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Do You Lose Air When Removing the Shock Pump? 27

You have the specs dialed on your mountain bike suspension. You pump it up to the proper pressure but then go to remove your Vital MTB digital shock pump and hear the hissing of air being lost. So begins the cycle of second-guessing your suspension, checking and adding air to achieve the proper setup. What does it all mean? Are you really losing air? Get the full story so you can get your mountain bike back on the trails!

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Credit: Lucent Productions

BHowell BHowell 5/23/2020 7:00 AM

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27 comments newest first

Interesting tidbit... I left my pump on as I equalized the positive and negative chambers... That resulted in approximately a 5psi loss in the positive chamber on the rear shock... It wasn't as dramatic on the fork..

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Well of course. You added air to the positive chamber, cycled it, the positive and negative chambers equalized in pressures. This is done by air transfer between the two, meaning you lost some air in the positive chamber to the negative one. Therefore you lost pressure.

Why do you think you need to cycle the suspension when you adjust pressures? Because of this.

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No, you couldn't. Because you don't lose air removing the pump.

And cycling the shock you're not losing air, you're redistributing it between chambers. You 'lose' it from the positive chamber, therefore 'losing pressure', but only because you only measure the positive chamber.

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Ah on the first part. Regarding your point of people understanding... I doubt it smile

I think I might make a video when I get my Shockwiz back.

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I guess I should have put quotation marks around lose since there isn't a pressure loss, but just an equalizing of the two chambers...

Yes, I know that you don't lose pressure removing the pump... I have spent 15 or so years explaining that concept to people...

Basically , we are both arguing the same side of the argument right now... However, doing what I did and leaving the pump on might make the concept of what's happening a little more clear to some people.

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There's a simple way to check this. Attach a shockwiz to your shock/fork and do a realtime pressure check there.

FYI, regarding the valve, my first fork installation of the shockwiz was all over the place, because i couldn't screw it on enough by hand and I didn't open the valve enough. I had to use pliers.

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Is it not even relevant if air escapes or not? The guage is a guide to get you to your sag number, then you fine-tune adjust based on riding feel and what you want. If the amount of air escaping or not escaping is consistent, the result should be the same. I would even venture that all guages are not equal as well. Regardless, I prefer to know that air is not escaping and do the air hose preload trick.
Awesome video! So well done. The significant other "listening" had us rolling.

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If you don't unscrew the pump fast enough you do lose air or if the screw is longer. Airing up my DVO Jade is a nightmare because of this. I'll put in 200psi and then when I check it again it'll be 185psi!

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You're losing that 15psi when you re-attach the pump - the 200psi in the shock expands into the hose and, thus, only reads 185 because of the expansion. Either that or there is something wrong with your pump or valve. A 15psi "loss" from the piggy back of that shock is totally normal. Try unthreading it REALLY slowly next time, and if your pump isn't messed up, it'll still read 185 next time you attach it, regardless of how fast or slow you remove the pump.

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The Schrader valve on the shock usually closes at about a 1/4 turn when disconnecting... Just something to keep in mind..

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Honestly, the easiest way to prevent people from constantly questioning the shock pressure gain/loss is to remove the gauge from the shock pump and integrate a the gauge into the shock design itself. But then again that would just cost more and add useless weight.

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This video is 100% correct. Pre-charge the line of the pump before it fully engages the valve of your suspension, and you will prove to yourself the PSI matches where you left it.

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You wouldn't believe how many debates I've had over the years with people over the fact that you don't lose air (unless of course there's something wrong with your valve or pump).

My favorite Pro Tip for those that want accuracy when checking their pressure: attach your pump, get the original reading (let's call it 180psi), then add in a few psi (your choice, but remember what pressure you're pumping to). Then unthread the pump until it "hisses" and reads zero, then thread it right back on. What's your pressure now? Do that 1-3 times until, when threading the pump back on, you get that 180psi reading again. Now you know exactly what your pressure was before you lost some air to the pump when you attached it. Now you can make small adjustments to your fork and shock w/o guesswork.

