Re: Chain skipping on large chain ring.

Marvin Meredith writes:

>>>> PS - If it would help with the diagnosis, I can easily remove the
>>>> chain rings and photograph them on a piece of white paper... Let
>>>> me know if this will help and I will do it...

>>> That would be handy, actually. Then we can see the wear on all
>>> the teeth much more clearly.

>>> Another point I forgot to mention in my mammoth last post: if you
>>> do ride with even a slightly worn chain, the wear is worst at the
>>> points where the chain gets the highest tension. If the cranks
>>> are horizontal, this is usually at the top and bottom of the
>>> chainring. Hence you can have some teeth absolutely perfect and
>>> some significantly worn, which can cause havoc trying to detect
>>> wear.

>> The highest stressed sprocket teeth on a driving sprocket
>> (chainwheel) are the first ones to engage the chain. Thereafter,
>> they make no load contact with the teeth and roll idly through the
>> root of the tooth profile before climbing over the back side of the
>> teeth if in contact at all. That contact depends on the tension
>> given by the derailleur. Crank position has little to do with
>> where the chain bears most heavily, although that affects how much
>> force a rider can put into the chainwheel.

> Okay, that's what I meant, the force on the crank changes which
> changes the amount of wear. So you can (and indeed usually do) have
> a difference in wear between teeth at different points on the ring.
> Your ring doesn't particularly show this, which, again, I suspect is
> down to a brand-new chain spreading the load over more teeth.

It doesn't because I rotate it to spread the wear. Unfortunately it
is a 5-pin system instead of something divisible by four, so I can
only approximate. If we saw the who;e CW, I think you could see some
teeth are less worn than the average.

>>> With a new (or properly worn) chainring and chain this doesn't
>>> happen, because the load is taken evenly on all teeth. Even a
>>> small stretch means the majority of the load is taken by the first
>>> five or so teeth, the rest of the chain/chainring going slack.
>>> You can see this by putting some load on the pedals with a
>>> relatively worn chain and then trying to lift the chain off the
>>> ring at various points.

>> I think you are not assessing the tooth profile of the sprocket
>> shown at:

http://home.comcast.net/~carlfogel/download/zzz_006.jpg

>> The root of the tooth gaps is relatively unchanged from new and
>> shows how far the contact pint has shifted up the pressure face of
>> the teeth by the load pocket near the end of the hook. The tooth
>> nearest the 50 was centered on that number and the attachment bolt
>> hole. Its apex is now substantially shifted by wear. Although
>> severely worn this sprocket still accepts a new chain without
>> problem and climbs the steepest hills as when new. I have others
>> like this.

> As I've said, your chainrings are atypical - I see *no* rings like
> that at the LBS where I work and many, many with sharper ramps like
> the one of Carl Fogel's I keep dragging up as a convenient example.
> I believe this is related to the amount of care taken replacing worn
> chains.

Maybe in your circle of riders, but for mine it is typical. We ride
bike to ride and don't mind riding on less than new components.
Carl's CW has no teeth. Those are only stubs of teeth that could not
drive a new chain. I also have stacks of rims with merely 0.5mm wall
on brake surfaces, also atypical in general, people always being on
the latest fad wheels.

> This might be a long example, so bear with me:

> Assume we have a new chain and chainwheel. As the chain rolls on to
> the chainwheel, each roller will come into contact at a certain
> point. If the chain is the correct pitch, it will contact at the
> same point as the previous roller, which should be the base of the
> tooth profile, and all is well.

> Now the chain wears faster than the chainwheel, so in due time we
> have a worn chain and new chainwheel. Now when each roller engages
> it is slightly further from the previous roller than it should be,
> so it starts taking load and wearing some way up the tooth profile.

That assumption of wear rate is immaterial because the pitch of the CW
remains constant while its pressure pocket in its teeth increases in
height above the base circle as chain pitch increases from wear.

> Assume we don't replace either, and continue riding. The worn chain on
> the worn chainring is now running steadily higher on the tooth profile,
> and as it wears there will be less and less teeth taking any load so
> the wear gets faster. The profile of the tooth is gaining a relatively
> straight ramp until it wears right to the top of the tooth, sharpening
> the top of the tooth quite nicely and slipping under any load at all.

>From elastic considerations only the first few teeth of the CW bear
any significant load. You can test this by lifting the chain off the
CW starting at its exit end.

> If, on the other hand, the chainring never gets used with an
> appreciably worn chain, the wear stays low on the tooth profile. It
> still moves up slightly as shown by your extreme example, but by no
> means as fast as with a worn chain.

> So, in conclusion, the wear moves up the tooth faster with a worn
> chain, and the wear is faster with a worn chain.

Yes??? and now what?

>>> Just as an aside, the difference in wear between heavily and
>>> lightly loaded chainring teeth is quite a good indicator of riding
>>> style - big difference = stomper, small difference = spinner. I
>>> regularly impress easily impressed customers with this particular
>>> sleight of hand :-).

>> I think you should explain how you believe this comes to pass.

> It's very simple, more load at a given point on the chainwheel equals
> more wear. Worn chains don't spread the load anywhere near as evenly
> between teeth and therefore wear chainrings faster. I don't see why
> this is contentious.

It isn't, but you seem to suggest one should ride the lowest gear
possible for the benefit of bicycle components, modifying one's riding
to something other than what might come naturally? I believe the
bicycle is the riders servant, not the converse. Besides, this does
not affect the problem at hand.

>> My worn chainring has about 100,000 miles on it, that is, it was in
>> use for about 10 years and looks much like many others I have seen
>> that were not changed out by equipment fanatics.

> I'll say it again, if someone is careful about changing their chain
> they can indeed expect their chainrings to last for ludicrous numbers
> of miles. If they aren't, well, 10,000 miles is a closer par for the
> course.

So what is wrong with riding a CW for 10x as long by replacing the chain
at 0.5-1.0% chain wear?

Let's get back to the failure analysis at hand. I find it mysterious.
In fact every time I read of chains jumping over CW's in this forum, I
wonder how it is done, not having had this occur on my bicycle or
those of my riding companions since I got my first derailleur bicycle
in the 1950's.

Jobst Brandt
.

Relevant Pages

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  • Re: Chain skipping on large chain ring.
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