I was thinking recently (be afraid.)... how does the chain get dirty? Let's just assume I mean a commuter/road bike in for this question. Is it mostly the water/road grime coming off of the rear tire splashing down on top of the chain? Or is it mostly the water getting splashed up from the front tire hitting the chain on the chainrings?

The "chain tube" thread got me thinking. "If I were to purchase an 8 inch section of chain tube, would it be more effective to place it on the top part of the chain or the bottom part? (Assuming I had a single speed.) The next question was "are traditional (old style) chain guards effective in keeping the chain clean?"

My last question was "I wonder if I can find a slow motion video of a bike riding across the camera's view in slow motion so I could see where most of the water was hitting. Even better if I could see it with and without fenders." Anybody know of any videos like that?

Chain contamination obviously comes from the road. I have a velomobile in which the entire drivetrain is enclosed inside the body. It requires lubrication very sporadically, the chain stays very clean and the chain lasts forever.

By contrast I have a trike with a very similar drivetrain, that is always filthy and IT wearS out chains regularly. Part of its problem is that the chain runs only inches from the road. It also has a substantial part of the chain inside tubes.

I have no experience with chain cases, but imagine they should help.

I would be interested to see any videos you might find or film.

As to your question about which wheel is "throwing up" the debris, my guess would be that at least in the case of the trike (tadpole), it has to be the rear, since the fronts are to the sides of the drivetrain.

My chains get dirty even if I don't ride in any wet weather.

I suspect it's microscopic dust and dirt from the road that sticks to the lube and eventually builds up into a black oily mud on my drivetrain.

Same place dust on your chain stays and lower parts of your bicycles comes from even if you're riding on a nice, dry day.

Aliens put it there. And I don't mean the kind from Mexico.

I have to search this but I did speak to an engineer about lubricants. He said that lubricants have detergents in them and over time it wears out and that too shows up as a change in color. Its not only a dirty chain but also just the way detergents wear out.

Once you use a chain cleaner like Park Tools, or something like that, the cleaning fluid shows residue and that's not the detergent. So its both.