Cassette ratchet problem or worn cog??

Bike is a Specialized Allez with less than 1500 dry road miles... Shimano
Ultegra STI drivetrain (8-speed).... cleaned regularly...

Replaced my chain the other day (working on theory of replace chain #before#
it shows signs of wearing out in order to get more life out of
cassette/chainrings)... neither front nor rear cogs appear worn in any way
and besides at this level of mileage so they should'nt be. However I now
have this odd problem where on ONE cog only on the cassette (3rd gear) I am
experiencing what I can only describe as the sort of thing that would happen
if a pawl on the ratchet slips or fails to engage and this is only
noticeable when I am applying a fair bit of effort to the pedals... the
crank will suddenly "give" and turn forwards a fraction of a rotation
without the wheel turning and annd alarming "clunk" as it re-engages. It
appears to be specific to this one gear. As stated all cogs appear fine.
trying to reason this one out logically gives me a couple of problems:

If a pawl on the rear ratchet is failing/having a problem of some kind -
then why not ALL gears?

If the chain (new SRAM PC68 as I previously fitted) has a problem - then why
not ALL gears?

One thing I did differently during the chain replacement this time was I had
to fix a link back on as I had taken off one link too many (my cycle seems
to prefer 104 links - the chain is supplied as 114 links). I was not too
keen on concept of refixing a link and re-using pin I had previously taken
out using the proper removal tool but a quick call to my LBS and they
re-assured me ok. Initially this link was stiff so I had to gently prise the
two plates apart VERY slightly to lose the stiffness after I had refitted
the pin. Besides the traditional slow reverse turning of the pedals reveals
no lifting links going through the jockey wheels etc. The re-fitted pin is
not protuding beyond the levels its neighbouring ones do so chain APPEARS
fine. After this "repair" I joined the chain properly using the supplied
Powerlink.

One idea could be that this particular cog on rear cassette is worn and
maybe I should replace it but unles my eyes are deceiving me it looks fine.
(Maybe someone could advise me on some of the more subtle signs of
teeth-wear I should be looking for?)

So am up a gum tree... any ideas???

On Mon, 5 Dec 2005 17:52:46 -0000, "Deep Thought"
wrote:

If a pawl on the rear ratchet is failing/having a problem of some kind -
then why not ALL gears?

That's the clue. It's just in that one sprocket.

If the chain (new SRAM PC68 as I previously fitted) has a problem - then why
not ALL gears?

By any chance, is the one that's troublesme also the one that you use
most?

One thing I did differently during the chain replacement this time was I had
to fix a link back on as I had taken off one link too many (my cycle seems
to prefer 104 links - the chain is supplied as 114 links). I was not too
keen on concept of refixing a link and re-using pin I had previously taken
out using the proper removal tool but a quick call to my LBS and they
re-assured me ok. Initially this link was stiff so I had to gently prise the
two plates apart VERY slightly to lose the stiffness after I had refitted
the pin. Besides the traditional slow reverse turning of the pedals reveals
no lifting links going through the jockey wheels etc. The re-fitted pin is
not protuding beyond the levels its neighbouring ones do so chain APPEARS
fine. After this "repair" I joined the chain properly using the supplied
Powerlink.

This might be the source of your problem, but IMO it's less likely
than a worn single sprocket; once again, if the chain was the problem,
I'd expect it to occur in multiple gears.

One idea could be that this particular cog on rear cassette is worn and
maybe I should replace it but unles my eyes are deceiving me it looks fine.
(Maybe someone could advise me on some of the more subtle signs of
teeth-wear I should be looking for?)

Look for burrs along the driven side of each tooth, and closely
examine it for bent teeth as well. The old, slightly sloppier chain
may have been more tolerant of a tooth that was slightly out of whack.

That's just my two cents' worth; others will have additional advice,
and some of it may be better than mine.
--
Typoes are a feature, not a bug.
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Words processed in a facility that contains nuts.

A shy person writes:

Bike is a Specialized Allez with less than 1500 dry road
miles... Shimano Ultegra STI drivetrain (8-speed)... cleaned
regularly...

Replaced my chain the other day (working on theory of replace chain
#before# it shows signs of wearing out in order to get more life out
of cassette/chainrings)... neither front nor rear cogs appear worn
in any way and besides at this level of mileage so they shouldn't
be. However I now have this odd problem where on ONE cog only on the
cassette (3rd gear) I am experiencing what I can only describe as
the sort of thing that would happen if a pawl on the ratchet slips
or fails to engage and this is only noticeable when I am applying a
fair bit of effort to the pedals... the crank will suddenly "give"
and turn forwards a fraction of a rotation without the wheel turning
and alarming "clunk" as it re-engages. It appears to be specific to
this one gear. As stated all cogs appear fine. trying to reason
this one out logically gives me a couple of problems:

If a pawl on the rear ratchet is failing/having a problem of some
kind - then why not ALL gears?

