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  1. #1
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    Reputation: Johnny Chicken Bones's Avatar
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    Calling all compatibility Gurus...

    I think I know the oh so bummer of an answer to this.

    Is it possible to use an IS (tab) front XTR disc caliper on a post mount fork WITHOUT having to increase the size of the rotor? Is there some magical adapter out there that I haven't been able to find?
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  2. #2
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    Reputation: vk45de's Avatar
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    I hope there is but far as I know, there isn't.

  3. #3
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    Reputation: Hardtails Are Better's Avatar
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    No. You'll need to go to at least a 180 rotor.

  4. #4
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    Post Mount XTR

    [SIZE=3]What condition is your IS tab? I think I have a new post mount caliper in my brake parts box. That I would swap if in like condition. That would be the easiest way to fix the problem.

    -ZX
    [/SIZE]

  5. #5
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    Caliper Swap?

    Crazy 8-
    the caliper was new last May- so a year old. No scratches, pretty perfect condition.
    One hitch though-
    It's a rear XTR caliper. If you run it on the front you can use a 180mm rotor w/o any adapters. It's actually a really cool solution if you like larger rotors. Only bummer is- the new post mount standard leaves me high and dry.

    I'd love to do a swap if you want it. Most recent generation XTR for same.
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  6. #6
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    I hate the new PM std too. Leaves me pretty much stuck w/ rockshox as my only choice of forks - as you can see I also have a set of RR calipers. Some ppl even want PM on frames.... that's just messed up

  7. #7
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    Post mounts...change for change sake?

    Does anyone know of a good reason for the new post mount system? Some local dingle here told me once that it stresses the bolts less. That may be true but the post mount system uses the same size bolts as IS so what is the difference? Ive never seen a IS bolt break. The way I see it is that you now need longer/heavier bolts and get the added benefit of stripping out your fork/frame instead of a cheap adapter or caliper body, the former Ive done once, the latter Ive never done.
    There is nothing more difficult to plan or more dangerous to manage than the establishment of a new order of things.

  8. #8
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    Problems with IS:

    1. The standard for front and rear is different, which is dumb.
    2. Caliper alignment can only be done with shims. PM is way easier to set up.
    3. It's harder to make adapters for different rotor sizes than with PM.

  9. #9
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    Problems with PM:

    1. Can't be adapted to IS w/o increasing rotor
    2. Thread can be stripped
    3. Removal of caliper requires realignment

    Stressing bolt less applies to weight weenies who want to use Ti or Alloy bolts. Your car brake caliper is held on by a setup similar to IS and it can take a couple thousand pounds of force.

  10. #10
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    Advantage with IS:
    - stripped thread => new adapter instead of new frame or new fork.

  11. #11
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    Quote Originally Posted by vk45de
    Problems with PM:

    1. Can't be adapted to IS w/o increasing rotor
    OK, but if IS is just done away with, that problem is moot.
    Quote Originally Posted by vk45de
    2. Thread can be stripped
    Sure, if you do strip something is a bigger issue, but if you're stripping brake bolts out, you're doing something wrong. With proper maintenance and torque there won't be any issues. Also, on the new Turners, which use PM rears, instead of threading the frame itself, there's just a cylidrical steel insert that goes into a hole perpendicular to the tabs. The hole for the insert is in the same orientation as the holes for an IS caliper. If something gets stripped, you can just replace the insert.
    Quote Originally Posted by vk45de
    3. Removal of caliper requires realignment.
    Again true, but it only takes about a minute, and with IS, you still need to keep track of your previous shim setup. Not necessarily easier or more convenient.

  12. #12
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    1. No, some ppl still use it
    2. Not all manufacturers are that careful to use an insert and not all people service their bikes themselves. We all know some bike shops keep gorillas just to strip bolts
    3. If you have IS mount w/ adaptor, you can remove caliper w/o worrying about shims or anything

  13. #13
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    1. Who? Which decent brakes still use IS mount?
    2. If your bike shop threads your fork, they sure as hell better replace it
    3. True, but it takes literally two seconds to align a PM caliper - even the first time you fit it. No faff with adaptors.

    Out of interest, how long did it take you to make the switch from v brakes to disc? That new standard must really have made your blood boil

  14. #14
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    switch to Discs? took ~ 3 yrs after I got my bike in 98. Had 1st set was Hayes HFX comp. What's so bad about adaptors? They cheaper to replace when you do strip them. Furthermore, it's not like you're getting rid of all adaptors - only those for people running 6". I've read a lot more about shop gorillas over torquing bolts than them taking responsibility for it.

  15. #15
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    This sure has degraded.
    I thought we just made fun of those guys w/ big wheels, or lycra, or recumbents. Now we have to fight over brake mount options?
    Man E-Biking is gruelling.
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