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Suspension Tech: How much air do you lose disconnecting a shock pump?

why does it sound like I lose air from my suspension when I remove the shock pump
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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.

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32 Comments
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TheStansMonster
TheStansMonster
5 years ago

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?

JBikes
JBikes
5 years ago

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.

JBikes
JBikes
5 years ago

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

Andrew E Bentley
Andrew E Bentley
5 years ago
Reply to  JBikes

do the math right
P1V1=P2V2 —> P1=P2(V2/V1), and V2>V1 so P2 is * a number greater than 1 so P2 must be < P1 so the pressure after must be less than pressure before is what I assume you were trying to show.

Patrick Cavender
Patrick Cavender
5 years ago

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.

JBikes
JBikes
5 years ago

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.

Patrick Cavender
Patrick Cavender
5 years ago
Reply to  JBikes

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

Peter Kundrat
5 years ago
Reply to  JBikes

@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.

Justin White
5 years ago

Spoiler Alert: it doesn’t!

jasonmiles31
5 years ago

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.

ninjichor
5 years ago
Reply to  jasonmiles31

Sounds like a pain in the ass to bleed air + get a consistent/precise pressure reading.

Mark
Mark
5 years ago
Reply to  jasonmiles31

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.

Charles
Charles
5 years ago

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.

Antipodean_G
5 years ago

Ah ha, this is a cool little article! I’ve always wondered but never cared enough to work it out 🙂

gringo
gringo
5 years ago

: 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.

Dennis
Dennis
5 years ago

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.

James
James
5 years ago

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).

Jason West
5 years ago

My pumps must suck or im tripping bc i loose air. I swear i am…

Peter Kundrat
5 years ago
Reply to  Jason West

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).

greg
greg
2 months ago
Reply to  Peter Kundrat

I do not agree. If the valve inside the schrader on the shock is still depressed and unseated when the seal inside the pump connection comes away from the head of the screw fitting it’s attached to on the pump, you lose air. My RockShox pump looses 50# on disconnect when the shock is at 300#. I know because I check the sag and I have to pump 50# overpressure to get the desired sag. Anyone that says no pump loses air on disconnect is full of S***. The diagrams above are substandard, misleading, and otherwise useless because they do not show the complete pneunemitic sealing mechanics in the fitting connections that are supposed to prevent air loss on connect/disconnect. Topeak uses what they call a Pressure-Rite connection that solves this problem.

Jim C Welch
Jim C Welch
5 years ago

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.

Skip
Skip
5 years ago

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….)

RobertW
RobertW
5 years ago

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.

greg
greg
2 months ago
Reply to  RobertW

And in my experience, disconnecting loses #50 +/-, depending on how fast I can unscrew the fitting. See my other comment above

webbovich
5 years ago

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.

jk
jk
4 years ago

absolutely wrong article, the thread is not sealing at all when you unscrew the connector from the valve!
air is lost during this operation.

KB
KB
3 years ago
Reply to  jk

Look inside the threaded end of your pump, you’ll see a rubber seal. If it didn’t seal you would never be able to obtain pressure in your shock.

TS
TS
3 years ago

I have the same air pump chuck (threaded end of pump) as shown in the first picture. The picture with article title. The black rubber seal inside the chuck appears to be proud of the center brass inflater piece. Makes sense as this should contact the valve stem first, creating an air tight seal prior to opening the valve plunger. On my pump, this rubber seal is smaller / loose / worn / not proud of the center piece. When I disconnect my pump, I loose ALL (100%) of air pressure in my shock. I believe this rubber seal is the culprit in my case.

brandon
brandon
1 year ago

when i pump up my rockshock recon when unscrewing the pump all the air comes out how do i stop this

greg
greg
2 months ago
Reply to  brandon

I believe you, my friend, whereas others say it can’t happen. It can. See my comments above.

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