RS Rev Dual Air – strange going ons. Advice needed

Hello,

Long story short – I had a seal pop out whilst riding, lost some positive air pressure, fork would not extend fully when pumped back up to normal pressure no matter what i tried. I could pull it up by hand but it would sink back in 20/30mm.

I decided it was time I serviced the fork spring and lowers anyway.

Performed a service on the air spring, clean – new o-rings – greased up. I am satisfied with the service quality.

However now (and this is the strange bit):

Before the service my pressures were 35+ and 45-. (I’m very light.)
Now, I have to have 80 psi to get the same sag, maybe more as I set without riding gear.

If I have the negative 10 psi more than positive, it sucks in like 20/30mm. (I can pull it out by hand but it sucks back in)

If I have the negative the same as positive, it sucks it about 1cm. Or rather, doesn’t extend the last 10mm.

Any ideas why this is? Should I just carry on reducing the negative until it doesn’t pull in?

Other than that the fork does feel fantastic thanks to the new oil etc.

If you had 35 psi you must be non existent!! I’m 12 stone and use around 115 psi. The negative thing is right, it pulls it into the stroke.

80 psi sounds about right, but you do not mention your exact weight?

re suckdown, try empty the neg and pos air chamers, cycle the fork a couple of times and put the air back in to the poitive side first. then the neg.

35/45 psi not near enough.
Look on fork leg for psi for your weight
Subtract 20 psi for +
Run – about 10 psi more
Good starting point.

35 is seriously low, not sure why that was working for you but 80 is still a lightweight setting.

I find mine suck the fork down a little if set equal- I’ve always used the approach of setting the positive more or less how I want it, then adding negative pressure until the moment it starts to suck. Usually about 10% less.

Oh yeah, make sure you empty the negative chamber first, set sag then add negative to the same pressure as the positive +/- 15 psi

Keep the pressures the same as well. All that happens if you run more negative air is that the pressures equalise, it sucks the fork down and you loose travel. It pointless. So pointless that Rockshox have done away with Dual Air for 2013 and gone to Solo Air which simply sets both chambers at the same time to the same pressure from one valve. 🙂

solo air is the way forward the new 2013 ones are brilliant and my new rev rl’s feel just as good as my Float FITs at more than double the price….

RS defo done the right thing doing away with dual air, such a pain to get setup

I like the neg air. It reduces the fork to where I need it at full extension.

each to there own i understand totally, im not great at setting up suspension though TBH

i ended up getting rid of the dual airs i had, as i simply coudlnt ever get them setup right, either felt to soft, crap over smaller stuff, or sucked down and didnt use all the travel…..

swapped them for fox’s and never looked back….until now that is, when i heard theyd done the solo air thing i thought they might just be the ticket for me now….glad i took the risk they are bloody good forks!

I weigh about 10stone with all my gear, maybe a bit more.

I will try letting it all out, cycling the forks, and sticking the 80 psi back in, then just adding negative until it starts to pull down.

It is odd but previously at 35psi it was working fantastically for me. I’ll try again tomorrow fiddling with the pressures 🙂

To the people who said run neg 10 psi more – look at my first post, if I do that the fork pulls in 20mm.

To the people who said run the negative the same, as I said in my first post – if I do that it pulls in 10mm.

I think I will just empty negative until it doesn’t pull

Cheers!

I like the neg air. It reduces the fork to where I need it at full extension.

No no no. Do that with spacers.

I will try letting it all out, cycling the forks, and sticking the 80 psi back in, then just adding negative until it starts to pull down.

No no no. That’s why you have a gauge on your shock pump. Let out all the negative air, set the positive, then set the negative to the same. Do it this way always and you’ll be fine. Think you are clever and can ignore it, and your forks will feel like shit. 🙂

PeterPoddy – Member

So pointless that Rockshox have done away with Dual Air for 2013 and gone to Solo Air which simply sets both chambers at the same time to the same pressure from one valve.

