Gears Misbehaving

Sheldon Brown should get you started...

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Quoted post said:

Shifting to larger sprockets is accomplished by tightening the cable; if such shifts are slow, the cable is not tight enough--turn the barrel counterclockwise to tighten it.

Sounds like the cable might not be tensioned enough. Not enough tension --> derailleur doesn't move as much as it should --> doesn't successfully downshift sometimes.

This is a common thing to happen to near-new bikes, or when you've just recently replaced the shifter cables; the wire has an initial amount of 'stretch' as you use it at the start. After 50-100km or so it should have stretched all that it will, and the adjustments shouldn't need any tweaking after that.

Try fiddling with it by turning the little tension adjust knob thingy (at the back of the rear derailleur, where the cable goes into it) anti-clockwise a couple of notches at a time (you should be able to feel a subtle notchiness to the knob at every 1/4 turn or so), and see if that helps.

Conversely, if you have trouble shifting to a faster gear, it means the cable has too much tension -- and turning that knob clockwise eases the cable tension slightly.

If your tension is seriously out of whack you might need to shift into the smallest cog (where the cable is slackest), un-clamp the shifter cable, and re-clamp it when it's fairly tight. Or if it's just all over the shop, consider getting a new cable (and cable housing).

If you want a full service document to read through, take a look at this one:

http://www.parktool.com/repair_help/FAQrindx.shtml

grayduncs said:

My rear gears are misbehaving. When changing from smallest to largest sprocket the first couple of gear changes are fine. The next change does not happen, and then it jumps two and catches up with where it should be although things are still not quite right until you get to the largest sprocket. Changing gears in the other direction is better but sometimes has problems too.

When the problem occurs the derailleur is moving but not quite enough to move the chain to correct sprocket. The limits to the movement are right (because the largest and smallest sprockets are ok) so is it the indexing that is off?

All advice on what to fiddle with welcome (the rear derailleur is a Shimano 105 9 – speed).

First, I assume you have checked the derailer for any foreign matter in the derailer body. Also, removed the cable to make sure the derailer moves freely.
If so, it sounds like you have a problem with the cable inside the casing. Derailers do not jump gears in the manner you are speeking of due to a stretching cable. If the cable was stretching it would be slow to react when moving from the small to the big sprocket and fast moving from big to small.
One of the wires in the cable my have snapped which will prevent the cable from sliding smoothly. Slide the casing back and forth on the cable, making sure it slides freely.
If it looks like all is working properly, it could be a bent hanger.