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stupid vibration under brakes
Ok - so i have a problem ... (all funny conotations aside - most of which are probably true 
)
I have a 2011 stumpy FSR 29er - running jetblack front and rear rims, 203mm discs front and rear, Avid elixir hydraulic brakes
the issue i have is this - whenever i apply enough pressure on the rear brake lever so that the pads grab the disc - the whole frame vibrates and makes a shrieking sound, the front brakes are perfectly fine.
this is what i have done so far to try and remedy the issue
- tried the stock 185mm rear (no difference)
- new pads (no difference)
- cleaned the pads (no difference)
- reseated the pads (no difference)
- re-adjusted the left/right alignment of the caliper (no joy at any setting)
- tried reseating the wheel in the dropouts (no difference)
- lightly sanding the disc (no difference)
- put my other set of wheels on (mavic with 203mm discs and 2.2 kenda's) still the same issue...
the discs are perfectly straight - no warps
the pads are not wearing unevenly
if i swap the front and rear pads around - the problem is still there.
I am running out of ideas - the only options i have left are
completely dissasemble and clean/reasemble the rear section of the frame - and i mean everything!
completely flush out the brake system + remove pistons to check for any dirt
--- and thats pretty much all i can think of thats left to do...
HELP!!
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At the risk of starting World War III, Shimano brakes could be a long term solution. Highly recommend XT, great stopping power, no noise and easy to bleed
I had the same problem with my back promax hornet brake and I spent about 3 months trying all sorts of things, and everything you listed. I even got this purpose-built gum from an autoparts store that you can put behind the pads to dampen the vibration. No success. Cleaned and then replaced the rotor. No difference. Bike store looked at them a few times and couldn't fix them. Finally they just replaced them with exactly the same model. Everything fine.
My guess is that the construction of the brake was faulty in some way that it became resonant at the frequencies experience under normal braking. Only thing to do is replace.
I fitted shimano discs which worked. I did have to space the calliper with a washer though. Google Avid Turkey Warble or Avid Turkey Gobble. There are heaps of solutions but the annoying thing is what works for one, doesn't work for another.
Hey Adza
I had the same problem for a while on my 575. I had Avid Juicy 7's.
There were lots of posts on blogs and forums about people with the same problem. Mine got so bad that I couldn't bear to ride the bike or when I did, I resisted using the back brake... Not too smart!
I tried all the things you did but none of them seemed to work for more than a few hundreds metres. In the end, the problem seemed to be fixed by using a new set of avid's lightweight race pads with the aluminium backing and organic pads. They cost more but the problem disappeared. Unfortunately, I know of other people who have tried the same fix but with no improvement. Maybe my brake gremlins just found somewhere better to live......
I never had the same problem with my Elixir R's but I had so many other problems I got rid of them.
Right now I'm using XT's which are excellent although the pads do seem to rattle a bit noisily .....
Good luck
CB
Hi - well known problem with the specialized frame - the resonant frequency of the brakes is the same as the frame - in the US mostly referred to a a "warbling" sound.
You need new thicker discs - i think the repalcement disc used by specialized is the avid G something "solid", AND (dont ever try just the one change) resin pads. Once you have new RESIN pads and a new disc, bed them in by doing the stopping hard to walking pace, let them cool a little, then do again etc etc etc - read up on this, it makes a big difference.
While you re at it, set the brakes up carefully by eye etc, and try get in the habit of eitehr using the brakes hard or not using them - ie dont drag brakes.
You are not insane, it is real, and it is not your fault. if you bought the bike brand new , specialized 'should' fix it for you
Thanks all for the updates
glad i am not completely nuts 
@ pharma - i have new thicker discs (not the same ones they come with) same problem
also i'm pretty sure it'd be outta warranty now (2yr old) - doubt specialized would look at it... \
also i always bed in the brakes properly and always set them up carefully... - it might be worth a brake systems flush first and then if all else fails i will replace the brake set. - chainreactioncycles have some good prices
Says that a brake flush has merit. Any air in the hydraulic system could make it slightly springy allowing the brake pistons to vibrate under braking.
Have you also checked the bearings in the suspension linkages (chain and seat stays etc)? If there is excessive play in the FSR this may change the stiffness creating a problem.
If it is the brakes and frame have resonances close together then replacing one so long as the mass and stiffness combo is different could work. Just disappointing and costly.
The only other thing I can think of is wrapping the seat stay or chain stay etc with an old inner tube to provide damping.
These are all just guesses, I haven't had this problem.
You could also just be unlucky with a combination of gear that just doesn't work. I had a fork with all parts in manufacturing tolerance. However it wouldn't work. They replaced all internals and checked all stanchion and follower tolerances. All within spec except the two did not work together. Fork always had no small bump compliance within 10 hours after every rebuild. Ended up with replacement fork that has never had a problem.
An interesting thread
I have also experienced what I can only describe as brake shuddering on my 2012 XT braked 29er hardtail recently & put it down to contamination of the pads although I have used the same cleaner on my Hope braked bikes for years with no problem.
I'm not convinced about "Resonant frequency" theory though as surely the frequency is too variable, changing as the wheel (disc) speeds up or slows down.
However, I do think that as 29ers have longer spokes the wheels may suffer more from flex than smaller wheels which could cause a fluttering effect as the brakes grab then release, a sort of bungey/occy strap effect.
Thankfully for me the old trail fix of spit and dirt liberally spread around the disc braking surface removed the contaminated layer & all was good after two applications but having read your list of fixes tried I doubt that this will help you.
Does it do this immediately or after a while & is it under hard braking or any braking?
Good luck mate, niggling stuff like this can really piss the most patient person off....
An interesting thread
I have also experienced what I can only describe as brake shuddering on my 2012 XT braked 29er hardtail recently & put it down to contamination of the pads although I have used the same cleaner on my Hope braked bikes for years with no problem.
I'm not convinced about "Resonant frequency" theory though as surely the frequency is too variable, changing as the wheel (disc) speeds up or slows down.
However, I do think that as 29ers have longer spokes the wheels may suffer more from flex than smaller wheels which could cause a fluttering effect as the brakes grab then release, a sort of bungey/occy strap effect.
Thankfully for me the old trail fix of spit and dirt liberally spread around the disc braking surface removed the contaminated layer & all was good after two applications but having read your list of fixes tried I doubt that this will help you.
Does it do this immediately or after a while & is it under hard braking or any braking?
Good luck mate, niggling stuff like this can really piss the most patient person off....
@ Cambo - got the same recommendation from my LBS guy.....
@ Cotic - at any time in the ride - if i pull hard enough to stop the noise - the wheel locks up (203mm rear brake rotors kinda do that)
Shimano disc brakes are fantastic....plenty of power and hassle free pad changing and easy to bleed. Running SLXs on my Trance and there great! Deore on my hardtail!
I used to have avid juicy 5s on an older bike....binned them!