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Metal shavings on rear dropout
To be honest, I don't know which category this belongs in. Probably wheels and hubs, now that I think about it. Anyway, my girlfriend was complaining of sloppy shifting on her bike, so since she skipped the country for a few days (without me, I might add...), I figured I'd take a look at it for her. Right off the bat I found that the derailleur hanger was loose, so I tightened that up. Then I noticed that there was a ton of grease on the inside of the rear drive side dropout that looked like it had some kind of fabric threads in it. When I started wiping it up I discovered that the threads are metal shavings, and I'm not sure where they're coming from. I suspect they're either from stripped threads in the nut that holds the derailleur hanger to the frame, or from the fretting that had to have been happening from shifting with a loose hanger, but I have a feeling there might be something messed up in the freehub body because of all the grease on the dropout. There's a little bit of grease on the end of the freehub, too. Has anyone ever run into anything like this?
Re: the grease
Hubs (well, bearings in general) can / will loose grease when it is warm. They also loose grease if you put way too much into it (which I tend to do, better than running dry). You can just clean it up or take a look at the bearings (easy if it is cup and cone bearings, but check that first!).

Re: metal shavings
No idea. Your guess sounds alright to me
The guys at the Trek store said the metal shavings were from the chain and the cassette wearing down. I've never seen it that pronounced, I guess. They also said that the blob of grease was just extra chain lube that has collected there throughout the year. I don't really know how well my girlfriend takes care of her bike, so I guess this is possible.
Metal shavings from the chain and cassette? I haven't seen this (yet) and find it hard to believe. The wear should leave more like a metallic residue because chain and cassette wear are more like a slow abrasive process with a suspension of particles (sand etc) in oil. Since I do not really know how the shavings look like they still might be correct (even if I find this explanation really weird).

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