More on disk brakes and quick release failure



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J

James Annan

Guest
I've eventually got around to finishing off a web page I started back in December following a crash
I had when my disk brake pulled my front wheel out of the fork. The aftermath of that crash was
discussed here, and there are pictures of it on http://www.ne.jp/asahi/julesandjames/home/fork/

The new web pages is on:

http://www.ne.jp/asahi/julesandjames/home/disk_and_quick_release/

and I'd appreciate any comment or criticism.

What prompted me to finish it off after a few months of procrastination is hearing about yet another
serious crash which seems to have been due to the same basic problem: a disk brake puts a huge force
on the front wheel, in the direction of the open fork ends, which may be far in excess of anything
the QR is designed to handle.

James
 
T

Tim McNamara

Guest
Well, my first thought is that if this was a real problem:

1. reports of this type of failure would be more widespread (as you note you couldn't find any)

2. manufacturers- who don't want to get sued and who pay huge liability insurance premiums- would
have changed the design.

Since neither 1 or 2 seem to be true, I can only speculate that either

3. you hadn't properly tightened your QR (e.g., the wheel was cocked in the dropout when you
clamped it down, then straightened out while riding) or

4. you were using a poorly designed QR with inadequate clamping force
(e.g., most of the aftermarket "cool" QRs with Ti skewers and/or aluminum jam nuts).

I'll leave others who know more to judge your engineering analysis. In the interests of disclosure,
I should note that I think "disk" brakes on bikes are a silly idea and offer only one significant
benefit (the braking surface is harder to get muddy) and a number of disadvantages (weight,
increased loads on spokes, etc). Bikes already have disk brakes if they use a rim brake.
 
J

Jon Isaacs

Guest
>and I'd appreciate any comment or criticism.
>

I looked over both your pages.

Some comments and questions:

1. It appears to me that there were no "Lawyer Lips" on the fork in question and that they might
have been filed off at some time.

Is this the case? This is an issue that should be addressed up front because it is a critical part
of any analysis.

2. I think your diagram of how a QR might slip past a retention lip is faulty. The forces required
to over come the friction of the QR are quite different that the forces required to either bend
the QR which is supported by the axle within a few mm of the end or to shear the retention tab.

3. It appears to me that this is a standard cantilever MTB fork that someone fitted disk brake
mounts to. Besides the issue of the tab, it is also important to remember that disk brakes put
considerably more bending moments and stresses on the crown. This means a cantilver/Vbrake fork
might not be adequate for use with a disk brake.

4. I believe that commerically made bikes equipped with disk brakes do have substantial retention
tabs and beefy forks.

On your web page your wrote:

"So was this just a one-off dangerous design? Perhaps it is worse than most, but I'm not convinced
that 'mainstream' forks are so much better."

I think you hit the nail on the head here. When doing any design, whether it is one-off or something
that will be used for a million bikes, one needs to understand the forces involved and make sure
there are no problems.

It seems to me that the problem here is not with the QR but rather with the lack of adequate
retension lips.

Jon Isaacs
 
M

Mike Jacoubowsk

Guest
The design of the fork that failed is not similar to what is found on any suspension fork that I
know of; if there was any retention device on the dropout, it's not evident in the photos. Current
mountain bike forks not only have a "lawyer lip" (as they're affectionately called) but typically a
recessed area in which the quick release resides. Short of complete failure (breakage) of the quick
release, I do not think it likely that any force created by a disc brake could liberate the wheel
from the fork.

As another poster pointed out, this method of failure appears to be rare (virtually unknown) in the
real world, and that's a world in which I have customers who often exert enough force on a wheel to
destroy a fork, but leaving the wheel in place.

Could we have a close-up photo of the dropout itself? I'd like to see what type of retention device
it actually had, and compare it to others.

--Mike-- Chain Reaction Bicycles http://www.ChainReactionBicycles.com

"James Annan" <[email protected]> wrote in message
news:[email protected]...
> I've eventually got around to finishing off a web page I started back in December following a
> crash I had when my disk brake pulled my front wheel out of the fork. The aftermath of that crash
> was discussed here, and there are pictures of it on
> http://www.ne.jp/asahi/julesandjames/home/fork/
>
> The new web pages is on:
>
> http://www.ne.jp/asahi/julesandjames/home/disk_and_quick_release/
>
> and I'd appreciate any comment or criticism.
>
> What prompted me to finish it off after a few months of procrastination is hearing about yet
> another serious crash which seems to have been due to the same basic problem: a disk brake puts a
> huge force on the front wheel, in the direction of the open fork ends, which may be far in excess
> of anything the QR is designed to handle.
>
> James
 
J

James Annan

Guest
Tim McNamara <[email protected]> wrote in message
news:<[email protected]>...
> Well, my first thought is that if this was a real problem:
>
> 1. reports of this type of failure would be more widespread (as you note you couldn't find any)

Did you actually read the web pages? I mentioned 3 other failures that happened on standard MTB
forks, as well as my rather unusual case. One of the other cases was a QR shearing, the other 2 were
almost certainly also wheel loss of some sort of another, but the precise cause has not been
diagnosed. These are all in the last few months, by the way. I've only been interestd in the problem
since it happened to
me.

