If you are planning to go a long way on the Great Divide Mountain Bike Route, you will depend a great deal on your gear. There are few services such as bike shops, outdoor stores, hospitals, and commercial accommodations available on the route. Since there are few opportunities to purchase equipment or have repairs done, you will depend largely on what you bring with you. Careful selection of equipment will pay off during the trip.
A standard mountain bike frame is your best bet unless you have prior off-pavement, self-contained touring experience on another style of frame. Mountain bike frames are the most suitable for this terrain as they are durable, stable, and versatile. There is a lot of talk about what frame material is strongest, and there is one simple fact -- the construction of the frame has a lot more to do with its strength than does the actual material. Durable frames can be made of steel, aluminum, or titanium. The key is to get one that is designed to be strong rather than lightweight. The welded aluminum frames made by Cannondale, Specialized and Trek are a safe bet.
Although it is not necessary, I do recommend front suspension. It will help smooth out miles of eroded, washboard route surfaces and can save you from a crash when you hit a pothole zooming downhill. There are a number of different designs available. Choose one that does not utilize air pressure. Some models rely on a sealed air cartridge to supply the spring resistance, but these are the most prone to failure and require the use of a special inflator pump, which you'd have to carry with you on your tour. The most reliable and serviceable models available today are made by Marzocchi and use steel coil springs instead of a compressed air. If you don't use a suspension fork, consider using a wide front tire (2.3-2.5) and run low air pressure. Many of the new freeride and downhill tires are designed to run with lower pressure without pinch-flatting. This will make your front end quite soft and take the edge off.
Full-suspension bikes add weight and can be more susceptible to mechanical failure. They often require the use of a trailer or racks that are specifically designed to mount on bikes with suspension. I would consider rear suspension to be unnecessary for this route unless you suffer from chronic back pain. These bikes are getting better all the time, but, while some designs are very durable, some are not. My suggestion would be a single-pivot design such as the Santa Cruz Heckler. Again, opt for a spring-coil shock rather than one that relies on air pressure. That way, even if the damping cartridge fails, you'll still have a spring that you can crank down by hand. Seatpost shock absorbers are a great way to take the edge off and are well worth the money if your bike does not have rear suspension.
The best money you can spend on a mountain bike is for wheels. For this trip, wheels should be professionally hand-built, if possible. These tend to be stronger and more durable than wheels that are machine-made. The more spokes in the wheel, the stronger it will tend to be. Thirty-six spokes are recommended, and, in some cases (especially for heavier riders), it may be wise to consider using a wider "downhill" rim as they tend to be more durable. Consult with a professional to find out if your wheels will be adequate for loaded off-pavement touring, and bring extra spokes for your wheels.
Many new high-end bikes come with high-tech, lightweight wheels that have fewer spokes or radial lacing and may require parts or spokes that are not common. They also often have hubs made by companies that don't have much experience in that area. Manufacturers often claim that these wheels are just as strong or stronger than traditional three-cross patterns. That is not true for touring purposes. The fact is, very few people have experience in loaded off-pavement touring, which creates a great deal more stress on the wheels than does single track riding. Weight should not be an issue when selecting wheels for touring. In fact, heavier is often better.
In recent years wheel size has also become an option when determining what type of mountain bike to purchase. The larger wheels roll over obstacles easier and allow lower tire pressure while providing a little extra comfort. A 29er mountain bike is a great alternative to a full suspension bike. In the 2007 Great Divide Mountain Bike Race, the winner rode an Orbea Alma 29er to victory and a new course record. Other models of 29er mountain bikes can be purchased from Cannondale, Trek, Kona, and others.
There are two main types of hub construction. The first type utilizes a traditional ball-and-cone bearing system. Bearings, cones, and axles for these hubs are readily available at most bike shops. The second type uses pressed-in sealed bearings similar to those found in in-line skate wheels. Sealed-bearing hubs are usually more expensive because they run smoother and have the potential to go farther between maintenance. The catch is, if you are using a sealed-bearing hub and it does have a problem (which I have seen happen with even the best), it might be difficult to find replacement parts in a timely fashion since most of these hubs require unique parts. Therefore, for maximum reliability, I vote for the traditional ball-and-cone hubs made by Shimano (the XT model is sufficient). Although they may require a bit more maintenance overall, they are easy to service, parts are readily available, and they will not likely leave you stranded.
Tires with decent-sized knobbies are recommended. Look for a model that has thick rubber on the sidewall and the tread. Again, many tires are designed to be lightweight and compromise durability. Look for tires that are as durable as possible. Semi-slick and inverted tread tires are not recommended. Although they work fine in ideal conditions, you want to be prepared for loose gravel and mud, conditions in which knobby tires work best. Don't worry about rolling resistance. When you hit pavement or hard-packed, smooth dirt, you'll be pleased by the easier pedaling conditions anyway, and you can always pump the tires up harder if you want to.
Trailers are an excellent way to carry your gear and are particularly useful off-pavement. Trailers distribute part of the load over their own wheel, thereby putting less stress on the bicycle frame and wheels. If you choose to use a trailer to carry your gear, it must be a model that is designed for off-pavement use. I recommend one-wheel models, like the BOB Yak and BOB Ibex trailers.
Old Man Mountain racks can be mounted to suspension forks and, in many cases, frames with rear suspension. Any racks you choose should be connected to the bicycle at no less than four points. Loctite should be used on all rack-mounting hardware. Low-rider front racks are generally inappropriate for off-pavement touring as they don't always allow adequate ground clearance for the panniers.
So now that I stuck my neck out and made my recommendations, I want to qualify a few things. Cyclists have used and will continue to use some of the very parts that I do not recommend with no problems at all. Maybe you rode the Great Divide with a 12-spoke wheel and never had a problem. Maybe you used semi-slick tires and swear by them. I know it has been done and that's great! The recommendations I made here are designed to give you the best chance to avoid mechanical problems on the Great Divide route.