Yet more on disk brakes, quick releases and lawyer lips

Discussion in 'UK and Europe' started by James Annan, Mar 28, 2003.

Thread Status:
Not open for further replies.
  1. James Annan

    James Annan Guest

    Another (perhaps the last) piece of the puzzle has now fallen into place.

    To recap: I'm trying to understand the persistent reports of front wheel loss on bikes with
    disk brakes (and quick release wheel fasteners). We all 'know' that the QR is infallible, but
    nevertheless the failures occur even to experienced cyclists who definitely know how to use a
    QR properly.

    Adherents of the Church of the Infallible Quick Release have two main points of faith:

    (1) A properly fastened quick release cannot slip
    (2) A properly fastened quick release cannot unscrew

    Point (1) is based on the assumption that the QR is never subjected to any significant forces. I
    think it is now well established that this is false, where disc brakes are concerned. There is a big
    force pushing the wheel down in the dropout slot. Slippage is widely reported, most riders keep it
    somewhat under control by the use of a good skewer at a high tension, but nevertheless it moves
    (sometimes). A quick release simply cannot be expected to reliably resist the pull that is generated
    in normal use. I think that just about every technically-minded cyclist who has seen the arguments,
    agrees with this point now. I can demonstrate it at will with the rear wheel of my Ventana tandem.
    At 'normal' QR torque, the wheel slips. At very high torque, it hasn't slipped for some time, but I
    wouldn't bet my life on it not happening again. (With my light stoker, the max braking force here is
    comparable to a typical single-bike front wheel.)

    This still leaves the puzzle of how a skewer can overcome the retention lips. I agree with the
    sceptics who argue that a really tight skewer almost certainly cannot pull over a decent retention
    lip, the physical obstruction is too great. The skewer would almost certainly break first, and/or
    the force required would break or shear off the lip. The disk forces are big, but not that big.

    Nevertheless, wheels do come out.

    So let's return to the axioms of the Church of the Infallible Quick Release. We've already seen that
    the first one is false. What is the basis for the second one anyway?

    I'm not an expert on vibration loosening of bolted fasteners, so I found a source that seems fairly
    authentic: http://www.boltscience.com/pages/vibloose.htm

    begin quote
    =========

    It is widely believed that vibration causes bolt loosening. By far the most frequent cause of
    loosening is side sliding of the nut or bolt head relative to the joint, resulting in relative
    motion occurring in the threads. If this does not occur, then the bolts will not loosen, even if the
    joint is subjected to severe vibration.

    Pre-loaded bolts (or nuts) rotate loose, as soon as relative motion between the male and female
    threads takes place. This motion cancels the friction grip and originates an off torque which is
    proportional to the thread pitch and to the preload. The off torque rotates the screw loose, if the
    friction under the nut or bolt head bearing surface is overcome, by this torque.

    There are three common causes of the relative motion occurring in the threads:

    3. Bending of parts which results in forces being induced at the friction surface. If slip occurs,
    the head and threads will slip which can lead to loosening.

    4. Differential thermal effects caused as a result of either differences in temperature or
    differences in clamped materials.

    5. Applied forces on the joint can lead to shifting of the joint surfaces leading to bolt loosening.

    Work completed during the 1960's in Germany indicated that transversely applied alternating forces
    generate the most severe conditions for self loosening.

    ========
    end quote

    But we've just shown that the joint surfaces (ie dropout face and skewer nut) can and do move
    relative to each other! So it turns out that point
    (6) of the dogma is not an independent axiom at all, it is directly dependent on point (1),
    which is false!

    At this point, I consider the case proven beyond reasonable doubt. The design is such, contrary to
    popular belief, unscrewing of the QR skewer can occur in normal use, and then the wheel with loose
    QR will fall or be pushed out of the fork ends by the force of the disk brake. That doesn't mean it
    will always fail, but we should not be surprised when it does occasionally - and in particular, a
    failure does not imply that the user has done anything wrong.

    It may be worth pointing out for the benefit of any remaining sceptics, that many other bolts in the
    vicinity of the disk brake work loose on at least an occasional basis (ie the bolts holding the
    calliper onto the frame, and also the rotor to the hub). Mine certainly do, I've witnessed it on
    other bikes and use of loctite is widely recommended (I now sometimes use it myself). I have heard
    that Shimano use various mechanical preventative measures on some of their disk brake bolts too. A
    disk brake can generate a lot of very powerful vibration, directly in the hub area, and there's
    nothing magic about the quick release that automatically makes it immune from unscrewing. There are
    various reasons why it might in practice often be less vulnerable to unscrewing than a standard
    bolt, but that doesn't mean it won't happen at all.

