pink poodle
気が狂っている男
He lives! Good to see you and the shock live to ride again.
Specialized rear shock "Brain" rebuild with pictures & now forks too
- Thread starter link1896
- Start date
SummitFever
Eats Squid
Just giving the shock a service can make it feel like it has less comp damping because of the reduced stiction. Does the brain have some sort of comp damping adjuster? Going to a heavier weight oil is very much like adding clicks on your comp (and reb) adjuster. Heavier oil has almost no difference on damping in high speed events, it really only has an effect on flow through the free bleed circuits and past the shims on very small shim deflections.
So changing oil weight can be a useful approach where you've run out of clicker adjustment, but a huck to flat can be a fairly high speed damping event that will not be affected much by going heavier on the oil.
Also, even though the IFP pressure does very little to affect the overall spring rate of the shock, reducing the IFP volume or upping the IFP pressure can assist with ramping up the spring rate just before bottom out.
link1896
Mr Greenfield
Are you Mr Ohlins by any chance?
I see where you're going with the rebuild has reduced stiction, that's much more probable then the published oil specs being off.
only damping adjust is rebound in the main chamber but it's only the metering rod.
I don't feel the bottom outs when the occur, the landings feel smooth, so it's not a major imbalance
The brain assembly has a click adjuster that sets the amount of free bleed into the chamber up against the IFP, it's a small hole all the way down to a VERY small hole for the low force impacts, comp shim stacks open for bigger events in parallel but not adjustable.
You'd suggested a comp shim stack mod in the past, maybe I need to look at an adjustable comp shim stack, or just tune. Seems to be comp and reb shim stacks in both main body and reservoir.
tweaking IFP position is an easy first step.
thank you summit, more food for thought.
link1896
Mr Greenfield
and o rings for the older X fusion shock,
| Position | Material | ID | CS |
| brain adjust knob | buna 70A | 3.68 | 1.78 |
| top outer of reservior can to machined head | buna 70A | 22 | 2 |
| IFP | buna 70A | 18 | 3.55 |
| nitrogen stopper | buna 70A | 22 | 2 |
| brain piston OD | buna 70A | 20 | 1 |
| hose end x 2 (polyurethane) | polyurethane | 4.75 | 1.8 |
| bearing (black) to damper body | buna 70A | 21.95 | 1.78 |
| bearing (bsilver) to damper body | buna 70A | 15 | 1.5 |
| bearing to damper shaft seal | ?? | ?? | ?? |
| aircan to damper body quad ring | buna 70A | 20 | 3.55 |
| metering rod | buna 70A | 2.57 | 1.78 |
| aircan to head | buna 70A | 34.5 | 1.8 |
| damping rod to head | buna 70A | 10 | 2 |
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stirk
Burner
link1896
Mr Greenfield
Yes. it's a third variant, I reckon the older shocks were 100% x-fusion, then there was a hybrid period, then 100% fox
link1896
Mr Greenfield
280psi feels like factory fresh WC tuned brain. Still going strong about 14 rides later. Only issue is the o ring to seal the brain fade shaft properly makes adjustment hard, but it's not a regular adjustment.Bad things happen when IFP pressure goes lower then air can pressure. It lets gas into the damping oil, game over
Had to do another bleed, managed to do it with assembly still on bike
paceman101
Cannon Fodder
Was directed to this thread from HuckingKitty whilst trying to find info about Brain rebuilds.
Great reading!
I have an s-works Enduro from 2004, and I think the 'brain' has finally gone (loss of the nitrogen I think), the shock itself seems fine (holds air pressure).
I've managed to get a second hand RP3 so that I can have a go at the brain, and this thread is certainly going to help - although I doubt the seals will be the same size! And I don't have the tools to build up a vacuum pump for bleeding, so I'll need to be creative there, but nothing lost, nothing gained!
Once again - good job sir. Out of interest - what is your day job?!?
Jason
Great reading!
I have an s-works Enduro from 2004, and I think the 'brain' has finally gone (loss of the nitrogen I think), the shock itself seems fine (holds air pressure).
