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Learning: mechanical disc brakes

andUKandUK Posts: 85
edited September 2015 in Workshop
First contact with mechanical disc brakes on my new CX bike that I'm going to use as a winter road bike and on trails & bridle paths has proved interesting.
Anyway, knowing nothing of mechanical disc brakes I set them up as per a few tutorials and videos all saying the same thing.
Problem: front brake pads adjusted to nearly touching the disc, but way too much travel on the brake lever, ie right back to the drop bars.
Looking through the pads it was obvious to see that the caliper and thus the bosses on the forks were not parallel to the rotor disc - disc nearly touching the front of the inner pad and nearly touching the back of the outer pad.
Trying to modify the rotor disc by bending it to suit turned out to be a poor choice.
One new disc later, I filed the caliper itself where it contacts the bosses on the forks and now the pads sit nicely parallel to the rotor and the brake lever travel is set to how I like it and it all works fine.
For sale: one non-flat, warped, bent, misshapen, angrily smashed with a hammer rotor disc ... unused!

Posts

  • sungodsungod Posts: 12,677
    filed the caliper! nononono

    sounds like it wasn't mounted right in the first place, plus maybe the cable is too long (assuming you ran out of room on the barrel adjuster)

    the mounting has shaped washers allowing the caliper to be set correctly, it's fiddly the first time, but once you've done it once it's simpler

    ensure at the very start is that the front wheel is fully seated in the drops, lean over bars to apply some body weight on the front, undo qr, tighten qr

    loosen caliper bolts, pull brake lever, the caliper will centre to some extent but there may be some friction/binding in the mounting still, so while still holding brake lever use the other hand to wiggle the caliper to fully centre it, then still holding the lever tighten the bolts making sure not to push the caliper off-line

    test, repeat if necessary to get just right

    ones like bb5 are a bit fiddlier as only one pad moves, shimming the static side with something thin (credit card for instance) helps get it right when doing the above
    my bike - faster than god's and twice as shiny
  • andUKandUK Posts: 85
    the mounting has shaped washers allowing the caliper to be set correctly, it's fiddly the first time, but once you've done it once it's simpler

    Calipers are Tektro Mira 400: there appears to be no shaped washers to allow for any adjustment re: parallelness to the rotor disc, just side to side movement to centre the caliper; the body of the caliper bolts directly onto the bosses on the forks.

    I figured to get the caliper parallel the safest option was to file the back of the caliper rather than file the bosses or make thin tapered washers. I filed out maybe 0.5 to 0.75mm on the one edge tapering to 0.0mm on the other edge of the cast alloy body, nothing really to compromise the strength of the caliper and it worked.

    I take it from your above comment that there are calipers with adjustment in two planes as opposed to my Tektros which appear to have adjustment only in one plane ... unless I missed something on the Tektros?
    Which calipers are you referring to with an adjustable washer?
  • sungodsungod Posts: 12,677
    seems weird to have no angle adjustment on the mounting as the set-up tolerances are so tight, but i looked at the tektro instructions linked on the mira page and it seems that's how they are...

    http://www.tektro.com/_english/03_support/download.php?f=down01386730401.pdf

    you can use the same loosen-clamp-wiggle-tighten method to get them at least centred, but then it looks like they're relying on pad clearance to handle any remaining misalignment

    this picture shows the washers on avid calipers, you can see a pair each side, these allow the angle of the calipers to be set

    Figure_11-59.jpg
    my bike - faster than god's and twice as shiny
  • veronese68veronese68 Posts: 22,788 Lives Here
    Only Avid calipers use those mounting washers as far as I know. You shouldn't have to file the caliper, if anything get the mount faced, but I've never had to.
  • sungodsungod Posts: 12,677
    ah, i was wondering if it was just avid

    i had a look at a few bikes with discs yesterday, some had adjustment screws, some washers, some appeared to have no adjustment

    though i guess if there's a mounting bracket between fork/frame and caliper that gives two different sets of wiggle room to get things lined up ok, plus the pad adjustment on the caliper
    my bike - faster than god's and twice as shiny
  • andUKandUK Posts: 85
    Only Avid calipers use those mounting washers as far as I know. You shouldn't have to file the caliper, if anything get the mount faced, but I've never had to.

    Having mounting washers seems to make sense as this then allows for any discrepancies in the caliper body and the bosses on the forks/frame. The rear caliper on my bike was fine, as I imagine most are, it just so happened that the front one wasn't.

    As an ex-engineer I would say that filing can be quite an accurate process by doing it carefully and marking the surface with indelible ink and then filing it away and repeating. I don't think I'd attempt to file a Bottom Bracket housing that needed facing, but a cast alloy caliper body lends itself to being tweaked with a file.
  • veronese68veronese68 Posts: 22,788 Lives Here

    As an ex-engineer I would say that filing can be quite an accurate process by doing it carefully and marking the surface with indelible ink and then filing it away and repeating. I don't think I'd attempt to file a Bottom Bracket housing that needed facing, but a cast alloy caliper body lends itself to being tweaked with a file.
    Most people are not accurate enough. That's why it's better to get the mount faced properly with a tool like this, much harder to make a hash of it.
    DT-4-move2ndcut.jpg
  • fudgeyfudgey Posts: 859
    Bending the rotor to suit is genius, i havnt heard that one before!
    anyway, all i do to mount callipers is slacken the calliper mounting bolts, hold the brake on hard and nip up.
    release brake and spin wheel to see if all is ok, and if it is hold the brake on and tighten the bolts properly.
    if not slacken the bolts and try again.
    never had any issues doing it like that
    My winter bike is exactly the same as my summer bike,,, but dirty...
  • grenwgrenw Posts: 785
    Bending the rotor to suit is genius, i havnt heard that one before!
    anyway, all i do to mount callipers is slacken the calliper mounting bolts, hold the brake on hard and nip up.
    release brake and spin wheel to see if all is ok, and if it is hold the brake on and tighten the bolts properly.
    if not slacken the bolts and try again.
    never had any issues doing it like that

    Been doing that on my mtb hydraulics for ages. I'm assuming mechanicals are no different when it comes to this sort of thing?
  • andUKandUK Posts: 85
    Bending the rotor to suit is genius
    It's not the first time I've done "genius" things! ... and probably won't be the last!
  • fudgeyfudgey Posts: 859
    Live and learn eh.

    Still if its all sorted then good, but if you still have trouble try slackening the bolts and then tighten while holding the brake on.
    My winter bike is exactly the same as my summer bike,,, but dirty...
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