What is the correct adaptor for a Big Mama post mount disc?- Mtbr.com
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  1. #1
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    What is the correct adaptor for a Big Mama post mount disc?

    The Big Mama has post mounts. My XT brake calipers are post mount. I am running a 180 mm disc. A Shimano 160 to 180 post mount adaptor for front forks DOES NOT WORK. It puts the caliper out too far and only about half of the pad runs over the disc with the Shimano adaptor. (Adapter pictured below).

    I ended up using washers to move the caliper up approximately 10mm. Not ideal at all.

    Pic of adapter that only works on the forks apparently:


  2. #2
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    It should work fine. Perhaps you oriented it the wrong direction. Post Mount are interchangeable front and rear.

  3. #3
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    Negative. Tried it. In either direction, this adapter holds the caliper too far out, where only about half of the pads cover the outer edge of the brake rotor. The one end is like 15mm thick, the other end is 7.5mm thick, so in theory it should hold the caliper out 10mm at the center between the posts. From my calibrated eyeball, what I see is that the posts seem to be equidistant from the outermost edge of the rotor... when I shimmed the caliper out with washers, It took the exact same space on each post (10mm) to get the caliper positioned optimally. I ended up using a set of spherical washer and matched conical seat washer that I had, they'll allow some minor adjustment in the position of the caliper, and it should work fine.

  4. #4
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    Shimano makes a rear post to post adapter, perhaps you should try ordering it?!
    I don't post to generate business for myself or make like I'm better than sliced bread

  5. #5
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    Quote Originally Posted by DeeEight View Post
    Shimano makes a rear post to post adapter, perhaps you should try ordering it?!
    Do you happen to know the part number? Thats my question and the purpose for this thread.

    Thanks.

  6. #6
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    The adapter is different for the front and rear brakes. The rear adapter P/N is SM-MA-R180P/S, it looks like you have the front brake version.

    http://bike.shimano.com/media/techdo...9830682461.pdf

    http://bike.shimano.com/media/techdo...9830682460.pdf

  7. #7
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    It's only different for IS, not post mount. Note how the rear does not even show post mount adapters. The P/N you referenced is for an IS to PM adapter.

    For Post Mount it establishes a set spacing for a 160mm rotor. Any changes in spacing for larger rotors will now be the same for the front or rear.

  8. #8
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    Quote Originally Posted by Schmucker View Post
    It's only different for IS, not post mount. Note how the rear does not even show post mount adapters. The P/N you referenced is for an IS to PM adapter.

    For Post Mount it establishes a set spacing for a 160mm rotor. Any changes in spacing for larger rotors will now be the same for the front or rear.
    My mistake, I didn't catch that.

  9. #9
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    Quote Originally Posted by GpzGuy View Post
    Negative. Tried it. In either direction, this adapter holds the caliper too far out, where only about half of the pads cover the outer edge of the brake rotor. The one end is like 15mm thick, the other end is 7.5mm thick, so in theory it should hold the caliper out 10mm at the center between the posts. From my calibrated eyeball, what I see is that the posts seem to be equidistant from the outermost edge of the rotor... when I shimmed the caliper out with washers, It took the exact same space on each post (10mm) to get the caliper positioned optimally. I ended up using a set of spherical washer and matched conical seat washer that I had, they'll allow some minor adjustment in the position of the caliper, and it should work fine.
    The problem is the caliper mount is at an angle to the radius of the rotor, therefore it isnt as simple as 'adding 10mm' because you have to account for the angle. Hence, the shimano adaptor is thicker at one end, to maintian the correct angle of the caliper ralative to the brake surface of the rotor. If your caliper is not sitting right it is possible you fitted the adaptor backwards, if not measure your rotor to check its 180mm not 170mm (an old shimano size). Failing all that, check it works as 160mm and then post us a picture of the issue at 180mm.

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