I've managed to get a second hand RP3 so that I can have a go at the brain, and this thread is certainly going to help - although I doubt the seals will be the same size! And I don't have the tools to build up a vacuum pump for bleeding, so I'll need to be creative there, but nothing lost, nothing gained!
Once again - good job sir. Out of interest - what is your day job?!?
Jason
link1896
Mr Greenfield
Thank you marpilli, welcome to our crazy little forum.Just wanted to drop in and say congrats on the rebuild!
link1896
Mr Greenfield
Welcome Jason.
Photos please. Lots of them. I'm unsure how the long arm extension bolts up to the damper tube end. Please take some photos. I fear it's proprietary.
Don't need to vacuum bleed the older shocks without hoses, you can assemble in a bucket of oil.
Parts needed for rebuild should be generic and available.
I'm a middle Managment paper pusher these days, but I developed my mechanical and electronic skills maintaining cinema equipment.
link1896
Mr Greenfield
Redbruce and I started on the rebuild of his 2011 Epic's rear brain assembly today.
We stripped down and measured all o ring grooves and bores.
Found very little IFP gas charge left. Found oil on the gas side of the IFP. Found the IFP had been rocking about and has some wear marks from striking the wall, so the oil was black around the IFP. Found the design is different again to the other two assemblies I've had apart, some o rings are the same. We found a few o rings had severely swollen, which were probably the cause of the oil leak into the air chamber, which then probably let the IFP bottom out.
Looks like a mix of X fusion and Fox parts to me.
Going to again make a custom fitting for the IFP gas charge with a Schrader valve so redBruce can tune the brain to his liking.
We stripped down and measured all o ring grooves and bores.
Found very little IFP gas charge left. Found oil on the gas side of the IFP. Found the IFP had been rocking about and has some wear marks from striking the wall, so the oil was black around the IFP. Found the design is different again to the other two assemblies I've had apart, some o rings are the same. We found a few o rings had severely swollen, which were probably the cause of the oil leak into the air chamber, which then probably let the IFP bottom out.
Looks like a mix of X fusion and Fox parts to me.
Going to again make a custom fitting for the IFP gas charge with a Schrader valve so redBruce can tune the brain to his liking.
link1896
Mr Greenfield
the following o rings should work for the 2011/2012 rear brain assembly. Again as best as I could grooves and bores were measured and stretch and crush were calculated instead of trying to measure old swollen o rings
| position | ID | CS |
| brain adjust knob | 5 | 1.5 |
| aircan to damper tube quad ring | 20 | 3.5 |
| IFP | 17.15 | 3.52 |
| nitrogen stopper | 20 | 2 |
| brain ass seal | 10.7 | 1.5 |
| hose end x 2 (polyurethane) | 4.5 | 1.8 |
| metering rod to inside of damper shaft | 3 | 1.5 |
| air can to head | 34 | 2 |
| damping body to bearing | 19.5 | 2 |
| bearing to damping rod (polyurethane) | 12.37 | 2.62 |
| IFP fill port | 3 | 1.5 |
| bearing to air can | 24.99 | 3.53 |
| travel indicator | 20.29 | 1.6 |
| travel indicator | 20.1 | 1.6 |
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link1896
Mr Greenfield
Before negative air chamber equalisation notches ( or auto sag), the process of removing and reinstalling the air can recharged the negative chamber. Something we take for granted these days
link1896
Mr Greenfield
Redbruce's air can is the black "fox" on the left. My spare on the right from an older 2011 sworks frame. I'm thinking of turning up a ring with room for a schrader valve (like a rp23's large air can), snap ring grooves in my air can ( it can be the sacrificial lamb) , drill a hole in the aircan, to make a negative fill port on Bruce's shock so we can tune negative air pressure, instead of removing the aircan to reset negative chamber air pressure, which on an epic is a major pita.
SummitFever
Eats Squid
I'd just machine a "dimple" on the inside of the air can at the point where you want equalisation to occur. Measure the dimple position off of one of your air cans that actually has the dimple. Use a dremel etc. with a small (2mm diameter) rotary burr. Give it a good polish afterwards to remove any sharp edges. You will not need much of a "dimple" because of the relatively high pressures involved. If you can, measure the wall thickness of the can so you know how much material you have to play with.