J
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
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
