Many noted experts have commented on this story
over the last few weeks. Here are their comments, in alphabetical
slight is intended on anyone who I might have inadvertently missed off!
is an experienced cycling campaigner and expert
witness/consultant in bicycle crash litigation:
"Your analysis of the forces imposed on the hub axle by the disk brake
is correct, and the alignment of the dropout slot with the direction of
this force guarantees failure if the quick release does not hold. With
a rim brake, weight transfer to the front wheel pushes the axle *into*
the dropout, and the deceleration force is partly taken up by the
forward angle and rake of the fork. (If the headset is loose, you will
feel it rattle at the level of braking at which the total force is in
line with the steering axis.)"
Jobst Brandt is best known as the author of
the classic 'The
'. He provided the following summary:
Disk brake wheel ejection
This subject was discussed at length on the technical newsgroup
"rec.bicycles.tech" where, as often occurs, users of the product who
have not experienced a failure defend the product in spite of clear
explanation why a front wheel disk brake with its caliper behind the
fork causes a large downward forces on the axle. Although not
denying that brake pads clamp onto an upward moving part of the disk,
that this causes an upward force on the caliper is denied. The
caliper, attached to the fork blade, receives lift from the upward
rotation of the disk with respect to the fork and tries to lift the
fork dropout, separating it from a quick release (QR) axle.
The magnitude of this force is the horizontal braking force by the tire
at the ground, multiplied by the ratio of wheel-to-disk diameter.
Therefore, with 1g retardation on dry pavement and a wheel-to-disk
diameter ratio of 27" wheel and a brake caliper at 6-3/4" inches, the
separating force on that end of the axle is four times the weight on
the front wheel. If that end separates, the wheel will lock askew
in the fork to cause an end-over crash.
This probability is denied by most participants in the discussion. Two
readily apparent solutions to the problem are to move the caliper ahead
of the fork blade or to use a motorcycle axle that is not a QR dropout
but rather a through bore in the fork end with a clamp.
wrote 'Effective Cycling', and is also a highly
experienced expert witness in bicycle litigation:
"I have not seen any of these front disk brakes. Apparently, there
are several with similar designs. All that I can say is gross
Any brake designer needs to know how the torque developed by the brake
is transmitted to the frame of the bicycle (or any other machine that
has brakes, for that matter). It is immediately obvious that with the
caliper pads at the rear of the brake disk the torque developed by the
brake is transmitted by a couple that consists of upward on the brake
frame to the fork, and downward through the axle, again to the front
fork. The near vertical
slots in the conventional front fork are to permit the front wheel to
removed by downward motion, and are quite secure against the normal
of the cyclist bearing down on them. But they are not designed to
significant downward pull. The problem is not with the axle fastening
but with the design of the brake. The brake pads should have been
so that the torque couple is transmitted, say, by forwards and
forces on the front fork, or even by an upward force on the axle
and a downward force resisted by some form of rigid fixing to the front
(In this context, I think it's fair to take 'brake designer' as
referring to those who place the brake mount on an unsuitable fork,
rather than those who build the callipers.)
This more general comment from one of the articles on his web page is
"Every advance in engineering brings with it new failure patterns
that must be understood before we can guard against them. I have
investigated quite a few accidents in which the cause stemmed from the
failure of new ideas
whose failure modes were not properly understood by the initial
Chris Juden is Technical Officer at the CTC
, the UK's main cyclists'
"I've been corresponding with James about this recently and although
his tandem fork is an oddity, I'm
convinced he's exposed a real problem here. That's backed up by all the
reports coming into this site of ejected disc-braked wheels and
loosened fasteners - some obviously rotated.
"It's not just scaremongering, but all hangs together and makes perfect
sense. In fact I'm kicking myself for not thinking of it before."
"changes must be made to the way disk brakes and front wheels
are attached to forks"
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