I have:
* a Fox branded pump that came with forks about 8 years ago
* a RockShox branded pump that is new, and came with Rebas a couple of weeks ago
* another pump that I don't know where came from
The Fox pump registers just under 20psi when disconnected, which makes sense because atmospheric pressure is 16psi at sea level I believe. The other pumps register 0.
On my forks, the Fox reads about 20 higher than the others, on the same valve. So they are consistent.
According to the 'air guide' on my Rebas, the +ve air for my weight is 120psi. By the RockShox pump, 120psi is concrete hard, I run the about 90psi by the RockShox pump, which is about 110psi by the Fox pump, which is about right. (25% sag).
Who's right? What gives?
Is this why everyone says Air Guide pressures are too high? RockShox pumps aren't calibrated to match RockShox Air Guide because they don't take into account default atmospheric pressure?
Yours, confused of STW
The pressures irrelevant really
All thats important is that they feel right and dont exceed the max or min psi the fork needs
Sounds like a broken pump rather than an atmospheric pressure measurement.The Fox pump registers just under 20psi when disconnected, which makes sense because atmospheric pressure is 16psi at sea level I believe. The other pumps register 0.
IMO the only way useful use of a shock pump is so you can record your own personal settings (and possibly so you can match +ve/-ve air pressures). It's far, far better to use sag and 'feel' rather than manufacturer-quoted pressures.
I don't think I've ever used pressures within 20% of what manufacturers state. Of course, my various shock pumps may also be inaccurate too. FWIW I'm about 80kg and I've found that although RS recommend 120 ish PSI in my Revs, I've found 90-95psi (on my Topeak pump) is about right for 20% sag (Sag measurements on forks is a bit of a rubbish measurement though since it depends entirely on how far forwards you lean...)
The pressure gauge will only measure pressure in the pump relative to the outside, so unless it was specifically calibrated to include atmospheric pressure (unlikely), then it's just inaccurate. Besides, atmospheric pressure is 14psi / 1 bar so 'just under 20' is wrong anyway.
The actual pressure doesn't matter, does the shock sag? does it work as it should? As long as you set it to the same indicated pressure on the pump and know what that same pressure reads on the others then that is all that matters.
What they said...RS pressure are way to high IMO/E anyway.
The other thing with quoted pressures is that RS forks are spacable for different travel. You remove a travel spacer, the chamber is bigger, less air is required to obtain similar amounts of sag.
Every gauge has automatic atmospheric compensation as one side is at the high pressure in whatever you are measuring and the other side is open to the air you are breathing.
Sounds like the fox one is knackered. Can you rotate the bezel? Maybe you can reset it.
I use a Topeak one and I just use that gauge so as long as I get it back to where I had it before a service then I know it's at my setting.
Quoting atmospheric pressure at sea level is a red herring. Pressure changes regularly, especially in the UK, and being at sea level does not mean that it will remain constant. Look at a synoptic weather chart to see areas of pressure and their movements across the UK.
The guage on a shock pump is measuring the pressure within your shocks, not what's happening outside them. So, you either need a third pump to regulate the other two or you use just one of them; find the ideal pressure [i]indicated[/i] on the pump and set them up in future according to that reading. What the [i]actual[/i] pressure is is irrelevant.
I run my rs fork at about 110+ve, the label states 140 or something like that.
Just go by sat %age....
DrP
Thx all, been away all day. Yup I do set by feel. @Superficial, yup I'm about your weight and that's what my RS pumps says when it feels right too.
Never had a Rockshox air fork whose settings were anything like correct tbh. But yeah, everyone up there is right, what it says on the pump doesn't matter at all. Choose the pump that seems the most consistent though.
The number is totally irrelevant.
Just put the amount in that feels right and remember what the dial says for whatever pump you want to use.