I'm stumped. I'm running a Tiagra 4503 crank, HG53 chain and cassette. Chain and cassette have 2000-3000 miles. Crank is pushing 13000 miles. Cassette is a 12-23 9-spd.

When I'm in the big chainring-central cog combination, I get quite a bit of popping and roughness. For those that have had a chain break slowly, it almost feels as if one of the plates in the chain is about to pop off, or that maybe I have a stiff link, but upon inspection all looks fine, and in any other gear combination, all feels smooth.

It's gotten progressively more noticeable over the past few rides, and seemed to get worse when I relubed my chain.

I do spend the majority of my time riding in that gear combination.

Any suggestions?

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.

That's my instinct, but I don't feel the catching and popping in the middle chainring, even when pushing equivalently hard in that combination.

In fact, it's ever more noticeable at medium speed under moderate tension. Under constant high speed, high tension, it's less noticeable.

Sounds to me like your big ring and/or that cog might be worn.

How much elongation on the chain?

And maybe the cassette has come a bit loose?

The chain shows little if any wear that I can measure.

No visible wear on that particular cog.

If the problem were in the big chainring, it seems like it would affect other combinations with the big chainring. In fact, the next smaller cog is silky smooth in combination with the big ring.

1. See my post above - can't be the chain, as worse chain angles and higher gears show no problems.

2. You cannot generally SEE the wear on a cog until it is extreme

3. Exactly the point I made above. The evidence points to the cog and nothing else.

Instead of focusing on what you think it isn't you need to focus on the only thing it can be. Replace the cog if you can, or the cassette, and the chain if things aren't smooth with a new cassette.

Harmonics couldn't complicate things a bit?

Why propose an arcane cause when the most obvious has not been tried? Besides, anything like harmonics would show as ....something harmonic, such high frequency vibration - not what the OP is describing.