broken crank arm

[cliff]

I sheared off my right crank arm near the pedal tonight while riding home. I was thankfully making a left turn, so my weight was on the left. The arm sheared as I was applying torque and it made me fall to the right. My backpack saved me from serious injury as I landed on my right side back. That crank is a 20+ years old Stronglight 105 (anyone remember this one?). The crank had seen heavy usage for about 5 years in the early 80's when it was on my only bike. Since then it's use has been relatively light. Maybe a total of 15k-20k miles. I've weighed the same all these years: 150#.

I know this has been discussed before, but can this be attributed strictly to fatigue, or age, or poor design. This is a grooved, forged "alloy" crank arm. Should I take any precautions to my other cranks such as replacement at a certain point? (for example a Ritchey non-grooved arm from the late 80's/early 90's that is on my primary CX bike, which I use to race CX)
Thanks for any input.

BTW, I was going to replace the broken crank with a NOS Stonglight 93...

 

[jim beam]

cliff wrote:
> I sheared off my right crank arm near the pedal tonight while riding
> home. I was thankfully making a left turn, so my weight was on the
> left. The arm sheared as I was applying torque and it made me fall to
> the right. My backpack saved me from serious injury as I landed on my
> right side back. That crank is a 20+ years old Stronglight 105 (anyone
> remember this one?). The crank had seen heavy usage for about 5 years
> in the early 80's when it was on my only bike. Since then it's use has
> been relatively light. Maybe a total of 15k-20k miles. I've weighed the
> same all these years: 150#.
>
> I know this has been discussed before, but can this be attributed
> strictly to fatigue, or age, or poor design.

it probably failed because of fatigue [although lack of photos make this
a guess!] fatigue in this situation is typically a function of
design/manufacture execution, any damage & material. age can come into
it with some alloy systems also.

> This is a grooved, forged
> "alloy" crank arm.

consider that neither campy nor shimano, the largest manufacturers with
the largest r&d budgets, use grooved cranks these days. a smooth oval
design helps mitigate fatigue.

> Should I take any precautions to my other cranks
> such as replacement at a certain point? (for example a Ritchey
> non-grooved arm from the late 80's/early 90's that is on my primary CX
> bike, which I use to -race - CX)

unless you have access to crack testing equipment, periodic replacement
is the the only safe way to go. consider the maufacturer's warranty as
an indicator! you can also undertake regular visual inspection, but
it's no guarantee, especially if you don't have much experience of what
you're looking for.

> Thanks for any input.
>
> BTW, I was going to replace the broken crank with a NOS Stonglight
> 93...
>

i'd use campy or shimano. search on this group for the link to a
gallery of broken cranks. there's no new campy represented, and only
one freak shimano. unfortunately, stronglight is there more than once.

 

[Zog The Undeniable]

cliff wrote:

> I sheared off my right crank arm near the pedal tonight while riding
> home.

Ah yes - they usually go at that end.

<snip>

> I know this has been discussed before, but can this be attributed
> strictly to fatigue, or age, or poor design.

The pedal gnawing away at the face of the crank, and some fatigue.

This is a grooved, forged
> "alloy" crank arm. Should I take any precautions to my other cranks
> such as replacement at a certain point? (for example a Ritchey
> non-grooved arm from the late 80's/early 90's that is on my primary CX
> bike, which I use to -race - CX)

I would consider changing those if they have been riddden every week.
There are no hard and fast rules - it depends on your weight, road
surfaces, mileage, riding style etc etc.

 

[David L. Johnson]

On Tue, 26 Oct 2004 06:43:18 -0700, jim beam wrote:

> consider that neither campy nor shimano, the largest manufacturers with
> the largest r&d budgets,

and the largest marketing departments. I don't think there is anything
magic about their cranks.

> unless you have access to crack testing equipment, periodic replacement is
> the the only safe way to go. consider the maufacturer's warranty as an
> indicator!

Oh, boy. Do you get a commission on this? Sure, things break.
Eventually. But it it ludicrous to replace every part based on the
manufacturer's warranty. Most spokes have no warranty at all, but in
properly-built wheels they can last for decades. I don't have any cranks
that old, but I have hubs and seatposts that are nearly 40 and going
strong.

