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  1. #1
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    My ghetto rigged XTR IS (M965) to post mount..

    I decided to post about this since it was kind of a struggle to find a solution to my problem without completely upgrading my shifters AND brakes since I'm rocking the M966 XTR system. Basically, I upgraded to a newer frame with a post mount rear, bought a Shimano SM-MA_F180S/P adapter hoping I'd just need to throw a 180mm rotor on the rear... well, that didn't work at all. So I went to a LBS and purchased a 203mm rotor and a couple shims to make it work. While I think it's kinda ridiculous (or awesome?) to have a 203mm rotor for my speed/skill/preferred riding style. I'm going to eventually pick up a shorter bolt, but for now, the washers are working, and the bolt is nice an torqued up!! Hope this helps folks looking for similar solutions in the future.
    My ghetto rigged XTR IS (M965) to post mount..-imag0599.jpgMy ghetto rigged XTR IS (M965) to post mount..-imag0588.jpg

    Keywords: Post mount to IS, Shimano Brake adapter, IS to post mount.

  2. #2
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    I have used longer bolts and sets of washers to set up the right caliper angle/radius. Sometimes it can be a creativity situation. From the pix it looks like the back bolt needs to be shimmed up a little.
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  3. #3
    rebmem rbtm
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    I'm glad you've posted a photo of a IS caliper mounted on rear Post Mounts, theory say's it can work but I've never seen anyone do it.


    A 1.5mm shim under each side of the adapter would probably pack it out correctly for a 203mm rotor.

    A 180mm front/160mm rear IS brake caliper is a 20mm increase in size.
    A 180mm adapter is a 20mm increase in size.

    You're setup is a 40mm increase in size, this means the caliper is correctly positioned for a 200mm rotor.

    There's 1.5mm difference in the radius of 200mm rotor and a 203mm rotor.

    If you really wanted to use a 180mm rotor the following 160mm KCNC adapter should work.

    Product KCNC Disc Brake Adapter Post Fork - IS Caliper


    bought a Shimano SM-MA_F180S/P adapter hoping I'd just need to throw a 180mm rotor on the rear
    A 160mm front IS caliper is the same as 140mm rear IS caliper.
    A 180mm front IS caliper is the same as 160mm rear IS caliper.
    The difference in sizes between front and rear is because there's a 20mm difference in fork and frame IS mounts, front IS mounts are set at 160mm and rear IS mounts are set at 140mm

    A 160mm front/140mm rear IS caliper doesn't increase rotor size when attached to front 160mm IS mounts or rear 140mm IS mounts.
    A 180mm front/160mm rear IS caliper will increase rotor size by 20mm when attached to front 160mm IS mounts or rear 140mm IS mounts.

    160mm front/140mm rear IS caliper + 160mm post mounts + 160mm PM to IS adapter = 160mm rotor
    160mm front/140mm rear IS caliper + 160mm post mounts + 180mm PM to IS adapter = 180mm rotor

    180mm front/160mm rear IS caliper + 160mm post mounts + 160mm PM to IS adapter = 180mm rotor
    180mm front/160mm rear IS caliper + 160mm post mounts + 180mm PM to IS adapter = 200mm rotor
    Last edited by cobba; 10-21-2013 at 09:42 PM.

  4. #4
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    If you can find a short arm caliper like your front caliper is, you can use a 180 rotor on the back with that adapter.
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  5. #5
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    Cobba- I actually have the same (Shimano) adapter on my Fox fork and it worked perfectly with the 180mm rotor. The KCNC never showed up on any of my multitude of searches, but it looks way nicer than the Shimano one! The biggest problem I ran into was what Bordershy said, my rear caliper has a longer rear mounting arm so it threw things off.

    I decided I didn't want to buy a new caliper and deal with bleeding, etc.After hearing awful noises from the rotor during my ride today, I came home and added a small shim to the rear mount and put a cup/cone washer system on the front one to allow some room to align everything a little better. All in all, it was a $50 solution, and hopefuly, much less noisy than before.