James
 
J

James Annan

Guest
[email protected] (Jon Isaacs) wrote in message
news:<[email protected]>...

> 1. It appears to me that there were no "Lawyer Lips" on the fork in question and that they might
> have been filed off at some time.
>
> Is this the case? This is an issue that should be addressed up front because it is a critical part
> of any analysis.

In my case, the builder chose to use fork ends without the lips. This obviously is directly relevant
to my crash, but not to the other 3 I know of where lips were present on standard MTB suspension
forks. In one of these other cases, the QR sheared, but in the other 2, this appears to have not
happened (being on the other side of the world makes it hard to examine the remains in person, and
I'm relying on reports from witnesses who don't necessarily know what they should be looking for).

I don't consider that the sheared QR is a significantly different problem from the others - it was
still demonstrably being put under a load substantially in excess of that which it is designed to
handle, due to the positioning of disk and fork end slot.

> 2. I think your diagram of how a QR might slip past a retention lip is faulty.

I wouldn't be at all surprised to find out that it is. It was only a random idea. _Something_
happened to allow the wheel to separate from the fork, apparently without any breakage. Obviously
I'd like to have a look at the parts myself, but that't not going to be possible.

The particularly horrifying thing about the most recent crash, apart from the resulting injuries
(which are very bad), is the fact that the victim is not just some random cyclist who has a friend
with net access, but is one of the small handful of people with whom I had recently discussed this
very 'problem'. He was an experienced cyclist, he understood the theoretical problem of the wheel
extraction force, and he knew how to use a QR. If wheel loss can be ruled out as a cause in his
case, that would be great. Otherwise, it seems to me that a method that fails on him is likely to be
insufficiently safe for everyone.

> 4. I believe that commerically made bikes equipped with disk brakes do have substantial retention
> tabs.

This is definitely false, as only a few days ago I saw a set of rigid no-name forks in a Tokyo bike
shop with very small retention tabs (with a rounded paint-filled corner) and somewhat
rearward-facing slots. I will try to go back and take some close-up photos. Of course, even with
the lip, there's still the possible problem of the QR breaking, and the resulting crash will be
just as bad.

What do you think of the Surly forks shown on my 'fork' web page? Also on

http://www.surlybikes.com/frames/images/karatemonkeyLG.jpg

FWIW, I've heard a report of front wheel slippage when using disk brakes on this frame, but not yet
a full wheel extraction. Clearing off the paint and using 'a high quality QR' probably helps, but
how is a consumer supposed to know that, and what measure is available of 'high quality' beyond the
ISO standard?

I realise that I'm talking about something that is quite rare, and the hypothesis is somewhat
improbable. This seems entirely consistent to
me. These relatively rare crashes must of course be explained by something that only happens
occasionally, perhaps under a combination of circumstances. Otherwise the crashes wouldn't be
rare! Therefore, a hypothesis cannot be dismissed simply on the grounds that it seems
'unlikely'.

James
 
J

James Annan

Guest
Jon Isaacs wrote:

>
> Still wondering what was the history of your fork..
>
> Were there retention tabs ie Lawyer lips?? Was this a caliper fork modified to take a disk brake??

Um, I think I just answered, but hasn't propagated yet. It was specifically built for the disk
brake, and had no lips from the outset. Not my deliberate choice, I just trusted the builder.

I realise that my case is a specific and unusual combination, but the 3 other recent cases known to
me, that appear to be also due to front wheel loss, were all using standard MTB forks.

James
 
J

James Annan

Guest
Jon Isaacs wrote:
>>What do you think of the Surly forks shown on my 'fork' web page?
>
>
> I can't see it well enough to say. But I am quite sure it has retention tabs.

Indeed it does, of some sort. However I'm sure there are plenty who scoff at them, at least before
they ride the bike.

> I think it is really on possible for me to comment on your crash because yours is the only one
> which is well documented.