Most parts, including cranks, don't go from pristine to failure without
some signs. Look at the cracks on that gallery of broken cranks; most
have a large area that is corroded, indicating the crack was there for a
long time before it completely failed. A simple visual inspection,
replacing parts that show cracks, is a better idea than replacing every
part when its warranty runs out.

> i'd use campy or shimano. search on this group for the link to a gallery
> of broken cranks. there's no new campy represented, and only one freak
> shimano. unfortunately, stronglight is there more than once.

And Campy has many, many representatives, mostly from the same time period
as the stronglight that broke in this story. Maybe a good idea to avoid
cranks of that era, and to avoid those with grooves cut into them, but not
a particular endorsement of shimano or Campagnolo.

--

David L. Johnson

__o | You will say Christ saith this and the apostles say this; but
_`\(,_ | what canst thou say? -- George Fox.
(_)/ (_) |

 

[Donald Gillies]

cliff <[email protected]> writes:

>I know this has been discussed before, but can this be attributed
>strictly to fatigue, or age, or poor design.

It's not poor design, there are still many stronglight 93/105 cranks
on the road (105 is just a 93 with rings that are missing a spider,
105bis is drilled as well.)

All cranks are aluminum. Aluminum has no lower fatigue limit. After
XYZ fatigue cycles, aluminum will fail. There is no way around this
problem.

About the only thing you can do is inspect the cranks for hairline
cracks regularly, possibly with a non-permanent magic marker which
will infiltrate the cracks and make them visible. For failures at the
pedal eye, you can use pedal washers and/or adopt jobst brandt's 45
degree chamfering (beveling?) technique on the pedal spindle, if you
have access to machine tools. search the usenet archives
(www.google.com, "Groups" link) for details

- Don Gillies
San Diego, CA

 

[[email protected]]

On Tue, 26 Oct 2004 15:23:50 -0400, "David L. Johnson"
<[email protected]> wrote:

>Most parts, including cranks, don't go from pristine to failure without
>some signs. Look at the cracks on that gallery of broken cranks; most
>have a large area that is corroded, indicating the crack was there for a
>long time before it completely failed. A simple visual inspection,
>replacing parts that show cracks, is a better idea than replacing every
>part when its warranty runs out.

[jim beam wrote:]

>> i'd use campy or shimano. search on this group for the link to a gallery
>> of broken cranks. there's no new campy represented, and only one freak
>> shimano. unfortunately, stronglight is there more than once.
>
>And Campy has many, many representatives, mostly from the same time period
>as the stronglight that broke in this story. Maybe a good idea to avoid
>cranks of that era, and to avoid those with grooves cut into them, but not
>a particular endorsement of shimano or Campagnolo.

Dear Cliff, Jim, and David,

This is probably the gallery of broken parts that Jim and
David have in mind:

http://pardo.net/pardo/bike/pic/fail/000.html

Carl Fogel

 

[[email protected]]

On 26 Oct 2004 12:48:48 -0700, [email protected] (Donald
Gillies) wrote:

>cliff <[email protected]> writes:
>
>>I know this has been discussed before, but can this be attributed
>>strictly to fatigue, or age, or poor design.
>
>It's not poor design, there are still many stronglight 93/105 cranks
>on the road (105 is just a 93 with rings that are missing a spider,
>105bis is drilled as well.)
>
>All cranks are aluminum. Aluminum has no lower fatigue limit. After
>XYZ fatigue cycles, aluminum will fail. There is no way around this
>problem.
>
>About the only thing you can do is inspect the cranks for hairline
>cracks regularly, possibly with a non-permanent magic marker which
>will infiltrate the cracks and make them visible. For failures at the
>pedal eye, you can use pedal washers and/or adopt jobst brandt's 45
>degree chamfering (beveling?) technique on the pedal spindle, if you
>have access to machine tools. search the usenet archives
>(www.google.com, "Groups" link) for details
>
>- Don Gillies
>San Diego, CA

Dear Cliff and Don,

To search, go here:

http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&group=rec.bicycles.tech

select "search only in rec.bicycles.tech," and search for
"jobst damerell pedal eye," which will lead you to:

http://groups.google.com/[email protected]&rnum=1

or http://tinyurl.com/6ymve

which in turn mentions where David Damerell has very nicely
hosted a detailed picture for us of Jobst's conical washers
and chamfered pedal eyes.

