[email protected] (Jon Isaacs) wrote in message
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[email protected]>...
> Again, your QR only slipped, it did not fail.
Its job was to keep the wheel in place, and it failed to do so. Of course it was not designed to
do that job.
> But failure of the QR is a whole different issue. You show a calculation in your page that the
> force on the dropout could be as high as 1875 newtons which is about 420 lbs force.
No, I don't think that 'as high as' is fair. It's a ballpark figure that could surely be momentarily
exceeded, for example by a juddering brake. Wouldn't a stick-slip scenario imply that the peak force
could be roughly double the average force?
(Actually, we were riding a tandem, so the force in our case might well have been much higher again.
However I think the calculations I have done for a single bike are still worrying enough.)
> A rough calculation shows the force required to shear a standard 5 mm diameter QR would be much
> higher than that, something on the order of 13000 newtons, a factor of 7.
The QR is already in tension to the tune of perhaps half that level (I've seen a value of 5600N
quoted). The force due to the disk might be 3600N (I'm assuming that my 'doubling' above is
plausible - please correct if I'm out of line here). Of course, the disk force isn't a direct
tension, it's applied perpendicular to the skewer and will bend it putting the stress
disproportionately on the edge. Consider a beam, maybe 3cm long (distance from disc to the QR nut,
at which end the beam is supported) and only 5mm diameter with a perpendicular force applied to the
free end. Or maybe, the left hand end of the hub is pushed down in the dropout, and then the stress
is focussed on the lower edge of the skewer, at the point where it exits the hub. Now what is the
safety margin for the QR, which is already notched with threads?
> And of course the shear load should not be taken by the QR but rather by axle.
By 'should', do you mean that you think it actually is, or that you think the fork end should be
designed to ensure this? If the latter, then I agree, but it's not the case with current designs.
The axle is not supported by the fork end, the force is (near enough) aligned with the slot.
> So, in my view, it is unlikely that in your case the QR would have sheared or other wise failed
> and furthermore it is important for you to clearly understand and ackknowledge this.
Oh, I'm happy to agree that it's 'unlikely', if QRs can routinely withstand the forces described
above. I'm not sure what value of 'unlikely' you are using, and I'm not convinced that it will be
adequate for such a critical component where a failure is so dangerous.
> Again, without knowing what actually happened, it is impossible to say whether it was a QR failure
> or something else.
In that case, can you think of any way that someone who suffers a QR failure (break) could convince
you of that fact? It really seems like you are the one with a closed mind here. I don't see why it
is so hard to believe, and it is certainly the most plausible explanation I can think of for this
crash. It's not as if skewer breakage is particularly rare even, but it tends to happen when a wheel
is being installed rather than ridden.
> I think you are the one who presumes that the disk brake-QR issue is the cause when indeed there
> may be some other cause.
Well all the 4 recent wheel separation failures I know of are on disk brake + QR bikes, all of them
occurred on steep descents, and in my case it certainly seems very clear that it wouldn't have
happened without the disk brake. Even if lips were present, the skewer is still not designed to
withstand the force although perhaps it would 'usually' be ok in practice.
> What you have clearly proved is that a bike equipped with a front disk brake better have
> retention tabs.
>
> I think this is a significant point that should not be overlooked because of the number of people
> that routinely file these off so that it is easier to take the wheel on and off.
Yes, I think this is certainly a conclusion that we can all agree on. Furthermore, I hope you will
agree that the retention tabs had better be designed to handle the large forces applied by a disk
brake, rather than simply be high enough to stop a loose QR from falling out due to gravity. Of
course, the same goes for the QR skewer.
[It seems to me that changing the dropout angle would be a more sensible alternative, since it
removes the problem at source.]
> If you want to establish that QRs can fail because of the loads applied by a disk brake mounted on
> the rear side of the fork, then IMHO, you need to attack this with an open mind and work on it.
>
> Your apparent belief that the QR would have failed had the QR not slipped indicates to me that you
> do not have an open mind.
It's not a 'belief that the QR would have failed'. I agree that quite possibly it would not have
done so. I'm just pointing out that lips or not, it was not designed for this application, and that
is as true on any MTB with standard forks as it is on my badly designed custom fork.
> Think about it. These are not just words I am writing or flames.
>
> This is scientific/technical issue that you have raised and it is an interesting one.
>
> But to resolve this issue you will have to resolve it in a scientific/technical manner and that
> means forgetting about your unfortunate experience and actually understanding the issue
> objectively.
I am trying to understand it objectively. I agree that is it difficult to see how a properly
adjusted QR can pull over a substantial retention lip. It seems to imply the QR stretching by about
1mm = 1%, even for a 1mm lip. That implies a tension of 40kN (I think - 2kN per sq mm, for 20sq mm
x-sectional area) which is 3 times the force you quote to break it. Nevertheless, wheels come out,
even apparently for experienced cyclists who know how to use a QR. I do think it's important to have
a good look at the aftermath of these crashes, but I would rather stop them from happening in the
first place. It is certainly clear that quick release skewers are not designed for this application,
even if they 'usually' cope.
James