Loading the bike for a tour is more of an art than a science. The principles are simple - balancing the bike from front-to-back, and from side-to-side. In practice, however, there are a few considerations.
Overview
A fully-loaded touring bike carries a rider (generally 100 - 250 lbs) and between 10 and 80 pounds of gear. Dedicated touring frames typically have wider tires and are built to handle a load up to 350 pounds. A touring bike will typically weigh 30-50 pounds including rack and basic gear, like lights, frame pump, and water bottles / cages. In theory, both tires should support the entire weight of the bike. Each item on the bike (each load) exerts force on the frame downward in a straight line towards the center of the Earth. If the item rests between the two wheels, the force will be distributed through both wheels - more towards whichever wheel the load is closest to. Most saddles place the rider behind the midpoint between the front and rear wheel, placing about 2/3 of the rider's weight over the rear wheel and about 1/3 over the front wheel. This is on purpose: when you brake, weight shifts forward, placing more of the force over the front wheel and removing some from the back wheel. If a person were centered between the wheels, enough weight might shift to catapult a rider over the handlebars. (and if you brake suddenly enough, that can still sometimes happen)
If a load is placed where the imaginary line downward is outside the space between the wheels, then all of the force of that weight is on the wheel closest to it, and some of the force acts to remove weight from the other wheel. For example - if you strap something onto the back of the bike behind the rear wheel, all of the weight of that item is on the rear wheel and some of the force of that weight will act to oppose the weight over the front wheel - making the front wheel feel "lighter". This effect is greatly magnified if traveling uphill, which shifts weight rearward.
(article in progess)
