bent rotor or duff disk?

shefbiker
shefbiker Posts: 255
edited December 2009 in MTB workshop & tech
Hi. My front disk is rubbing at one point each revolution, initially i supposed that the disk is slightly bent... the one thing that's confusing me is that I get a significant 'surge' in brake power each revoloution, even when braking fairly heavily. if it was just a slightly bent disk (i can't see any major bend b.t.w.) i would have thought that there'd be a light rubbing without the brakes applied, but under braking that the disk would just run through between the pads without any drama. The surging is like i'm applying the brakes harder (which i'm not) so suggests that the disk is actually slightly thicker, and hence giving the illusion that the brakes are applied harder as it squeezes between the pads.

Has anyone come across this before? it's an avid 180mm disk.

if it's bent, what's the best way to straighten it? Is it worth fixing something to the fork to show where the disk is bent? or maybe getting one of the park disc truing tools?

Any advice gratefully received,
Thanks

Comments

  • biff55
    biff55 Posts: 1,404
    can be done with adjustable spanner.
    rotate wheel by hand to locate rubbing point on rotor
    mark warped area with chalk / pencil
    make slight bends with spanner
    repeat until rubbing on pads has gone
    trail and error , tiny adjustments , and patience.
    hope that helps , it is a faff though
  • nicklouse
    nicklouse Posts: 50,675
    new brakes?

    old brakes?

    see the how toos in the Tech links sticky if an old disc. if new just use it it should sort itself.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • shefbiker
    shefbiker Posts: 255
    old juicy 5s, but with the larger rotor fitted within the last year with farily few miles on it.
  • *AJ*
    *AJ* Posts: 1,080
    Is the wheel true?

    The wheel not being true can stop the disc running true.
  • bails87
    bails87 Posts: 12,998
    The disc is attached to the hub. The condition of the rim shouldn't affect it. If the hub is damaged/not in straight then yes, possibly
    MTB/CX

    "As I said last time, it won't happen again."
  • shefbiker
    shefbiker Posts: 255
    the hub is an almost new mavic crossride. I don't think there's anything wrong with it.

    is my theory of the rotor being of non-uniform thickness not valid?
  • sniper68
    sniper68 Posts: 2,910
    shefbiker wrote:
    the hub is an almost new mavic crossride. I don't think there's anything wrong with it.

    is my theory of the rotor being of non-uniform thickness not valid?
    Possible I suppose,but more likely its a slight warp of the rotor.Take it off and lay it on a flat surface(kitchen worktop etc)and see if it lays flat.
    Lynskey R480

  • Soul Boy
    Soul Boy Posts: 359
    shefbiker wrote:
    the hub is an almost new mavic crossride. I don't think there's anything wrong with it.

    is my theory of the rotor being of non-uniform thickness not valid?

    Ah Mavic Crossrides. I have these and I had a problem with alignment on the front wheel (but, never the rear wheel!?!). I think because it has a bolt at the end, depending on where I clamped the wheel in my forks it would line up slightly differently. So the brake (Juicy 5's) would rub, I undo the quick release, rotate the hub around a little, reclamp....I would eventually find the sweet spot. This never happens on the Shimano hubs, I just put the wheel in and do up the QR, lines up every time.

    What I did was fit my Shimano hubbed wheel, lined up the caliper properly, and now when I fit my Mavic Crossrides, although I still have to fiddle to get it aligned properly, as least now I know when I do the whole set-up is stright and correct.

    I wouldn't have thought you have a disc of varied thickness.
  • Soul Boy
    Soul Boy Posts: 359
    Oh, and I slightly warped my rotor in the Alps in the summer. I bent it back with a pair of pliers, little by little. A bit of patience and perseverance. Of course this was easier once I had established when the alignment was true, see above.
  • *AJ*
    *AJ* Posts: 1,080
    bails87 wrote:
    The disc is attached to the hub. The condition of the rim shouldn't affect it. If the hub is damaged/not in straight then yes, possibly

    You'd be very suprised!... I had a buckled wheel come in to true and the disc was really warped... removed the disc and it sat perfectly flat on my work surface.

    Trued the wheel and put the original disc back on and no warp at all.....
  • The Rookie
    The Rookie Posts: 27,812
    Of course....still taking the tablets? Not physically possible that truing the wheel rim fixed the disc, but it may have been mismounted or stressed at the mounting bolts.

    Simon
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • *AJ*
    *AJ* Posts: 1,080
    Im just explaining what happened?...

    Ive worked in a bike shop as a mechanic for years, so I do know what im talking about.

    I doesn't make sense i realise that, but it's what happened.

    Of course im not denying that it could have been mounted incorrectly in the first place but nothing looked untoward as i dismantled it.

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