broken crank (was: crank crack)

Tom Paterson writes:

> http://technology.open.ac.uk/materials/mem/mem-ccf4.html

> Good one there (IMHO)! Looks like the common Campagnolo "spider crack" might have found full
> expression on this crank.

Nice picture. I've had cranks like that.

This is a common failure and the one most easily prevented but the bicycle industry doesn't want to
bother. One solution is to take the Campagnolo C-Record approach and make the crank one of the five
spider arms. Ritchey cranks do that also. Shimano doesn't do that because their cranks are hollow
and might crack from the chainring bolt entering the hollow crank. However, with the crank in the
picture, Campagnolo and others who use this design need only to machine away the thin web between
spider and both sides of the crank so that it no longer looks like the skin between index and middle
finger spread apart.

Jobst Brandt [email protected] Palo Alto CA

>[email protected]

>However, with the crank in the picture, Campagnolo and others who use this design need only to
>machine away the thin web between spider and both sides of the crank so that it no longer looks
>like the skin between index and middle finger spread apart.

Hmmm...just curious. Your statement seems to suggest that the web can contribute to a crank's
failure rather than its strength. How is this so?

Sig Chicago

<[email protected]> wrote in message news:[email protected]...
>

> This is a common failure and the one most easily prevented but the bicycle industry doesn't want
> to bother. One solution is to take the Campagnolo C-Record approach and make the crank one of the
> five spider arms. Ritchey cranks do that also. Shimano doesn't do that because their cranks are
> hollow and might crack from the chainring bolt entering the hollow crank. However, with the crank
> in the picture, Campagnolo and others who use this design need only to machine away the thin web
> between spider and both sides of the crank so that it no longer looks like the skin between index
> and middle finger spread apart.
>

Dremel.

Or needle file and about 20 minutes...

anonymous snipes:

>> However, with the crank in the picture, Campagnolo and others who use this design need only to
>> machine away the thin web between spider and both sides of the crank so that it no longer looks
>> like the skin between index and middle finger spread apart.

> Hmmm... just curious. Your statement seems to suggest that the web can contribute to a crank's
> failure rather than its strength. How is this so?

Thin webs that are at the extremes of a bending element carry many times the stress of the main
member. That is why I-beams have thick flat "caps" connected by a thin web less than half the
thickness of the caps. Cutting the beam in half lengthwise through the web and putting the caps back
to back so that the half-webs are outermost is what we have in the crank. Structural engineers are
aware of this but the bicycle industry is not.

In the collection of failed cranks, we also see evidence of failures at embossed or cast-in logos.
Formerly, most cranks had a lengthwise vanity groove on the outside that reduced crank stiffness by
at least 30%. There are still I-beam like cranks on the market even though this shape has
notoriously poor torsional stiffness. It may be a fashion statement because many people recognize
the I-bean shape and think cranks are stressed mainly in bending. Failures are as much from torsion
as bending. Where do these guys go to school, if at all?

http://pardo.net/pardo/bike/pic/fail/FAIL-001.html

Jobst Brandt [email protected] Palo Alto CA

<[email protected]> wrote in message news:[email protected]...

> Thin webs that are at the extremes of a bending element
carry many
> times the stress of the main member. That is why I-beams
have thick
> flat "caps" connected by a thin web less than half the
thickness of
> the caps. Cutting the beam in half lengthwise through the
web and
> putting the caps back to back so that the half-webs are
outermost is
> what we have in the crank. Structural engineers are aware
of this but
> the bicycle industry is not.
>
> In the collection of failed cranks, we also see evidence
of failures
> at embossed or cast-in logos. Formerly, most cranks had a
lengthwise
> vanity groove on the outside that reduced crank stiffness
by at least
> 30%. There are still I-beam like cranks on the market
even though
> this shape has notoriously poor torsional stiffness. It
may be a
> fashion statement because many people recognize the I-bean
shape and
> think cranks are stressed mainly in bending. Failures are
as much
> from torsion as bending. Where do these guys go to
school, if at all?

Art school.

Matt O.

On Wed, 21 May 2003 21:43:32 GMT, [email protected] wrote:

>Where do these guys go to school, if at all?

Sloan ?

The bike trade is about selling things, not making things. If you want to see a better crank, try
educating the bike mag journos who rave about a product, despite some obvious design horror
(certainly for the UK MTB scene).

When a logo is seen as a fashionable branding, then it's going to be on there. Only when the punter
starts to see it as a potential stress riser, and not buy the product, will they disappear.

Andy Dingley writes:

>> Where do these guys go to school, if at all?

> Sloan?

> The bike trade is about selling things, not making things. If you want to see a better crank, try
> educating the bike magazine journalists who rave about a product, despite some obvious design
> horror (certainly for the UK MTB scene).

> When a logo is seen as a fashionable branding, then it's going to be on there. Only when the
> punter starts to see it as a potential stress riser, and not buy the product, will they disappear.

I still have hope for Shimano. Their DA cranks no longer have a vanity groove, they don't emboss
their logo and they are continuing in their pursuit of a reliable and easy to install BB. The
current ones don't meet that criterion and that's why we see the one on the Lance Armstrong bicycle
with the double pinch-bolt cranks. They may be thinking about pedal attachment on the back burner.

http://www.lancearmstrong.com/lance/OnLine2.nsf/Docs/D37499CB86AB278C86256D15006F21CD

Jobst Brandt [email protected] Palo Alto CA

[email protected] wrote in message news:<[email protected]>...

>... Their DA cranks no longer have a vanity groove, they don't emboss
their logo and they are continuing in their pursuit of a reliable and easy to install BB. ...>

I haven't follow bike trends for ~ 10 years. When was the last generation that Shimano cranks have
"vanity" grooves? In the 70s?

> [email protected] wrote in message
news:<[email protected]>...
>
> >... Their DA cranks no longer have a vanity groove, they don't emboss
> their logo and they are continuing in their pursuit of a reliable and easy to install BB. ...>

"Richard Chan" <[email protected]> wrote in message
news:[email protected]...
> I haven't follow bike trends for ~ 10 years. When was the last generation that Shimano cranks have
> "vanity" grooves? In the 70s?

The Dura Ace EX before it became New Dura Ace (7400) in 1983?4?

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