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'dual air' set-up advice

26 posts & 12 voices | Started 2 years ago by bowglie | Latest reply from The Great Stonk

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bowglie - Member

I'm a newbie to dual-air forks, so apologies for my ignorance.

Just to give some background, I've bought a new 150mm Revelation dual-air to replace my current DT XMC130 - reason for getting the taller fork is to slacken the head angle of the bike a little.

What I'd like to do with the Rev. is to run a maximum of 30mm sag, but keep the fork supple for small fast bumps. I gather I have to put more pressure in the negative air chamber to achieve this..er...? If this is the case, can anyone advise what sort of pressure difference is needed between the two chambers?

Thanks

Posted 2 years ago #
PeterPoddy - Member

Lots of people will disagree, but if you add more -ve then +ve pressure, all it does is drag the fork down a bit into it's travel and equalize the pressures anyway, so I reckon it's a waste of time.

Why do you want to run a maximum of 30mm sag? I usually reckon on running 25% sag, which is 35mm on your forks, but it's very hard to set be that accurate anyway.

What I would do, is set 30-35mm sag for a start, keep +ve and -ve pressures the same, then see how it goes. If it feels too stiff, drop 5psi out of both chambers, too soft, add 5psi, and so on. I've found different forks work better with different amounts of sag

FWIW I think people get too hung up on the 'numbers game' with suspension. TBH I couldn't tell you waht settings I'm running, but I undersand how the settings work and how I arrived at the set-up I use and it works just fine.

One tip - When messing with pressures, when you adjust the +ve pressure, let the -ve chamber down first as you run the risk of a big imbalance of pressures which will drag the fork down if you don't

So, to make a change

Let out -ve
Adjust +ve
Reset -ve

Posted 2 years ago #
Alex - Member

+1 for PP. That's the way I did my 140 revs and now with my Rebas. I also extended the fork to max travel before adding air but that might have been entirely pointless!

Posted 2 years ago #
bowglie - Member

Thanks for the tips, especially the one about reducing pressure in -ve first. I'd also wondered about the 'dragging down' effect of unequal pressures. The reason i'm trying to keep the fork quite 'tall' is to get the benefit of its head slackening effect. When I ran the fork with about 35-40-ish mm of sag, the bike felt the same as with the shorter fork.

TBH, with all the snow & ice I've haven't had chance to get any serious riding done on the fork yet - but I was just after some sort of starting point for set-up (which you've provided - Ta:) We're having along weekend away, so will hopefully be able to get a few miles on the fork.

p.s.
FWIW I think people get too hung up on the 'numbers game' with suspension. TBH I couldn't tell you waht settings I'm running, but I undersand how the settings work and how I arrived at the set-up I use and it works just fine.
I can appreciate where you're coming from here - & it's the main reason that the other 3 bikes in the household have coil sprung forks.

Posted 2 years ago #
CaptainMainwaring - Member

Agree with most of what PP said. However IME with air Pikes, you can have the -ve pressure up to 10psi more than the +ve without any drag down and it does make them more small bump sensitive which you will need if you are not using much sag. It does not equalise the pressure because the chambers are not connected.

Once you've found what works, note the settings and leave it forever

Posted 2 years ago #
crazy-legs - Member

I don't know what it's like with Revs but my Dual Air SIDs took a fair bit of playing with to set up right and what PP says is correct - if you're adding air +ve first then -ve. Removing air, -ve first then +ve. SIDs really don't like the air pressures being too similar, it drags them down. The figures written on the forks were reasonably accurate though as a starting point.

Posted 2 years ago #
CaptainMainwaring - Member

SID's are completely different fork to Revs/Pikes which are basically the same.

FWIW the figures on the Pikes are much too high

Posted 2 years ago #
robhughes - Member

CaptainMainwaring.on my new 426 dual air pikes this happens.
if i put 75psi in the +ve.
th -ve pulls down the travel at 65psi???
this goes the oposite to what you said.
got me wondering now...

Posted 2 years ago #
neilnevill - Member

peter, yes it drags the fork down and eqalises the presures....but think abouot what that means to the initial volumes of the +ve and -ve chambers and therfore how the spring rate is affected. pump more air in the -ve to get a falling rate, more air in the +ve to get a rising rate.
Neil

Posted 2 years ago #
robhughes - Member

neil-does this go against srams advice that you should put equal pressures in both and still get 140 travel.if i did that then i,d be down to 125mm..confused.com.