    So does this mean that every skewer is automatically going to unscrew, and every front wheel will
    fall out? Obviously not. There are several reasons why a particular skewer might not come loose on
    any particular occasion, or even not at all. For instance, if there is substantial knurling on the
    faces of the bolt and head, and the fork dropout is soft enough for this knurling to dig in, then
    the disk brake might not apply sufficent force for the skewer to slip at all (in which case it
    cannot unscrew). If the bolt threads are dirty and it is stiff to turn, and/or the disk brake is
    smooth-running, that probably helps too. But I'm not going to ride one again until this faulty
    design is changed.

    James
     
    Tags:


  2. Pete White

    Pete White Guest

    "James Annan" <[email protected]> wrote in message news:[email protected]...
    > Another (perhaps the last) piece of the puzzle has now fallen into place.
    >
    > To recap: I'm trying to understand the persistent reports of front wheel loss on bikes with disk
    > brakes (and quick release wheel fasteners). We all 'know' that the QR is infallible, but
    > nevertheless the failures occur even to experienced cyclists who definitely know how to use a QR
    > properly.
    >
    > Adherents of the Church of the Infallible Quick Release have two main points of faith:
    >
    > (1) A properly fastened quick release cannot slip
    > (2) A properly fastened quick release cannot unscrew
    >
    > Point (1) is based on the assumption that the QR is never subjected to any significant forces. I
    > think it is now well established that this is false, where disc brakes are concerned. There is a
    > big force pushing the wheel down in the dropout slot. Slippage is widely reported, most riders
    > keep it somewhat under control by the use of a good skewer at a high tension, but nevertheless it
    > moves (sometimes). A quick release simply cannot be expected to reliably resist the pull that is
    > generated in normal use. I think that just about every technically-minded cyclist who has seen the
    > arguments, agrees with this point now. I can demonstrate it at will with the rear wheel of my
    > Ventana tandem. At 'normal' QR torque, the wheel slips. At very high torque, it hasn't slipped for
    > some time, but I wouldn't bet my life on it not happening again. (With my light stoker, the max
    > braking force here is comparable to a typical single-bike front wheel.)
    >
    > This still leaves the puzzle of how a skewer can overcome the retention lips. I agree with the
    > sceptics who argue that a really tight skewer almost certainly cannot pull over a decent retention
    > lip, the physical obstruction is too great. The skewer would almost certainly break first, and/or
    > the force required would break or shear off the lip. The disk forces are big, but not that big.
    >
    > Nevertheless, wheels do come out.
    >
    > So let's return to the axioms of the Church of the Infallible Quick Release. We've already seen
    > that the first one is false. What is the basis for the second one anyway?
    >
    > I'm not an expert on vibration loosening of bolted fasteners, so I found a source that seems
    > fairly authentic: http://www.boltscience.com/pages/vibloose.htm
    >
    > begin quote
    > =========
    >
    > It is widely believed that vibration causes bolt loosening. By far the most frequent cause of
    > loosening is side sliding of the nut or bolt head relative to the joint, resulting in relative
    > motion occurring in the threads. If this does not occur, then the bolts will not loosen, even if
    > the joint is subjected to severe vibration.
    >
    > Pre-loaded bolts (or nuts) rotate loose, as soon as relative motion between the male and female
    > threads takes place. This motion cancels the friction grip and originates an off torque which is
    > proportional to the thread pitch and to the preload. The off torque rotates the screw loose, if
    > the friction under the nut or bolt head bearing surface is overcome, by this torque.
    >
    > There are three common causes of the relative motion occurring in the threads:
    >
    > 1. Bending of parts which results in forces being induced at the friction surface. If slip occurs,
    > the head and threads will slip which can lead to loosening.
    >
    > 2. Differential thermal effects caused as a result of either differences in temperature or
    > differences in clamped materials.
    >
    > 3. Applied forces on the joint can lead to shifting of the joint surfaces leading to bolt
    > loosening.
    >
    > Work completed during the 1960's in Germany indicated that transversely applied alternating forces
    > generate the most severe conditions for self loosening.
    >
    > ========
    > end quote
    >
    > But we've just shown that the joint surfaces (ie dropout face and skewer nut) can and do move
    > relative to each other! So it turns out that point
    > (2) of the dogma is not an independent axiom at all, it is directly dependent on point (1), which
    > is false!
    >
    > At this point, I consider the case proven beyond reasonable doubt. The design is such, contrary to
    > popular belief, unscrewing of the QR skewer can occur in normal use, and then the wheel with loose
    > QR will fall or be pushed out of the fork ends by the force of the disk brake. That doesn't mean
    > it will always fail, but we should not be surprised when it does occasionally - and in particular,
    > a failure does not imply that the user has done anything wrong.
    >
    > It may be worth pointing out for the benefit of any remaining sceptics, that many other bolts in
    > the vicinity of the disk brake work loose on at least an occasional basis (ie the bolts holding
    > the calliper onto the frame, and also the rotor to the hub). Mine certainly do, I've witnessed it
    > on other bikes and use of loctite is widely recommended (I now sometimes use it myself). I have
    > heard that Shimano use various mechanical preventative measures on some of their disk brake bolts
    > too. A disk brake can generate a lot of very powerful vibration, directly in the hub area, and
    > there's nothing magic about the quick release that automatically makes it immune from unscrewing.
    > There are various reasons why it might in practice often be less vulnerable to unscrewing than a
    > standard bolt, but that doesn't mean it won't happen at all.
    >
    > So does this mean that every skewer is automatically going to unscrew, and every front wheel will
    > fall out? Obviously not. There are several reasons why a particular skewer might not come loose on
    > any particular occasion, or even not at all. For instance, if there is substantial knurling on the
    > faces of the bolt and head, and the fork dropout is soft enough for this knurling to dig in, then
    > the disk brake might not apply sufficent force for the skewer to slip at all (in which case it
    > cannot unscrew). If the bolt threads are dirty and it is stiff to turn, and/or the disk brake is
    > smooth-running, that probably helps too. But I'm not going to ride one again until this faulty
    > design is changed.
    >
    > James
    >

