Broken Fork Steerer Tube

bicycle_disciple said:

Hi,

Please view these pictures. http://www.flickr.com/photos/tree_trunks/sets/72157600514688243/

Its a carbon fiber failure in a bicycle fork at steerer tube-crown
junction. Any ideas on what could have gone wrong?

My observations :

- Steerer tube seems to have been bonded to an alloy or ALU crown of
fork, and then maybe sanded down.

- Cylindrical base of steerer tube must not have provided adequate
surface area for bond. This way, any stress risers can really spell
trouble.

- I also wonder if difference in coefficients of thermal expansion of
the bonding sites somehow played a role (?)

- Manufacturing defect a likely possibility. I dont know too much
about how these things are exactly made, so I can't offer an
explanation in this area.

Ron
http://cozybeehive.blogspot.com

yes, it's a manufacturing defect. yes, the aluminum sleeve is a stress
riser. but failure, even though it occurred to you, is very rare.
google this group for carbon fork failures and you'll only find a
handful for all the years they've been used. your remedy is warranty
and an injury claim.

regarding events that preceded the failure, were there any creaking or
cracking noises? you need to be very aware of this with carbon
componentry as this almost always is a warning ahead of actual failure.

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

Quoted message said:

Hi,

Please view these pictures.
http://www.flickr.com/photos/tree_trunks/sets/72157600514688243/

Its a carbon fiber failure in a bicycle fork at steerer tube-crown
junction. Any ideas on what could have gone wrong?

My observations :

- Steerer tube seems to have been bonded to an alloy or ALU crown of
fork, and then maybe sanded down.

- Cylindrical base of steerer tube must not have provided adequate
surface area for bond. This way, any stress risers can really spell
trouble.

- I also wonder if difference in coefficients of thermal expansion of
the bonding sites somehow played a role (?)

- Manufacturing defect a likely possibility. I dont know too much
about how these things are exactly made, so I can't offer an
explanation in this area.

Ron
http://cozybeehive.blogspot.com

It might be something that was defective, but it also broke exactly where
I'd expect it to, if it had been previously involved in a crash. The
pictures don't tell us enough, because we can't see if there's any evidence
of distortion of the aluminum, nor a directional component to the torn
carbon fibers. If you could take some additional photos with the camera in
its "macro" setting we might be able to tell quite a bit more.

By the way, did this actually happen recently? Just curious where, if it
did, since it appears to be pretty dry for this time of year. Southern
hemisphere?

--Mike-- Chain Reaction Bicycles
www.ChainReactionBicycles.com

bicycle_disciple said:

Hi,

Please view these pictures. http://www.flickr.com/photos/tree_trunks/sets/72157600514688243/

Its a carbon fiber failure in a bicycle fork at steerer tube-crown
junction. Any ideas on what could have gone wrong?

My observations :

- Steerer tube seems to have been bonded to an alloy or ALU crown of
fork, and then maybe sanded down.

What's the point of that aluminium sleeve? Presumably the forks
themselves are made of carbon-fibre?

I'm wondering if the sleeve is there to have something to mount the
front brake to.

Just a guess but perhaps a loose headset would help promote that kind of
failure.

On Sun, 3 Feb 2008 06:07:09 -0500, [email hidden] (Michael Baldwin)

Quoted message said:

How could frame size effect the "failed' component?

Dunno. But steel wins again!

Mike Jacoubowsky said:

It might be something that was defective, but it also broke exactly where
I'd expect it to, if it had been previously involved in a crash.

If the steerer tube is really the location most likely to be damaged
in a crash, then that in itself strikes me as evidence of poor
design. The steerer tube is invisible to the user without some
disassembly of the bicycle. Therefore any initial damage from a crash
that only shows up on the steerer is likely to go unnoticed if the
rest of the frame (esp. top tube, down tube, and fork blades) is
unaffected. The bike then continues to be used until there's the
possibility of the initial damage site growing and resulting in a
serious injury. Much better to have a design where the likely
locations of damage that might later fail catastrophically are in
plain sight for even the most casual examination of the frame by the
cyclist.

And it seems to me that most frames and forks are made in such a way
that crash damage to only the steerer tube is unlikely. I see many
frames that show clear evidence of previous crashes by bends in the
top/downtubes and/or fork blades that are bent back. OTOH, failures
of steerer tubes are rare events - which is fortunate given the high
likelihood that they will result in injuries.

still just me said:

On Sun, 3 Feb 2008 06:07:09 -0500, [email hidden] (Michael Baldwin)

Quoted message said:

How could frame size effect the "failed' component?

Dunno. But steel wins again!

but steel fatigues...