It's raining. Matter of fact, there's rain in the forecast pretty much every day for the better part of a week. What are you going to do? Sit at home on your ass in peak cycling season? Skip 5 days of workouts during a build period when you're supposed to be building toward an A race, or some big century ride, or a big mountain biking trip?
Hells no, you're going to man the f*** up and go for a ride. Here's 6 tips to help you do it, based on what I've learned the very hard way.
1. Fenders. You know I'm an incorrigible Fenderphile, but you don't have to go whole hog with full fenders. A pair of SKS Raceblades, Planet Bike SpeedEz or similar clip-on partial fenders do a good job at the critical tasks - keeping a blast of spray from shooting off the front wheel, up between your legs, and keeping a similar blast off the back wheel from painting a skunk stripe up your back, and muddying your riding partner's face. They attach to the forks and rear seatstays by means of simple rubber straps, and take about a minute to install both. ~$30 - $50.
2. A cheap plastic rain jacket. Now check this out - it seems you can actually get rained on, and you won't melt. Yeah, I know, whodda thunkit? It's true though. So a rain jacket may actually be unnecessary if it's warm enough out. And here's another oddity - if you ride with a cheap plastic rain jacket, you'll probably be as wet at the end of the ride, as if you hadn't. The wet will be from your sweat (eeeeewwww). The difference is you won't be hypothermic - a constant flow of cool rain will chill you like nothing else. You can live with a little sweat pooling under the jacket and making you clammy; four hours of icy spew from God's Garden Hose, on the other hand, will have you crying for mommy. I find if it's about 65 or cooler when the rain hits, that I prefer to have the rain jacket on. ~$10 - $25
3. Remember to drink. You probably don't need to drink quite as much in the rain, since you will likely be cooler than you would be in warm weather. However, you still need the calories from your sports drink, and in all likelihood you will still be sweating and getting dehydrated - it's just that you won't notice it because the rainwater dripping from your nose dilutes the sweat.
4. Remember those old road shoes you were about to throw away? Don't. The biggest aggro I've found from rain rides is trying to dry out my shoes. When nice new shoes spend four hours in the dunking tank, the aggravation of waiting for them to dry is coupled with worries that prolonged immersion damaged the insole or the leather. Avoid the worries, and keep the old pair around for rainy days. When you return from you ride, hose them off, undo all the fasteners, remove the insole, lift the tongue, and let them air out to dry. You can ball up some newspaper in them for a few hours, this will help draw some of the moisture out of the leather. You can also put them in front of a fan overnight, which will get them mostly dry. Cost: free.
5. Immediate cleanup. The best thing about riding in the rain, other than the fact it beats not riding, is that wet dirt washes off your bike much easier than dried-on dirt, which is colloquially known as "cement." As soon as you end your ride, take the garden hose to your bike. Pay special attention to the parts that rub other parts. Rinse off the pedals and chain/chainring/cassette really well. Loosen the quick releases and spray in between the QR and the dropouts - grit always gets in there somehow, no matter how tight you had the QR. Get under the seat, and underneath the bottom bracket, where your derailleur cable probably runs. One cautionary note - don't spray high pressure water into areas that have bearings and/or seals of dubious quality. So don't jet water directly at your headset, at the cones on your hub, directly into the bottom bracket seals, etc. Use a sponge or cloth to remove that dirt then rinse. Same thing with your shoes and helmet - rinse them off right away, and if you're feeling particularly froggy, wear the helmet in the shower and use this as an opportunity to shampoo the pads and the straps.
6. I don't recommend riding in insanely heavy rain on a nice bike. I've done this and it takes a complete disassembly and cleaning to get all the grit out. If you're planning on doing that kind of riding regularly it merits investment in a dedicated ratty rain bike or cheap fixie, which are, respectively, beyond worrying about, or much easier to clean. But if you do a long ride in a downpour, in seriously heavy rain, you may have water inside your rims, and inside the bottom bracket. You get this from rolling through puddles with deep water, e.g. regular trips through 2"+ deep puddles, or from being out for a while in a torrential downpour. If that's the case you need to take off your tires & tubes, turn the rim so that the stem hole is downward, and let the water drain out. You might want to leave the wheel in that state until the rim tape dries as well. Some older styles of bottom brackets or bottom bracket shells may need to be drained out as well, and as long as you're going this far, you might as well check out the hubs to make sure there's no water in there, especially if you had to pedal through some hub-deep water. Doesn't that cheap fixie sound better now? Performance sells one for about $300, though your LBS would be happy to set you up on something much more stylish and with nicer quality components for not much more than that.
Those are just some things I've learned by riding in the rain. It's not hard, as long as you're not talking about a downpour. When do I skip rain rides? On days like today, for instance, where it's absolutely pelting down. There is a puddle outside my window, and one raindrop per second lands in each square inch of that puddle. That's way too much rain. But the moderate drizzle we had yesterday? No problem. It was 75 outside, fairly calm, a little overcast, and all the trees have just turned green. The weather was too nice to not ride, and I wasn't going to let just a little rain stop me. Nor should you.