Throbbing front brake unsafe?

Discussion in 'Cycling Equipment' started by Jeff Potter, Sep 5, 2003.

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  1. Jeff Potter

    Jeff Potter Guest

    For the past couple years my frontbrake on roadbike hasn't braked smoothly. It's kinda smooth but
    has a throb to it. I'm guessing that there are bumps on my rim. It kind of chatters sometime but not
    with much noise. If I look down I'll see the fork ends vibrating. When I went to the mountains this
    summer I had to do lots of braking. When I ride around here in the wide open flats I don't brake
    much. But this summer I did and it made me wonder if something bad was going to happen. I do keep
    the brake bolt tight---if it gets loose this throb of course gets much worse.

    --

    Jeff Potter
    ****
    *Out Your Backdoor * http://www.outyourbackdoor.com for modern folkways and culture revival...
    ...offering "small world" views on bikes, bows, books, movies... ...new books featuring: XC ski
    culture, a Gulf Coast thriller folding bicycles ... with radical novels coming up! ...original
    downloadable music ... and articles galore! plus national "Off the Beaten Path" travel forums!
    HOLY SMOKES!
     
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  2. Jim Beam

    Jim Beam Guest

    yes, very very unsafe. you need to fix it asap, and you also need to consider entirely replacing the
    front brake caliper.

    http://technology.open.ac.uk/materials/mem/mem-ccf2.html

    this is exactly the scenario you are describing. if this has been going on for some time, you could
    have initiated a crack in the mounting bolt that is just waiting for a dangerous place to fail and
    severely injure you.

    ensure you use a wheel whose rim has a machined braking surface to eliminate this problem. and when
    you do, you'll still need to replace the caliper! fatigue cracks don't go away on their own.

    jb
     
  3. Peter Cole

    Peter Cole Guest

    "jim beam" <[email protected]> wrote in message news:[email protected]...
    > yes, very very unsafe. you need to fix it asap, and you also need to consider entirely replacing
    > the front brake caliper.
    >
    > http://technology.open.ac.uk/materials/mem/mem-ccf2.html
    >
    > this is exactly the scenario you are describing. if this has been going on for some time, you
    > could have initiated a crack in the mounting bolt that is just waiting for a dangerous place to
    > fail and severely injure you.
    >
    > ensure you use a wheel whose rim has a machined braking surface to eliminate this problem. and
    > when you do, you'll still need to replace the caliper! fatigue cracks don't go away on their own.

    If machining solves the problem of uneven rim surfaces, then it does so at the expense of creating a
    non-uniform wall thickness. In this case, it's difficult to know when to retire a rim. A cracking
    rim can have as bad consequences as a cracking brake/brake bolt.
     
  4. Art Harris

    Art Harris Guest

    Jeff Potter wrote:

    > For the past couple years my frontbrake on roadbike hasn't braked smoothly. It's kinda smooth but
    > has a throb to it. I'm guessing that there are bumps on my rim.

    Using the brake pads as a guide, spin the wheel and see if the rim moves closer to the pad at some
    point. The wheel may be out of true, or the rim may have bottomed out on a pothole and spread out at
    the point of impact. Also, make sure the rim is free of gunk.

    See:

    http://www.sheldonbrown.com/creaks.html#wheel

    Art Harris
     
  5. Jim Beam

    Jim Beam Guest

    > non-uniform wall thickness

    that's not logical. if you have an unmachined rim with wall thickness "X" and thickness deviation of
    "Y", machining off a fraction subtracts from the X and half the Y because you're machining just one
    of the two faces, the inner and the outer side of the braking surface. so machining makes
    dimensional accuracy better, not worse.

    and you don't end up with a "thinner" rim because it's extruded with machining in mind and is made
    slightly thicker in the places that are going to be machined.

    jb
     
  6. In article <[email protected]>, Jeff Potter
    <[email protected]> wrote:
    >For the past couple years my frontbrake on roadbike hasn't braked smoothly. It's kinda smooth but
    >has a throb to it. I'm guessing that there are bumps on my rim. It kind of chatters sometime but
    >not with much noise. If I look down I'll see the fork ends vibrating. When I went to the mountains
    >this summer I had to do lots of braking. When I ride around here in the wide open flats I don't
    >brake much. But this summer I did and it made me wonder if something bad was going to happen. I do
    >keep the brake bolt tight---if it gets loose this throb of course gets much worse.

    Assuming the caliper pivot is properly adjusted, you should be able to locate the portions of the
    rim's braking surface that are causing the brake pad to grab and sand them with super fine
    sandpaper. You can also reduce the grabbing effect a little by using a harder brake pad, assuming
    you are happy with the braking performance of a harder pad.
     
  7. Dianne_1234

    Dianne_1234 Guest

    jim beam <[email protected]> wrote in message news:<[email protected]>...
    > > non-uniform wall thickness
    >
    > that's not logical. if you have an unmachined rim with wall thickness "X" and thickness deviation
    > of "Y", machining off a fraction subtracts from the X and half the Y because you're machining just
    > one of the two faces, the inner and the outer side of the braking surface. so machining makes
    > dimensional accuracy better, not worse.
    >
    > and you don't end up with a "thinner" rim because it's extruded with machining in mind and is made
    > slightly thicker in the places that are going to be machined.
    >
    > jb

    However, illogical as it may be, the machined rims I've measured have greater variation in their
    wall thickness than unmachined rims. My opinion is that the rims aren't always fixtured perfectly.
     