All of which leads up to my question: has anyone else on
rec.bicycles.tech actually modified their pedal eyes like
this?

Carl Fogel

 

[cliff]

Thank you everyone for your input on this. I only wish that I had inspected the crank more closely and with regularity as I'm not sure if I would have seen the crack. I certainly would like to know what to look out for in the future...

Cliff

 

[jim beam]

David L. Johnson wrote:
> On Tue, 26 Oct 2004 06:43:18 -0700, jim beam wrote:
>
>
>>consider that neither campy nor shimano, the largest manufacturers with
>>the largest r&d budgets,
>
>
> and the largest marketing departments.

that's not mutually exclusive to research or q.c.

>I don't think there is anything
> magic about their cranks.

"magic", no. "quality", yes.

>
>
>>unless you have access to crack testing equipment, periodic replacement is
>>the the only safe way to go. consider the maufacturer's warranty as an
>>indicator!
>
>
> Oh, boy. Do you get a commission on this? Sure, things break.
> Eventually. But it it ludicrous to replace every part based on the
> manufacturer's warranty. Most spokes have no warranty at all, but in
> properly-built wheels they can last for decades. I don't have any cranks
> that old, but I have hubs and seatposts that are nearly 40 and going
> strong.

one broken spoke in a wheel of many is not a catastrophe. a broken
crank is. the op has concerns about failure, and also has concerns
about not being able to see something similar in the future. short of
recommending they spend money on a dye penetrant kit & leaning how to
use it properly, the /safe/ advice is replacement. new cranks are
cheap, particularly if you have uncertain mileage, weight, strength &
habits of leaning up against nice scratchy stone walls.

>
> Most parts, including cranks, don't go from pristine to failure without
> some signs. Look at the cracks on that gallery of broken cranks; most
> have a large area that is corroded, indicating the crack was there for a
> long time before it completely failed. A simple visual inspection,
> replacing parts that show cracks, is a better idea than replacing every
> part when its warranty runs out.
>
>
>>i'd use campy or shimano. search on this group for the link to a gallery
>>of broken cranks. there's no new campy represented, and only one freak
>>shimano. unfortunately, stronglight is there more than once.
>
>
> And Campy has many, many representatives, mostly from the same time period
> as the stronglight that broke in this story. Maybe a good idea to avoid
> cranks of that era

agreed - maybe i should qualify - i'd use /new/ campy or shimano.

>, and to avoid those with grooves cut into them, but not
> a particular endorsement of shimano or Campagnolo.
>

 

[David L. Johnson]

On Tue, 26 Oct 2004 21:32:30 -0700, jim beam wrote:

> one broken spoke in a wheel of many is not a catastrophe. a broken crank
> is. the op has concerns about failure, and also has concerns about not
> being able to see something similar in the future. short of recommending
> they spend money on a dye penetrant kit & leaning how to use it properly,
> the /safe/ advice is replacement. new cranks are cheap, particularly if
> you have uncertain mileage, weight, strength & habits of leaning up
> against nice scratchy stone walls.

Replacement how often? Based on what criteria? By that logic, it would
be "prudent" to regularly replace brakes, seatposts, forks, frames,
stems, as well as bars and --- well, every single part, as a preventative
measure. The failure of a brake could be disastrous; the potential
risk of impalement due to failure of a seatpost is too awful to
contemplate. Bars and stems _do_ fail.

This all seems too scary to be "safe", and yet millions of cyclists manage
not to impale themselves on broken seatpost shards year after year.

Occasionally, some part may fail. For most riders, that is a very rare
event, and usually one that is not unpredictable. You should periodically
inspect every part for cracks as well as wear. Cranks, stems, bars, and
brakes should get a close examination, since they are known to fail --
depending on design. Most failures do not result in a catastrophe.

But if you consider it prudent to replace parts that show no signs of
failure on a regular basis, what you should be doing is replacing your
entire bike every two years, and that is neither cheap nor reasonable.

The amount of risk-aversion you are displaying suggests that you consider
cycling too dangerous to engage in.

