi. People see that and think they “lost” air when disconnecting as they hear the loud hiss (and are “confirmed” in their belief when reconnecting that they lost air as it immediate show lower pressure)

      My fairly intelligent but non-mechanical friend overinflates his tires by about 10 psi because of this “belief”. After explaining how a valve works he still does it(???). People learn things in different ways and some people won’t actually know how a valve works unless they are instructed…and even then. Its a good article.

  2. The comment after figure B is the key one, but I think it could be made more clear.

    On shocks, the volume of air is tiny. When you connect the pump and “charge it”, the hose volume is relatively large. This bleed off and increase in relatively system volume reduces pressure at least 10+ psi in my experience. PV=NRT —> P1V1=P2V2 —> P2=P1(V2/V1), and V2>V1

  3. Nice write up on a niche question.
    Latetly, I have been screwing on the shock pump to the point of sealing, but not opening the valve. I then pump the pressure of the pump (only) to a value close to the fork/shock value, then continue screwing in to open the valve. This just gets me closer to actual pressure (equalization) and I can see closer to what the actual psi was at.

    • That’s quite clever, and a great idea for checking pressure after a fork or shock has been sitting for a while. One thing we didn’t mention in the story is that if you’re using a “floor pump” style shock pump, the difference will be even greater when checking it because there’s a lot more hose to fill with air!

    • I’ve done this, but what I found a more useful method is to re-connect my pump and check pressure immediately after setting to a known amount. I ensure I always have the pump handle all the way in (although I am not sure this really matters given where the pump valve is).

      I record my set point fill pressure.
      I also record my indicated pressure on re-connect.

      After the initial calibration I can use the indicated value to track actual air pressure and somewhat track if poor performance is because my fork/shock is losing air or something else. Not entirely foolproof, but with the same pump and done with the shock “cold”, it seemed pretty consistent. Just remember to top off whenever you check.

      • @JBikes I think you have the more accurate and more efficient solution, now if I could just write down my numbers… 🙂

      • @JBikes You actually lose some air into the hose when reconnecting the hose – so you dont gain anything, by rechecking immediately after pumping. If you re-check multiple times, you lose even more pressure. Patrick’s trick reduces this loss.
        If the value was correct, you still need to pump up the lost air.

  4. Someone should design a shock pump that has a one way valve and the end of the hose to prevent the hose from being charged by the shock. I suppose you would also have to integrate the air bleed valve ahead of this one way valve.

    • But then how would you know what the pressure is? /rhetoricalQ

      This is all nice and tidy, theoretically, but what I’d like to see is an actual physical experiment:
      Thread a pressure gauge on a shock and put a schrader valve on that. Pump it up normally, remove the pump and see if there was any difference.

      If you really want to, you could also compensate for the extra air volume taken up by the pressure gauge and plumbing by adding more oil to the fork. All you’d need is a screw on schrader head, hoses, pressure gauge, schrader valve, a 3 outlet manifold, barbed to threaded hose adaptors, and some hose clamps.

      Unfortunately, I don’t have any suspension forks (or fork pumps) otherwise I’d do the experiment myself. I think I’ve got most the other stuff here at work.

      Actually, I’m sure somebody has already done this, but my Google Fu isn’t good enough to find anything.

  5. A simple hypothetical way to determine if your pump is letting air out or not is to create a second valve stem that is attached to a pressure gauge. Then the original valve stem would be attached to the pump. Ideally both the pressure gauge and the pump will be calibrated with each other. Pump away at the original valve stem while the pressure gauge is attached to the second auxiliary stem. If there is no pressure loss when the pump is removed then the pressure gauge will stay stable. Then when you reattach the pump to the main stem the pressure gauge will then go down due to charging of the pump tubing. The longer the pump tubing, the more air is lost while an insanely short pump tubing will have no air loss.

    Also remember air loss is relative to the volume that is being pumped. A car tire has many more liters of volume compared to a fork or shock setup. So while the charging of the pump tubing is the same, its more of a relative loss with the shock than a car tire.

  6. @tyler: Interesting that there is no mention of 2 stage, no loss pump heads in this post. They have been around forever and many of the better pumps have them…….
    One of the many benefits being you can deactivate the shock or fork valve and check sag with the pump still attached, which can protect the delicate aluminum valve threads over time.

  7. If you attach a shockwiz gadget you can witness this because the shockwizz will update the pressure in the fork real-time. When you detach the shock pump, the pressure the shockwiz registers stays the same. Voila, proof.

  8. This is systematic instrumental error, should be the same every time you add air. Since you’re comparing to your previous measurement this won’t make any difference. An exception would be if you’re using a standard pressure table (e.g.. the sticker on the back of your fork).

    • You do lose air, but when connecting the hose, not disconnecting. So if you connect, then pump up then disconnect – all is fine. If you then reconnect and measure – the loss will be measured (see Patrick’s trick above to reduce the loss).

  9. If you REALLY wanted to know the amount of “lost air pressure”, you could install a gauge between the fill valve the shock for test purposes.

  10. Better question. How much air did I lose out of my shock when I hooked my pump up to it before it could get a reading at what my shock WAS at?
    Did your head explode?
    How will you ever know what your ideal setting was?
    (this is where the shock wiz guy chimes back in….)

  11. Experimentation has shown that on my Fox DPX, attaching the pump reduces the pressure by 10psi. This is important to know when setting sag or you could really confuse yourself.

  12. Everyone is assuming their pump is accurate. At the very least, you want your pump to consistent, then accurate. Much like your oven – get a high quality gauge to calibrate against.

What do you think?

This site uses Akismet to reduce spam. Learn how your comment data is processed.