  8. Jeff Potter

    Jeff Potter Guest

    Thanks for the tips everyone. --The rim sanding, esp. I will try it and be judicious.

    Should I be able to see hairline cracks? Do cracks have to be visible to be dangerous?

    Oops, now I see you mean a crack in the bolt, not in the fork crown hole that the bolt goes
    thru. Hmmm...

    --

    Jeff Potter
    ****
    *Out Your Backdoor * http://www.outyourbackdoor.com for modern folkways and culture revival...
    ...offering "small world" views on bikes, bows, books, movies... ...new books featuring: XC ski
    culture, a Gulf Coast thriller folding bicycles ... with radical novels coming up! ...original
    downloadable music ... and articles galore! plus national "Off the Beaten Path" travel forums!
    HOLY SMOKES!
     
  9. Paul Kopit

    Paul Kopit Guest

    On Fri, 05 Sep 2003 21:34:13 -0400, Jeff Potter <[email protected]> wrote:

    >Thanks for the tips everyone. --The rim sanding, esp. I will try it and be judicious.
    >
    >Should I be able to see hairline cracks? Do cracks have to be visible to be dangerous?
    >
    >Oops, now I see you mean a crack in the bolt, not in the fork crown hole that the bolt goes
    >thru. Hmmm...

    Maybe just borrow another front wheel for a short ride and see if it is the rim or the brakes.
     
  10. Rick Onanian

    Rick Onanian Guest

    On Fri, 05 Sep 2003 18:29:37 GMT, jim beam <[email protected]> wrote:
    >> non-uniform wall thickness
    >
    > that's not logical. if you have an unmachined rim with wall thickness "X" and thickness deviation
    > of "Y", machining off a fraction subtracts from the X and half the Y because you're machining just
    > one of the two faces, the inner and the outer side of the braking surface. so machining makes
    > dimensional accuracy better, not worse.

    You, of course, are assuming that the rim is perfectly straight. If the outside and inside faces
    both wobble in the same direction, then you remove material from one surface, then where that
    surface stuck out, you now have a thinner wall.

    Of course, if your rim does that, you've got major problems anyway...at least, if it does it a lot.

    > and you don't end up with a "thinner" rim because it's extruded with machining in mind and is made
    > slightly thicker in the places that are going to be machined.

    I thought the suggestion was to machine an existing, non-machined rim?

    > jb
    --
    Rick Onanian
     
  11. Jim Beam

    Jim Beam Guest

    > However, illogical as it may be, the machined rims I've measured have greater variation in their
    > wall thickness than unmachined rims. My opinion is that the rims aren't always fixtured perfectly.

    yes, your milage my vary. all i can say is that measuring two of the unbuilt rims, cxp14
    [unmachined] and ma3 [machined] i have in the bike closet right now, i get the following, in mm:

    ma3 cxp14
    21.10 19.88
    22.10 19.92
    23.10 19.90
    24.10* 19.66*

    * at the rim joint.

    this is measuring at 90 degree intervals. this is /not/ a wall thickness measurement because i don't
    have a suitably anvilled micrometer to measure under the hook right now, but i would expect wall
    thickness to be more consistent on the ma3 based on the above.
     
  12. Peter Cole

    Peter Cole Guest

    "jim beam" <[email protected]> wrote in message news:[email protected]...
    > > non-uniform wall thickness
    >
    > that's not logical. if you have an unmachined rim with wall thickness "X" and thickness deviation
    > of "Y", machining off a fraction subtracts from the X and half the Y because you're machining just
    > one of the two faces, the inner and the outer side of the braking surface. so machining makes
    > dimensional accuracy better, not worse.
    >
    > and you don't end up with a "thinner" rim because it's extruded with machining in mind and is made
    > slightly thicker in the places that are going to be machined.

    Depends on how the rim is machined. There are obvious (and likely) ways it can be done to yield
    non-uniform wall thickness (thin spots).

    The rim I just retired, a Mavic Reflex "SUP", "UB-control", had about 1 mm of brake surface wear.
    There were clear signs of cracking (circumferential) at the joint. This was the only place cracking
    was evident. There are also clear signs of grinding on the inside surface and some degree of
    misalignment of the joint. I don't think the cracking at that exact spot is a coincidence. Any
    misalignment that's "squared up" with a post-weld machining is likely to leave a thin spot. After
    all, it's a subtractive process, isn't it? If the joint were not welded and machined, any
    misalignment would stand out, and the buyer could reject the rim. This process allows sloppy
    fabrication with a cosmetic dressing up to hide it. What you see is not what you get. Despite my
    displeasure with this, I had to fork out another $60 for a nearly identical new rim. Try to find any
    not made this way any more.
     
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