  6. #6
    rebmem rbtm
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    Quote Originally Posted by tuffguy View Post
    Cobba- I actually have the same (Shimano) adapter on my Fox fork and it worked perfectly with the 180mm rotor.
    Yes but.................

    A 160mm front caliper is the same as a 140mm rear caliper, it's also called a 'short arm' caliper

    A 180mm front caliper is the same as a 160mm rear caliper, it's also called a 'long arm' caliper.

    Notice the difference in the 'short arm' and 'long arm' calipers.

    My ghetto rigged XTR IS (M965) to post mount..-p4pb8600158.jpg

    You couldn't use a 'long arm' caliper with a 180mm adapter and a 180mm rotor on the fork, you've found this out on the rear, it'll only work with a 200/203mm rotor.

    If you swap your brake calipers around so the 'short arm' caliper is on the rear and the 'long arm' caliper is on the front, your brakes would be setup for a 200/203mm front rotor and a 180mm rear rotor.

    It would be a better setup with a larger rotor on the front.


    I came home and added a small shim to the rear mount
    The top surface of the post mounts is flat and the surface of the adapter that sits on the post mount is flat too, by jacking up one of the adapter with shims, the other end the adapter wouldn't be seated flat against the post mount, that's why I said it's probably be better to put a even amount of shims on both sides of the adapter.
    Last edited by cobba; 10-22-2013 at 09:14 PM.

  7. #7
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    Ahh, now it make sense what you were saying before about the size caliper comparisons. For some reason it didn't register with me the first few times I read that. It sounds like I'd need a new fork with IS mounts in order to pull of the 203 front rotor and a 180mm rear rotor (paired to front caliper) with my current adapter then. I'll be checking back to this thread once the cash is available for a new fork (i wanted a 160 anyways) just to refresh my brain.

    Thanks Cobba, that's a lot of solid info and helped a lot!

  8. #8
    rebmem rbtm
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    Quote Originally Posted by tuffguy View Post
    It sounds like I'd need a new fork with IS mounts in order to pull of the 203 front rotor and a 180mm rear rotor (paired to front caliper) with my current adapter then. I'll be checking back to this thread once the cash is available for a new fork (i wanted a 160 anyways) just to refresh my brain.
    What makes you think that you'd need a IS mount fork to use a 203mm rotor?

    If you have a post mount fork and frame with IS mount brake calipers, you're running a 180mm front rotor / 203mm rear rotor and you want to swap the rotors around then you only need to do the following:
    * remove your front and rear calipers
    * swap the brake lines around
    * bleed the calipers
    * put the long arm caliper on the front
    * put the short arm caliper on the rear
    * put the 203mm rotor on the front
    * put the 180mm rotor on the rear

    Post Mount fork/frame + 160mm IS to PM adapter + 'short arm' caliper = 160mm rotor
    Post Mount fork/frame + 180mm IS to PM adapter + 'short arm' caliper = 180mm rotor
    Post Mount fork/frame + 160mm IS to PM adapter + 'long arm' caliper = 180mm rotor
    Post Mount fork/frame + 180mm IS to PM adapter + 'long arm' caliper = 200/203mm rotor




    With a IS mount fork:
    'short arm' caliper = 160mm rotor
    'long arm' caliper = 180mm rotor

    180mm IS to IS adapter (+20mm adapter): http://www.bikewagon.com/shimano-fro...s-is-to-is-180

    IS mount fork + 180mm IS to IS adapter (+20mm adapter) + 'short arm' caliper = 180mm rotor
    IS mount fork + 180mm IS to IS adapter (+20mm adapter)+ 'long arm' caliper = 200mm rotor


    With a IS mount frame:
    'short arm' caliper = 140mm rotor
    'long arm' caliper = 160mm rotor

    IS mount fork + 180mm IS to IS adapter (+20mm adapter) + 'short arm' caliper = 160mm rotor
    IS mount fork + 180mm IS to IS adapter (+20mm adapter) + 'short arm' caliper = 180mm rotor
    Last edited by cobba; 10-23-2013 at 08:50 PM.

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