Hi all,

I’ve got a cheap shock pump here that worked okay on my Rebas but by time I unscrew it from my Fox 32’s, it loses around 20 psi. I can overinflated to compensate, but I’d like something more accurate. I’ve seen one from Leyzne that has some sort of stopper valve on, but it’s quite expensive. Any ideas?

Thanks,

FM

I always thought that the air you hear coming out is that from the pump to the valve, rather than the forks?

It is. You don’t lose the air in the shock/fork/tyre as it has a one way valve.
The ‘travel’ in the valve is such that it is closed before you can detach the head.

If you look at the threads on most shock pumps, they are quite long, and so they take longer to remove, introducing more travel allowing the one way/non return valve to fully close.

You lose the air in the pipe, and then when you reattach the pump, the pipe fills with air from the shock/fork/tyre before you pump and so of course you lose some pressure initially.

Caveat to this can be clip on type Schrader valves or old school flexi pump valves where you fumble and remove them slowly from the tyre (you only have screw on ones in high pressure situations)

I’ve got a (possibly Topeak but can’t check as I’m at work) one that you undo a separate thread which retracts the valve pin, before you unscrew the pump head and theoretically reduces the air leak.

I’ve got a digital Topeak one, and I really like it.  It’s not cheap, but then nor are the forks and I reckon the pump will last years (if I don’t lose it)…

https://www.extrauk.co.uk/topeak/shock-pumps_~2236/product/160280-65421-pocket_shock_digital.html

Usually it’s just the stored air if you’ve got a pump where you dial back the part that opens the shraeder valve, but i do struggle to find a decent shock pump that does this well and confidently, and can pressure a shock up to 250 without taking 10 minutes of pumping the damn thing, i wish there was a foot style pump that did shocks as well up to 300psi

SKS make one with a valve head that screws open and shut specifically to avoid this air loss.

I bought a ‘cheap’ (looks the same as most them do with a analogue dial) shock pump. I had a suspicion it was letting air out so I stopped mid unscrew and sure enough all the air came out.

As above, the escaping air is coming from the pump, not the forks, or at least it should be. When you attach the pump, it draws air from the forks to pressurize the pump. When you detach the pump, that air escapes but the forks should stay at the preset pressure. There’s no need to overpressurize the forks to compensate, just find a pressure that works for the riding you do and set them to that pressure each time.

It’s not impossible that a badly designed pump head will actually allow air to leak out of the forks. The way to check this is to slowly unscrew the pump until you hear air starting to escape, then stop unscrewing it. If the air is leaking out of the forks, it’ll keep leaking until there is no pressure left in the forks. In this case, you need a better pump. If the air quickly stops leaking and the forks remain pressurized, then it’s just the air in the pump and you have nothing to worry about.

valve head that screws open and shut specifically to avoid this air loss.

You get one like this if you buy a Canyon Strive, too.

On man, this again..does no-one search!

I’ve got a Giant branded digital one. It’s great and pumps up well hard easily.

but i do struggle to find a decent shock pump that does this well and confidently, and can pressure a shock up to 250 without taking 10 minutes of pumping the damn thing

SHOCK DIGITAL DRIVE

As a heavier rider, I find this is a godsend to get shocks up to the pressures I require.

Except that won’t fit in a backpack…😉

smatkins1
I bought a ‘cheap’ shock pump. I had a suspicion it was letting air out so I stopped mid unscrew and sure enough all the air came out.

they do all tend to be clones, so much of muchness. I’d wonder if your shock valve insert was screwed all the way in, or did a new pump fix it?

To the OP even if it lets out a little air out (I go with the accepted wisdom on this), as long as the sag is correct, & you keep using the same pump, does it matter?

When I worked in a workshop I had to explain how shock pumps work at least once a week to people coming back with ‘faulty suspension ‘.

Sometimes you see the light bulb coming on, sometimes they go away more confused.

I always thought that the air you hear coming out is that from the pump to the valve, rather than the forks?

According to my Shockwiz no shock pump I have used has ever let any air out of the suspension on removal, at least not enough for the Shockwiz to register.  When I connect the pump, then the suspension loses some pressure in all cases as the air in the pump is at a lower pressure as I connect it.