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  1. #1
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    XT (M785) - Pad play and knocking

    Hi everyone!

    My xt disc brakes (M785) have a weird problem right from the beginning: The pads slide beweet piston and disc. When i apply the brakes, i can still move the wheel a few milimeters. This gets annoying when braking while going over roots and steps - the front wheel knocks very noticeable.

    Does someone else have this problem or is this just normal?

  2. #2
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    Normal, mine do this too. The pads don't lock into the pistons like some other brakes, only the axle screw holds them in so some movement is normal. I'll take the slight clicking over constant squealing or turkey gobble any day!
    "That's a niiiiiiiice biiike boy! That a Huffy!?"

  3. #3
    Chamois Dropper
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    I took me a couple rides to get used to it, but it's fine. the pads float around a little bit. I find it is more pronounced with the SLXs that are on my wife's bike. Anything is better than the Avid gobble, IMO
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  4. #4
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    The slx's use a cotter pin right? I bet that's why it's louder, the cotter pin probably allows more slop than the axle. But again, much better than other brakes noises!
    "That's a niiiiiiiice biiike boy! That a Huffy!?"

  5. #5
    Chamois Dropper
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    Quote Originally Posted by Nickbm3 View Post
    The slx's use a cotter pin right? I bet that's why it's louder, the cotter pin probably allows more slop than the axle. But again, much better than other brakes noises!
    Yeah,that makes sense. Probably a little more play with the pin. Both brakes work awesome.
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  6. #6
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    Quote Originally Posted by B.BL View Post
    Hi everyone!

    My xt disc brakes (M785) have a weird problem right from the beginning: The pads slide beweet piston and disc. When i apply the brakes, i can still move the wheel a few milimeters. This gets annoying when braking while going over roots and steps - the front wheel knocks very noticeable.

    Does someone else have this problem or is this just normal?
    Is your hub snug?
    Does it knock on it's own-meaning is there side to side play in the hub itself?
    A loose hub or hub with incorrect bearing preload can cause the situation you are describing.
    Wiggle the front wheel without the brakes applied to check for hub slop.

  7. #7
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    It's the only thing I don't like about these brakes. Drives me nuts!

  8. #8
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    I bought the 2012 XT M785 brakes a year ago, and I've never had any knocking problems. My brakes have a bolt that goes through the pads and screws into the caliper, and there is a clip on the end of the bolt so that it doesn't fall out. I notice the replacement pads have a cotter pin, which I throw away. What do your brakes use?

  9. #9
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    Quote Originally Posted by happyriding View Post
    I bought the 2012 XT M785 brakes a year ago, and I've never had any knocking problems. My brakes have a bolt that goes through the pads and screws into the caliper, and there is a clip on the end of the bolt so that it doesn't fall out. I notice the replacement pads have a cotter pin, which I throw away. What do your brakes use?
    My brake pads are held in by exactly this bolt. But i highly doubt the bolt plays any role in fixing the pads when braking, the forces would be way to much for the tiny tab.

    Could you try applying the front brakes moderately, picking up the front wheel with the other hand, and rocking it back and forth. Wenn i do that, it is quite noticeable that the pads move. I even see the finned part moving relative to the caliper a tiny bit. If you brakes dont do that, i would like to find out why.

  10. #10
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    I find it is more pronounced with the SLXs that are on my wife's bike
    The slx's use a cotter pin right? I bet that's why it's louder, the cotter pin probably allows more slop than the axle
    A tip for the SLX users out there. Although the SLX comes with a split pin instead of the XTs screw-in pad axle (what Shimano call them), the SLX caliper still has the screw thread machined in it so you can retro-fit the XT pad axles in there. Just more proof if you need it that the SLX and XT use most of the same parts just sprayed a different colour!

  11. #11
    West Chester, PA
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    The rattling of the pads is the only minor fault I can find after several months with my XTs. You can use 2 pairs of needle-nose pliers and expand the pad springs a little, it will reduce the rattling but not totally eliminate it.

  12. #12
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    Hopefully this will help your problem, I just bought an XT and found one of these tips.
    XT Brake Tips
    Unfortunately i don't have the time to install mine yet but I will definitely implement this during installation.
    The only thing I'm curious about is if it will disrupt heat dissipation on the fins.

    Anyway worth to try I guess.
    Last edited by Dirthugger; 09-25-2012 at 01:04 PM.

  13. #13
    J-Flo
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    My new XT brakes don't knock as described, but they rub. I have a moderate rub between the pads and rotors on both front and back. I thought it would go away with the bedding in and an initial ride, but it hasn't. Seems the pads are running too close to the rotors. I thought the pistons should retract into the calipers -- perhaps there is too much pressure in the system from when I cut the cables (I just installed the brakes yesterday). Anyone know how to fix?

