From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Broken Fork
Date: 23 Sep 1998 15:28:55 GMT

Paul Butler writes:

> After a few minutes I discovered a crack about 75% through the top
> of the right fork blade starting at the back of the blade (to
> separate the crack would have hinged the wheel forward).

> I rode home on egg shells but made it home in one piece.  A crash
> would seem unavoidable if the crack had gone the rest of the way
> through and I often ignore creaks.  Has anyone here ridden a broken
> fork until it separated?  Did it cause a crash?

Most people choose not to ride on one fork blade and find some other
way to reach home.  Yes, this is the classic fork failure and I have
experienced them.  The only difference is that I was in the mountains
in both occasions and noticed the loss of control.  Fortunately,
applying the brakes stabilizes the condition by closing the gap so the
fork becomes rigid again until stopped.

The important point is that forks break forward from vertical road
vibration induced fatigue, not backward, the way most people visualize
fork failure.  The crack begins at the rear of the fork blade at the
fork crown.  Internally lugged (aka sloping fork crown) forks fail
more often than ones with external lugs because the transition inside
the blade of the crown extension cannot be feathered, leaving a sharp
discontinuity.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Does the frame make a difference in ride?
Date: 17 May 2000 01:20:45 GMT

Robert Perkins writes:

>> In particular downtubes with large bending stiffness is an invitation
>> to fork failure.  A frontal collision with an obstruction on a good
>> bicycle traditionally bends the top and downtubes with sufficient
>> force, also the fork.  This is as it should be because the fork should
>> not be the weakest link, something that would lead to fatigue failure
>> if it were the highest stressed member.

> I personally was glad to have only my steel fork bend in a collision
> in September; my Aluminum frame survived intact and unbent.


That may be true for that one incident but for the long run, fork
failure is not preferred in fatigue because that is crack development
and fork separation when it fails.

> Where are the cases of Alu frame bicycles with steel forks having
> increased fork fatigue failure?  All early Cannondales came with steel
> forks, and have not read of any failures.

I have had two forks fail but not for that reason, but rather because
they were sloping crown forks (ala Cinelli) that are internally lugged
and cannot be feathered at the fork blade juncture.  This was a great
styling disservice that mainly Cinelli introduced to the bicycle and
subsequently became fairly standard.

>> This is not the case with some frames we see today with tall cross
>> section and fat aluminum downtubes that absorb no significant road
>> shock to spare the fork.  Still, this rigidity is not perceptible for
>> the rider.

> How did you determine that some frames do absorb fork-damaging road
> shock, but that this shock absorption is imperceptible to the rider?

I think I explained that.  If the frame tubes take a bend first in a
collision, then they are more highly stressed than the fork, otherwise
the fork would bend.  Forces below the plastic deformation level are
the ones that cause fatigue failures but they are caused by similar
loading, (braking and bumps encountered).

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jobst.brandt@stanfordalumni.org
Subject: Re: Why Are Modern Stems So Short?
Newsgroups: rec.bicycles.tech
Message-ID: <pNgde.59$T3.431@typhoon.sonic.net>
Date: Mon, 02 May 2005 03:04:53 GMT

David L. Johnson writes:

>>> The point was that names like Cinelli used to mean something in
>>> terms of craftsmanship, but the name has been bought and paid for
>>> in order to stick on a frame that had nothing to do with that
>>> tradition.  Most of the old Italian marks have gone through that
>>> transformation, and none have been the better for it.

>> I'll argue this. The old Cinelli bikes may have been well conceived
>> and fine machines, but the workmanship was never anything
>> special. They truly did not suck, but they were never particularly
>> well crafted either.

> I demur.  They were indeed well-crafted.  The finishing was awful,
> with visible file marks, poor chrome and paint.  But nice lugs,
> well-designed, and well-built.  I was going to make a comment about
> the finish, but decided to let that lie, until you brought it up.

That was my experience as well except that Cinelli made the sloping
fork crown popular and that was a bad choice in my experience.  This
fork crown looked elegant but had two major faults.  First, it
connected the steertube to the fork blades by a heavy solid steel
forging.  Second and worse was that it used internal lugs to engage
the fork blades.  Being internal, these could not be given a feathered
transition to the fork blades and thereby caused a stress concentration
through this discontinuity in elasticity.  I had two of these forks
break at that junction and subsequently demanded external feathered
lugs for my forks.

In my estimation that fork crown was a major disservice to bicycling
and it still holds many adherents who are not aware of its weakness.
I hope they don't ride as much as I did with my forks and find out
about it.  Fortunately fork blades generally fail one at a time and
not from braking either, but rather from vertical shock (forward).

Jobst.Brandt@stanfordalumni.org

From: jobst.brandt@stanfordalumni.org
Subject: Re: Why Are Modern Stems So Short?
Newsgroups: rec.bicycles.tech
Message-ID: <Hfsde.76$T3.661@typhoon.sonic.net>
Date: Mon, 02 May 2005 16:08:07 GMT

Phil Lee writes:

>>>>> The point was that names like Cinelli used to mean something in
>>>>> terms of craftsmanship, but the name has been bought and paid
>>>>> for in order to stick on a frame that had nothing to do with
>>>>> that tradition.  Most of the old Italian marks have gone through
>>>>> that transformation, and none have been the better for it.

