Reeks of aged cotton duck
Join Date: Oct 2007
Location: Middle Georgia, USA
Posts: 1,176
Bikes: 2008 Kogswell PR mkII, 1976 Raleigh Professional, 1996 Serotta Atlanta, 1984 Trek 520, 1979 Raleigh Comp GS
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If I was a betting man, I'd bet that this problem is being caused by your shifting technique.
You said that your derailleurs are adjusted correctly and that it only happens when you're riding uphill. It's your technique... I'd bet the chickens on it... and here's why:
When you pedal, your bike's drive chain has a span of chain that is under tension... the top span... and a span of chain that is relatively slack... the bottom span. And your derailleurs shift gears by moving different parts of the chain... the front derailleur moves the top span of chain and the rear derailleur moves the bottom span of chain.
Because the bottom span is never under tension (well... no tension beyond the tension that your derailleur springs apply) you can shift rear cogs even while you're cranking and grinding on the pedals. The rear derailleur can move that chain any time because the bottom span never gets tight.
But the front derailleur is different. It has to move that top span, and when you're grinding away up a hill... that span of chain quickly gets pulled under such tension that the FD can't make the chain hop chainrings. This is especially true when you're grinding up a hill and suddenly realize that a cadence of 30 rpm isn't the easiest to push. Heck... if your FD has weak springs in it, the least little hill can put you into a situation in which you're trying to shift the FD while putting pressure on the pedals and it just won't go. Does it shift as soon as you let up on the pedals and let them spin for a couple of revolutions?
Try shifting while spinning the cranks under no pressure... I'll bet the problem goes away.
As far as the problem worsening over the day:
Riders tend to let their cadence drop as they tire over the day, and I'll bet that you were tending to grind a little bit lower cadences as you approached those hills. So before you really started climbing the hill, it put you right in the chain tension situation that I described.
The problem did get worse, but probably not because of any mechanical issue.