The relevant forces and dimensions on the wheel
indicated. We have the braking force B and the ground reaction force R
at the contact patch. The disk calliper exerts a force D as indicated,
tangential to the disc at the point of calliper contact. The radii
of the disk and wheel are r1 and r2, and finally the angle of
the dropout exit is 'a' in front of vertical. For this fork and
calliper, the force D is virtually vertical. If it wasn't, there
would be another angle 'b' for the angle the disk force makes behind
Some typical fork end geometries
|Marzocchi Dirt Jumper 1 - the
disk force is even slightly forwards of true vertical here! The fork
opening would be lined up with the `quick release' lever on the
qr version of the fork (this is the 'QR20' with a metal cap bolted over
the fork end).
|Stratos FR4(T), with the disk
force at about 10 degrees behind vertical, perhaps 30 degrees from the
stanchion angle. Again, I've got the bolt-through version, but in most
QR forks, the fork end opens in line with the stanchion or perhaps
fractionally forward of it.
|Both of these
forks are actually 20mm bolt-through axle versions, but the geometry of
stanchion and disc mount is typical of quick release forks too. The
brakes are Hope 4 piston models (2 opposing pairs of pistons, the
position of which is clear). I believe that most disk brakes would
forces in much the same direction, although perhaps a 165mm disk would
exert a force even more closely aligned with the stanchion than my
185mm disks do. The smaller disc generates a larger force too.
Possibly the DJ is slightly worse than most other forks, however
changing this angle by 10 or even 20 degrees won't affect the result
enough to turn a poor design into a good one.
1825N pull - is that a lot?
The most recent ISO standard (1996) specifies that the quick release
should be able to withstand a direct pull of 2300N, symmetrically
applied, without slipping. That sounds like a big number, but then you
realise that this figure only works out at 1150N per dropout. For that
the standard specifies a symmetric pull and no requirements are set
for withstanding an asymmetric force. Compare that to the ballpark
figure of 1825N on the left dropout alone, and you'll understand
why I am worried. The limited testing I've seen (which is available on
the web as this pdf
) found that all the skewers tested do meet the ISO standard
quite comfortably, but only when carefully installed with the correct
torque (various 'rules of thumb' were found to be unreliable). It's
obvious to me that the ISO specification was designed with conventional
rim brakes in mind. Prior to 1996 the ISO standard was only about 500N
force, which is still far more than a rim braked-bike will see in
It is very easy to find anecdotal evidence that the wheel frequently
slips under hard braking, as predicted by the analysis. There are one
or two examples of such stories here.
Part 2. The self-extracting quick release
Up to now, all I've demonstrated is that the skewer can slip in the
dropouts, even if correctly used. Although clearly undesirable, this
isn't obviously all that dangerous at a first glance. Almost all forks
have significant retention lips at the fork tips and it's hard to see
how a tight skewer could pull over these
even under the force of a disk brake. For starters, it implies the
skewer stretching by about 2mm, at which point it would probably snap
or show obvious signs of damage. The massive force would either shear
or smear off the lips, and although the disk brake generates large
forces, they aren't this big.
And we all know that the quick release cannot unscrew, don't we? That
belief is every bit as fundamental as the belief that the skewer cannot
slip. It now seems clear to me that it is every bit as false, too. From
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
1. 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.
2. Differential thermal effects caused as a result of either
differences in temperature or differences in clamped materials.
3. 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
But condition 3 is precisely what we have just shown to occur with
quick release skewers! The belief that a correctly fastened skewer
cannot unscrew, is based on the premise that it cannot slip, and when
the latter fails to be true, so too does the former. It's not clear how
technically precise it is to label this
as 'vibration loosening', since it may be forced more by intermittent
slip rather than regular vibration. However, disk brakes do generate
a lot of vibration, especially under the hard sustained braking of
steep fast descents when these failures tend to occur. In any case, the
important point is clear: if the skewer slips, it cannot be regarded
as a secure fastener, and will tend to unscrew.
This should not be particularly surprising. After all, many other bolts
connected to the disk brake system (ie holding the calliper to the
frame, and the rotor to the hub) work loose
regularly. It happens on all of our disk-brake equipped bikes, I've
seen it happen on others, and loctite is widely recommended as a
The disk brake can generate a huge amount of vibration and it's all
focussed directly in the hub area. There's no reason why the quick
skewer should be immune from this, it's just a bolted fastener.
Again, stories of quick release skewers loosening in use are common.
Some of them could theoretically be due to 'operator error', but I find
most of them very convincing. Read for yourself
and make up your own mind.
tests show that once the QR is loose, dropout failure may result even
if the QR is not actually loose enough to be forced over the lawyer
lips. Some of the failures I have heard about definitely involved the
rear of the left dropout being sheared off, but in other cases this did
not happen. A tight QR apparently shares the load better across the
dropout so that failure is less likely (but they only performed a
single test of this).
Some experimental and anecdotal
Since first presenting this theory at the end of March, I've found a
large number of anecdotes that fit the description very well. Often for
any given story it is difficult to prove beyond any possible doubt that
there was not some user error involved, as although the cyclist is
generally confident that they fitted the wheel properly, they cannot
swear on oath that this is the case - it's such an everyday and simple
event that they do not remember every precise detail. Given the
strength of the belief in QR infallibility, they are persuaded by
friends that perhaps it was their fault all along. However along with
weight of entirely consistent stories, there are many completely
convincing descriptions of the failure, including several real gems of
observation and description that put the matter beyond reasonable
This tale comes from this
thread on singletrackworld.com
. I have marginally edited the
text to clarify a couple of points that were subsequently explained
by the original author.
