Hi all,

I was changing a tire tonight, and I think I may have done something bad to my rear derailleur. I had the wheel partway back on, and I realized I hadn't shifted into the smallest cog, so I tried to do so. The derailleur didn't move, so I tried shifting to a bigger cog instead of a smaller one. The shift lever gave some resistance when I tried to go that way, so (here comes the Bad Move) I kinda forced it. A couple of times. Nothing bad seemed to happen, so I finished putting the wheel on.

I was testing the shifting afterwards, and things were quite messed up. I couldn't reach the smallest cogs, nor could I reach the largest. The shifter clicks, and pays out or pulls in cable for each shift, but the derailleur does not move as it should. Towards the smaller cogs, the cable goes completely slack, yet the derailleur does not move any further outward. Towards the larger ones, the cable is taut, but I still cannot shift into the largest cog unless I pull on the cable by hand to increase the tension even more.

I've tried playing around with the cable tension adjustment, but the result is that I can make the biggest cogs accessible by increasing tension, but no amount of loosening will let me reach the smallest ones.

So: what's my next step? Is there something I could have overtightened and stripped or bent when I forced the downshift? What should I look for?

The bike is a Specialized Sirrus, the rear derailleur is Sora, with trigger shifters.

It sounds like the rear derailer housing, having been heavily compressed, may have pulled thru a ferrule a bit, impeding the smooth movement of the inner wire.

Another possibility is that a single strand or two of the inner wire broke, partially unravering within the housing and similarly impeding smooth cable movement. Broken strands inside the shifter are most common and would require a bit of care to extract the cable's end.

I would start by removing the cable anchor bolt and sliding the rear-most cable housing off of the cable, then test that the shifting movements are actuating the cable through the front-most cable housing and around the bottom bracket guide.
Use care to keep the end of the wire from fraying about, this will make reassembly easier.

A new spare inner cable wire and a length of the rear-most cable housing (plus ferrules) are really good items to have around as spares.

All praise be to Allah, your cable's onset of failure occurs when the bike is in your workshop instead of out on the road.

I, too, would carefully examine the cable and housing. I'm guessing you crimped something when you were installing the wheel. You might end up taking it to a shop and asking them- sometimes it takes an expert eye to spot what's wrong.

Did you take the wheel back out , shift back and forth and see if it works
without the wheel in, just going in and out?
then, put the wheel back in , put it in a repair stand ,
[or hang from a cord so you can work the drivetrain with the wheel off the ground]
and shift it back and forth while turning the pedals etc..?

Check to see if the chain is sitting off the upper pulley...

Good thinking, and remionds me of how sometimes the rear cable housing gets hung up on certain quick-release nuts or a rack strut, either of which can somewhat foul the shifting. The housing should follow it's own path and not tucked in or stuffed under, etc.

Thanks everyone for the advice!

I took out the wheel and unclamped the shifter cable from the derailleur, and quickly found the problem. The cable was not sliding freely through the last piece of housing, either when pulled by the shifter or by hand. Looking more closely at the housing, I can see a spot where it looks like it buckled under the compression, and when I try to pull the cable through by hand, it grinds at that spot.

Looks like I have a nice walk to the shop ahead of me this morning!

Yes indeed! Thanks again.

Ah, yes - mechanic's number one rule when things don't go right:

STOP
OBSERVE
THINK
REDO
then and only then
WHACK IT HARDER

Great.
And if you haven't done the install yet, and are re-using the inner wire, please cut the new housing a quarter inch shorter than the old one.
This way, the previously-clamped (and damaged to a degree) spot on the inner wire will be pulled past the binder bolt washer and not be subject to normal cyclic tension in use that might likely lead to a fractured strand.

And if that doesn't work: Get A Bigger Hammer.

Thanks for the tip! In the end I had the shop do the work, and they replaced both the housing and the shifter cable, which was rusty in several spots and probably overdue for replacement anyhow. Now it shifts silky-smooth

Why walk? You might not be able to shift effectively, but if it's in a mid range gear you can ride it as a single speed.