Posted 2 years ago #
neilnevill - Member

No it doesn't go against it. equal presures (or more in the +ve) and you get full travel. put more in the -ve and it starts to pack down by reducing the volume of the +ve. The ration of volumes +ve to -ve is thus different and you get a differnt spring rate (falling rate)

Posted 2 years ago #
robhughes - Member

(equal presures (or more in the +ve) and you get full travel).
still a bit confused on that one.
is it equal presures or more in the +ve?

Posted 2 years ago #
neilnevill - Member

its both - equal pressure, and more pressure +ve both give full travel. more in the -ve will begin to pack the fork down and make the spring rate change to more falling rate.

Posted 2 years ago #
robhughes - Member

thats it you see.equal presure and it packs down.

Posted 2 years ago #
neilnevill - Member

? even after letting the -ve air out before setting +ve as PP said? Okay then - fecked +ve/-ve chamber seal - send fork for service. probably (confirm with TFT first) If you've noticed the fork packing down (and staying packed) when riding then it's a fecked seal....air leaks +ve to -ve past the dodgy seal when the fork compresses, which pumps up the -ve pressure and packs the fork.

Posted 2 years ago #
robhughes - Member

neil-if i use them with 10psi less in the -ve they work spot on.
don,t pack down when in use.
it,s setting them up that equal presure pulls down the fork to 125mm.
they don,t leak from -ve or +ve they just stay at those presures.
bit bewildering.

Posted 2 years ago #
Jolly Green Giant - Member

Qucik question.How are you measuring pressure?Before removing pump?

Posted 2 years ago #
neilnevill - Member

Okay I'm baffled, unless it's a measurement/pump accuracy issue as JGG suggests. Try a mates shock pump?

Posted 2 years ago #
robhughes - Member

jolly.i know there is a pressure loss on the -ve side when you remove the pump(small -ve chamber)
so buy repeating the proses a few times you can determine what it is.with my pump it,s 15 psi.
iv,e tried 3 different pumps on it.
but to get full travel i still need to put 10psi less in the -ve.
got me absolutly baffled.
as i said before they do not loose any presure nor do the equalize.
guese i,ll just have to live with it or send them to tft.

Posted 2 years ago #
Suggsey - Member

Tried all sorts on my Revs too, settled on 20 psi under the quoted pressure for rides where I want a bit more sensitivity and minimal drops/jumps. Run them at full 170 psi in both chambers for harder riding (I am 17 stone hence the pressures).
To be honest I do prefer the plush travel of coil forks but loive the weight of the dual airs.
Does anyone know is it a real problem for the -ve chamber top loose a little air when left standing for a couple of days? (No oil loss, just about 10-20 psi over a week).

Posted 2 years ago #
Rorschach - Member

Probably not losing air-the -ve chamber is really small (volume).When you re-connect the pump it takes a small amount of air to pressurize the connecting tube/guage and so reads less than you put in.
Contrary to what some people think you DO NOT lose an appreciable amount of air (unless you've got an equalizer shock running 400psi) when you disconnect shock pumps-its just the air from the valve/guage.
IM(semi professional)O of course.
Let the naysayers begin.

Posted 2 years ago #
Rorschach - Member

TBH rock shox internals are so stunningly simple its in most peoples (I use this term loosely and all usual caveats apply) scope to take em apart and have a look themselves.
You'll be annoyed that they cost £400 after you do tho.

Posted 2 years ago #
Jolly Green Giant - Member

Suggsey.Are you actually losing 10-20psi from the -ve?
You can lose this much just by screwing the pump hose on.
More noticeable on the -ve of course because the air volume is so small....

Posted 2 years ago #
Rorschach - Member

Echo?

Posted 2 years ago #
Purplefunkymonkey - Member

Jolly Green Giant - Member
Qucik question.How are you measuring pressure?Before removing pump?

Most shock pumps have a gauge ................ don't they?

da funk

Posted 2 years ago #
The Great Stonk - Member

Rorschach...

has your fork been serviced during its life at all ?

the reason i ask is because it is possible to tweak the amount of air in the lower legs when the fork is re-assembled. ive noticed this myself when servicing my own rockshox forks.

the proper way to do it is after mating the threads on the shaft bolts, to pull the fork to full extension and then tighten them up, so the lowers contain air.

the other way to do it is to copress the fork and tighten the shaft bolts while compressed, so then when the fork is extended, a vacuum is formed inside the lower legs which sucks the fork down, like having too much air in the -ve spring.

the oil seals/bushings tend to seal tight enough to trap in the air/maintain this vacuum.

try inverting your fork, loosening the shaft bolts and tap them loose to brake the seal (but dont fully unthread them!), check the fork is fully extended, and then bolt them up again and see if your problem persists.

Posted 2 years ago #

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