Worn out chainrings?

wrote:
I've spent hours searching in groups. Maybe there's a NEW approach to
this.

I've a pile of used chainrings. Just as if I had a pile of used
chains, I'd like to describe their wear. How to do it?

I tried putting a 5 inch length of new SRAM PC58 chain on each ring,
holding down the last links and pulling up; also moving the length of
chain around the rings. For comparison with used chainrings I had 3
new rings. A circa-1995 Campagnolo 53t ring and a new 1985 Takagi 52t.
Both rings were ground and should represent quality machining. I also
had a new old stock 52 t 144bcd ring by ???

Aside from visually obvious wear to the face of the teeth of some used
rings, I could not say that the movement of the chainon the used
chains differentiated them from the movement on the new ones. There
was as much difference between the new chainrings as there was between
new and used.

This isn't academic if I'd like to predict how the ring will perform,
for me or someone else, or if I'd like to be sure the ring will not
excessively wear or damage a new chain, and then cassette sprockets.

So what is the metric, besides "chain skips on it under load" and
"teeth look hooked?

Andrew Muzi wrote:
Yes, it's hard to evaluate chainring wear! A new chain will generally
run acceptably well on old rings but shift response may suffer. The
teeth have to be down to ugly little stumps before you get actual
failure. That limit is a smaller stump on big rings than on small
rings because fewer teeth engage on small rings. (32t and 36t triple
middle rings being the most commonly replaced 'worn out' sizes)
Definitive overview he
http://draco.acs.uci.edu/rbfaq/FAQ/8d.2.html
short answer: No definitive measurable limit off the bike.

Tom Sherman wrote:
So how does the professional bicycle mechanic decide when to recommend
new chainrings [1] to the customer?
[1] Or is a new crank with rings included less expensive?

There are indeed cases where 2 or 3 rings on an older crank cost more
than a complete current model crank set (complete cranks are a zero-duty
item for some reason; other factors come into play as well).

We normally replace chainrings after customer report of wear symptoms as
we cannot see that they are dysfunctional except in extreme cases.
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

A Muzi writes:

Tom Sherman wrote:
So how does the professional bicycle mechanic decide when to recommend
new chainrings [1] to the customer?
[1] Or is a new crank with rings included less expensive?

There are indeed cases where 2 or 3 rings on an older crank cost more
than a complete current model crank set (complete cranks are a zero-duty
item for some reason; other factors come into play as well).

We normally replace chainrings after customer report of wear symptoms as
we cannot see that they are dysfunctional except in extreme cases.
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

http://i20.ebayimg.com/04/i/000/e6/06/5cd8_1.JPG

Here are some chainrings on Ebay that are probably 'at least' 70%
gone.

Some signs of severe wear are (a) Chamfering has worn away from the
sides of each tooth, ( see http://en.wikipedia.org/wiki/Chamfer ) (b)
between each tooth, it looks more like a "U" than a semicircle, and
(c) the slope of the space between each tooth looks like \_|, not |_|,
i.e. the teeth are 'leaning back' on one side only.

The width of each tooth is really how we should measure chain wear.

- Don Gillies
San Diego, CA