Daniel Kelly \(AKA Jack\) said:I fear my bike has a fracture in its front cog. I first
noticed something was wrong when I heard the front cog
hitting against the front derailleur once every
revolution of the pedals. It turned out the front cog was
actually bent and so "wobbled" as it rotated, relative to
the front derailleur. Closer inspection showed that the
front cog was a little loose and could be jiggled a
little. The join between the front cog and the right
pedal was clearly loose.
I took the right pedal off and found a little crack in the
front cog. Some photos are here:
[snipped]
Quoted message said:
Is this a fracture?
Yes. It looks like you've got a typical non-repairable crank
set there. The large and small sprockets (also called
chainrings) are both riveted to the middle-size one, and the
middle sprocket has the splines broached into the center to
mate with the hub. Unfortunately, these almost never can be
disassembled nondestructively, and no replacement parts for
the individual pieces are sold in any event.
Quoted message said:If so, how dangerous is it?
It will continue to wear away at the teeth of the splined
stub on the crank, and will get wobblier as it does so,
eventually failing in a rather final manner if your patience
isn't tried to the point of launching it into the North Sea
first due to the chain rubbing and such.
Quoted message said:Can I ride my bike with this fracture?
For a while, with increasing inconvenience, probably yes. It
is, however, very difficult to forecast the rate of
deterioration.
Quoted message said:Do I need a new front cog & right pedal?
You need what is called a "crank set" by some vendors, a
"chain set" by others. The terminological uncertainty is
vexing. What you will buy is two cranks (the arms into which
the pedals are screwed), of which one will have a set of
sprockets on it.
Count the teeth on the old sprockets and shop for a crank
set whose tooth count and crank length (probably 170mm, but
measure to make sure) matches what you have now. Your old
one looks like a Sunrace Apex set; you won't find the same
make and model available as a spare in all likelihood, but
there are a number of alternate choices available.
Bear in mind that you may need to replace the bottom bracket
(this is the confusing name used in the industry for the
bearing and shaft assembly on which the cranks are mounted)
if the length of your existing shaft is not a close match
for the requirements of the replacement cranks.
If there is a shop in your area which acts as a breaking
yard for old bikes, you might be able to secure a used
replacement crank and sprockets from them inexpensively. If
this is a bike that you rely upon for transportation, I
would advise buying new.
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