    I have a new trek 4900disc (which is why I've been a bit quiet in these 'ere parts for the last two
    weeks!) and I have noticed both these two phenomena. The bolts holding the caliper onto its mount
    holes on the rear seat-stay are coming loose rather quickly (the brake rattles under light braking)
    and the rear QR skewer is rather looser after a 20 mile bike ride than it was on my pre-ride
    check... I don't feel that either of these two thingies are a problem to a vigilant cyclist, but it
    goes with out saying, it is a _bit_ worrying ...

    Pete White
     
  3. Mike Spence

    Mike Spence Guest

    "James Annan" <[email protected]> wrote in message news:[email protected]...
    > Another (perhaps the last) piece of the puzzle has now fallen into place.
    >
    > To recap: I'm trying to understand the persistent reports of front wheel loss on bikes with disk
    > brakes (and quick release wheel fasteners).

    Mounting the caliper in front of the fork would make the forces hold the wheel in place. It's a bit
    late to change the IS mounts now though.

    Mike
     
  4. James Annan

    James Annan Guest

    "Pete White" <[email protected]> wrote in message
    news:<[email protected]>...

    > I have a new trek 4900disc (which is why I've been a bit quiet in these 'ere parts for the last
    > two weeks!) and I have noticed both these two phenomena. The bolts holding the caliper onto its
    > mount holes on the rear seat-stay are coming loose rather quickly (the brake rattles under light
    > braking) and the rear QR skewer is rather looser after a 20 mile bike ride than it was on my
    > pre-ride check... I don't feel that either of these two thingies are a problem to a vigilant
    > cyclist, but it goes with out saying, it is a _bit_ worrying ...
    ^^^^^^^^

    I think you mis-spelt 'lucky'. I hope you are right, but judging from the number of anecdotes
    confirming the theory, it may require more luck than you think. I'm amazed by the number of people
    recounting similar stories (on the forum at singletrackworld.com).

    James
     
  5. "James Annan" <[email protected]> wrote in message news:[email protected]...
    > Another (perhaps the last) piece of the puzzle has now fallen into place.
    >
    > To recap: I'm trying to understand the persistent reports of front wheel loss on bikes with disk
    > brakes (and quick release wheel fasteners). We all 'know' that the QR is infallible, but
    > nevertheless the failures occur even to experienced cyclists who definitely know how to use a QR
    > properly.
    Big snip
    > James
    >
    James I've been reading this with interest and did the sums on the forces today, which agree with
    your calculations. It's clear that the low number of catastrophic failures suggest there are other
    variables at play however a set of disc brakes just fell off my wishlist. I don't fancy splashing
    out on new forks as well. I would expect that to thoroughly quantatively understand this would need
    a test programme using rig tests,data capture equipment and collecting information about how
    individual owner behaviour affects the effectiveness of the bolted joints. By the way, to help get
    my head around this, I worked out that the wheel is essentially acting as a cranked lever - at least
    in my head it does. A good one to fix. Graham header is munged. replace nospam with bbowers36
     
  6. James Annan

    James Annan Guest

    "Graham Bowers" <[email protected]> wrote in message
    news:<[email protected]>...

    > I've been reading this with interest and did the sums on the forces today, which agree with your
    > calculations. It's clear that the low number of catastrophic failures suggest there are other
    > variables at play

    Yes I agree, there will surely be some combinations of fork/skewer/disk that work better than
    others. None of them are actually designed to be safe, however, and it's not immediately obvious to
    me how to distinguish a priori the safer from the less safe, given that they all share the same
    basic design flaw. A proper design would seem a better alternative than trusting to luck!

    James
     
Loading...
Thread Status:
Not open for further replies.
Loading...