--

David L. Johnson

__o | The lottery is a tax on those who fail to understand
_`\(,_ | mathematics.
(_)/ (_) |

 

[H]

[email protected] (Donald Gillies) wrote in message news:<[email protected]>...
> [...]
> About the only thing you can do is inspect the cranks for hairline
> cracks regularly, possibly with a non-permanent magic marker which
> will infiltrate the cracks and make them visible. For failures at the
> pedal eye, you can use pedal washers and/or adopt jobst brandt's 45
> degree chamfering (beveling?) technique on the pedal spindle, if you
> have access to machine tools. search the usenet archives
> (www.google.com, "Groups" link) for details
>
> - Don Gillies
> San Diego, CA
>


I get concerned everytime this subject comes up. I have a bike with
old nuovo record cranks. I know that these have been used a lot before
I got them, and I am now using them.

I try to inspect the cranks regularly (once a month or so), with
careful attention to the pedal holes. However, there are so many scuff
marks all over the cranks and especially near the pedal holes-- it
makes me wonder if I am missing any cracks which may be in progress.

So my question:
Is it adviseable to "polish out" scuffs and scratches? I think if I
had a nice surface finish, I would be able see any cracks better.

-H.

 

[Jay Beattie]

<[email protected]> wrote in message
news:[email protected]...
> On 26 Oct 2004 12:48:48 -0700, [email protected] (Donald
> Gillies) wrote:
>
> >cliff <[email protected]> writes:
> >
> >>I know this has been discussed before, but can this be
attributed
> >>strictly to fatigue, or age, or poor design.
> >
> >It's not poor design, there are still many stronglight 93/105
cranks
> >on the road (105 is just a 93 with rings that are missing a
spider,
> >105bis is drilled as well.)
> >
> >All cranks are aluminum. Aluminum has no lower fatigue limit.
After
> >XYZ fatigue cycles, aluminum will fail. There is no way
around this
> >problem.
> >
> >About the only thing you can do is inspect the cranks for
hairline
> >cracks regularly, possibly with a non-permanent magic marker
which
> >will infiltrate the cracks and make them visible. For
failures at the
> >pedal eye, you can use pedal washers and/or adopt jobst
brandt's 45
> >degree chamfering (beveling?) technique on the pedal spindle,
if you
> >have access to machine tools. search the usenet archives
> >(www.google.com, "Groups" link) for details
> >
> >- Don Gillies
> >San Diego, CA
>
> Dear Cliff and Don,
>
> To search, go here:
>
>
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&group=rec.bicycles.tech
>
> select "search only in rec.bicycles.tech," and search for
> "jobst damerell pedal eye," which will lead you to:
>
>
http://groups.google.com/[email protected]&rnum=1
>
> or http://tinyurl.com/6ymve
>
> which in turn mentions where David Damerell has very nicely
> hosted a detailed picture for us of Jobst's conical washers
> and chamfered pedal eyes.
>
> All of which leads up to my question: has anyone else on
> rec.bicycles.tech actually modified their pedal eyes like
> this?

The OP had a failure near the pedal, but apparently not at the
pedal eye. Pedal eye failure was not uncommon on the old
Campagnolo NR cranks, and Jobst made the same modification to an
80s vintage Dura Ace -- the same crank I raced/rode for 20+ years
and many miles with no problems (at various weights around
200lbs). I see no reason to worry about crank failure absent
signs of cracking, crash abuse or known defects. Periodic
replacement certainly is not necessary, and that pedal eye
modification is expensive unless you own machining equipment and
can DIY. -- Jay Beattie.

 

[jim beam]

David L. Johnson wrote:
> On Tue, 26 Oct 2004 21:32:30 -0700, jim beam wrote:
>
>
>>one broken spoke in a wheel of many is not a catastrophe. a broken crank
>>is. the op has concerns about failure, and also has concerns about not
>>being able to see something similar in the future. short of recommending
>>they spend money on a dye penetrant kit & leaning how to use it properly,
>>the /safe/ advice is replacement. new cranks are cheap, particularly if
>>you have uncertain mileage, weight, strength & habits of leaning up
>>against nice scratchy stone walls.
>
>
> Replacement how often? Based on what criteria? By that logic, it would
> be "prudent" to regularly replace brakes, seatposts, forks, frames,
> stems, as well as bars and --- well, every single part, as a preventative
> measure. The failure of a brake could be disastrous; the potential
> risk of impalement due to failure of a seatpost is too awful to
> contemplate. Bars and stems _do_ fail.

actually, replacement of those items /is/ prudent. ttt for example are
kind enough to give mileage & use replacement recommendations on their
handlebars & stems. others rely on the old catch-all "inspect after
every ride". there are only 2 options: inspect or replace. inspection
requires experience and/or expensive equipment. i doubt anyone here
would consider it viable to spend a few hundred dollars doing ultrasound
or x-ray on a crank that can be replaced for less.

