There are several things you want to look at when choosing a bike for a bicycle tour. A touring bike is a certain type of bicycle, but it can also be any bike that you can outfit for bicycle touring. In the end you will see that buying a bike with certain features, and with touring in mind, from the very beginning will give you a much better experience on your bike tour than trying to convert a bike that was never intended for the job.
General Touring Bicycle Requirements
You want a bike that is reliable and strong. Look for components that are simple, easily available and not expensive to repair or replace while on tour. You need a bike that you feel confident will carry you from day to day towards your destination. Obviously things can go wrong with any bike on any tour, but choosing the right bicycle and components will bolster your confidence while traveling.
Choose a bike that can be repaired easily if something were to go wrong. An example is that having a heavier steel frame can be repaired on the side of the road with a portable welder on the backside of nowhere, whereas a lightweight carbon fiber frame will cost hundreds, if not thousands, of dollars to replace. And, that is only if you can get one delivered to you.
Touring Bicycle Frame
Choose a frame that is both strong and flexible. While aluminum is strong, it is very rigid. It is more likely to break than it is to flex under a heavy load. Aluminum is lighter than steel, but it is not as easy to repair. If you are buying a new touring bike from a store most will be a chromoly steel. That is steel with chromium and molybdenum mixed in for strength and flexibility.
A touring bicycle frame is notably longer than a standard mountain bike which is longer than a racing bike. This allows for good clearance between your feet and panniers. Longer bikes are also more comfortable over extended time in the saddle because of its flexibility. The longer wheelbase gives the bike more stability and better handling when loaded down.
Choose a frame that is the right size for your body. A bike shop will be able to tell you what size frame is right for you. This is usually about the length of your inseam minus 10". Also choose a frame that has a standard diamond geometry.
A good frame can last for many years and is the basics of any bike construction. If you have to save money on the fancy components and accessories to get started, at least put as much money as you can afford to get the right frame for you. Other accessories can be upgraded over time if necessary.
Touring Bicycle Wheels and Tires
Choose wheels that are strong and built for touring. This may mean that there are more spokes than a standard bike (from 36 spokes up to 48). The spokes may also be thicker.
There are two main wheel sizes four touring bikes: 700c and 26". The 700c wheels are popular and easy to find in the United States, Europe and Australia. However you may have trouble finding a replacement wheel if you plan to travel outside of those countries or happen to be in remote places needing a new wheel. The 26" wheel is standard around the world and is easy to find in an emergency. If your bicycle touring will be confined to countries where 700c wheels are available then that is certainly the preferred size. The larger 700c wheels roll easier than smaller wheels.
While no one likes to pay a lot of money for their tires, consider that a good set of tires can last for many more miles than a cheaper set. Puncture resistant Kevlar lined tires are available. These are not completely puncture proof, but they give good protection while cutting down on bulk and weight.
Touring Bicycle Components and Add-ons
If a bicycle is made for touring there will be plenty of places to attach racks and water bottles. These are usually brazed on connection points near the chainstays, seatstays and wheel dropouts. Most will be sold with racks, but if not, you need to be able to attach both a rear and front rack. This is where converting a bike that was designed for a different purpose begins to fall down as a touring bike.
Simplicity in components is important. Having an older style friction shifter on the down tubes is preferable to the newer "click" shifters built into the brake levers. The more complicated a system is, the less likely even well-equipped bike shops in developing nations will be able to repair your ride.
The call for simplicity goes for the braking system too. Rim brakes are simpler than disc brakes. And cantilever are simpler than caliper. There are many newer touring bikes being sold with disc brakes. Unless you are doing light touring, these should be avoided. Disc brakes provide more stopping power but also put much more stress on the spokes, hubs and wheels. This is even a greater problem the heavier your load.
You should choose a bike saddle that works well for you. Keep in mind that you will be sitting on it for many hours a day. Your touring bike should also have bike fenders. It only takes riding in the rain one time to realize the weight savings of not having fenders is not worth the muddy mess you will be.
Touring bikes should have drop handlebars. These are the type of bars seen on racing bikes. Drop bars allow you to use many different hand positions which keep your hands from getting cramped and potentially suffering paralysis of the ulnar nerve. Drop bars allow you to change hand positions often.
These suggestions should help you find the best bike for your bicycle touring adventures. As you prepare to buy a bike and do your first bike tour, you will be helped by reading good books and asking lots of questions from other tourists.