Rain shouldn't slow you down too much. Your bicycle, however is a machine that is best kept dry. There are a few things you can do to keep it in good working order. Some prevention, some clean-up, and some repairs are all you will need to keep your bicycle running smoothly in the worst weather. Keeping yourself clean and dry is the subject of an upcoming blog entry.
Fend off that water
Keeping yourself dry in a downpour is difficult but having the right equipment can be a big help. Water comes from up and down when one is riding a bike. The wheels throw up rooster-tails of water which land on the rider's back and front, especially on a slight turn. Unlike rain, this water is full of grit and grime, getting the rider and bike nice and wet. The most obvious fix for this is fenders. Fenders can range from minimalistic blades to wrap-around wheel-covering jobs. Some are mounted permanently and others can be attached at will.
For the removable fenders, I recommend something like the SKS X-tra Dry rear fender. It is light, the angle is adjustable, and it attaches to the seat-post with a secure strap (thus it attaches to any sized bike), no permanent mounting is needed. Just be sure not to leave it in a shady area, as it is easily detached. If it is stolen, no worries, they are generally available for $15 or so. Now, that being said, these are only useful because they are convenient and sexy, they do not provide the protection of full fenders.
For full, more permanent fenders, you have more choice (as well as more protection). First, one must figure out if a bike has fender mounts. Look for small eyelets (holes for screws) near the dropouts (where the wheels attach to the frame). Not all bicycles have fender mounts. With fender mounts, look for how much clearance is between the brake mount and the wheel. Look online for fenders to match your wheel size. The fenders to be used with mount points will have longer metal rods tracing the radius of the fender curve. Don't despair if a bike does not have these mounts, you can still have relatively full, permanent fenders. If your bike does not have fender mounts, they can be attached to the seatstays (tubes from seat post to rear axle) and fork (arms that hold the front wheel), usually with quick-release wrap-around mount solutions. They can be made more permanent with zip-ties.
Another option I have used is mounting a plate (of plastic or other lightweight, waterproof material) on the underside of a rear rack. It functions similarly to a proper fender.
Where the Rubber Meets the Road
Another consideration is tires. Traction can be an issue riding in the rain. Therefore knobbier tires might seem like a good idea. On the other hand, the knobbier the tire, the more water that will be trapped on the tire and thrown up on the rider. Slick road tires will throw up less water, but may provide less traction. So it is a give-take relationship. I ride slick tires and have never had much of a problem but every rider is different.
Keeping Your Bike Happy
How is a bike affected in the rain? If a bike is ridden in a rainy area, it will need to be serviced more often than a bike ridden in a dry one. The chain is the first and most exposed part of the works. Dirt is thrown up from the front wheel directly onto the chain. There are two basic types of lubes: petroleum- and wax-based (such as tri-flow or white lightning). I will not go into cleaning chains here (that belongs in another blog) but here is how these lubricants are affected by rain. The petroleum products tend to get washed off more quickly. Not only that, they are dirty, especially when wet. Wax-based lubricants (including hot waxing) tend to fair better in the rain, plus they are not dirty when wet. Remember, do not switch lubricants without degreasing the chain first as the oil and wax repel one another.
The bottom bracket and axles are also prone to water damage. The bottom bracket gets attacked from the inside and out. Water can run into the bottom bracket from the frame tubes on many bikes. It is also taking on water from the spindle and external joins. Prevent water getting into your bearings by using a good waterproof grease (such as Phil Wood). I grease all parts of the spindle which are internal (i.e., not the crank-mount area). I also run grease around the entire inside of the bottom bracket to repel water. Learn how to overhaul your bottom bracket, don't be intimidated.
Depending on the material your frame is made out of, you may have to deal with water damaging your frame. Keep your frame dry, park under an overhang if you can. Have a cloth by your bike parking spot at home and give it a quick wipe down when you get home wet. Be careful with rust. If you have it, and it is only slight, sand it out and paint over it.
Careful with that pretty leather seat; comfortable but less weather resistant. The easiest solution is parking under an overhang or tying a plastic bag over it. Be aware of the water being thrown up from underneath (especially if you don't have fenders). The back underside of your seat can get gritty quickly. Also, that water can run down the seat post into the seat tube and then into the bottom bracket. Some, such as Sheldon Brown, suggest lightly greasing the seat post, which would also form a seal to keep water from running down as well.
A further issue is the gear train. Whether you have a derailleur, fixed gear, or internal hub, rain and water can be an issue. With a derailleur, all of your works are exposed. Expect to clean gunk off with a toothbrush periodically. Check the cog wheels and in between the sprockets of the cassette. Park makes a tool specifically for cleaning this. I use a piece of rag twisted to fit between the gears. With a fixed gear, it is a bit easier, just keep the cog wheels clean and wipe down the surfaces, done. Internal hubs are also easier than derailleurs, because they are so robust. Just make sure they stay oiled. Shifting problems are usually due to cable issues, not internal ones.