My road bike\’s fourth-largest cog and large ring are a bad match, all right. They just don\’t seem able to hook up.

Today I tried to shift up to the big ring (50 teeth) while I was on the 24-tooth cog. I pushed the brifter; held it; and the chain rattled and clattered, but did not get onto the big ring. I shifted back to the small ring, or tried to. That was futile; the chain had fallen off.

I had to stop and put the chain back on the small ring; then hold the rear wheel off the ground while I turned the pedal with one hand and shifted back to the biggest cog with the other hand.

I don\’t know why that happens, unless it has something to do with the fact that the tooth-count ratio between my big ring and the 24-cog is very close to 50/50. If so, I say it\’s weird. Fifty-fifty is so often considered a good thing.

I suppose it might be a quirk of my particular bicycle. I\’m glad the bicycle in general isn\’t a bad match.

Later addendum: the problem turned out to be worn pulleys. I got them replaced, and now the shifting works fine.