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  1. #1
    mtbr member
    Reputation: Wombat's Avatar
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    It seems that recent threads about non-IS

    disc hubs and forks and poor manufacturing tolerances causing rotor rub are mostly missing the point. The variations are unlikely to be in the manufactured items (especially if you are talking about Shimano, CK or the major fork companies) as they will have a very close eye on their tolerances.

    It is much more likely that discrepancies arise because the width of the axles, from nut to nut, varies from one bike to the other and this directly affects the distance between the rotor and the calipers.

    When you tighten the QR on every axle you will move each fork leg in slightly until they hit the axle nuts. Because the discs always sit inboard of the nuts the amount the "shorter" the nut-to-nut length is the closer the caliper will be to the rotor.

    It's quite easy to see this effect when you replace an axle and use different spacers. If you don't get the total distance spot on you will get rotor rub. Of course this also applies to the rear disc as the drop outs will move inwards just as the forks do.

    The beauty about this is that even if I am wrong, changing the nut-to-nut length of the axle will compensate for any hub or fork variations that causes the rotors to rub.

    Wombat

  2. #2
    Sublime Absurdity
    Reputation: sodade's Avatar
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    hmmm...
    specific to the issue of rotor placement, it seems to me that the drop out spacing is irrelavant. The nut of the axle sits flush against the dropout, so it seems to come down to the spacing between the nut and the rotor mount.

    Am I missing something?
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  3. #3
    Lay off the Levers
    Reputation: Bikezilla's Avatar
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    Quote Originally Posted by sodade
    hmmm...
    specific to the issue of rotor placement, it seems to me that the drop out spacing is irrelavant. The nut of the axle sits flush against the dropout, so it seems to come down to the spacing between the nut and the rotor mount.

    Am I missing something?
    Question: How much of a role does self-centering calipers play in this?

    It would seem to me that such a feture could offset a fair bit of variance in axle nut thicknes/placement.

    IMO the bigger issue in rub could be vertical boss alignment (flatness) if the boss on the fork itself is not absolutely flat, due to mfg tolerance or variance in paint thickness, the top part of the brake calipar can be further inboard/outboard than the bottom part. Then even with a centered rotor it's hard as heck to control brake rub.

    That seems to be the problem I'm having with my setup anyway. To get the rotor (closer to) dead-center in the caliper slot I use different shim combinations under the top and bottom caliper bolts. I still get a little rub. This is my second set of calipers as I had the first set replaced by Shimano for their famous XTR brake rub. I have seen, on other setups where the caliper/bolt can dig into the boss paint and some of it gets compressed and even removed. The thickness of the paint chips seem to be similar to the shims used.

    I'm guessing it's a vertical alignment issue b/c I never had rub problems on my unpainted rear triangle brake mounting. Too bad not everyone has access to a boss facing tool eh?
    Last edited by Bikezilla; 03-15-2004 at 07:44 AM.
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