If the chain (new SRAM PC68 as I previously fitted) has a problem -
then why not ALL gears?

The one that skips is more worn than the others. Replace the sprocket.
The same pawl holds the load independent of where the chain is running.

Jobst Brandt

On Mon, 05 Dec 2005 17:52:46 +0000, Deep Thought wrote:

Bike is a Specialized Allez with less than 1500 dry road miles... Shimano
Ultegra STI drivetrain (8-speed).... cleaned regularly...

Replaced my chain the other day (working on theory of replace chain #before#
it shows signs of wearing out

Umm, what are "signs of wearing out"? Measure the chain. If 12 links are
more than 12 + 1/16 inches long, it's worn.

in order to get more life out of
cassette/chainrings)... neither front nor rear cogs appear worn in any way
and besides at this level of mileage so they should'nt be. However I now
have this odd problem where on ONE cog only on the cassette (3rd gear) I am
experiencing what I can only describe as the sort of thing that would happen
if a pawl on the ratchet slips or fails to engage and this is only
noticeable when I am applying a fair bit of effort to the pedals... the
crank will suddenly "give" and turn forwards a fraction of a rotation
without the wheel turning and annd alarming "clunk" as it re-engages. It
appears to be specific to this one gear. As stated all cogs appear fine.
trying to reason this one out logically gives me a couple of problems:

If a pawl on the rear ratchet is failing/having a problem of some kind -
then why not ALL gears?

Your logic is faulty because you are ignoring other possible causes.
These symptoms are classic. Let me guess: that one cog in back is the
one you use most.

Your chain is skipping. The cog is worn, whether or not it looks that way
to you. The old chain, even at only 1500 miles, had worn too much. As a
chain wears, it becomes longer, "stretches" (it doesn't really stretch,
but the worn pins give more space between links), and so the cog wears so
that the slighttly-more-than 1/2" spacing between links fits the cog. New
chain does not fit, so climbs out of the sprocket, usually when you are
applying a lot of force.

It has nothing to do with the ratcheting mechanism, which affects all
sprockets equally.

Replace the cassette. In a just world you could replace only the sprocket
that is worn, but 1) they are all riveted together, so you would have to
grind off the head of the rivets (no big deal), and 2) a single sprocket,
even if it is available, would cost darn near what a full cassette will,
and it probably isn't available, and 3) others are probably worn, too.

Next time, clean and re-lube your chain more often. That will extend
chain life.

One idea could be that this particular cog on rear cassette is worn and
maybe I should replace it but unles my eyes are deceiving me it looks
fine. (Maybe someone could advise me on some of the more subtle signs of
teeth-wear I should be looking for?)

Your eyes deceive you. The best test to see if a sprocket is worn is to
try it with a new chain. If it skips, and yours did, it's worn.

--

David L. Johnson

__o | As far as the laws of mathematics refer to reality, they are not
_`\(,_ | certain, and as far as they are certain, they do not refer to
(_)/ (_) | reality. -- Albert Einstein

This is a case where logic is more reliable than what you *think* you see
with your own eyes. As Jobst says, everything points to a problem with that
one cog.

....looks like new cassette then. I am staggered regards sprocket wear at
this mileage however - I mean I cleaned the chain so often I could have been
described as becoming rather anal about it... 8-speed HG90 Shimano 12/21
cassettes are becoming hens teeth - I assume a Dura Ace 12/21 8 speeder
should suffice?


"David L. Johnson" wrote in message
news
On Mon, 05 Dec 2005 17:52:46 +0000, Deep Thought wrote:

Bike is a Specialized Allez with less than 1500 dry road miles... Shimano
Ultegra STI drivetrain (8-speed).... cleaned regularly...

Replaced my chain the other day (working on theory of replace chain
#before#
it shows signs of wearing out

Umm, what are "signs of wearing out"? Measure the chain. If 12 links are
more than 12 + 1/16 inches long, it's worn.

in order to get more life out of
cassette/chainrings)... neither front nor rear cogs appear worn in any
way
and besides at this level of mileage so they should'nt be. However I now
have this odd problem where on ONE cog only on the cassette (3rd gear) I
am
experiencing what I can only describe as the sort of thing that would
happen
if a pawl on the ratchet slips or fails to engage and this is only
noticeable when I am applying a fair bit of effort to the pedals... the
crank will suddenly "give" and turn forwards a fraction of a rotation
without the wheel turning and annd alarming "clunk" as it re-engages. It
appears to be specific to this one gear. As stated all cogs appear fine.
trying to reason this one out logically gives me a couple of problems:

If a pawl on the rear ratchet is failing/having a problem of some kind -
then why not ALL gears?

Your logic is faulty because you are ignoring other possible causes.
These symptoms are classic. Let me guess: that one cog in back is the
one you use most.

