A bicycle is made up of a frame and components. The most important parts of a bicycle for touring are:
1. Frame & Suspension: The bike has to be comfortable. This means you need to fit on the bike and the bike has to have suspension to keep your body fresh.
A. Fitting on a bicycle frame
1. In order to fit on a frame the rider needs to make sure his/her sit bones ( back of the butt ) are rested flat on the back of the saddle. There should not be any weight on soft tissue in front of the butt. If there is any weight on soft tissue there will be irritation & pain, and the cause is NOT the saddle. A comfy saddle with NOT solve the underlying problem.
2. The ball of the riders foot should be right in the middle of the pedal for optimal power transfer and optimal fit.
3. The rider should have a slight knee bend at the furthest point from his/her hip. An exaggerated knee bend will make cycling more difficult.
4. The wrist should be straight when engaging the brake levers. If this is not done wrist problems are more likely to occur.
B. Frame material and Suspension - Please make sure you buy a frame with braze-one for a rear rack, and multiple water bottle braze-ons. Bicycles without frame mounted rear racks should only be used for credit card touring ( short tours or tours with support ).
1. Titanium - This is a super soft and strong metal. The suspension will be provided in the frame. It is ideal for touring, unfortunately it is very expensive.
2. Chromoly Steel - This is very strong and somewhat soft. It can be used without additional suspension. A carbon fork on 700C bikes will help with suspension.
3. Carbon - A well made carbon frame will flex in the vertical plane, making the ride more comfortable for the rider. Hence additional suspension may not be necessary.
4. Aluminum - Aluminum is a very stiff metal and requires suspension, ideally front and rear
5. Hi-ten Steel - This is considered junk in the industry, it is very heavy. Hence your bike would be ridiculously heavy when loaded with stuff. I would recommend you get a different frame material.
C. Frame Style- People have toured on road bikes, mountain bikes, folding bikes and much more on loaded tours. Your choice should reflect your style. Keep in mind that smaller wheels are stiffer and need more suspension than larger wheeled bikes. The suspension can be in the frame or in the tires, by using balloon tires at low pressure. Classical geometry and compact geometry bikes feel different. the classical geometry will be taller, and flex more ( more comfortable ) than the compact geometry models.
2. Wheels - The rear wheel is the weakest point on a touring bicycle, since most of the weight is supported by that wheel. I would recommend riders use double wall rims with plenty of spokes at least 36 ( more spokes makes a stronger wheel ). Keep in mind smaller diameter wheels are stronger than larger ones. Hence using less spokes is acceptable on folding bike with 20" or 16" wheels.
3. Shifters - STI ( brake lever shifters ), and downtube shifters are excellent on road style bicycles. Trigger shifters are perfect for all other kinds of bikes. Grip shifters are not a good idea on a tour, they will most likely cause wrist problems.
4. Gear range ( Chainrings/ Cogs ) - A triple crankset with an 11-34 cassette has been the standard. However you may be able to do a single chainring with a super wide 11-42 cassette depending on your route ( i.e. mountain climbs ). In general I don't think a single chainring is a good idea for a tour, the super wide cassette has huge gaps between gears, and this will likely cause knee pain/irritation. A double chainring is always an option, and probably best for small wheel bikes. Keep in mind that small wheel bikes will have lower gears than bigger wheeled bikes. Hence a granny gear triple 20" wheel bike would serve no use for most people.
5. Racks & Bags - A solid rear rack & rackpack is required. Panniers can hold lots of gear. However, I don't like them, because they act like sails in the wind. A normal rider will average 1.5mph faster without panniers. I can up with an interesting solution to carry my stuff and keep it all behind my back to minimize wind resistance. Please read about my super rack concept.