Why does my shifter say the wrong gear?

Question

I had a slow puncture in my rear tyre. I fixed it by flipping the bike upside down, removing the rear wheel, patching the inner tube and replacing the wheel. However, I seem to have caused a strange problem.

The first time I shifted the rear gear after putting the wheel back, the chain skipped into the wrong gear. Now the shifter says I'm in gear 6, but I'm actually in 4. I tried lifting the chain onto gear 6 to make the shifter correct, but as soon as I pedal it skips back into gear 4 while the shifter still says 6.

I also now get a rattling sound when pedalling, which seems to come from the front cogs. The chain has also skipped over the cog teeth a couple of times (I think that's what happened; both times I just heard a noise and the pedals shot forward a inch or so).

Does anyone know what's happening and how to fix it please?

Some context:

• Before removing the wheel I changed the rear gear into 7 (the smallest cog). I did it by lifting the rear wheel off the ground with one hand, clicking the shifter up to 7, then turning the pedals by hand to make the chain shift gears.
• When I first got the bike 8 months ago, it had a problem with the chain coming off when changing gears; the guy at the shop fixed this by adding tension to the chain.
• The bike is a Cannondale Quick 6. The shifters say 'Promax' and 'Microshift' on them, and the rear derailleur says 'Microshift 26' on it. I've attached a couple of photos below taken after the problem started.

Any help would be much appreciated!

Answer 1

In the pictures, it looks like the outer shift cable tube is not properly seated in the barrel adjuster at your rear derailleur.
Shift into your highest gear and try to move it back into place.

Since you were in your highest gear, you had the least, maybe eve no, tension on the shift cable. By working near the derailleur when removing and re-inserting the rear wheel it's quite possible you accidentally pulled the outer cable out of the barrel adjuster without noticing.

[EDIT] You say you shifted to highest gear, and then turned the pedals. This means your derailleur could not at first move completely to the outside. As a result, the cable went completely slack. That is most likely when the outer came out of the barrel adjuster. Normally, the moving derailleur keeps the cable stretched, but when you are shifting far outward from a very easy gear without turning the pedals, the chain prevents the derailleur from moving as it normally would. Since you appear to have an outer all the way from the shifter to the barrel adjuster, there is nowhere the slack can happen, so the outer being pushed out is the only possibility.
This also means in a setup like yours, whenever you shift far out without pedaling, you should expect the outer cable to be pushed from the barrel adjusters, either at the shifter or at the derailleur.

Answer 2

I would suspect that at some point, the outer cable has dropped free of a braze-on and is "pulling" more inner cable than expected.

The common point would be either end of the downtube, or perhaps right under the shifter itself.

To fix it, while stationary change to a lower/harder gear at the shifter. This should slacken the inner cable, and you can insert it back to where it goes in the frame.

Answer 3

The bowden guiding the cable from the frame mount and deraileur is out of the adjustable nut. It's clear from both images you provided. So your indexes show the correct gear, your cable track shifts the alignment to lower gears. I would bet my pants you couldn't shift to the lowest gear.

While standing still, shift to the smallest cog (highest gear). Just pull the bowden out from the deraileur gently and align it well inside the nut. You can also push the deraileur to the lower gear to release the tension.

You may have pulled the cable while getting the wheel out or in and the cable was pulled back wrong way.