Bicycle Mechanics - Bad to leave bike dirty?

From mountain biking? It's covered more or less in red sandy dirt and some mud on the downtube. The red stuff all over the components, chain, etc.

Is it bad to leave it there? I'm not riding it now. Use the road bike most of the time. When I finish building the new front wheel I'll ride it to work and give it a bath and cleaning, but for another say - 2 weeks - is it fine being dirty?

Aside from aesthetic reasons, dirt, mud, etc. can work it's way into bearing-cups and races, damage paint from any rubbing through it's abrasive action (ever heard of sandblasting?), and attracting more of the same. So it's an all-around good idea to wipe your bike down with a damp cloth after you are through mucking it up for the day.

let it dry, brush it off...oil things. Keep after bearings, keep the grease in them fresh. It is a LOT easier to brush off dry dirt than wipe off wet dirt. Personal experience there from the days of college mountain bike trails (insane). I also would redo my bearings a few times throughout the season. toothbrushes work great for derailleurs, then lightly oil after getting as much dirt and dust off as you can.

I rarely wash my mountain bike, maybe once a year at most.

I do however, use a dry brush to knock off dust off the cassette, derailleurs, and chain rings. I use a dry lube so dirt doesn't cake on these components.

Otherwise, my bike is always covered in a dusting of earth. It's a full suspension (Fisher Cake) that I've owned and ride weekly for over 5 years.

One advantage to washing is that you can examine the frame and look for cracks or potential problem areas. You'd think that would give me motivation to wash more often.

Just brush, sponge, vacuum, feather-duster, whatever - the poor thing. How would you like it if you crapped your diaper and your mom didn't change you?

(Sorry - couldn't help it! :D)

Well I know about the bearings and all, and I know the chain needs to be cleaned and oiled - but when stationary with no front wheel - does it matter?

The only time storing it dirty really matters is if there's salt or something else reactive in the dirt.

A heavy coating of ordinary dry dirt will eventually wick the oils out of adjacent areas but if you plan on cleaning and re-lubing it after setting it aside for awhile, it hardly matters.

If it were my bike, I'd give the chain a quick wipe and a shot of spray lube, just to keep it from rusting in the interim.