Re: Missy Giove's QR pops open (long)
In
news:c96ea403.0305180122.3029c7e8@posting.google.com, James Annan
<still_the_same_me@hotmail.com> typed:
>
> Ok then, have a go at challenging it. But let's stick to the scientific method, and avoid simply
> saying "I don't believe it". _Why_ don't you believe it? There are essentially two aspects to the
> failure, the slipping and the unscrewing. Both are predicted by elementary theory, both have been
> repeatedly observed. I'm at a loss to see what more you can need.
OK. There are three element not two, the proposition that the disk brake will eject an unconstrained
wheel from the drop out, the proposition that the QR is insufficient to constrain the wheel against
those forces and the proposition that a mechanism exists to loosen the QR sufficient for it to pass
the other constraining mechanism, the lawyers lips.
The first proposition is reasonable and as can be easily demonstrated by taking the QR out an
unconstrained wheel will eject in many designs of drop out. This is undesirable because people
sometimes forget to do up the QR. It could be easily fixed by having for example a forward facing
drop out or as on my old forks a drop out direction and disk mount that put the braking force
essentially at right angles to the exit path.
The second proposition may be true for some designs of QR's and is easily checked by static load
tests - the University of Kansas study gives some data in that respect. For Shimano QR's there is
evidence that used normally they will easily resist the same or greater loads than you calculate in
the disk brake scenario. An example is the rear wheel on a fixed. With horizontal dropouts the chain
is trying to pull the axle along the drop out. The force on the chain when standing on the RH pedal
on the down stroke is the weight of the rider times the ratio of the crank arm to chain ring radius.
Typically this is 2.3 times bodyweight, maybe more if pulling up on the bars, and repeated regularly
on each of many pedal strokes when climbing. In my case this force is approx 2700N and is held
without trouble by a Shimano QR done up normally and considerably more than the figures you are
estimating for wheel ejection. I cannot speak for other QRs but until some have been tested under
normal usage with static loading its not possible to say whether a slipping wheel in a dropout is a
generic QR problem or one of specific QRs.
The third proposition is that the QR can loosen enough to get past the lawyers lips. You have
proposed a mechanism by which that may happen. What remains to be shown is that that or another as
yet unidentified mechanism is actually happening in real life with QRs. It has to be a disk brake
specific mechanism otherwise QRs on the many road bikes would be mysteriously loosening on the road
and I have not heard of that as a recognised problem since Mr Campag introduced them to cycling.
Anecdotal evidence is not sufficient. Lawyers lips were introduced because people were not doing up
their QRs properly and having wheels fall out long before disk brakes came on the scene. I have
forgotten to do up my QR on occasions and noticed pretty quickly because the handling was "loose".
"My QR was undone/loose and I am certain I did it up before I set out" anecdotes are not proof of
your proposed mechanism happening. It is proof that people, as they were in the days before disk
brakes and lawyers lips, are human and that there are other reasons that Brant Richards refers to
that can cause QRs to undo. I see for example a surprising number of QR's with the lever pointing
forward for a wayward stick or rock can catch and undo it. The two tests I would suggest that need
doing are to see if you can simulate a done up QR loosening in the lab by the mechanism you propose.
The other easy to do one would be to take a mass event such as one of the 24 hour races. Have
testers checking the QR's on every bike setting out to ensure they are done up properly and check
them again when they come back in, noting any crashes/punctures or loose wheels that the rider had
while out. You should pretty quickly get some statistics on whether QRs confirmed as done up are
loosening in normal use in the absence of other interventions. Incidentally try riding cautiously a
bike with the QR undone enough to exit the lawyers lips. Its pretty noticeable.
If proposition one and two are happening a study of accident comparing Pace forks with all other
makes should show a significantly higher rate of "failures" with the Pace forks since they have no
lawyers lips. This means they only need propositions one and two to be happen for an ejection which
is considerably more likely than all three propositions being true which is a necessary condition
for all other forks. Also Pace forks are generally ridden by enthusiasts who will ride more
frequently, aggressively and brake harder on average than the users of most other makes. I've not
heard of Pace forks having a reputation for "failing" which wheel ejection on the carbon legs would
almost certainly cause but maybe Carlton can comment from an industry perspective. For the rest of
the forks proposition 3 has to occur as well
Overall I think it would be sensible to rethink the drop out design because people do forget to do
up their QRs quite frequently. The more normal case you are proposing of propositions 2 and 3 are,
at this stage, propositions remaining to be proven. There are some relatively simple tests that can
be done - static holding tests on QR's, investigation of fork failure statistics between Pace and
other makes, controlled tests at a mass event and lab demonstration of a done up QR loosening as you
suggest. Until then we should treat it as an interesting theory that deserves further investigation
but not proven
Tony
--
http://www.raven-family.com
"All truth goes through three steps: First, it is ridiculed. Second, it is violently opposed.
Finally, it is accepted as self-evident." Arthur Schopenhauer
