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  1. #1
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    SLX Brake Rubs on rotor after descent

    I have SLX M666 brakes, and have a weird issue that the front rotor starts rubbing after a descent. At first I thought the rotor was warped, but after removing and inspecting it, it's not. I have taken out the pads and reset the pistons/aligned the caliper, but as soon as I head out again the rubbing starts after coming down any hill where I have to use them heavily.

    Any suggestions? The only thing I can think of is that in order to slow me coming down a hill, I am pushing the piston further out than simply slowing on flats, and the piston is then not retracting fully.

    The brakes work fine, I feel plenty of power and no sponginess to the lever. The pads have a decent amount of life left in them, and are worn evenly. Just having this one annoying issue.

    Thanks.

  2. #2
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    It might be possible that the heat generated by a long descent is causing everything, including the mineral oil, to expand just enough in order to push the pistons/pads out just enough to rub. Magura which also uses mineral oil has cautioned that over filling their brakes (say, in order to reduce lever toss) might cause the oil to expand enough under high heat such that the brakes can lock up. If there is still rubbing after everything has cooled down, then it sounds like the pistons might have a little trouble retracting. They often need to be cleaned like other moving parts.

  3. #3
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    My Deore 596 brakes also do that. However once the brakes get cool again the slight rubbing stops. It seems knl2stls theory of too much mineral oil in the system is the case here. I have to say though I'm not really comfortable with the idea of getting some oil out of my reservoir..

  4. #4
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    Try a larger rotor. If there is oil expansion spin the wheel (by hand) when hot to check for drag.

  5. #5
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    Mine do the same thing after a long downhill. Usually I just have to wait 15-20 seconds and then give the brakes a quick squeeze and then they're fine.
    I'm looking forward to regretting this.......

  6. #6
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    +1 to thermal expansion of the oil. How close are your pads to the rotor? Might need to back them off a bit.

  7. #7
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    Quote Originally Posted by Jernas View Post
    I have to say though I'm not really comfortable with the idea of getting some oil out of my reservoir..
    Me either, I would have no idea how much to remove, and would likely end up initially removing too much.

    Quote Originally Posted by johnnyspoke View Post
    Usually I just have to wait 15-20 seconds and then give the brakes a quick squeeze and then they're fine.
    That's typically what I do, just a quick squeeze of the lever and it stops.

    Quote Originally Posted by keen View Post
    Try a larger rotor.
    I might give this a try, but already have a 180 up front, and not sure I want to go any bigger.

    Quote Originally Posted by watts888 View Post
    +1 to thermal expansion of the oil. How close are your pads to the rotor? Might need to back them off a bit.
    As far as I know the pads are pretty close, this weekend I might strip the brakes down and give everything a real good clean, re-align pads/rotor/caliper etc.

    Thanks for all the replies.

  8. #8
    El CicloPath!!!!!!!
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    I have the issue too, but in my case I dont think it's the thermal expansion of the fluid or heat. This happens if I just do some really hard braking, even for a short decent. I think what happens is the the wheel/hub/disc is getting torque during the extreme braking and things are flexing in a way that push the disc to the inside piston more than the outside. I think it has to do with the rotational force and friction. This is with a RS Sektor/15mm thru and an SLX hub 32 on a mavic 521.

    FOr an experiment, find a short but steep downhill, something like a parking garage ramp or similar, ride down as fast as you can and hit the front brake hard (don't endo! safety first) then check the disc spacing between the pads and see if its shifted to one side more than the other.

    For me, after a few seconds, everything returns to center.

  9. #9
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    I had the same problem with the aluminium backed pads. When I changed to steel backed pads it stopped. My guess was like those above - thermal expansion.

  10. #10
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    This may be relevant, it may not...but I know if you're running a standard QR front fork/hub and your QR skewer isn't tight enough it can shift the axle slightly in the dropout under hard braking, causing some rubbing. Once you get off the brakes and hit a bump or rock hard enough it can knock the axle back in place. I had this issue on a bike a few years ago and have seen it before on other bikes. Once I tightened my QR skewer a bit more I didn't have any issues.

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