What exactly is "grime" anyway?

It is black and icky like grease, but is it really grease? Where does it come from? I mean, you can get grime build-up even on things that you do not lube, so it must be coming from the road.

Is it oil and grease from automobiles being flung up from the road onto our bicycles?

Is it creasote from the roads?

What exactly is this nasty goo?

I don't think its coming from the road - around here the street department is kind enough to put a protective layer of salt between us and the road. Its thick enough to keep anything from migrating up.

A mixture of carbon and rubber from tyres, particulate exhaust fumes, oil.
Add a pinch of salt and season with dog poop.

Hee hee. I knew things were bad when I caught a porcupine chewing on my chainstay.

I think MichaelW hit it about right. Vehicles leak oil some leak a lot. Some also burn oil, the exhaust of which settles on the road. Then there is the everyday dirt and grit that tires pick up and fling off as they roll merrily along. Once it starts to drizzle a little all of these quality ingredients blend into the lovely grey/black concoction called road grime. You also get a certain amount of the grit clinging wherever the tires tossed, but that rinses off pretty easily. Road grime is more tenacious. On your rims, it makes your brakes play pretty music, too.

Don't forget hair, especially in the rear derailleur pulleys.

I think I pick up the equivalent of a cat every year.

Ya, hair and "other" fiber. That is another big question; what is with all that wierd fiber that gets packed around the freewheel sprockets?

Road grime can sometimes be the egg yolks from some rednecks in pickup trucks that you use as form of entertainment.