They didn’t do that because it’s pointless- they did it because people find anything with more than 1 control to be too complicated. I’ve never run more negative than positive, but running more positive than negative has always worked best for me

They didn’t do that because it’s pointless- they did it because people find anything with more than 1 control to be too complicated. I’ve never run more negative than positive, but running more positive than negative has always worked best for me

🙂
Same here. But most people think the opposite, as is obvious in this thread, or have no clue, like at least 50% of MTBers I talk to. People love bells and whistles to justify the price of the fork but the truth is that three quarters of the ‘features’ are pointless to most people and they don’t understand them or use them [properly] and IMO a good quality fork, properly designed, needs no more than a way to change spring rate, adjustable rebound and maybe lock out.

Mmm. Nah, don’t agree with that, there’s no right damping setup, you can’t have one fixed option that’ll suit everyone. And as soon as you start into one-size-fits-all you end up catering for the single biggest group. Which, going by Fox’s latest “improvements” means soft spongy divey forks that feel nice for mincing.

Though… I think Rockshox did pretty well with the new single air.

To the people who said run neg 10 psi more – look at my first post, if I do that the fork pulls in 20mm.

that is because you are running FAR too low pressure. Your 35psi is against the atmosphere. Your negative pressure is against the pressure in the fork.

You only need to read the manual/fork leg to see that you are way off on pressure. I wouldn’t go on what they advise though (well not on my 2008 forks, they might have better suggestions now) but you normally need to be about 100 psi in the positive.

Empty the negative. Then add the positive. Never have the fork with lots more pressure in the negative than the positive – ie don’t pump up the negative first or let all the air out of the positive first.

there’s no right damping setup

Correct. But all you need is rebound to control it, as I said. Get the spring rate correct and adjustable compression isn’t really necessary, if well done. Which is my point. 🙂

Empty the negative. Then add the positive. Never have the fork with lots more pressure in the negative than the positive – ie don’t pump up the negative first or let all the air out of the positive first.

Correct. 🙂

Well, if 35 psi has worked fine before then that still makes you wonder what’s up (or what was up before), right?

I’m not sure how travel affects air pressure (I’m thinking the volume increases but the pressure remains the same?), but FWIW I’m 10 stone as well and has gotten 25% sag at 50 psi on both the 120mm Rebas I’ve owned. 60-70% of the recommended pressure seems about right, and then adjust with negative air.

If adding negative air still sucks the fork into it’s travel (is this even when you’re off the bike?) when + air is at atlest say 50 psi, something is wrong. Having more – than + will increase the sag slightly though.

PeterPoddy – Member

if well done. Which is my point.

Biggest if evarz 😉

But nah, still disagree, lots of room for personal preference in damping setup. As long as there’s a clear “standard” option for simplicity, like Rockshox do with the Lyrik, anyway.

Ah I didn’t think the dual air could be spacered down. I’ll look into that.

better documentation on setup from manufactureres would be nice. but hey there is always STW/tinternet!
good case for adjustabily in rear shock units as linkage design as well as rider preference need to be catered for.

No no no. That’s why you have a gauge on your shock pump. Let out all the negative air, set the positive, then set the negative to the same. Do it this way always and you’ll be fine. Think you are clever and can ignore it, and your forks will feel like shit.

Whilst i appreciate all the help – this is exactly what I am doing as I said above and the fork is pulling in 10mm.

also perhaps I should of stated in my first post that I am familiar with setting up the air chambers, IE – positive first, never negative more or full without positive etc etc

This thread isn’t (or wasn’t) supposed to be a “how do I set up” – it’s a “I’ve set it up with the proper methods and not getting the expected results”.

But I appreciate everyone’s help 🙂

[quoteWhilst i appreciate all the help – this is exactly what I am doing as I said above and the fork is pulling in 10mm.[/quote]

With respect, no, you’re not. 🙂
If its pulling in 10mm, you’re putting too much negative pressure in. And that’s a fact my friend. Use the gauge. Do it properly:

1) let out negative air
2) set positive air
3) set negative air to same values as positive air
4) check seated sag
5) if sag is incorrect return to step one. NOT step 2!

This is the way it’s done. It’s not MY way, it’s the CORRECT way. 🙂

This thread isn’t (or wasn’t) supposed to be a “how do I set up” – it’s a “I’ve set it up with the proper methods and not getting the expected results”.

Sorry, crossed post, missed that. 🙂
To me it sounds like the results you are NOW getting are correct. 35/45psi is waaaaaaaaaay too low for anyone.