But know that the pressure loss varies depending on the size of the air chamber AND it's pressure. So at 190psi you might go down to 180 when you attach the pump (10psi loss), but at 300psi you might go down to 280 (20 psi loss). So don't just assume, "oh yeah, this pump makes your fork/shock lose 10psi every single time". And just as importantly, you lose fewer PSI at the fork than the shock because the air chamber in your fork is WAY bigger than the one in your shock. So mess around with both and see what you get.

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As someone who used to work for a suspension company and someone that has explained this 1000s of times, thank you...

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@Brian_Peterson When you left said company, did your employer make you leave all the smutty pics we emailed you and not let you take any posters or pics of the ladies as souvenirs?
I mean...you left on good terms, but we sent you some high quality smut back then & I know you had an extensive collection. Would like to think they didn't just freeze such assets as you left.

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@Bizutch I believe I still have some posters... The email files I left behind to stay out of trouble at home.. But, alot of that stuff got shared, so I'm sure someone might still have it. But, I'm not sure if anyone would fess up since nobody wants to get in trouble. I did just find a ton of jerseys from those days that I'm thinking of throwing online to see if they might sell...

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Is there really no loss though? This video reminded me of Nigel Reeve's pro tool box check where he talks about buying a fancy gauge that has zero loss which he says most don't have?

https://www.vitalmtb.com/photos/features/Pro-Toolbox-Check-Nigel-Reeve-Stevie-Smith-Mark-Wallaces-Mechanic,10361/Slideshow,0/sspomer,2?mobile=false

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He says that most gauges aren't zero loss, not that most pumps aren't zero loss. But either way, the whole idea of air loss just doesn't add up.
Think of it this way. When you attach a shock pump, you thread it on a ways and it forms a seal (i.e. pump still reads zero, but the shock/pump interface is now airtight), and then you continue threading it on (those last 1-3 rotations on the threads) until the pump opens the shock's valve core and you get a reading.
The whole idea of "air loss" hinges on the idea that there's an overlap between those two instances I just mentioned above, basically saying that, as you unthread the pump, the airtight seal between the pump and the shock is broken while the valve is still open. This is why people are so scared of that little "hiss" when you detach the pump. And there are two ways to disprove it:

1) Why doesn't the shock lose air when you put the pump on (as opposed to when you take the pump off)?? If this overlap was actually real, the valve core would open before an airtight seal is formed while you attach the pump, and you would hear that hiss just as you do when the pump is being removed. But it NEVER happens when you attach it, does it?

2) Try unthreading your pump VERY slowly from your fork/shock once you've pumped it up. Right when you hear the hiss, stop. If there's actually air loss, the air would drain completely out of your suspension when the pump is threaded on halfway like that. But this also doesn't happen. The hiss is just the air leaving the pump, and once the pump is empty, the hiss stops. Thread the pump right back on and you'll see that your suspension is still at full pressure (with the disclaimer that, when you reattach it, just like this video explains, your reading will now be lower because the air in the fork/shock must expand slightly into the pump's hose in order to get a reading).

Like some people have said, pre-charging definitely helps with accuracy, but is only 100% accurate if you inflate to exactly the same pressure as what's in your suspension in the first place. If you're low, the shock loses a little bit of air and it'll read low. And if you're high, you add air to it during attachment and it'll read high. In the end just do two things: always use the same pump for consistency, and remember that air loss isn't real.

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I get it, the air you hear coming out is from the pump not the shock. I was just questioning whether it's truly zero loss, and throwing a bit of Stevie Smith content for fun.

IIt would be in the noise but is there a volume difference between when the valve core is open and closed?

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That's a really good question. I've wondered myself. Like @Brian_Peterson said, the valve closes after about 1/4 turn when disconnecting the pump. So any (incredibly small) increase in volume from that 1/4 turn could drop the pressure, but I'd be surprised if it's more than a fraction of a PSI, especially when we're talking about a fork rather than a shock.

Can't go wrong with some Stevie content - thank you for that!

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