>>>> I'll argue this. The old Cinelli bikes may have been well
>>>> conceived and fine machines, but the workmanship was never
>>>> anything special. They truly did not suck, but they were never
>>>> particularly well crafted either.

>>> I demur.  They were indeed well-crafted.  The finishing was awful,
>>> with visible file marks, poor chrome and paint.  But nice lugs,
>>> well-designed, and well-built.  I was going to make a comment
>>> about the finish, but decided to let that lie, until you brought
>>> it up.

>> That was my experience as well except that Cinelli made the sloping
>> fork crown popular and that was a bad choice in my experience.
>> This fork crown looked elegant but had two major faults.  First, it
>> connected the steertube to the fork blades by a heavy solid steel
>> forging.  Second and worse was that it used internal lugs to engage
>> the fork blades.  Being internal, these could not be given a
>> feathered transition to the fork blades and thereby caused a stress
>> concentration through this discontinuity in elasticity.  I had two
>> of these forks break at that junction and subsequently demanded
>> external feathered lugs for my forks.

> Is this the crown you speak of?

> http://www.retroraleighs.com/catalogs/1976/pages/04-76-professional.html

That's the one.  In olden times, a fork crown was made by overlapping
the steertube and fork blades and wrapping a formed three holed
fitting around them.  The ends of the blades were closed by a thin
flat cap.  To shorten the fork blades and reach down to them with a
solid steel crown was externally svelte but had no basis in function.
Unfortunately it caught on for its appearance and there it is.

Old US fat tires bicycles of the 1940's etc used three or four flat
three hole plates where the steertube and fork blades overlapped.
Although this left stress concentrations, it was straight forward in
design and made no style statement while addressing the basic problem.

Current unicrown forks with fork blades curves inward to the steertube
where they are welded is a crude heavy manufacturing shortcut using
far heavier fork blades that road bicycles used.

Jobst.Brandt@stanfordalumni.org

From: jobst.brandt@stanfordalumni.org
Subject: Re: Bike shimmy question
Newsgroups: rec.bicycles.tech
Message-ID: <Uhuh9.24660$Ik.581284@typhoon.sonic.net>
Date: Tue, 17 Sep 2002 00:01:56 GMT

Bill who? writes:

>> I have a Cannondale frame with a Look (off of a Litespeed) carbon
>> fork with carbon steerer tube.  I'm using a standard X3 lacing
>> pattern wheelset whch are true, and am running 23 foldable Michelin
>> tires.  Recently on a ride, I experienced a very severe shimmying
>> doing about 40 mph down a hill.

> Thanks to everyone for the suggestions.  Ultimately, I took the fork
> off to examine things and make sure the headset was properly
> tightened, and discovered that the carbon fiber steerer tube had
> many major cracks just above the crown, and the tube was actually
> bent.  Thanks to a rainy day and not much to do...

Why are we expected to do the life tests for these fork folks?  The
fork is no place to do idle inventions because a failure at the fork
crown is nearly always a forward crash onto ones head.

In the days of yore when the steel racing frame was perfected, fork
crown failures were recognized as the worst of possible failures and
subsequently frames were made so that top and downtubes bent before
the fork bent.  With the onset of large diameter aluminum frame tubes
and deep cross section aero tubes, all this was lost and we hear of
fork crown failures.  Carbon is doubly bad due to its incompatibility
with a steel steertube while steel forks are at risk from excess
frame rigidity.

Once again the fork has become the weakest point.  I wonder what frame
designers have learned from history, if anything.  Oops, they don't
look at history, so they are doomed to repeat it.  The trouble is the
riders are the ones that suffer.

Jobst Brandt  <jobst.brandt@stanfordalumni.org>  Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: Bike shimmy question
Newsgroups: rec.bicycles.tech
Message-ID: <YMxh9.24768$Ik.582546@typhoon.sonic.net>
Date: Tue, 17 Sep 2002 03:59:52 GMT

Mike DeMicco writes:

>> Once again the fork has become the weakest point.  I wonder what
>> frame designers have learned from history, if anything.  Oops, they
>> don't look at history, so they are doomed to repeat it.  The
>> trouble is the riders are the ones that suffer.

> It's pretty hard to learn from history if it isn't documented. These
> concepts are often passed down from master to apprentice - an
> obsolete way to do things in this day and age. Perhaps that is the
> subject for your new book - a book on frame design.

I don't know enough about frame building to write a book.  As you see,
as exhaustively as I wrote about every detail of spoked bicycle
wheels, detractors continually attack it with spurious arguments that I
can expose as such through a thorough understanding of the subject.
Why steel frames were built the way they were, and how they got that
way is one thing, but brazing and welding are out of my domain.

The loss of frame expertise came largely through hubris.  New builders
knew everything and didn't need to ask old timers about anything.  Not
only that, but most were not demanding bicyclists themselves.  My
riding companions Joe Breeze, Peter Johnson and Tom Ritchey (expert
frame builders) know how critical reliability is because we descended
rocky passes here and in the Alps with precipices that left no room
for failures, even small frame failures on some descents meant fairly
certain death.  This makes a believer of a frame builder rapidly.

Their comments included, that those new frame builders ought to ride
here (in the Alps) to realize what their frames should withstand.

Jobst Brandt  <jobst.brandt@stanfordalumni.org>  Palo Alto CA