Bicycle Repair/Rear derailleur
I have a problem with the rear derailleur on my rocky mountain slayer 30, it's a shimano gear. I have adjusted everything so that it shifts down the gears (up to the biggest cog at the back) perfectly but when i try to go to the smaller cogs nothing happens, i've found that when i change down i can push the derailleur over manually but it's not going on it's own. I've had a look at the spring inside and it looks like it's under tension but it's behaving as if it's too weak to change up, or not even attached at all. Any idea what's gone wrong and how to fix it? (It's quite a new bike so I don't think it can just be general wear and tear).
My guess is there is something binding the release of cable tension in the shift cable system.
Try disconnecting the shift cable from the rear derailer attachment point.
Next, push the derailer manually towards the inboard(Low) side of the cluster (while pedaling, of course.) With the rear derailer in the Low gear position, release your hand pressure, and if everything is okay with the derailer return spring tension, whilst pedaling again, the derailer should return to its normal/High gear position.
If so, then the problem is likely to be found in the cable system (kinked cable/housing and/or delaminated housing-where one or more of the housing wire sheaves has crept out of the bundle and is impinging on the cable's movement.) Cable housing delamination can be confirmed by removing the ferrules at each cable housing junction, and looking for any individual wires that form the bundle or shell of the housing that have poked out from the others. This will cause enough cable drag to impede the cable's travel. It is a common effect of housing that has been cut too short.
If it is not the cable, then the next likely cause is the shifter unit itself. There is not much that can be serviced on Shimano shifters, so if it's less than two years old, there may be recourse through Shimano's warranty program.