>
> This all seems too scary to be "safe", and yet millions of cyclists manage
> not to impale themselves on broken seatpost shards year after year.

yes, and millions of bicycles never wear out even /one/ set of tires.

>
> Occasionally, some part may fail. For most riders, that is a very rare
> event, and usually one that is not unpredictable. You should periodically
> inspect every part for cracks as well as wear. Cranks, stems, bars, and
> brakes should get a close examination, since they are known to fail --
> depending on design. Most failures do not result in a catastrophe.
>
> But if you consider it prudent to replace parts that show no signs of
> failure on a regular basis, what you should be doing is replacing your
> entire bike every two years, and that is neither cheap nor reasonable.
>
> The amount of risk-aversion you are displaying suggests that you consider
> cycling too dangerous to engage in.
>

no, you're trying to make it sound that way to sensationalize the issue.
unless you have experience in non-destructive testing, can you
/safely/ say a component will not fail? judging from the number of
fatigue-initiated vintage campy cranks i've seen at bike swaps, i'd say
either the whole planet is populated by unscrupulous crooks, or, much
more likely, most people simply have no clue what they're looking for
when they inspect. again, the _cheap safe_ option is to replace.

 

[jim beam]

Jay Beattie wrote:
> <[email protected]> wrote in message
> news:[email protected]...
>
>>On 26 Oct 2004 12:48:48 -0700, [email protected] (Donald
>>Gillies) wrote:
>>
>>
>>>cliff <[email protected]> writes:
>>>
>>>
>>>>I know this has been discussed before, but can this be
>
> attributed
>
>>>>strictly to fatigue, or age, or poor design.
>>>
>>>It's not poor design, there are still many stronglight 93/105
>
> cranks
>
>>>on the road (105 is just a 93 with rings that are missing a
>
> spider,
>
>>>105bis is drilled as well.)
>>>
>>>All cranks are aluminum. Aluminum has no lower fatigue limit.
>
> After
>
>>>XYZ fatigue cycles, aluminum will fail. There is no way
>
> around this
>
>>>problem.
>>>
>>>About the only thing you can do is inspect the cranks for
>
> hairline
>
>>>cracks regularly, possibly with a non-permanent magic marker
>
> which
>
>>>will infiltrate the cracks and make them visible. For
>
> failures at the
>
>>>pedal eye, you can use pedal washers and/or adopt jobst
>
> brandt's 45
>
>>>degree chamfering (beveling?) technique on the pedal spindle,
>
> if you
>
>>>have access to machine tools. search the usenet archives
>>>(www.google.com, "Groups" link) for details
>>>
>>>- Don Gillies
>>>San Diego, CA
>>
>>Dear Cliff and Don,
>>
>>To search, go here:
>>
>>
>
> http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&group=rec.bicycles.tech
>
>>select "search only in rec.bicycles.tech," and search for
>>"jobst damerell pedal eye," which will lead you to:
>>
>>
>
> http://groups.google.com/[email protected]&rnum=1
>
>>or http://tinyurl.com/6ymve
>>
>>which in turn mentions where David Damerell has very nicely
>>hosted a detailed picture for us of Jobst's conical washers
>>and chamfered pedal eyes.
>>
>>All of which leads up to my question: has anyone else on
>>rec.bicycles.tech actually modified their pedal eyes like
>>this?
>
>
> The OP had a failure near the pedal, but apparently not at the
> pedal eye. Pedal eye failure was not uncommon on the old
> Campagnolo NR cranks, and Jobst made the same modification to an
> 80s vintage Dura Ace -- the same crank I raced/rode for 20+ years
> and many miles with no problems (at various weights around
> 200lbs). I see no reason to worry about crank failure absent
> signs of cracking, crash abuse or known defects. Periodic
> replacement certainly is not necessary, and that pedal eye
> modification is expensive unless you own machining equipment and
> can DIY. -- Jay Beattie.
>
>

rhetorical: if the crank did not fail at the pedal eye, how would this
modification have mitigated the failure?

