Ben Pfaff said:On my way home, a few hundred yards from the office, I managed to
get my shoelace stuck in the chain. Somehow it managed to rip the
front derailer out of position. I was able to limp to the side of
the road and cut my shoelace off, then lift up the chain and get the
remaining piece out of the chain, then bend the derailer by hand
until it was functional again.
Quoted message said:Then, a mile or two from home, I lost a pedal. I coasted to the
side of the road, waited for the line of cars to go by, and
retrieved the pedal. At this point I realized that the pedal hadn't
fallen off; rather, the threaded part of the crank had split. I had
to make it the rest of the way home pedaling with my left foot and
using the right foot to kick the remaining part of the crank over to
start the next stroke. Challenging, but not exactly fun.
Quoted message said:I'm glad that I have a new bike on order. I'm expecting it to be
delivered tomorrow.
You are not safe from another crank failure, but it is good to hear
others admitting that this occurs. Most people are intimidated into
silence by accusations that the bicycle was improperly used. I heard
lots of that over those years that I had at least one crank failure
per year. I broke more than two dozen cranks that way before I said
ENOUGH!
Consider that left cranks have left hand threads so they won't unscrew
and fall off. Consider that tightened as tight as practical, tandems
that do not use tandem cranks (threading on the left front and rear
cranks (that carry the transfer chain [on right hand cranks]) unscrew.
Somehow this interface that violates a basic mechanical design rule is
not recognized in the bicycle industry for what it is, a joint asking
to fail. Cars used to have left hand threads on left wheels for the
same reason. "Do not center loads on threads!" Today lug nuts have
conical faces and lock into place securely. No more left hand threads
on passenger cars.
I modified my cranks to have a conical seat for a conical shoulder on
the pedal shaft. Because I must use existing pedals, I emulated the
conical shoulder with a split conical adapter that is pressed securely
into the thread relief of the pedal shaft by the taper. Formerly I
inspected both cranks with a loupe under bright light before every
longer ride (finding cracks on occasion). Today I no longer do that
and have not had a failure since I modified the cranks more than ten
years ago.
If you don't believe your pedals are moving, just look at the face of
the crank when the pedal is removed. The erosion you see is fretting
damage caused by movement and this movement generates crack both on
the crank face and in the threads.
I cannot interest anyone in the bicycle component business with this
modification, because as Carl Fogel says, "just because Jobst said so
is not proof that it occurs".
http://www.chiark.greenend.org.uk/~damerell/bikes/brandt-cranks.jpg
http://pardo.net/bike/pic/fail-001/FAIL-016.html
Others have had their cranks modified after such a failure, but what
do they know.
Jobst Brandt
