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Rear brake screech/vibrate through entire frame

daveaasmithdaveaasmith Posts: 223
edited October 2010 in MTB workshop & tech
Folks

My Scott Scale 30 has started to wail like a banshee when applying the rear brake. The sounds resonates through the entire carbon frame. It's not a squeak, it's an almighty wail.

Brakes are Avid Juicy 7, 185mm rotors, pads (original Avids) are 1/4 worn. Rotor is fine.

Took pads out, cleaned them up removing all traces of rust and dust. Checked everything (not just caliper related) is nice and tight througout. Still wails :(

Swapped front and rear rotors. Still wails :(

Only thing which stands out to me is that the brakes really really bite - more than any brake I've had before.

Any ideas?

Dave.

Posts

  • abarth_1200abarth_1200 Posts: 370
    The brake mounts might need facing, I dont know if that possible on a carbon frame, are the frames brakes mounts alloy?
  • nicklousenicklouse Posts: 50,675 Lives Here
    some times it just happens, have a read of the brake topics in the FAQs.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • daveaasmithdaveaasmith Posts: 223
    The brake mounts might need facing, I dont know if that possible on a carbon frame, are the frames brakes mounts alloy?

    The brakes are post mount, and sit on an IS adaptor - they have the Avid tri-align thingy, so sit perfectly in line with the rotor.

    So, I'm guessing everything should be ok in terms of alignment.

    Unless anyone thinks I'm talking [email protected] ?!
  • richg1979richg1979 Posts: 1,087
    pads contamination makes noise and vibration but usually have less powerfull brakes not vice versa.
  • abarth_1200abarth_1200 Posts: 370
    No, the surface between the mount and the frame, it might have alyer of paint/clearcoat which is making the surface uneven thus reverberating.

    The caliper to adapter is fine like you said also its not anything to do with being in line with the rotor thats sounds alright to me.

    Give me amin and ill find a pick of the area that migh be the problem, also are the screws torqued according to specs.

    http://www.bikeradar.com/forums/viewtop ... c&start=80

    Last 2 pics in first post by Nick.

    That is what im talking about but might not be possible with your carbon frame unless you have alloy brake mounts
  • Teflon DaveTeflon Dave Posts: 29
    I had the same problem on my Cannondale, i traced it to the pads vibrating in the caliper (Avid juicy 3). It was so bad that i was on the verge of selling the thing .The only way i found to cure this is to wipe a very fine smear of copper grease on the back of the pad between pad and piston, Hey presto quiet brakes.
    Ride bike, eat cake, drink tea, ride bike, eat cake, dri...........
  • abarth_1200abarth_1200 Posts: 370
    yup thats a good suggestion, I had to do that to the brakes on my car after it came out the garage getting the brakes fixed, copper grease is your friend, tiny layer is all that is needed
  • daveaasmithdaveaasmith Posts: 223
    Copper grease does sound like an idea - I've got some from when I sorted the car brakes out.

    Think I'll try swapping the front / rear pads first.
  • BG2000BG2000 Posts: 517
    If all else fails, consider rubbing down your rotor with emery paper (600 grit) then clean heavily with isopropyl alcohol and a rag. And install new pads (Clarks Sintered).

    Make sure you bed the pads in. Follow the FAQs.

    It worked for me.

    I can only imagine you've got an over-effecient build up of pad material bedded into the rotor, and it just makes the brake too powerful. Sometimes a frame/caliper combination just isn't sturdy enough and will resonate (vibrate heavily) when the brakes reach their biting point.
  • daveaasmithdaveaasmith Posts: 223
    Swapped front pads to rear and still wails.

    Checked the rotors closely and they are definitely grooved - I'd bought these both used - maybe they are just at the end of their life?

    Think I might bite the bullet and get some new ones - am also wondering if 185mm rotors are really needed - I'm sure 160mm is plenty for my XC riding.

    Obviously I'll get new pads as well.

    Dave.
  • BG2000BG2000 Posts: 517
    Swapping the pads won't help if your front were contaminated, and also they were presumably bedded into the front rotors from new ?