"Following my mates accident and the forum
discussions of the previous
week, I marked my skewers and kept an eye on QR's during my merry
Having gone for a tootle round the Peaks, I arrived at
the top of the
first hill- I sat down to take in the view and sunshine...and looked
at my QR - no movement, tight as a bell.
['looked at' later clarified as 'I physically checked as well as
checked the QR at the top - no rotation, tightly done up'.]
Rattle rattle rattle down a Peak rockfest - and I had a strange
feeling to the front of the bike(clunk). - I glanced down and the QR
was rotating slowly forward! Splash into stream...bugger, stop quickly
- look down and QR was now completely undone - i.e. unlocked! Wheel
was loose, but wouldn't drop out because of the dropout shape...
A few of points:-
1. QR WAS done up - I had checked it at the top and had not stopped,
crashed or clipped anything that may have undone it.
2. I could see the QR lever had rotated forward when I
[lever had been vertical just in front of fork stanchion on LHS, so
'rotated forward' is the unscrewing direction]
3.QR was hot to touch - not red hot but hot. Disc was the usual hot,
4. Was a descent that meant often trailing brakes, followed by hard
braking for around 10 minutes.
5. 'Twas a Rocky descent in places. (Whinstone Lee Tor
mmm more food for thoughts. Its making me think about bolt up
Spaceman Spiff (well, it's just a handle)
has also been generated by bike shop owner Ben Cooper, on this page
"I retightened the skewer using the “90 degree
rule”. This rule is often quoted for quick releases -
you tighten the nut so the lever starts to get tight when the lever is
at 90 degrees to the wheel (straight out)."
[...]I then rode the bike on my usual commuting journey - 6 miles
per day, on and off road, including cattle grids
and speed bumps taken at speed. Every day I loosened then tightened the
lever, and recorded the angle
at which the lever began to “bite”.[...] The results were
Disc bike: after 3 days, the “bite point” was 80
degrees - the experiment was halted.
V bike: After a week of use, the “bite point” was still 90 degrees."
I understand he has repeated the experiment at least
one more time, with the same result: the skewer loosened, when disk
brakes were used.
Axle slip has also been measured by Ernst Brust of the "privately-owned,
industry-respected testing lab velotech.de"
reported on bikebiz and STW
There are plenty more personal descriptions of the failure. For
example, on the subject of axle slip, we have this
"I have briefly experienced what you
describe. A while back, I installed a
2002 Rock Shox Psylo on my XC bike with Hayes disc brakes and a 6-inch
rotor. One of the tests I often do to test fork stiffness is to shift my
weight back and lock up the front brake at about 5-10mph on asphalt,
it to skid (be careful if you try this). The first time, the front wheel
quickly cocked to the left, with the left side of the axle (same as
dropped in the slot. The lawyer tab stopped further movement.
how tight I had the skewer, I opened and closed it again to what I would
consider reasonably tight. I performed the test again,
and the wheel again
cocked sideways. I continued to tighten the skewer with the same, albeit
better, results. Finally, when it took wearing a glove
to reduce the pain of
the skewer in my palm, I was able to stop the wheel from slipping in the
Of course the pull down on the left hand side is exactly as expected
from the positioning of the disk brake. He got it to stay put
eventually, but how do you think "wearing a glove to reduce the pain of
the skewer in my palm" compares to the manufacturer's recommended
installation torque? This sort of story is very common, but I don't see
the point of putting up a pile of them.
Skewer unscrewing is also regularly reported, here
particularly clear description:
"Both my G/f and I have had problems with
1) They were done up f@cking tight.
2) Every time they've come loose, the lever has been shut, but instead
of being next to the fork leg, its pointing straight down to the floor,
implying they've "wound" loose. Spin the lever
back through 180° to its usual position and its "normal tight"
Here the evidence of unscrewing is absolutely crystal clear in the
rotation of the lever. It's not always so obvious,
as when the lever is on the right, the rotation of the nut on the
left cannot easily be observed. However loosening of the skewer is
quite a frequent occurrence.
whole thread from which that last comment is taken
was a bit of an
eye-opener to me. Seems like there are a lot of people with skewers
loosening, and wheels dropping out, due to the disk brake. The stories
are convincing, consistent and worrying. Of particular note also within
that thread is the comment
by Chris Juden
, Technical Officer of the CTC
, the UK's national 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."
Chris Juden, Technical Officer of the CTC
For some more comments from the experts who have looked at this
problem, click here
Just put the calliper on the front of the fork (RH leg) and be done
with it. This will result in a generally upwards force applied to the
hub through braking, into the fork ends with no tendency to eject the
have already adopted this design with their tandems (I'd be grateful
a photo from an owner). As an alternative, any proper restraint on the
axle that supports it (not just the QR, but the axle itself) against
braking force, like on 20mm through-axle fork models. Perhaps some
clamp over the fork ends would work, that could be manufactured and
freely to users.
There are two common ideas that I'm not so keen on:
Several ideas have been suggesteded for threadlocking the skewer. This
still leaves it under a stress far greater than it is designed for,
with a massive pull in the direction of the open fork ends. If it
breaks under this force (and I would not be surprised to find that
skewer breakage is more
common with disk brakes than rim brakes) then the wheel will be
ejected. There is precious little room for error here, even if these
solutions do actually mean that a genuine operator error or some other
breakage is required for failure.
Simply changing the dropout angle (to a much more forwards-opening
position) will reduce the tendency to eject the wheel. However note
that the unscrewing phenomenon only requires a very small amount of
movement, and the alternating up/down forcing will still be present.
There may not be a 'magic angle' at which this problem suddenly
vanishes, so although this might reduce the risk, it may not completely
eliminate the problem.