What about the case of the snapped QR? I suppose I could say that the rider in that case was
definitely using a standard cannodale fork with lips, the QR was the Coda-branded one that came with
the bike. I'm not sure what more documentation would be helpful. Possibly part of the QR could be
examined closely (I think one end was lost), but it broke anyway - what more is there to say?

> The lack of Retention tabs on your fork is clearly the important issue.

If my fork had had a lip and the QR had snapped instead of pulling out, the crash would hardly have
been any better. A changed dropout angle would make the lips irrelevant though, and also eliminate
the massive loading on the skewer.

> I do hope you make this clear on your page so that someone does not have to guess this fact.

Sorry, I'll change that. The page was originally put up following conversations with several people
who already knew about the lack of lips - I hadn't realised it was not obvious to new readers.

James
 
D

David Kunz

Guest
James Annan wrote:
>
>
> Jon Isaacs wrote:
>
>>> What do you think of the Surly forks shown on my 'fork' web page?
>>
>>
>>
>> I can't see it well enough to say. But I am quite sure it has retention tabs.
>
>
> Indeed it does, of some sort. However I'm sure there are plenty who scoff at them, at least before
> they ride the bike.
>
>> I think it is really on possible for me to comment on your crash because yours is the only one
>> which is well documented.
>
>
> What about the case of the snapped QR? I suppose I could say that the rider in that case was
> definitely using a standard cannodale fork with lips, the QR was the Coda-branded one that
> came with the bike. I'm not sure what more documentation would be helpful. Possibly part of
> the QR could be examined closely (I think one end was lost), but it broke anyway - what more
> is there to say?

Didn't Cannondale recall this QR? Also, some people put titanium QRs in front -- and this is what
happens (why not to use titanium there).

>> The lack of Retention tabs on your fork is clearly the important issue.
>
>
> If my fork had had a lip and the QR had snapped instead of pulling out, the crash would hardly
> have been any better. A changed dropout angle would make the lips irrelevant though, and also
> eliminate the massive loading on the skewer.
>
>> I do hope you make this clear on your page so that someone does not have to guess this fact.
>
>
> Sorry, I'll change that. The page was originally put up following conversations with several
> people who already knew about the lack of lips - I hadn't realised it was not obvious to new
> readers.
>
> James
 
J

James Annan

Guest
David Kunz wrote:

>
> Didn't Cannondale recall this QR?

I haven't heard that, and presumably neither did the victim. Do you have any more details of which
skewers were recalled, and when?

> Also, some people put titanium QRs in front -- and this is what happens (why not to use
> titanium there).

Well, I agree that a Ti skewer seems like a bad idea, but this Cannondale one certainly wasn't that.
I don't believe it is reasonable to put the onus on the customer to work out which skewers are fit
for purpose, and which are not - AIUI there is a single ISO standard for pull retention, and it is
provably inadequate for disk brake-equipped bikes.

I do understand the scepticism on whether a QR can really get pulled out of a fork with a solid
lawyer lip. It's hard to understand how that can happen without some sort of mechanical failure.
Nevertheless, wheel/fork separation of some sort continues to occur occasionally, even when
installed by experienced cyclists who are aware of the danger. And even without a direct pull-out,
skewers do snap under the force which is far in excess of that which they are designed to handle.
With the spread of disk brakes to road bikes, there's a whole new set of frame builders (and
cyclists) who have been brought up to believe that retention lips are entirely unnecessary, and who
may not even realise there is the potential for a problem.

(NB the person who built my forks was actually an experienced MTB builder, although it seems like he
had limited experience of disk brakes and tandems - I didn't actually communicate with him at all,
he was subcontracted by my frame builder.)

James
 
J

James Annan

Guest
Jon Isaacs wrote:
>>If my fork had had a lip and the QR had snapped instead of pulling out, the crash would hardly
>>have been any better
>
>
> The point is that the QR slipped. You have no evidence that the QR would have snapped had it not
> slipped.

No, but I do have evidence that no QR is designed to handle this stress. At least, a Shimano
engineer has emailed me to say that their skewers are designed to satisfy the ISO standard, and by
testing and popular acclaim, they are as good as any and better than most, probably including the
one I was using. That doesn't mean that my QR would necessarily have broken in that incident, of
course. It might have waited until I was going faster over rougher ground.

The person with a snapped skewer said that it snapped, while he was descending a steep slope. One
end was lost in the crash. It was all as supplied with the bike. Not clear what he might have been
doing to 'abuse' the QR, it was almost certainly above the recommended torque because - ta da! - it
has to be to keep the wheel in place! Ah, off the hook again, that's his fault for doing it up too
tight, and if it had slipped, it would have been his fault for doing it too loose. When one starts
off from the presumption that the QR is always innocent, it's easy to 'diagnose' any
failure...however that does not actually _cure_ the problem, does it?

