Rear Post mount adapter query

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  • Rear Post mount adapter query
  • Premier Icon jimfrandisco
    Subscriber

    I have a Ragley Big Wig that is post mount on the rear – i want to run a 180mm disc rather than 160mm.
    Most 160mm to 180mm adapters seems to be front, and struggling to find anything rear specific.
    I have heard that if you’re going PM to PM on the back you need an adaptor one size bigger so if going 160mm to 180mm i actually need a 160mm to 203mm PM adapter.
    Anyone know if there’s any truth in that?

    cheers

    Premier Icon mikewsmith
    Subscriber

    the mount sizes are 20mm increments (generally but there are exceptions)
    If the frame is PM and runs 160mm with no extra hardware then to go 20mm up you will (probably) need the +20mm 160-180. If you already have something there then you will need a +40 (160-200).

    Nothing looks comparable on the websites, depending where you are the quickest and simplest solution is to ride the bike into a shop and go got an adaptor for this mate 🙂

    Premier Icon Tracey
    Subscriber

    Had the same problem with Abigales new Enduro build. Fitted 203 mm rotors front and rear. Treated it as if it was a front and just fitted the equivalent front adapter. Works a treat.

    Premier Icon jimfrandisco
    Subscriber

    Thanks – in theory its simple, but the fact that a 160-180 doesn’t fit and with what you’ve both said about makes me think it’s luck of the draw and depends on the brake and frame combo?
    Unfortunately its not the kind of thing my local shop stocks so its a bit trial and error at the moment.
    cheers

    STATO
    Member

    IS Mounts were different front and rear, so bolting the same IStoPost mount on the front resulted in a 20mm bigger rotor compared to the rear.

    Post mount frames/forks ‘should’ be the same, as a post mount 160 is a post mount 160 irrespective of if its front or rear. Of course frame/fork designers complicated this by then designing dedicated 140mm, 180 and 203mm post mounts.

    So..
    To fit a bigger rotor you need to identify the frame mount and buy the appropriate +xx adaptor (no front/rear specific versions exist?), of course some frame designers will not consider you might want to use adaptors or only consider one brand, so you might find some wont fit very well or at all.

    it’s luck of the draw and depends on the brake and frame combo?

    yup!

    Premier Icon jimfrandisco
    Subscriber

    Thanks Stato – trial and error it is then!
    That, and asking what other owners are running
    cheers.

    crofts2007
    Member

    Hope H adaptor.
    Using two on my turner, PM rear, 160 standard,. now running 180 hope disks with tech m4’s.
    Same on rockshox forks on the front.

    http://www.sjscycles.co.uk/hope-disc-brake-mount-h-postmount-to-postmount-160-mm-(f-183-mm)-prod27511/

    STATO
    Member

    Had a quick look at the frame on google and cant see why a shimano adaptor wouldnt fit (make sure you use the right bolts mind!) but that hope one might interfere, apart from the fact its also 183mm not 180mm.

    Premier Icon Tracey
    Subscriber

    Shimano XT Trail brakes with 203mm rotors. Same post to post mount front and rear

    Premier Icon jimfrandisco
    Subscriber

    Thanks all for the advice.
    Seems to defy logic, have Tried a shimano 160 to 180, but no joy.
    Will have to double check i’m not doing something stupid.
    Cheers

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