[Internal14]

I did a search and must not be using correct something er other in my search language, as I know I've seen this discussed before....

But I'm going to tear into my DHX Air tomorrow and see if I can make it work better again. It's a little over a year old and has gotten air into the oil, you can hear it hiss as it comes back up on rebound, and the rebound damping is spotty, as in almost nill. So I'm wanting to dump out the old oil and refill/rebleed with some fresh oil.

So does anyone have a link to somewhere showing the procedure? I figure I can't make it much worse, so I might as well try, and if all else fails send it off to Fox and have then charge an arm and a leg.
~~wish PUSH serviced these~~

 

[Udi]

I was thinking about this today - and nope, it hasn't been covered here before.

My first bit of advice would be, don't do it if you haven't at least rebuilt a coil rear shock in the past, preferably a DHX.

If you've done that (and by done I mean done well, built a dead silent shock), you'll hopefully have the basics down pat - like knowing where air can hide, the importance of a bath, knowing what tools you need like strap wrenches etc etc.

The DHX air is a little different, because the main oil chamber and damping piston are located inside the large shaft you see from outside the shock, and the actual shock shaft resides in the "body" of the shock - ie. orientation is reverse to normal. What makes this complex is that to reach the propedal circuit / piggyback, the oil has to travel through the internal shock shaft (around the rebound needle).

Now I haven't rebuilt one either, but I presume this complexity of oil path means that it's pretty damn hard to bleed the shock air-free, and probably involves a little more than a coil shock which you can just partially assemble under oil and get a pretty good result.

I've also noticed there's one countersunk screw in the piggyback arm that might be there to help with bleeding.. not sure how though. The grubscrew below it isn't related (it's just for the rebound adjuster detent ball).

Anyway if anyone's done it before I'd love to hear about it too.

 

[Udi]

After staring at the cutaway for a while, it almost looks like they could just use the bleed port at the end of the shaft (towards top left hand side of pic - it'll have a torx fitting once you remove the white delrin plug) to force oil right through the damping circuit if they removed the piggyback endcap and IFP (or just the IFP bleed screw).

Removing the bleed screw in the resovoir arm would allow easier removal of air from the main damper area (air would only have to flow upwards, if you held the shock upright with the air shaft pointing down), and then after closing it off again you could fill the propedal / piggyback area with oil as well.

You'd need some good fittings and a tough syringe though!

Anyway someone like transcend probably knows how they do it at factory, I'm interested in hearing.

 

[dhkid]

i had a hard look at one of them too, i reckon the best way to bleed one is mount it the the piggy back open and upright. add oil from there as you cycle oil into the main chamber by making a vacuum, compress the shox, close the bleed hole on top, suck oil in. open bleed screw at the top and let oil cycle out when you compress it again.

as long as you set the propedal at max, it should provide enough resistance to stop all the oil flowing back into the piggy back. other wise you could always hold the boost valve closed by sticking a screwdriver against it.

 

[dhkid]

i am all for ghetto way of working on bike stuff.

i reckon it would work fine, once you have the main piston chamber full of oil, you can cycle it a few times to make sure there is no air anywhere between the main piston and the piggy back. that was the oil is moved through all the circuits on the shox anyways. let it settle for a while and then just force the last bit of air out through the top bleed screw again.

otherwise if you just have a full oil bath, you can do it the same way, just get the oil to cycle through from the piggy back side to the main piston by sucking oil through the shox by extending the shox with the bleed screw closed and letting it out by compressing the shox with the bleed screw open.

that way the lowest point is the little channel between the main piston and the piggy back and the highest points would be the open piggy back and the bleed screw. i dont see why it wouldn't give a good bleed at all.

 

[Udi]

Haha, I think that's a REALLY dodgy way of doing it. You would wind up with a lot of air bubbles in there, and using the propedal assy as a semi one way valve is pretty questionable too.

