From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: What about chainring wear?
Date: 31 Aug 2000 18:00:54 GMT

Doug Huffman writes:

> Grasping another opportunity to be wrong!

Why don't you stop doing this and report only those things you know
instead of guessing.  This is a disservice to readers who are not
aware that your authoritative postings are largely BS as in the
example here.

>> I know the chainring wears differently than the cogs, since it is
>> loaded differently, and of course there's more teeth.

> Loaded differently?  How so?  The fact that there are more teeth is
> a mere distraction.  Only the last tooth is loaded unless you can
> yield a chain (but that's another rant).

A new chain on a new sprocket rides practically on the root diameter
of the tooth spaces, riding up on the tooth to the point that the
chain pitch lies on the pitch diameter of the sprocket.  As the chain
wears, it rides higher on the tooth because the sprocket pitch remains
unchanged while the chain pitch elongates, defining a new pitch
diameter on the sprocket.  The longer a worn chain is used, the more
wear occurs at the larger pitch diameter making wear pockets.  Under
load the chain rises up to the point that all rollers (actually every
other roller) bear on the false pitch diameter.

A new chain, under load, rolls into the wear pocket of the driven
sprocket and because this is for a chain of larger pitch, is unable to
enter the sprocket on the slack side because it cannot reach the next
tooth.  In severely worn sprockets there is even a visible hook at the
top of the wear pattern that prevents engagement.  However, the wear
pockets at a larger pitch diameter are what causes a new chain to skip
on worn sprocket.  No manner of filing or Dremel tool can fix this
unless all teeth are uniformly reshaped, something that is both
impractical and worthless because the case hardening of the sprocket
is usually worn through if you can see misshapen teeth.

Interestingly there is a difference between driven and driving
sprockets, something that is less apparent because chainrings (driving
sprockets) are seldom changed.  A worn chain will not engage a new
driving sprocket although it will readily accept a new chain, just the
converse of driven sprockets that we must replace when installing a
new chain.  This occurs because under load the chain rides on the base
circle of the sprocket where the pitch is correct but the chain's
pitch is too long and cannot engage the incoming tooth.  Because most
people use a new chain when installing a new drive sprocket, this
seldom comes to light.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: chain slippage
Date: 4 Oct 2000 00:05:47 GMT

Sheldon Brown writes:

> That's it!  Time for a new cassette.  Although only the one sprocket is
> badly enough to skip, others will be worn enough to cause accelerated
> chain wear to your new chain until it "stretches" to match them.

A worn sprocket will not cause accelerated chain wear because if the
chain is running anywhere but at the right pitch diameter it will
skip.  If it doesn't skip it works and won't cause wear.  It is a worn
chain that damages sprockets, not the converse.

>> although they do also appear to look fine to me.

That's because it is fine... for a worn chain with a greater pitch.
Since the base circle and number of teeth hasn't changed, you cannot
see whether the sprocket is worn until the front and back sides of the
teeth are perceptibly different.  That is the most apparent indication
of a worn sprocket.

> Sprockets have to be very, very worn before you can tell just by
> looking at them.

If you look closely, you may see that the shiny pocket where the chain
roller bears is no longer near the root of the tooth.  That's about
all that changes.  In the past we have seen all sorts of hypothetical
exercises of filing the teeth, as though one could manually put the
original pitch circle back into the teeth.  I am sure that those who
proposed these remedies never tried them.  Even if it worked, it would
not work for long because the case hardening is gone from the tooth
face and wear progresses rapidly after that.

One remedy is to reverse the sprocket on the hub so the back side of
the teeth bear the load.  Most newer sprockets do not lend themselves
to this because they have unilateral ramps on the teeth and side cuts
one one side to facilitate shifting under load.  Riding older
freewheels, I get two sided wear from my sprockets that are also all
individually replaceable.

Jobst Brandt      <jbrandt@hpl.hp.com>