Stress concentration due to severe cross sectional change. Look at
historic handlebar design for methods of reducing the cross-sectional
change by adding darts to the gooseneck. Looking down on a gooseneck,
you see something like this:
I am the OP of the discussion Rick refers to. Unfortunately I know little
about Mechanical Engineering, and have not been able to find any useful
information on the web to clarify your post. Would it be possible for you to
explain what you are saying in a more layman-compatible format, as I am
really quite interested to understand what you mean.
Specifically, I am not sure what you mean by "severe cross-sectional
change", "darts", "gooseneck", nor what your ASCII diagram represents.
Thanks in advance,
I believe the cross section is constant through the broken area. The
bars are broken at the stem opening (about 1/2 inch from center), yet
the bar's OD is constant for about 2 inches from center.
Yes, these bars show such a "smooth" tapered OD in the region away
from the break, near the blue colored handlebar tape. See the first
Not mentioned is that the handle bar has a nominal OD of 26mm, and the
stem a nominal ID of 25.8mm. I wonder if such a small mismatch could
cause local plastic deformation, maybe at the pinch closure, and
initiate a crack?
Many 26.0mm handlebars are installed in even smaller 25.4mm stems,
though this hardly means they won't also fail someday...
Most such failures I have looked at were fatigue, often from scratches
caused during assembly, combined with poor design and high stresses in
service. I have also seen SCC and corrosion related failures although these
have been more common on seat posts.
A picture face on to both sides of the fracture would help.
Looks like it's failed in High Cycle Fatigue where you'd expect it to in
a bending mode.
Can I ask five questions?
a) Have you ever crashed it on that side?
b) Are the roads you ride on very bumpy?
c) Do you ride with a high saddle and tend to put a lot of weight on the
d) What's the typical ambient temperature when you go out riding?
e) We can't see from the pictures, but is the tube inside the clamp
distorted or show any evidence of being over-tightened?
I'm sure I could think of some more if I tried.
Probably not enough to cause damage that would initiate a fatigue
Yeah, that one would be a bit subjective.
So you're not Colonel Blimp and you haven't caused the bars to yield!
Rules another one out
How deep was the scratch? I would have thought that minor scratches
wouldn't be enough to initiate a failure. Could you define it as a
'score' rather than a scratch?
Could still do with some more photo's of the fracture face.
It's difficult to say as I only had a momentary look at it. It was
wide and deep enough to catch my fingernail in it, so it was
definitely more than a minute superficial surface stratch.
I can't work out what would have caused that though -- surely there's
not much movement there, and certainly not any rotational movement
(the clamping mechanism has a series of ridges which lock with ridges
on the handlebars, eg.
also elliminates the possibility of a mechanic twisting the bars in a
relatively tight clamp).
I'm afraid that will not be possible as the bars are now in the
posession (or garbage bin, as the case may be) of Modolo. They had to
send them off in order for me to get a warranty replacement, and I
never got them back. I even had the bike back the very next day, but
I've not actually had a chance to ride it yet because of the
spontaneous snow-storm! I hope to go out for the club ride tomorrow
morning though, weather conditions permitting.
Dani / asqui
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