Westie said:JLB wrote:
[snip]
Quoted message said:Quoted message said:This seems an odd statement if you actually accept the calculations
given on the web page. <snip> It also points out that the accepted
standard for QR systems leaves little or no margin with forces of this
magnitude.
Yes, the forces applied are enormous and excessive, but if I've read it
correctly these calculations talk about the force required to break or
tear the skewer from the dropouts.
The calculations describe the forces in various directions, and resolve
them to calculate the force acting directly in line with the dropout.
The page compares that force with the standard. The standard is a
minimum acceptable for a QR skewer. It says that in practice reasonably
(not necessarily perfectly) adjusted skewers easily exceed the minimum.
However, a skewer at the minimum leaves next to no margin.
In practice, it would seem that the
Quoted message said:QR skewers are NOT breaking or being torn from the dropouts. They are
coming open or loose first, and THEN the wheel is ejected. At least
that anectdotal account that you refer to indicates that the QR came
loose. Why is that? What is going on here to loosen the QR if that's
what happens?
Brakes go on, brakes go off; the wheel grips the ground, the wheel
skids, the wheel lifts off. The most difficult thing to model here is
what happens when forces of this magnitude rapidly build up and then
disappear. The forces are fluctuating wildly. What does this do to the
adjustment on the skewer? It's not clear. All we know is the skewer is
not made for that sort of battering; the forces act in an undesirable
direction; and riders are reporting their skewers coming loose or even
their wheels falling out.
Quoted message said:
Quoted message said:Finally, if it is not connected with disc brakes, why is it riders of
bikes with disc brakes that are reporting this problem?
Good point. Trouble is that much of it is anectdotal evidence that
isn't easily replicated under controlled conditions to eliminate bias.
Maybe it has been happening with rim brakes? Maybe it is incorrectly
adjusted QR locks? Many features on modern mountain bikes are
relatively new. There are larger numbers of people riding modern
mountainbikes than ever before. I'm just not convinced that this
problem is entirely unrelated to incorrectly adjusted equipment for the
most part. I'm not sure how big this problem is in real life.
I agree with you that the data is inconclusive. It is also obvious that
whatever the actual problem here, people do ride around on bikes with QR
skewers and disc brakes without disaster. In so far as there is a
problem, it is still unusual. But I would not want it to happen even
once when I'm riding. Among other things, it's one of those problems
that will only happen when you least want it, not when you're riding
nice and gentle along flat smooth ground.
[snip]
Quoted message said:Quoted message said:There is at least one carefully written post in this thread from
someone who was clearly very conscious of the potential for this
problem. It describes a skewer that was correctly adjusted at the top
of a hill being loose before the bottom of the hill. QR skewers are
made to standards that do not take adequate account of the forces
involved in common disc brake designs. So, its the combination of the
two things: QR skewers that are designed for, and perfectly adequate
with, rim brakes; and disc brakes that produce forces that
dramatically increase the possibility that the skewer fails to retain
the wheel.
So why is that? The calculations went to the trouble of demonstrating
forces that are capable of ripping a correctly adjusted QR from the
drop-out or breaking it. But that doesn't seem to be what is happening
in reality. WHY did the QR become loose in this example and NOT be
ripped out? Maybe it's not the fault of the disc brakes, but a fault of
QR in general that is just becoming widely acknowledged?
There is a substantial demonstrable difference in the forces from a disc
brake, as opposed to a rim brake, that make it credible as part of the
problem. My suggestion (I have no further evidence to back this up) to
explain the observations is above. The fact that the standard for
skewers is a minimum and the forces produced by the disc brake are
around that minimum does, however, explain why the skewers do not simply
break and are not simply instantly ripped out of the forks. The forces
are still a considerable way short of breaking a skewer that is in good
condition. Because a typical skewer can stand forces well in excess of
the minimum the disc brake cannot not rip out the wheel until the skewer
has been loosened to some extent. However, threaded fasteners loosening
under violent vibration is not unknown.
Maybe QRs have
Quoted message said:been coming loose for years? Now that brakes have been developed that
will eject those loose skewers it's time to examine options to secure
the skewers and prevent them from coming loose in the first place.
Regardless of the fault it does sound like it is time to take another
look at the design of systems used to hold the front wheel on.
Agreed.
--
Joe * If I cannot be free I'll be cheap
