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  1. #1
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    Piston retraction, sticking, and drag on top-end XC brakes

    So there are lots of comments and debates going on about hope mini x2 pro, marta sl/sl mag, R1, xtr brakes, but most of it that I have seen has focused on weight, power and modulation (all important things)...but one big concern of mine is drag.

    On that note, I've been surprised to read both lots of stories about xtr sticky pistons (for example), and a few claims that xtr pistons retract more than many others (when they aren't stuck at least). I haven't heard much about retraction and drag among the rest, but see occasional reports about drag.

    So how do these top-end brakes compare in terms of avoiding brake drag without lots of fuss and service?

    Also, I could use some educating on what makes for more and more effective piston retraction (other than clean pistons and seals that are in good shape).

  2. #2
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    I own 2 sets of Oros, Marta SL and Mini and Mini Pro. All the brakes have a min of 2 years on them, by me. The Mini Pros have the least power, but the best retraction. The others worked great when new, but retraction went away quickly with the martas (they don't have retraction springs), and then the Oros. I thought I liked the Oros the best, but the eventual retraction issues and the small pad area take away that top spot. I have not tried R1 and x2. I also run and like the Hayes Strokers. Retraction on those are minimal, but so far they work and retract. I only have 6 mos on those.
    Last edited by rensho; 05-05-2009 at 08:44 PM.

  3. #3
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    retraction

    Man, it's nice to get such a right-on-the mark response right off the bat, thanks. Do the x2's have the same design features that effect this?

    Other comparisons?

    More on what makes for more and more dependable piston retraction?

  4. #4
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    piston retraction

    c'mon brake guru's, some explanation of what makes pistons retract and why some would have more/better retraction than others?

  5. #5
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    Isn't the spring there to keep the pads on the pistons? I cant see it being strong enough to actually retract the piston.
    Lead by my Lefty............... right down the trail, no brakes.

  6. #6
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    I'm no guru, but this is my understanding:

    The piston seals, which are seated in the caliper body, are responsible for the movement of the pistons. The square-edge design of the seals, combined with the shape of the aperture they are seated in in the caliper, is the main component of piston retraction. It ensure that the piston is always returned to a specific position relative to where it stopped (ie when the pad touched the rotor) when the brake lever was applied. Basically, however far the piston extends when the lever is pulled it will always only return by a set distance, for example 1mm. As the pads wear, the pistons have to extend further from the caliper to press the pads against the rotor, but they will always only return by the 1mm that the seal has determined.

    There is also a retraction force (as the system is sealed) from the seals and spring in the lever assembly. This goes some way to 'sucking' the piston back to its initial position.

    What would affect retraction? Materials; tolerances; brake fluid (type/condition/volume); spring rate; size of lever piston/seals...all will contribute something.
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  7. #7
    PCC
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    Quote Originally Posted by SteveUK
    What would affect retraction? Materials; tolerances; brake fluid (type/condition/volume); spring rate; size of lever piston/seals...all will contribute something.
    I believe that the piston seals hardening with age would cause retraction problems. You are correct that the seals themselves are designed to retract the pistons and allow for compensation for pad wear.

  8. #8
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    Quote Originally Posted by SteveUK
    I'm no guru, but this is my understanding:

    The piston seals, which are seated in the caliper body, ... and so on...
    What would affect retraction? Materials; tolerances; brake fluid (type/condition/volume); spring rate; size of lever piston/seals...all will contribute something.
    Nice try Steve. All I could come up with as an explanation for self adjustment for another post is that there is "give" in the system and the spring separates the pads accordingly.

  9. #9
    meh....
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    Quote Originally Posted by kmacon
    Nice try Steve. All I could come up with as an explanation for self adjustment for another post is that there is "give" in the system and the spring separates the pads accordingly.
    Are you saying he's wrong? Hard to tell from your post, but he is correct, the piston seals retract the pistons, not the little springs between the pads, if there are any.

    The seals sort of stretch when the piston if forced out be the fluid behind it/them. Then when the pressure is released the seals relax and pull the pistons back a tad. The seals will move out as the pads wear.

  10. #10
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    Quote Originally Posted by Monte
    Are you saying he's wrong?
    Sorry you couldn't interpret the post, so here .. YES, I AGREE. Must be a northwest/southeast thing.

    I do believe that the springs help with that "tad bit of retraction" though. At least they do not hurt.

  11. #11
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    "The seals will move out as the pads wear."

    No, the seals remain in a fixed position in the caliper. They can only flex outward a certain distance, then they are prohibited by the edge of their aperture. To compensate for pad wear, the pistons are effectively forced beyond the limit of the seal. The distance is, of course, virtually unmeasurable, but this is essentially what occurs. When the lever is released and the outward force is removed, the seal simply relaxes to its 'neutral' position within its aperture, drawing the piston back with it.
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  12. #12
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    "I do believe that the springs help with that "tad bit of retraction" though. At least they do not hurt."

    The springs do nothing more than keep the pads pressed against the pistons.

    Oh, and I understood what you meant in your previous reply.
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  13. #13
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    Quote Originally Posted by Monte
    The seals sort of stretch when the piston if forced out be the fluid behind it/them. Then when the pressure is released the seals relax and pull the pistons back a tad. The seals will move out as the pads wear.
    thus the "give" in the system

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