Fractured front cog?

[Daniel Kelly (AKA Jack)]

Dear Bikers,

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:

http://www.xlk.org.uk/forum/modules.php?set_albumName=Bike&op=modload&name=gallery&file=index&include=view_album.php

Is this a fracture? If so, how dangerous is it? Can I ride my bike with
this fracture? Do I need a new front cog & right pedal?

Thanks,
Jack


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[Arthur Harris]

Post by Daniel Kelly (AKA Jack)
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
Unless this is a brit slang I'm not familiar with, I think you're using the
wrong term.

A cog is a tooth in a cogwheel, but often used to mean a rear cogwheel
(e.g., "a 14 tooth cog").

Your pedal threads into a crank. Your picture link isn't working for me, but
if you've got a crack where the pedal threads into the crank, that is VERY
SERIOUS! Your pedal could break free of the crank while you pedaling hard,
causing a bad crash.

For dramatic photos of this type of failure, see:
http://pardo.net/pardo/bike/pic/fail/FAIL-001.html

You need a new crank.

Art Harris



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[Daniel Kelly (AKA Jack)]

Sorry guys, it looks like my web host has gone offline.

Here are some photos that work:

Try these links:

Loading Image...
Loading Image...
Loading Image...

Thanks!
Jack

Post by Daniel Kelly (AKA Jack)
Dear Bikers,
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
http://www.xlk.org.uk/forum/modules.php?set_albumName=Bike&op=modload&name=gallery&file=index&include=view_album.php

Post by Daniel Kelly (AKA Jack)
Is this a fracture? If so, how dangerous is it? Can I ride my bike with
this fracture? Do I need a new front cog & right pedal?
Thanks,
Jack
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posting problems, or contact the moderators, please see http://rbor.org/
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[Werehatrack]

On Mon, 5 Jul 2004 11:03:27 CST, "Daniel Kelly \(AKA Jack\)"

Post by Daniel Kelly (AKA Jack)
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

[snipped]

Post by Daniel Kelly (AKA Jack)
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.

Post by Daniel Kelly (AKA Jack)
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.

Post by Daniel Kelly (AKA Jack)
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.

Post by Daniel Kelly (AKA Jack)
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.

[Gwood]

An excellent summary of the problem and solution.

I might add that frequently the chain stretches over time and begins to chew
into the side of the chainring teeth. It also chews into the teeth of the
rear cassette (the cluster of rings on the back wheel). So if you replace
the chainrings, the old chain may give poor performance - grinding, chain
suck and such. I found that I had to replace them as a set on my bike.

Luckily I had a newer cassette to replace my old one on the back wheel.
Otherwise the new chain probably would have had bad shifting and skipping on
the rear cassette. Hope this doesn't happen to you, but be aware that if
the new crankset causes noise in the rear of your drive train, that's likely
the cause.

Cheers
Gary

Post by Werehatrack
On Mon, 5 Jul 2004 11:03:27 CST, "Daniel Kelly \(AKA Jack\)"

< snip >

Post by Werehatrack
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.

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[Bernie]

Post by Daniel Kelly (AKA Jack)
Dear Bikers,
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
http://www.xlk.org.uk/forum/modules.php?set_albumName=Bike&op=modload&name=gallery&file=index&include=view_album.php
Is this a fracture? If so, how dangerous is it? Can I ride my bike with
this fracture? Do I need a new front cog & right pedal?
Thanks,
Jack
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rec.bicycles.off-road is moderated by volunteers. To find help solving
posting problems, or contact the moderators, please see http://rbor.org/
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Jack - your crankset appears to be worn out.
You need at least a new crankset, but more likely a new drive train...
chain, crankset (front chainrings & pedal crankarms in your case), rear
casset. It's a pity, but metal wears down.
Best, Bernie


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