    NOTE: The rotors are brand new and perfectly true. I installed them correctly. I already adjusted the caliper mounting position (three times) to no effect. The rub is more of an annoyance than anything else, as it doesn't produce enough drag to slow the bike down, but it makes noise and will slow a free rotating wheel (wheel in the air) reasonably quickly, and I think is not good for the long term.

  14. #14
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    Jfloren, you may have too much fluid in the system as a result of your hose shortening efforts, or just accidentally pushed the pistons out too far at some stage of the process (they will automatically extend, but not automatically retract).

    If so - take pads out, level levers on bar, undo bleed port on top of levers. Push pistons in with flat plastic implement (like a plastic tyre lever). Note if any oil overspilled out of bleet port (sign it was overfilled) & wipe it up if it did. Fit screw back into bleed port. Re-fit pads. Put orange brake spacer (should've come with new brakes) between pads and pump levers to get pistons out to right position. Re-fit wheel. Shouldn't rub!

  15. #15
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    Before resorting to bleeding, I would take the pads out and push the pistons back in with a plastic tire lever, then put the pads back in, and place the orange plastic insert between the pads and pump the brakes until they squeeze firmly agains the orange insert. Then remove the insert and try again to center the rotor.

    Personally, any rotor rub ruins the ride for me. I experienced some rotor rub last week, and I couldn't figure out what was causing it. The rotor seemed centered between the pads, and although the rotor wasn't perfectly true, it wasn't bad enough to hit the pads. But when I was riding I could here the rotor rubbing. It turns out that one of my pistons was stuck, so the other piston was extended fully, and the rotor was actually rubbing against the caliper body.

    To push the pistons in, position the end of the tire lever in the center of the piston, then position the middle of the tire lever against the caliper body, and push on the end of the tire lever using the caliper body to lever the piston in. However, before pushing a piston back in, make sure to clean the sides of the piston with a Q-tip soaked in isopropyl alcohol to remove the brake dust and dirt (from MBA magazine--Garage Files). Get it good and clean.

    You can also hold one piston in with the tire lever, then pump the brakes to make the other piston extend, clean the sides of the piston again, then push it back in. Then repeat on the other side. That will ensure that both pistons are operating normally and that you don't have a stuck piston (from MBA magazine--Garage Files).

  16. #16
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    Quote Originally Posted by B.BL View Post
    My brake pads are held in by exactly this bolt. But i highly doubt the bolt plays any role in fixing the pads when braking, the forces would be way to much for the tiny tab.

    Could you try applying the front brakes moderately, picking up the front wheel with the other hand, and rocking it back and forth. Wenn i do that, it is quite noticeable that the pads move. I even see the finned part moving relative to the caliper a tiny bit. If you brakes dont do that, i would like to find out why.
    I paid attention to my brakes while riding yesterday, and the front brake does click when I apply the brakes. I have an F03C(metallic) pad on the front. The rear brake does not click when I apply the brakes. I have on F01A(resin) pad on the rear.

    I think most of the time the click of the front brake is drowned out by other noise.

    There is a tip on youtube that might help silence your brakes:

    XT M785 Brake Tip - YouTube
    Last edited by happyriding; 09-30-2012 at 02:45 PM.

  17. #17
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    My front brake (SLX) clicks when I go downhill and try slow down quickly. There is still plenty of stopping power but the clicking annoys me. I thought my rotor was warped because it is so hard to align the caliper.

    Are XT pad axles an answer to my problem?

  18. #18
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    I'd like to revive this thread - my original problem (knocking, etc) is gone.

    I'm no longer sure the brakes were the reason, it might have been headset-related (alhough i adjusted the headset several times in this time frame)

    The M785 work really great now.

  19. #19
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    My XT M785 brake pads (shimano F03C metal pads with fins) also slide relative to the caliper pistons on the front but not the back wheel. This is annoying since it has the same feel as if the headset is loose, a metallic click if you squeeze the front brake and rock the handlebars forward and backward. The basic problem seems clear -- the friction between the piston and the pad is less than the friction between the pad and the rotor. So the piston-pad interface slips before the pad-rotor interface slips. Since the contact areas for piston against pad back and pad against rotor are similar it is not too surprising that this problem arises. I tried to fix it by cleaning very well the piston face and the backs of the pads to try to improve the friction between them -- maybe dirt was lubricating it. I sanded both surfaces lightly with 220 grit sand paper then scrubbed them with a toothbrush with isopropyl alcohol. Now the pads do not slip on the pistons except under very light brake pressure. It is much better than before but not 100% -- the back brake does not have pad slip against piston even for light brake pressure, so it should be possible to do better.

    I can imagine a ton of factors causing the piston-to-pad friction being low -- dirt, oil, a history of slipping smoothing both piston and pad back surfaces, piston and pad not being exactly parallel, the 4 radial indentations in the piston surface being aligned with the direction of slip in an unfavorable way. Maybe the piston-pad friction isn't unusually low, and instead the pad-rotor friction is especially high. I'm using an Ashima airotors 185 in front, 160 in back.

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