Your chain is skipping. The cog is worn, whether or not it looks that way
to you. The old chain, even at only 1500 miles, had worn too much. As a
chain wears, it becomes longer, "stretches" (it doesn't really stretch,
but the worn pins give more space between links), and so the cog wears so
that the slighttly-more-than 1/2" spacing between links fits the cog. New
chain does not fit, so climbs out of the sprocket, usually when you are
applying a lot of force.

It has nothing to do with the ratcheting mechanism, which affects all
sprockets equally.

Replace the cassette. In a just world you could replace only the sprocket
that is worn, but 1) they are all riveted together, so you would have to
grind off the head of the rivets (no big deal), and 2) a single sprocket,
even if it is available, would cost darn near what a full cassette will,
and it probably isn't available, and 3) others are probably worn, too.

Next time, clean and re-lube your chain more often. That will extend
chain life.

One idea could be that this particular cog on rear cassette is worn and
maybe I should replace it but unles my eyes are deceiving me it looks
fine. (Maybe someone could advise me on some of the more subtle signs of
teeth-wear I should be looking for?)

Your eyes deceive you. The best test to see if a sprocket is worn is to
try it with a new chain. If it skips, and yours did, it's worn.

--

David L. Johnson

__o | As far as the laws of mathematics refer to reality, they are not
_`\(,_ | certain, and as far as they are certain, they do not refer to
(_)/ (_) | reality. -- Albert Einstein

On Mon, 05 Dec 2005 22:06:37 +0000, Deep Thought wrote:

...looks like new cassette then. I am staggered regards sprocket wear at
this mileage however - I mean I cleaned the chain so often I could have been
described as becoming rather anal about it...

Maybe it's a technique issue. I don't think those on-the-bike scrubbers
do much good, and wiping the chain down with a rag does harm by
pushing grit inside. I always remove the chain, soak and agitate in
solvent, scrub, dry, then measure. Wear is not entirely unavoidable, but I
agree that 1500 miles is low.

--

David L. Johnson

__o | And though I have the gift of prophecy, and understand all
_`\(,_ | mysteries, and all knowledge; and though I have all faith, so
(_)/ (_) | that I could remove mountains, and have not charity, I am
nothing. [1 Corinth. 13:2]

David L. Johnson writes:

...looks like new cassette then. I am staggered regards sprocket
wear at this mileage however - I mean I cleaned the chain so often
I could have been described as becoming rather anal about it...

Maybe it's a technique issue. I don't think those on-the-bike
scrubbers do much good, and wiping the chain down with a rag does
harm by pushing grit inside. I always remove the chain, soak and
agitate in solvent, scrub, dry, then measure. Wear is not entirely
unavoidable, but I agree that 1500 miles is low.

Hold it. How worn was the chain. Chain cleaning only helps the
chain, not the sprockets on which it runs. If the chain was
significantly out of pitch it would readily cause skipping sprockets.
Was the old chain ever measured for wear?

Jobst Brandt

Have replaced the cassette since I last wrote to you folks (had a new HG90
12/21 I bought from a US dealer a year or so ago and was keeping for spare -
thank god for the global marketplace that is eBay) - seems to have cured the
problem. Close look at the old cassette shows the problem sprocket in
particular had tiny little "lip" on each of the teeth...compared to all the
other sprockets this was the only one showing a "lip" on the right hand side
on each tooth.

Will have to search for other 8-speed Ultegra spares now (hate not having
something in reserve) - while I think about it - I thought the Dura-Ace
8-speed cassette made a viable replacement but a seller of one has advised
me the following:
Quote: only really works with Dura Ace 8 speed shifters and rear mech. This
is due to the pitch of the cassette, which is the distance between the
middle of two sprockets which was unique to that groupset.

Is this a myth as I was sure all 8 speed Shimano was interchangeable??


wrote in message
...
David L. Johnson writes:

...looks like new cassette then. I am staggered regards sprocket
wear at this mileage however - I mean I cleaned the chain so often
I could have been described as becoming rather anal about it...

Maybe it's a technique issue. I don't think those on-the-bike
scrubbers do much good, and wiping the chain down with a rag does
harm by pushing grit inside. I always remove the chain, soak and
agitate in solvent, scrub, dry, then measure. Wear is not entirely
unavoidable, but I agree that 1500 miles is low.

Hold it. How worn was the chain. Chain cleaning only helps the
chain, not the sprockets on which it runs. If the chain was
significantly out of pitch it would readily cause skipping sprockets.
Was the old chain ever measured for wear?

Jobst Brandt

All the cogs and gear rings will wear down at different rates. This is
because of terrain factors and rider gear preferences. If you are using
the third cog for most of your riding, then this cog will wear out the
earliest. Try varying your gear / cog combination when riding as to not
be to complacent in using one combination most of the time.