 

[jim beam]

H wrote:
> [email protected] (Donald Gillies) wrote in message news:<[email protected]>...
>
>>[...]
>>About the only thing you can do is inspect the cranks for hairline
>>cracks regularly, possibly with a non-permanent magic marker which
>>will infiltrate the cracks and make them visible. For failures at the
>>pedal eye, you can use pedal washers and/or adopt jobst brandt's 45
>>degree chamfering (beveling?) technique on the pedal spindle, if you
>>have access to machine tools. search the usenet archives
>>(www.google.com, "Groups" link) for details
>>
>>- Don Gillies
>>San Diego, CA
>>
>
>
>
> I get concerned everytime this subject comes up. I have a bike with
> old nuovo record cranks. I know that these have been used a lot before
> I got them, and I am now using them.
>
> I try to inspect the cranks regularly (once a month or so), with
> careful attention to the pedal holes. However, there are so many scuff
> marks all over the cranks and especially near the pedal holes-- it
> makes me wonder if I am missing any cracks which may be in progress.
>
> So my question:
> Is it adviseable to "polish out" scuffs and scratches? I think if I
> had a nice surface finish, I would be able see any cracks better.
>
> -H.

i wouldn't. i'd just inspect, but much more regularly than once a
month. this http://pardo.net/pardo/bike/pic/fail/Dscn2410_640.jpg
is where i most often see cracking in cranks at swap meets.

reference pages:
http://pardo.net/pardo/bike/pic/fail/FAIL-008.html
http://pardo.net/pardo/bike/pic/fail/000.html

 

[[email protected]]

Jay Beattie writes:

> The OP had a failure near the pedal, but apparently not at the pedal
> eye. Pedal eye failure was not uncommon on the old Campagnolo NR
> cranks, and Jobst made the same modification to an 80s vintage Dura
> Ace -- the same crank I raced/rode for 20+ years and many miles with
> no problems (at various weights around 200lbs). I see no reason to
> worry about crank failure absent signs of cracking, crash abuse or
> known defects. Periodic replacement certainly is not necessary, and
> that pedal eye modification is expensive unless you own machining
> equipment and can DIY.

Just because you had no failures does not mean others won't. As I
outlined in the articles describing the modification I would like to
see become common on cranks, I also mentioned that I regularly broke a
crank per year, Campagnolo and Shimano and that since modifying the
cranks have had none.

My Campagnolo Nuovo Record cranks broke through the pedal eye in a
record distance of less than 2500 miles of use. They were the all
time worst.

As another item, my friend who weighs 165 lbs had a Shimano DuraAce
crank fail at the pedal eye and took a serious dive to the pavement.
The absence of failure on your bicycle does not prove that they do not
occur and often.

Jobst Brandt
[email protected]

 

[Jay Beattie]

<[email protected]> wrote in message
news:[email protected]...
> Jay Beattie writes:
>
> > The OP had a failure near the pedal, but apparently not at
the pedal
> > eye. Pedal eye failure was not uncommon on the old
Campagnolo NR
> > cranks, and Jobst made the same modification to an 80s
vintage Dura
> > Ace -- the same crank I raced/rode for 20+ years and many
miles with
> > no problems (at various weights around 200lbs). I see no
reason to
> > worry about crank failure absent signs of cracking, crash
abuse or
> > known defects. Periodic replacement certainly is not
necessary, and
> > that pedal eye modification is expensive unless you own
machining
> > equipment and can DIY.
>
> Just because you had no failures does not mean others won't.
As I
> outlined in the articles describing the modification I would
like to
> see become common on cranks, I also mentioned that I regularly
broke a
> crank per year, Campagnolo and Shimano and that since modifying
the
> cranks have had none.
>
> My Campagnolo Nuovo Record cranks broke through the pedal eye
in a
> record distance of less than 2500 miles of use. They were the
all
> time worst.
>
> As another item, my friend who weighs 165 lbs had a Shimano
DuraAce
> crank fail at the pedal eye and took a serious dive to the
pavement.
> The absence of failure on your bicycle does not prove that they
do not
> occur and often.

