Let's see - that's the only cog it happens on, the stress on it is higher on the large chainwheel, but the stress and chain deflection are both larger on the chain and chainwheel with the smaller cogs, and that's the combo you ride in the most. The only logical conclusion I can reach is that the cog is worn.
Last edited by cny-bikeman; 10-01-12 at 10:33 AM.