I think it's best to think about the easiest/cleanest way of doing it, because that'd be how they do it at factory... I like my previous idea (fluid forced through shock via main bleed port), the fluid path just means all the air gets pushed in one direction. The smaller bleed screw in the piggy arm coincides with the exact point the air would have to travel in a direction other than upwards, so it makes even more sense.

Failing that, the other method would be unscrewing the seal head (like a conventional shock) and building + cycling + rebuilding it under a bath. And I guess just leaving both bleed ports open during that, for easier oil access / air escape. That's a pretty boring explanation, thinking about it again though that might be the right one.

Surely the real procedure has been leaked by now though, the shock's in its 3rd generation. By the way - has anyone got the other cutaway pic? I swear it was on NSMB but couldn't find it this time.

Dhkid maybe that's the pic you looked at? Oh yeah sorry - I accidently deleted my post too, so now this looks funny.

 

[Udi]

otherwise if you just have a full oil bath, you can do it the same way, just get the oil to cycle through from the piggy back side to the main piston by sucking oil through the shox by extending the shox with the bleed screw closed and letting it out by compressing the shox with the bleed screw open.

Yeah that would work fine I think. Just that the vacuum method without a bath would probably produce really messy results (in terms of both the bleed, and your table/floor whatever haha).

But if I was going to the trouble of having a bath, I'd just unscrew the seal head like I said in my post just above... would make things easier by letting you get air out of one half at a time.

Slightly off topic, you might find this interesting - the DHX coil's body ID is the same as the piggy ID, so you can use a spare DHX piston/shaft if you have one to help pump oil through piggy back assemblies very neatly.

 

[MobileChernobyl]

Not really beneficial to your rebuilding question, but why do you think fox went with the "reverse" design on the DHX Air. It seams like it has way more restrictions than benefits.

The air canaster seems like it could be bigger with a normal design, the oil flowing through the shaft seams like a good way to heat it up faster - shaft suface area is directly exposed to the heated constantly compressing pressurized gas (considering its a thin layer going around the rebound needle and would heat up even faster) or is the oil heating up issue negligible?

Theres most likely a good reason why they went with the design, but what is wrong with the current design of air shocks (coil dampeners with smooth body and air canister on it) or does fox like to be innovative/different.

 

[Torrendo]

Another one.

 

[dhkid]

how would the air spring work if it was the other way round? this is pretty much the only way it would work. unless the air spring is inside the piggy back like the milliard racing shox.

 

[MobileChernobyl]

how would the air spring work if it was the other way round? this is pretty much the only way it would work. unless the air spring is inside the piggy back like the milliard racing shox.

The airspring/gas spring would work just like all the other airshocks out there, the only thing that would be different is the location of the piggyback - it would stem off the main oil chamber like a coil shock (not blead through the shaft and stem off the shaft mount side of things), and like every other air shock out there. I'm just wondering why fox chose their design over others, its fox - so they prolly have a solid reason as to why. To me it just appears overly complicated for its purpose.

 

[Udi]

Good question, but I think you've got the wrong idea on the heat thing. I was thinking about the same thing, and if anything you've got LESS metal exposed to the heated air in this config because there's only the thin shock shaft exposed (for the majority of the time), as opposed to a part with a much thicker cross section (if you had the oil chamber inside the air chamber, as opposed to in the air shaft). I hope that makes sense.

Also, oil moving through the rebound shaft (designed as a flow path, ie. as unrestrictive as possible) is barely going to heat the oil, it'll experience a lot more heat just running through the rebound/compression damping circuits (in any shock).

Either way, the heat thing is a bit of a wank IMO - it's a relatively large volume shock, the damper section doesn't get much warmer than on the DHX coil from what I've felt. Most people just don't touch coil shocks because the body is tucked behind the spring, but try it one day. No huge differences in results IMO. Good shock oil has a very high viscosity index too, so damping changes based on temperature are between miminal and intangible... the only "real" factor I can think of is springrate increasing with heat, but again in reality it'd be minimal. I think with some tuning the shocks work incredibly well for DH, I've been playing around with oil volumes in the air chamber on mine with some good results. Would love to try bleeding it with a lighter weight and higher VI oil someday if there was a hassle-free way to do it.