    I don't think any grooves will cause screeching. I'd really consider sanding them down so that you're effectively starting again with a new rotor surface (without any brown pad material embedded into the grooves). Make sure they're really clean afterwards. If you get some decent sintered pads and bed everything in really well see how that goes.
  • Teflon DaveTeflon Dave Posts: 29
    When my brakes were vibrating, i swapped pads front to rear, they still vibrated, swapped rotors the buggers still vibrated, fitted new pads, still vibrated, i checked spoke tention, wheel bearings, wheel alignment, nothing cured it until out of desparation i put a post on here asking for help and someone suggested copper grease. Instant cure.
    Ride bike, eat cake, drink tea, ride bike, eat cake, dri...........
  • I have the sme problem with my Avid Juicy 3's on a Cannondale, will try the copper grease tonight
  • JonesyMJonesyM Posts: 14
    I swapped my rear pads out of my Juicy 5's for new and they developed a high pitched squeel everytime I was cruising along a smooth surface. Basically the self adjusting pistons were too far out. All I needed to do was push the pistons right back to reset them. You can do it with a flat head screwdriver with your wheel out, or just push your disk hard against the pad, although that makes some people nervous. Fixed it for me.
  • I've had the rear brake severe vibration problems for weeks, just like some of the other posts state, and tried EVERYTHING TWICE to solve the problem. I was just about to give up and change out the brakes, when I read one comment on another site from a guy who said "Pad contamination". He solved his problem by baking his pads in a toaster oven. I had tried this before with minimally positive results. I also tried the Copper grease thing, again, with only slight improvement. After experimenting and experimenting and experimenting again, I believe I've solved the problem, at least in my case. My brakes are now completely vibration and noise free and have been for a good 400+ miles now. Here's what worked for me:

    My setup is as follows:

    2009 Avid Juicy Three front & rear
    G3 Avid rotors front & rear
    Rear pads are stock avid sintered from BB7's
    Front pads are Organic Avids that came with the Juicy Threes
    SETT'E Edge Xtrolite G7 Alum. Hard tail frame
    Rock Shox Tora XC Solo Air Fork 2009
    FSA DH Pro Headset
    Mavic Crossride rims with Mavic hubs

    Try to follow this procedure exactly as I describe below, and I think it will work for you also.

    Both my pads & rotors were glazed I did NOT sand anything, leave the glaze on both rotors and pads.

    Remove the pads and thoroughly clean them with a clean cotton cloth soaked with Denatured Alcohol. Rub them clean front and back until absolutely no black comes off on the white rag. DO NOT TOUCH THE PAD FACES WITH YOUR FINGERS.

    Clean the rotor equally as well, in the same manor.

    Pre-Heat your toaster oven to 450 Deg. F and bake the pads at this temperature for 45 Min. Let them cool slowly.

    At your local auto store get this:
    CRC synthetic brake & caliper grease - It contains Graphite & PTFE
    www.crcindustries.com Tech. Info. 800-521-3167
    (DO NOT substitute Silicone brake lube, tried it, and it does not work)
    If you have trouble finding it email me at: [email protected]

    Smear some of the grease across the BACK ONLY of the pads and reassemble into the calipers. This is kind of a pain because of how you make the pad "Sandwich" with your fingers to insert them. Just be careful not to get any grease on the face of the pads.

    Mount and align the calipers as per usual and ride. You should see and feel immediate improvement. After a few stops you may hear an occasional SLIGHT vibration but if you just squeeze and release the leaver it should go away. Just keep riding and after the first 20 to 30 stops, just clean only the rotor with a CLEAN cloth and Denatured Alcohol.
    I promise you, it works...

    If I could post a picture of the graphite grease package I would, but I can't. If you email me, I'll send you a photo.

    Hope this helps at least some of you, I know how utterly frustrating this problem can be and how great it feels when it goes away.
    Good Luck,

    Ken S.
  • konadawgkonadawg Posts: 447
    Try a different make of pads, usually works.

    Then make sure your hub is not loose.

    Basically the cause is pad to disc vibration and the frame resonates. Looking at it on the macro scale, your pads would be sticking to the disc and moving forwards with it, flex anywhere from the calliper assy through to the frame making this possible, then releasing, all a zillion times a second, rather than smoothly allowing the disc to rub against them.

    The reason why this happens however is something else, some frame designs are very susceptible to this, Kona Dawgs in particular. Anything loose will allow the vibration to build up rather than kill it.

    I had this once, it improved greatly when I replaced the rear suspension pivot bushes, and completely went away when I subsequently changed brake pad make.
    Giant Reign X1
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