Ah I didn’t think the dual air could be spacered down. I’ll look into that.

Oh yeah. I do it all the time, one way or another. My own personal ones are set at 120mm (don’t ask why I didn’t get Rebas, I have my reasons!) just that way.
It’s all in the RS technical manuals 🙂

So,

Just to humor myself I tried again making sure I follow the correct method as stated above:

1) let out negative air
2) set positive air
3) set negative air to same values as positive air
4) check seated sag
5) if sag is incorrect return to step one. NOT step 2!

The fork sucks in with both chambers at 80psi. If I let out negative, the fork jolts back to full travel. I measured, and it’s sucking in 20mm.

The fork starts to suck in at around 50psi in the negative, with 80 in the positive. I set the chambers again and put 50psi in the negative and it sucks in about 5mm, which I could live with, but obviously something isn’t right.

My questions are –
1: Why before my service was 35psi +ive, 45psi -ive giving me correct sag if those pressures are so low? Do you think friction from lack of oil & lube? The fork performed brilliantly at those pressure.

2: Why now when I set the chambers using the correct method does the fork suck in 20mm when both chambers are the same psi?

We will get there eventually…

They are broken.

Give them to me, I’ll fix them.

My dual air revs suck down about 10mm at equal pressure too. Theyre too soft/divey for me at that setting anyway though.

As someone else has already said ‘equal pressure’ isn’t a single fixed point, the positive and negative air equilibrate. As you put in more -ve pressure, the forks ‘suck’ down. As this happens, the volume of the positive side decreases so the pressure in the positive chamber increases to equal the negative spring pressure. So although the pressures are equal, you’ve still got unwanted suckdown.

In order to get optimum feel without this sucking down effect (and losing travel), inflate the positive air to the desired pressure. As you inflate the negative pressure, it will increase until it meets the positive pressure value. At that point, you can put in a lot of air without the pressure ramping up (because of change in size of the negative pressure chamber). You need to pump the negative side only to the first point at which the negative pressure equals the positive, before any suckdown occurs. This is quite hard to do as the negative chamber is relatively small (ie one pump from a shock pump is too big for the finer adjustments) – I often inflate & deflate the negative side a few times until I hit the right point. It’s harder than you think but it does work.

On the sram rockshox site they have a file for tuning Revs and they pretty much say what has been said already about letting air out etc. They do recommend different +/- values for specific effects but only up to 15%. However for sag they recommend 10-40% of travel. That seems like a very vague number to me and they don’t give any clues as to when you might want to use 10% and when 40%. Big difference between 10% and 40%!

plyphon – Member

2: Why now when I set the chambers using the correct method does the fork suck in 20mm when both chambers are the same psi?

Mine do that too, have done from new (my old 426s didn’t though).

if you have 2 pumps, connect them both and check out what happens to +ve chamber pressure as you increase the -ve chamber peressure, just an idea.

So, Superficial, if I’m understanding you correctly you are saying essentially:

1. Pump up positive to desired pressure to give me correct sag
2. Pump negative up until it starts to “suck into” the fork

Yes/No?

Pos first then neg balance pressures. This Is after letting all out of the neg them pos

I’ve done some more reading and I now understand what the negative piston does more, and why it exists etc.

From what I’ve read I’ve gathered that as long as the negative is pulling down the fork around 3/5mm that should be optimum setting.

So, even tho I have no idea why my fork is being sucked down so much with equal pressures – I can’t see any harm pumping until it moves a tiny bit? (around 50 PSI against 80 in the positive if I remember)

It’s also just occurred to me that my pump might be a bit crap and might be un-reading the true value in the negative.

Unfortunately I don’t have access to anything else that does high pressure low volume to check.

So, Superficial, if I’m understanding you correctly you are saying essentially:

1. Pump up positive to desired pressure to give me correct sag
2. Pump negative up until it starts to “suck into” the fork

Yes/No?

Sort of.

Ideally you want to pump -ve up until just before the fork starts to suck down.

It’s quite hard to explain simply so I’ve explained it in a complex way with graphs and stuff 😀

http://singletrackworld.com/forum/topic/dual-air-revs-my-thoughts?replies=1#post-4794944

Oh wow – haha I love a good graph.

I’ll take a read through that post – thanks for your time!