> But from what you wrote, this was not clear, one had to infer from the photo plus the fact that
> you tried to claim that the QR could slipout of a dropout with retention tabs.

As I said, the origin of the web-page (written several months) ago meant that this was known to the
original audience. It was clearly visible on the pictures too. I certainly wasn't trying to deceive
anyone, it would have been a rather feeble effort to show a close-up of the fork end if I was.

> It seems to me that whoever built that fork for you did not understand the issues involved and
> since it already had cantilever pads on it, it seems likely that this was modified fork rather
> than a new fork specifically built for this purpose.

No, it was specifically built for the disk brake, and this bicycle.

> I am still curious about the exact history of your fork.

It had no 'history'. It had been ridden for about an hour previous to failure. It was supplied with
the frame, complete with disk mounts, a fresh coat of paint and no retention tabs. Oh, much the
same goes for the skewer, except it was just a standard widely availabe one and not in any way
custom. The skewer I now realise is not the best available. Frankly, I'm glad it wasn't slightly
better, as it would in that case have failed on a faster and steeper hill a couple of miles further
along my route.

James
 
E

Eric Murray

Guest
This morning I tried a test. I've got a bike with Shimano XT disc brakes and a Marzzochi
"bomber" front fork, lawyer lips filed off. This fork has a dropout with the opening parallel to
the fork tube.

My test consisted of loosening the QR until the wheel was barely held in. Then I held the front end
off the ground, spun the tire and clamped on the brake. The wheel didn't budge.

I tried running alongside the bike (I wasn't going to ride a bike with a loose QR) and braking. The
wheel didn't budge then either.

I'm not going to worry about my wheel coming out on braking.

Eric
 
D

Dallas

Guest
"Eric Murray" <[email protected]> wrote in message news:[email protected]...
>
>
> This morning I tried a test. I've got a bike with Shimano XT disc brakes and a Marzzochi
> "bomber" front fork, lawyer lips filed off. This fork has a dropout with the opening parallel to
> the fork tube.
>
> My test consisted of loosening the QR until the wheel was barely held in. Then I held the front
> end off the ground, spun the tire and clamped on the brake. The wheel didn't budge.
>
> I tried running alongside the bike (I wasn't going to ride a bike with a loose QR) and braking.
> The wheel didn't budge then either.
>
> I'm not going to worry about my wheel coming out on braking.
>
> Eric
>

My Rock Shox Psylo sounds like a similar set-up, but with an Avid brake. The fork in the photos
seems to have dropouts that angle rearward with respect to the fork blades.

Incidentally I also noticed that the postitioning of the caliper on the
Psylo is much higher on the fork blade as well, torquing the wheel axle against the side of the
dropout instead of the opening. I wonder though if using a larger than stock disc might increase
the risk of a wheel pulling out (the mounts requiring the caliper to be positioned further out
from the fork rather than higher up the blade)?

Mike Dallas
 
J

James Annan

Guest
[email protected] (Jon Isaacs) wrote in message
news:<[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
 
M

Mark Hickey

Guest
[email protected] (James Annan) wrote:

>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.

Why would skewer tension increase under braking? Even if the skewer slips in the droput, the preload
should remain nearly the same unless the dropout width isn't uniform.

If the axle slips enough for the QR nut to contact the lawyer lips, it seems to me that an effective
impasse has been reached

And personally I don't put a lot of faith in the fact that an "experienced cyclist who knows how to
use a QR" can't simply forget to tighten it up. The one front wheel departure I helped clean up
(called in the ambulance) was clearly due to the rider not tightening the skewer properly.

Mark Hickey Habanero Cycles http://www.habcycles.com Home of the $695 ti frame
 
S

S. Anderson

Guest
"Jon Isaacs" <[email protected]> wrote in message
news:[email protected]...

<<snip..>>

> I personally wonder why they just don't mount the brake on the front of
the
> fork so that applying the brake forces the wheel into the dropout rather
than
> out of the dropout. Rear mounting might reduce the forces on the fork a
bit.
>
> Jon Isaacs
>

Usually calipers are mounted behind the fork blade to put their mass as close as possible to the
steering axis so that handling isn't affected to any great degree. It's a small point but that's why
it was done on motorcycles and probably copied on bicycles.

Cheers,

Scott..
 
J

Jon Isaacs

Guest
>This morning I tried a test. I've got a bike with Shimano XT disc brakes and a Marzzochi
>"bomber" front fork, lawyer lips filed off. This fork has a dropout with the opening parallel to
>the fork tube.
>
>My test consisted of loosening the QR until the wheel was barely held in. Then I held the front end
>off the ground, spun the tire and clamped on the brake. The wheel didn't budge.

