Gears not engaging
 

Hey --

I've got an old Trek with Sora components. Every once and awhile (usually after I somehow or other manage to bang it around) the gears won't engage. I can set it on the ground and pedal forward and the chain (for lack of a better word) floats over the gearing and provides no resistance to propel it forward. What is happening here?

When I put pressure on the rear derailleur it'll engage and the gearing will work, but after I take my hand off it'll usually stop functioning.

I noticed the cassette seemed to be shifting, so I took the wheel off and hand-tightened the nut that holds the cassette to the wheel and that seemed to help. Was a loose nut the cause of the problem or was it a temporary fix?

Thanks in advance.

If this was all there was to it I'd say that the shifter, or the shifter cable is gunked up, causing the RD not to settle properly in the right place. Cables are easier, so I'd start with replacing that.

The cassette lockring should be torqued down fairly hard. If the casette is free to slide along the body that would certainly mess upp the shifting too. Hand tight isn't enough, you need the splined tool bit and some sort of leverage. A torque wrench is nice.