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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