I am not against a beneficial design change, but absent evidence
that modern aluminum cranks are breaking regularly at the pedal
eye, I just do not see the need for us all to go out and get
expensive custom machine work. Also, custom machining carries its
own risks; it probably voids any warranty, and it is expensive. I
will let the manufacturer make that change rather than relying on
an unknown local machinist. -- Jay Beattie.

 

[Donald Gillies]

jim beam <[email protected]> writes:

>i wouldn't. i'd just inspect, but much more regularly than once a
>month. this http://pardo.net/pardo/bike/pic/fail/Dscn2410_640.jpg
>is where i most often see cracking in cranks at swap meets.

Bad advice. Some materials, such as glasses, gain significant,
significant tensile strength if their surfaces have no imperfections
(e.g. scratches). For example, some glasses can have higher strength
that steel if they are formed as "float glass" on a mercury bed with
absolutely no surface imperfections.

It is common knowledge that this applies to aluminum cranks as well.
Campagnolo cranks often get hairline cracks that begin developing on
both sides of the crank arm where it joins the spider. If you gently
sand and polish the crank in this area, you can virtually eliminate
all failures in this area using this very same principle. You can
effect this repair even after the cracks start forming, if you catch
them early and polish out the hairline cracks.

- Don Gillies
San Diego, CA

 

[[email protected]]

Jay Beattie <[email protected]> writes:

> I am not against a beneficial design change, but absent evidence
> that modern aluminum cranks are breaking regularly at the pedal eye,
> I just do not see the need for us all to go out and get expensive
> custom machine work. Also, custom machining carries its own risks;
> it probably voids any warranty, and it is expensive. I will let the
> manufacturer make that change rather than relying on an unknown
> local machinist.

I'm not suggesting that, only that it can be done and those who
experience such failures have a way out, expensive as it may be. My
interest is that manufacturers look at this with as much interest as
went into the threadless steertube. I had enough fretting stems,
frozen (oxidized) un-removable stems and stupid head bearing
adjustments. As you see on Sheldon's web page, I modified the old
Shimano DuraAce head bearing to work with a new threadless fork and
stem. That is what needs to be done by manufacturers. I am not sad
to see the quill stem go away nor will I mind when conical faces are
the standard for pedal shafts.

Jobst Brandt
[email protected]

 

[jim beam]

Donald Gillies wrote:
> jim beam <[email protected]> writes:
>
>
>>i wouldn't. i'd just inspect, but much more regularly than once a
>>month. this http://pardo.net/pardo/bike/pic/fail/Dscn2410_640.jpg
>>is where i most often see cracking in cranks at swap meets.
>
>
> Bad advice. Some materials, such as glasses, gain significant,
> significant tensile strength if their surfaces have no imperfections
> (e.g. scratches). For example, some glasses can have higher strength
> that steel if they are formed as "float glass" on a mercury bed with
> absolutely no surface imperfections.

float glass is a quick cheap [but highly capital intensive] way to make
glass flat - and it happens that the natural "blob" thickness of molten
glass on molten tin [NOT mercury] is about 1", the maximum thickness
typically used in sheet. it's got nothing to do with strength. you are
correct that glass failure is due to [surface] defects, but glass with
what appears to be a perfectly smooth surface is still massively flawed
on a scale that seriously impacts strength. the way to address this is
tempering which puts the surface in compression thus not allowing the
inherent surface flaws to propagate. float is not tempering.

>
> It is common knowledge that this applies to aluminum cranks as well.
> Campagnolo cranks often get hairline cracks that begin developing on
> both sides of the crank arm where it joins the spider. If you gently
> sand and polish the crank in this area, you can virtually eliminate
> all failures in this area using this very same principle. You can
> effect this repair even after the cracks start forming, if you catch
> them early and polish out the hairline cracks.
>
> - Don Gillies
> San Diego, CA

well, you're right about cracks being dangerous, but what is a scratch
and what is a crack? and at what point does removal of protective
anodizing expose a 2000 series aluminum alloy to stress corrosion from
chlorides or other agents? just removing material in the hope that
because you can no longer see it, you've removed the crack is a
dangerous way to approach this problem.