When I hung up my bike last night, I realized the drivetrain needs cleaning. I'm going to be removing the chain and cleaning it separately; while I'm at that, I'll have the rear wheel off as well, and should be able to clean the cassette pretty easily.

For cleaning the chainrings up front, I was thinking of just spraying them with WD40; I don't usually use WD40 for anything, so I'd like to make sure it won't damage the paint on the frame if I get some overspray, or the plastic chain guard. I'll be careful to avoid spraying it in/near the BB so as not to disturb the bearings. Any comments on this plan?

Also, my rear derailleur wheels are pretty grody. I figure that spraying those with WD40 could affect the grease inside -- will just brushing them off be enough?

WD40 will not hurt your paint nor will it clean your gunk. It is very easy to take the cage apart on your RD and clean the pulleys the right way. As for your chainrings a toothbrush and some cleaner agent of some sort would be your best bet. If it were me I would pull the crank arm off, scrub it good with paint thinner, wipe all the crud off with a rag and be done.

take a plastic card, like an expired credit card, and just scrape it off.

For rear cassette or freewheel cogs all you need to do is fold a rag to make it the correct thickness then "floss" between the cogs. You can add some solvent to the rag if necessary. With a little practice this is quick and easy.

lots of simple greens and tooth or bottle brush

toothbrush is no good, because the bristles are too soft to clean off the grime.
not even the toothbrush that you're not supposed to use on your teeth, the hard ones, are any good.

The wd-40 will also make it get dirty again more quickly.
I've seen it used in engine bays at car dealerships to make it look shinier, but the oil also attracts dirt and makes it dirty again faster. It will also degrade rubber, not that that matters much on a bike.

Yea wash it down with a bucket of soap, Dawn dishwashing stuff, water and a brush ..

dirt wears things out sooner , so if you are lazy it costs more in the long run ..

+1, dead credit cards are great for this job. With a carefully cut slot you can clean both sides of a chainring at once.

Two easy choices.

1) Spray it down with an aerosol engine cleaner/degreaser and then wash it off with a hose.

2) or same as above and then use an electric power washer afterwards.

3) or, same as 1 & 2, then spray some Fantastik or 409 to remove the engine degreaser and then power wash it again.

Take it for a spin around the block and it will be dry in this heat. I'm fortunate to have an air compressor so I can use that to blow the water off the parts.

If a toothbrush is no good, then your cassette's are way beyond dirty to begin with.

About every 300 -500 miles I clean the cassette with Mr. Clean in a spray bottle, a toothbrush and water rinse. They come out super-shiny clean.

The chain also gets run through a rag soaked with Mr. Clean, dried, lubed, and wiped down to remove any excess lube at the same mileage intervals.

After every ride, the chain, cassette, pulley wheels and rings get run through/wiped with a rag until no dirt shows on the rag.

Chains and cassette's last me a very long time, and the major benefit is how much easier it is to pedal a clean drive train. Free watts for everyone.

Depending on how much your time is worth, installing a cheap new Shimano crankset ($26 here: https://www.niagaracycle.com/product_...ucts_id=449761) is probably the laziest approach, and possibly cost effective too.

#1 is essentially what professional race mechanics did for decades. Many used gas or 50/50 gas/diesel instead of carb cleaner. Either works. I've never seen it harm bicycle paint, but I wouldn't use it on a carbon bike.

Air compressor? Power washer?? Don't use these, you'll wash and compress dirt and water into all your bearings.

When I want to clean cassettes/chainrings/rear derailers I disassemble them and clean them individually. A rag and mineral spirits is all I use. Mineral spirits cuts through even the nastiest old grease.

Just wanted to say thanks for all the suggestions and tips! FJ - I don't have access to a hose, much less a compressor or power washer, so my bearings should be safe.
I had hoped for some magical easy solution like spraying car brake cleaner all over the drivetrain, but it looks like the proper way should be doable once I set up a space for it.

To the best of my knowledge, this is exactly what pro mechanics do with team bikes these days. Granted, they have all the spares they want...

I do the same with an old t-shirt ripped into strips. Fast, easy and, with solvent, keeps the cassette very clean.

As a cleaner, WD-40 is just very expensive kerosene in a handy spray container. You can use it as a cleaner/degreaser but wipe off all of the excess or, as noted, it will be a dirt magnet in the future.

To clean the pulleys of a rear derailleur, all you have to do is take out the two axle bolts and separate the cage. Wipe off the cage plates and scrub the pulleys clean with a solvent dampened cloth. You can remove the dust covers and bushings to get the interior clean. Add a few drops of oil and reassemble your now sparkling clean derailleur.

After I removew the chain for cleaning I take an old piece of sheet and wrap it aroud a thin ruller. On the cogs I turn the wheel backwards and hold the ruler between the cogs. I do the same with the chain wheels.

I found Riga to be an amazing place! Lots of fun and the women are incredible!

As far as the chain rings, if it is very dirty, use a stiff plastic bristle brush with your choice of cleaner. Personally, I stay away from water-based cleaners. If you can protect the paint with some cardboard, use mineral spirits.

Once clean, I find a cheap paint brush (3 for $1 in my case) and MS to be a very good choice. Dry with a scrap t-shirt.

Geeeez, I must be the last person on earth that lovingly disassembles, cleans, lubes and reassembles his bike parts when they get crudded up.

Second to the last maybe. I use some of the methods described above throughout the season for interim cleanings and do a complete thorough cleaning as you described during the downtime when the snow is flying here.

An occasional good blast of compressed air will help keep it from getting all that cruddy in the first place. The air can be directed in such way that nothing is forced into the bearings (or your eyes). When the time comes for dis assembly a more thorough cleaning can be done.

spray on the degreaser, dunk it in a lake, or stream.. perhaps?

Sheldon Brown's method was to take it off, put it in a plastic bottle with degreaser and shake it up, and then rinse it and let it dry. You will find ti on his site.