The moment on the QR contact that causes it to come free is caused by the position of the brake and
the contact patch on ground. The inertial loading you applied is a different situation and for that
reason is not a valid.
>
>I tried running alongside the bike (I wasn't going to ride a bike with a loose QR) and braking. The
>wheel didn't budge then either.
>
>I'm not going to worry about my wheel coming out on braking.
>
>Eric

You ought to be concerned because James Annan has raised a valid issue here and if your disk brake
are mounted behind the fork, then there is indeed a force trying to slip the front wheel out of
the drop out.

Those "Lawyer Lips" are there for a reason and it is not just to keep the companies out of
law suits.

I suggest reading his pages because they should raise some concerns.

Jon Isaacs
 
J

Jobst Brandt

Guest
James Annan writes:

> What prompted me to finish it off after a few months of procrastination is hearing about yet
> another serious crash which seems to have been due to the same basic problem: a disk brake puts a
> huge force on the front wheel, in the direction of the open fork ends, which may be far in excess
> of anything the QR is designed to handle.

To make this more direct, lets assume the bicycle is capable of braking with 1g acceleration. That
means, for a low rider position, the entire weight of rider and bicycle is bearing on the front
wheel... and that the rotational torque of the front wheel is its radius times bicycle and rider
weight. The radius at which the brake pads act is about a 6 inch while wheel periphery lies at about
13 inches.

That means the upward force on the brake pads is slightly greater than twice (13/6) the horizontal
force (which is equal to the vertical force at 1g) on the front wheel. Therefore, there is a net
disengagement force of half the rider-and-bicycle weight. This is not a safe condition when repeated
often, especially on rough terrain where this force is hammered onto the wheel retention.

This example is accurate because the ratio of forces is equally true for lesser braking forces. The
example of 1g is to simplify visualization by avoiding arithmetic.

Jobst Brandt [email protected] Palo Alto CA
 
J

Jose Rizal

Guest
[email protected]:

> To make this more direct, lets assume the bicycle is capable of braking with 1g acceleration. That
> means, for a low rider position, the entire weight of rider and bicycle is bearing on the front
> wheel... and that the rotational torque of the front wheel is its radius times bicycle and rider
> weight. The radius at which the brake pads act is about a 6 inch while wheel periphery lies at
> about 13 inches.
>
> That means the upward force on the brake pads is slightly greater than twice (13/6) the horizontal
> force (which is equal to the vertical force at 1g) on the front wheel. Therefore, there is a net
> disengagement force of half the rider-and-bicycle weight. This is not a safe condition when
> repeated often, especially on rough terrain where this force is hammered onto the wheel retention.
>

If you look at the geometry of the dropouts in front suspension forks (which arguably most disc
brakes are used with), and the position of the disc brake caliper (regardless of model and rotor
size, as the caliper locations are all behind and above the QR), the QR will tend to be pushed
towards the rear half of the dropout rather than along the slot where the QR slides in and out at
removal. In other words, the retention of the QR axle in place when using disc brakes is not at all
dependent on the tightness of the QR, but rather on the strength of the dropout. The safety of this
arrangement is not as questionable as you imply.
 
A

A Muzi

Guest
"Eric Murray" <[email protected]> wrote in message news:[email protected]...
>
>
> This morning I tried a test. I've got a bike with Shimano XT disc brakes and a Marzzochi
> "bomber" front fork, lawyer lips filed off. This fork has a dropout with the opening parallel to
> the fork tube.
>
> My test consisted of loosening the QR until the wheel was barely held in. Then I held the front
> end off the ground, spun the tire and clamped on the brake. The wheel didn't budge.
>
> I tried running alongside the bike (I wasn't going to ride a bike with a loose QR) and braking.
> The wheel didn't budge then either.
>
> I'm not going to worry about my wheel coming out on braking.

As I understand this problem, the fork that began thuis discussion was erroneously built with
the opening in the tip exactly lined up with the direction the axle wants to go when the
brake is applied. No quick release could hold against those forces, being amplified and
directed out the slot!

If your fork tips are pointing along the fork blade's centerline, the axle wants to press against
the side of the tip when the brake is applied, not out the slot. There's no particular reason for a
Q/R to fail in that setup because it isn't the only thing holding the axle in the fork. I wouldn't
worry about your fork at all. Tens of thousands of forks are running just fine in that format.

--
Andrew Muzi http://www.yellowjersey.org Open every day since 1 April 1971
 
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