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I have a new dropper, and when pressurising it it seemed to be loosing a lot of air on removal of the pump. So, given the pump is probably 10 years old and quite hard to remove from the dropper due to the valve position, I bought a new Rock Shox one.
Yesterday I tried putting the dropper up to pressure again with the new pump, and it still seems to be loosing a lot of air on release.
Today, I pressurised my new forks with the new pump and again seem to be loosing a lot of air on removal. I tried my old pump, same result.
Now everything I read says I am wrong and the sound is just air leaving the pump. But the dropper return speed is slow, and the forks seem to have a lot of sag (can't check properly until I fit them to my bike, but I can nearly bottom them out putting all my weight on them).
With the forks, I put them up to 85 psi, and when I reattach the pump they read more like 20 psi -- that is surely way more pressure loss than you should get reattaching a shock pump.
So what is going on? Surely I don't have a faulty new dropper (KS LEV) and faulty new forks (Pike's)?
It does sound like a problem with the pump and / or user error.
I had a RS pump that after a few years, it became difficult to unscrew it quickly enough to avoid escaping air. I replaced it with a Topeak Pocket Shock DXG pump, which has a 'Pressure-rite connector'; basically, you screw the end on onto the thread of the valve, then there is a secondary dial which you screw up, and it is that that depresses the pin in the valve ... Pump up to required pressure, unwind the secondary dial (so the pin in the valve is then closed), and then you can undo the pump head from the valve with no air loss.
So, I'd recommend one of those (or a digital version)
My old KS LEV has always been a PITA to get the pressure right on, I over-inflate it quite a bit to account for the escaping air. Never had that issue on shocks though
If your pump comes with a check valve and you're not familiar with it, it will lose more air than a standard pump. Is this a possibility?
Can you actuate the dropper or try and compress the forks with the shock pump attached and set at your desired pressure to compare how they perform? The air volume will be larger so the fork wont ramp up as quickly but it should give you a reasonable estimate of how the fork feels during the sag/first 0-30%?
In theory when detaching your shock pump the pin that actuates the valve should release, sealing the pressure in the fork/dropper, before the rubber seal in the pump fully decompresses. The air loss you hear should then be from the shock pump. The normal way to check this, as you have done, is to pump up to pressure and remove the shock pump before reattaching it and witnessing the new pressure shown before repressurising it to your desired pressure. If you do this a couple of times unscrewing slowly once or twice and then fast you can check that the pump is not causing leakage when removing as the pressure when reinstalling will be the same each time. For a fork I would expect a change of no more than 5-10psi, on a dropper or small shock it can be greater due to the smaller air chamber.
Some shock pumps you can adjust the pin that pushes on the valve by turning with a small screwdriver, you could try recessing the pin further if your shock pump allows. This would mean the rubber seal should make contact sooner than the valve actuating pin.
If you haven't fitted your forks yet have you been able to balance the positive and negative air chambers? Could this be a factor in the air pressure dropping if the air has moved from the positive to the negative reducing the overall pressure?
Some fork/dropper valves can have different valve pin depths which can mean the pump works fine on one model of fork and not on another. Are the components new and are you having the same issue on previous sets?
I would still say it's unlikely to be air from the seat post, it's just air from the pump and hose.
You can prove it to yourself by screwing it on slowly, the air (shouldn't) hiss out of the head of the pump as you screw it on, it just pressurizes the pump and the gauge moves up.
The reason it looks like you lose several psi when connecting / disconnecting is because seat posts and shocks have very low volumes, so having to expand into the pump again each time loses a significant amount of pressure. You don't notice it on tyres because they're relatively large compared to the pump hose/guage.
The forks will have a negative chamber which needs pressurizing, usually this required the fork to be cycled through it's travel a few times, check the pressure, and repeat 2-4 times until it's stable (ish, as I said it'll lose a few psi just pressurizing the pump).
Like most components the seatpost might just need to bed in a bit and loosen up. Give the shaft a wipe with some silicone oil (sold in bike shops as stanchion lube for a huge markup) as well.
Some shock pumps you can adjust the pin that pushes on the valve by turning with a small screwdriver, you could try recessing the pin further if your shock pump allows. This would mean the rubber seal should make contact sooner than the valve actuating pin.
The valves are standard Schrader cores usually so they should all be the same size, but if air is escaping then this should solve it.
On releasing the pump head, it should close the shock valve before the head gets removed from the valve - so the air releasing is just the amount left between valve and pump head. But when you put the pump back onto the valve - the air comes out of the valve to fill the volume in the pump head, so the pressure will drop then (but you obviously add air with the pump at this stage).
Can you actuate the dropper or try and compress the forks with the shock pump attached and set at your desired pressure to compare how they perform? The air volume will be larger so the fork wont ramp up as quickly but it should give you a reasonable estimate of how the fork feels during the sag/first 0-30%?