Back on topic though, I think the design is mostly to help with frame compatibility, this ensures that the DHX Air will fit most frames that fit the DHX Coil. For one example of the opposite design (piggy at air shock shaft end), a manitou evolver/swinger air doesn't fit the Sunday to my knowledge.

The Roco Airs (WC/TST) are also both in the same configuration as the DHX Air, so those are probably compatible with a lot of bikes too. The 08 DHX Air is a bit of a bummer with the new propedal lever though IMO, for one it's kinda unnecessary but more importantly it doesn't fit in the Sunday now without mods. Probably the same for a few bikes.

And Torrendo - cheers for the pic!

 

[MobileChernobyl]

and if anything you've got LESS metal exposed to the heated air in this config because there's only the thin shock shaft exposed (for the majority of the time), as opposed to a part with a much thicker cross section (if you had the oil chamber inside the air chamber, as opposed to in the air shaft). I hope that makes sense.

- It'd still be about the same surface area exposed, maybe a little less with the other design, but like you said - heating of the oil is negligible compared to the air being heated & changing "spring rate". So the differences in designs in relation to heat exchange is almost a moot point.

Back on topic though, I think the design is mostly to help with frame compatibilaty.

- Yea I was figuring that's the main benefit fox has going for them with their design.

Anyone ever fool around with a spool valve design in a shock or is there a reason why they wouldnt work so well thats been proven? Off the top of my head i can think of a few designs but I'm sure there has been some already made/tested in the various forms of racing out there.

 

[dhkid]

the only thing that would be different is the location of the piggyback

ahh, misunderstood you over there. and whats a spool valve?

 

[DawnVoyager]

i had a hard look at one of them too, i reckon the best way to bleed one is mount it the the piggy back open and upright. add oil from there as you cycle oil into the main chamber by making a vacuum, compress the shox, close the bleed hole on top, suck oil in. open bleed screw at the top and let oil cycle out when you compress it again.

as long as you set the propedal at max, it should provide enough resistance to stop all the oil flowing back into the piggy back. other wise you could always hold the boost valve closed by sticking a screwdriver against it.

Reviving this ancient thread, because I have 3 of these shocks and the above suggestion turned out to give an excellent bleed-- many thanks!

First though, about getting that pesky delrin plug on the top bleed hole out-- I tried filing a slot, prying with a knife, filing flats and gripping with pliers, none of these worked. What did the trick was making a dimple with a pointy punch, drilling a pilot hole very slowly with a tiny bit, light pressure, and a hand-crank drill, then finishing with a bit almost as big as the plug. Obviously stopping before hitting metal is good.

I filled the boost reservoir repeatedly (but never let it run dry), sucking the oil through by extending the shock slowly and smoothly with a thumb on the top bleed hole, then compressing the shock with the thumb off the bleed hole. Setting Propedal to max helped slow the gravity flow back to the reservoir, but I can confirm the reservoir will overflow if you don't close the bleed hole when the phone rings. Once I couldn't suck up any more oil, I added a final step:

Install the IFP (since you have to anyway) with its bleeder open, and push it down very gently 'til oil flows out to fill the bleed hole and cover the IFP, and it drops to about 10mm below the rim, then close the IFP bleeder. Now take a wooden chopstick or similar and apply pressure to the centre of the IFP, and move fluid (and bubbles) back and forth in the system by alternately pushing the IFP down while extending the shock, and vice versa, with both bleeders closed. Finish by maintaining pressure on the IFP, cracking the top bleeder open, and slowly pushing the IFP down to the recommended height. (33mm on mine, but check the charts). Oil and any last bubbles will come out of the top bleeder, then close the bleeder, preferably tilted a bit so the hole is facing up slightly and has no air gap. This produced a completely silent shock for me.