From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Mechanism for chainring wear?
Date: 24 Mar 1998 00:49:42 GMT

Tim Holland writes:

> I overhauled the bike this weekend, and noticed the familiar
> "sharktooth" pattern on my 40t inner chainring.  The chain came off
> too, replaced with about 1/16" "stretch."  Cleaning the year-old
> Dura-Ace ring off and inspecting it closely, it was not what I
> expected to see.

> I had assumed that the hooks were worn into the ring from the
> abrasive action of grit and the sliding engagement of the chain
> rollers.  However, the rearward edge of each hook had small bumps
> raised on each side, as if the material yielded in plastic
> deformation.  So, what's the mechanism for chainring wear - abrasion
> or compressive yield?

Two things play a roll here.  As the chain wears its pitch matches
that of the sprocket more poorly and rides higher on the drive
sprocket as it engages.  Subsequently the chain rolls to the root of
the tooth before exiting high but under almost no load.  Larger
chainwheels of the 40-50t size can withstand the load unless they have
been worn thin from running with an angled chainline.  I have seen 50t
sprockets with upset tooth faces from high torque.  Even they have
insufficient cross section to avoid yield when heavily driven.  The
hooks arise primarily from wear, stating at the engagement point that
is usually below the tip of the tooth.  The tip escapes wear and is
left as a hooked nose.  The more chain wear the worse the ramp and
smaller the tip of the nose.

> Just say no to aluminum 22's.

That is probably the main contributor to rapid wear.  Aluminum doesn't
last acceptably long with less than about 30t depending on the use.

Jobst Brandt      <jbrandt@hpl.hp.com>