broken crank (was: crank crack)

Tom Paterson said:

technology.open.ac.ukmem ccf4.htmlOpen ↗

Quoted message said:

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 hidden] Palo Alto CA

Quoted message said:

[email hidden]

Quoted message said:

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 hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

Quoted message said:

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:

Quoted message said:

Quoted message said:

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.

Quoted message said:

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?

pardo.netFAIL 001.htmlOpen ↗

Jobst Brandt [email hidden] Palo Alto CA

<[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

Thin webs that are at the extremes of a bending element

carry many

Quoted message said:

times the stress of the main member. That is why I-beams

have thick

Quoted message said:

flat "caps" connected by a thin web less than half the

thickness of

Quoted message said:

the caps. Cutting the beam in half lengthwise through the

web and

Quoted message said:

putting the caps back to back so that the half-webs are

outermost is

Quoted message said:

what we have in the crank. Structural engineers are aware

of this but

Quoted message said:

the bicycle industry is not.

In the collection of failed cranks, we also see evidence

of failures

Quoted message said:

at embossed or cast-in logos. Formerly, most cranks had a

lengthwise

Quoted message said:

vanity groove on the outside that reduced crank stiffness

by at least

Quoted message said:

30%. There are still I-beam like cranks on the market

even though

Quoted message said:

this shape has notoriously poor torsional stiffness. It

may be a

Quoted message said:

fashion statement because many people recognize the I-bean

shape and

Quoted message said:

think cranks are stressed mainly in bending. Failures are

as much

Quoted message said:

from torsion as bending. Where do these guys go to

school, if at all?

Art school.

Matt O.

Quoted message said:

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 said:

Quoted message said:

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

Quoted message said:

Sloan?

Quoted message said:

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).

Quoted message said:

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.

lancearmstrong.comD37499CB86AB278C86256D15006F21CDOpen ↗

Jobst Brandt [email hidden] Palo Alto CA

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

Quoted message said:

... 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?

Quoted message said:

[email hidden] wrote in message

news:<[email hidden]>...

Quoted message said:

Quoted message said:

... 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 hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

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 yellowjersey.orgyellowjersey.orgOpen ↗ Open every day since 1 April 1971