That's a good idea. I can def check the fork sag with the pump attached, then again then after removal. Not sure I can do that with the dropper without risking damaging the valve/pump, given the potentially unpredictable return speed.
I don't understand fork internals well enough to have thought about +/- air chambers, but if there is a chance moving it through the travel a few times then repressuring will sort it, I will try that, too.
There are also bleed valves on the fork legs (which, being a dogmatic rigid rider who is now seeing the light, I first thought were wierd mudguard mounts). Anyway, I wonder if doing anything with them would help? These are brand new forks like I say, but potentially a 2023 model.
The reason it looks like you lose several psi when connecting / disconnecting is because seat posts and shocks have very low volumes, so having to expand into the pump again each time loses a significant amount of pressure. You don't notice it on tyres because they're relatively large compared to the pump hose/guage.
This may be it, but I assume that the amount of pressure lost attaching a pump is basically the same as the pressure increase you get from one stroke? I mean, if it takes 8 pumps to get my forks to 80 psi, one pump is 10 psi, so I would expect to loose 10 psi reattaching the pump. But I think I'm loosing more like 60 psi.
Another check would be if you had access to a shockwiz. That will display the pressure on your phone so you can check without the pump.
when you take the shock pump off you will get a whoosh of air - this is the pressurised air in teh pump coming out
when you screw it on (to get a pressure reading) you will get a whoosh of air from the pressurised seatpost/shock back into the pump as the air equalises the 2 air chambers (shock and fork) to give you a pressure reading.
i 'lose' just around 5-10psi just fitting a shock pump to my forks.
does the fork/seatpost go noticably softer on removal?
so I would expect to loose 10 psi reattaching the pump. But I think I'm loosing more like 60 psi.
Yea, I'd expect it to be more like 10-20psi in the fork. Try equalizing the positive and negative pressures by bouncing the fork, check and top up the pressure, bounce it to see if that resolves it. If not then see if the pin in the shock pump head can be screwed in a bit.
This may be it, but I assume that the amount of pressure lost attaching a pump is basically the same as the pressure increase you get from one stroke? I mean, if it takes 8 pumps to get my forks to 80 psi, one pump is 10 psi,
Gas pressures don't work like that, and there's a check valve between the cylinder of the pump and the bit with the gauge. It only has to pressurize the volume of the hose and the gauge which I'd guestimate is about 5% of the seatpost cartridge.
There are also bleed valves on the fork legs (which, being a dogmatic rigid rider who is now seeing the light, I first thought were wierd mudguard mounts). Anyway, I wonder if doing anything with them would help? These are brand new forks like I say, but potentially a 2023 model.
Those are to let excess pressure out of the lowers. Either because the oil in there gets hot and evaporates, or because you go up a mountain on a chairlift, or because the air spring seal leaks slightly.
I've never been convinced they were necessary but I guess Fox / RS had to justify why their forks had an RRP of £1000+
The topeak digital shock pump does a good job of managing this. The way the valve works means you end up closing the valve on the shock while the pump is still pressurised.
Most pumps do this anyway, but have a much lower tolerance for not being positive, so the topeak just makes the whole thing easier.
You could prove whether the pump is only losing it's own pressure or that of the shock/post.
Detached it slowly and when it starts leaking, leave it until it stops. Then compress the shock or post and see if you can hear additional hissing. If you can (or you bottom out the shock!) then the pump is allowing pressure to leak.
This type of query is something I get asked regularly by customers. More often than not (and I'm confident it's what you're experiencing) is that you're not screwing the pump far enough onto the valve. This will mean that the pressure gauge is only reading the pressure thats built up in the rubber hose tube, and there's no air being pumped into the fork/dropper.
This issue is sometimes made worse/more common by people thinking there's a leak, and over tightening the valve core which makes its sit too low in the valve stem, and makes the scenario even worse as you'll now have to screw the pump on even tighter to achieve the needle making contact with the valve pin.
A fork thats empty of air will take about 100 strokes of the pump to get it up to 80psi or so. If your pump is showing you an 80psi pressure reading after just 10-20 stokes thats a sure sign that air isn't making its way past the valve and its just building up pressure in the tube of the pump.
As others have said too, the smaller the air chamber size, the greater the pressure loss will be when re-attaching the pump. On a Pike fork at 80psi it would be normal for the pressure gauge to show a drop of about 5psi when re-attaching. On a much small air chamber size such as a dropper post, the drop in pressure when re-attaching the pump will be much more.
There's little need to buy over-elaborate pumps with fancy heads. The majority of professional suspension workshops use bog standard pumps from Rockshox and they are absolutely fine for almost all suspension inflation. I've used my current Rockshox pump 10 times a day, 5 days a week, for the past year without any issues.