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Mysterious rear derailleur (?) problem
#1
I have a two-year-old K2 roadbike with Shimano Sora derailleurs and shifters (partial STI) that currently have a mysterious problem. While it has only rarely had perfectly smooth shifting, on a ride the other day, the rear derailleur suddenly refused to shift up or down. Luckily, it stuck itself in a more or less manageable gear.

So, I've been trying to troubleshoot with not much luck. I've narrowed it down to a problem with either the rear derailleur cable (broken or frayed?) or the rear derailleur itself. It may also be a problem with the right shifter, but I'm doubtful.

The shifter is only able to move the chain on the rear derailleur between two cogs. Imagine if somebody had tightened both the limit-screws to make the range of the derailleur extremely narrow (though this is not the case, obviously!). Beyond these two cogs, the shifter doesn't even click, just moves to the end of its range without much resistance. I can change which of the two cogs the derailleur is stuck on by tightening or loosening the rear derailleur cable using the cable-fixing nut and the two barrel adjusters (making the cable looser moves the derailleur onto the smallest cog, making the cable as tight as possible moves the derailleur only up onto the fourth/fifth cog--I can't get it to go any higher than that).

My next great plan is to replace the rear derailleur cable. Is this the problem, or just a shot in the dark?

Thanks in advance!
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#2
Disconnect the cable at both ends and check if each component has full movement. Find the one that sticks and fix it. The system has 3 parts: shifters, cable,derailleur, one of them sticks.

Isolate your problem by testing each component separately.Thats the key to any troubleshooting.
Never Give Up!!!
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#3
I have a similar problem with a Sora front mech, which after only a few months has decided to freeze on the top ring, I have eliminated the cable and the shifter, and can only conclude that for some reason the pivot points have worn and tightened up, even lubing and working it by hand has not freed it to a usable condition.
As suggested above, try each part separately, although you can assess it without disconnecting it by pulling on the cable a the mid point and get some one to hold the bike off the ground and turning the pedals, or use a work stand if available.
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