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  1. #1
    mtbr member
    Join Date
    Oct 2006

    Adaptor for M975 XTR Post Mount Caliper with 180 mm Rotor

    I would like to use a 180 mm XTR centerlock rotor with my 2008 Fox 32 Series F100 RLC fork. The 2008 Fox fork uses a post style caliper mount. I have been able to determine that the XTR M975P caliper is intended to be used with post mount forks and 160mm rotors. I am trying to determine what modifications I need to undergo if I want to use a 180 mm front rotor. The exploded view of the caliper on the Shimano website shows using an IS caliper with a post mount adaptor in conjunction with a 180 mm (or 203 mm for that matter) front rotor.

    The part is #15 in the diagram. Is this really the correct way to do this? It seems to me that the post mount caliper should have an adaptor that offsets it 20 mm for use with the larger rotor as well.

    I have yet to purchase the calipers so if I need a front IS caliper along with the appropriate adaptor for use with my fork I can do that. It just seems strange that an IS style caliper would be intended for use on a post mount fork for this conversion. If this is true than the only way for a bike that has a post mount fork & front caliper to switch from a 160 mm rotor to a 180 mm rotor would be by buying a new IS caliper, the appropriate adaptor, and the 180 mm rotor.

    And I am assuming that the calipers do not come with any adaptors. If this is true, where do I get them? A google search came up with a lot of related things but no actual sellers of the adaptors. After a series of e-mails to some web retailers, it does not appear that even the sellers of these items know the correct way to perform this conversion.

    Thanks in advance

  2. #2
    -> <-
    Reputation: mtb_biker's Avatar
    Join Date
    Jan 2004
    part #15 is the wrong adapter if you have a post mount caliper.

    If you have a post mount caliper, you will need a post mount adapter to post mount.
    You need item #SM-MA-F180 P/P (160mm post mount to 180mm post mount)

    Also it won't need to space it out 20mm since the rotor is in a circle.

    Ebay / Bike shops can get you the part you neeed.

  3. #3
    Nightriding rules SuperModerator
    Reputation: crisillo's Avatar
    Join Date
    Jul 2004
    mtb_bike nailed it.... the SM-MA-F180 P/P is the right adapter...which needs to offset the caliper only 10mm (the difference in radius from the 160mm to the 180mm rotor)

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