The Bike
My bike, a 2006 Kawasaki Vulcan 750 did as well on the trip as I could have hoped. While many others taking similar trips use "dual sport' bikes, bikes that are a cross between an enduro and a street bike, that have nobby tires, stronger shocks, a larger gas tank, higher ground clearance and are generally built to withstand off road use as well as rougher streets and highways better than a street bike could. Even so, my bike "Mr. Spock", held up well, though the 10,000 mile trip definitely put it through some stress.
I had new tires put on before the trip, changed the oil twice on the trip as well as the spark plugs, and had a sheared off radiator drain plug replaced on my way back. Outside of that I didn't do any work to it, though after getting it out again for the summer this year for the first time after the trip I noticed the front end probably will need to be rebuilt soon, both mufflers had come loose, and the battery needed to be replaced. The shocks probably will need work at some point, all things a new dual sport may not have had problems with since they are built more for rough roads. Also many of the speed bumps in Mexico were high enough that I often bottomed out with them, something a dual wouldn't have a problem with.
I saw and heard of Harley's making the trip, as well as Goldwings and even small bikes like 125 cc's believe it or not. I think the only thing more important than what kind of bike you choose, is how strong your desire is to do it. While choosing the best bike and equipment for the job should not be understated and can make a big difference on how your trip goes, one needs to remember for the most part it will be the parts of the bike itself, the shocks, frame, and moveable parts that may go, not the engine itself. It would be nice to have all the time and money to go prepared with the best and newest equipment possible but at some point one needs to make a realistic assessment of what you do have, and if you have a realistic chance for success, than go. I felt reasonably sure, mainly because my bike was newer that it would make it. If I was continuing on around the world I would have had to plan on more repairs, or using a different bike. I have heard of a brand new KL650 going around the world, 30,000 miles with one oil change and two flats, so................ and that is a $6-$8,000 bike fully loaded for travel, perhaps less. Not too big of investment for reliable around the world transportation.
For me, I had 3 luggage rack bags, 2 saddle bags, and my camping gear tied between me and the back rest. I also had a tank bag which was the one thing I really loved. It made getting at maps, a camera, money or paperwork for the bike so much easier. Things like a windshield, a engine crash bar with highway pegs, a rain fly for the bike and a water holder on the handle bars were all things that made the trip easier. A helmet with a face shield and sunglasses was the way to go, and much better than trying to use goggles in a rain storm, or having ones face exposed. And the windshield was a must, not only for deflecting wind and water, but anything else that might come flying your way.
Maps
Get them before you go. I had a local bookstore order them for me and also found a good map place out of Vancouver Canada, though ordering them from another country is not cheap. Make sure and have a back up, and something nice about my tank bag is it has a waterproof map holder on it for convenience and protection of the map. I found few gas stations or other places even carried them, and it became a hassle to try to find them while travelling.
A note about maps and travel in third world countries. While GPS's can be used instead of regular maps, as far as I know one still runs into the same problems one does with a map. For the most part there are no addresses, even main highways aren't always labeled, or labeled in way as to easily be found. Bringing a compass and going in a general direction of the way you need to go is often the best you can do to try to find your way. Not speaking the language at times was a handicap, but not always. Even if I had understood the place I was looking for was across from the statue of a revolutionary, it wouldn't have done me any good since I didn't know where that was either. I often spent an hour or two a day lost, especially around big cities in Mexico. And this was one reason being on a time budget I avoided most of the bigger cities. Without street signs and crowded congested roads getting lost in a huge city was a worst case scenario for me. And ironically I found the smaller medium size cities to be the worst when it came to getting lost.
Roads
The interstate highways in the U.S. are luxuries you seldom find in Central America. For the most part roads from El Salvador on through Honduras and Nicaragua were in as good or better shape than most rural highways in the U.S., often better. I mostly traveled on the Pan American Highway through these countries due to time, but I'm assuming many of the main highways throughout the country are mostly the same or better, depending on the road. Mexico had a variety of road conditions, some excellent that you had to pay for, and others absolutely horrible. In Guatemala the roads while not always great, were ok, it was the bus drivers that made the driving there dangerous.
In many places, road hazards if they were marked at all were marked only at the hazard itself, one seldom got any advanced warning. In Mexico one might come around a corner only to find half the road had been washed out and if you had proceeded to stay in your own lane you could well find yourself tumbling down a huge cliff. In Nicaragua as elsewhere, animals, mostly alive, were often to be found on or near the roads. Road delays, especially during the rainy season in Costa Rica are quite normal.
In general the maximum travel in a day in Mexico and Central America should be around 200-300 miles. While certainly one can do more depending on the roads, and I did have some 400-450 mile days, they were for the most part the exception. Unlike in the U.S. where one can just get on an interstate and never get off going 80 mph around the clock till you get where your going, Mexico and Central America has way too many variables. And like everything else down here, travel requires more patience, and a different attitude altogether than driving in the states. One can have great cruising in the morning, only to find terrible conditions in the afternoon, and than good conditions again in the evening. Sometimes 20 minutes of 60 mph driving on great roads is replaced by two hours of 10 mph in a dirt construction zone.
Consider Mexico, coming down the gulf coast side through town after town there are no traffic lights, rather there are speed bumps, dozens of huge bottom scraping bumps in each town that make you almost come to a complete stop at each one. Sometime towns are only a few miles a part so your stopping hundreds of times a day for each bump you cross. Then there are construction zones that put you in a place where you don't even feel your on a road. One place coming off the gulf side in Mexico it took about 4 hours to drive 30 miles on terrible roads, bumper to bumper with other traffic in very high heat. When you throw in traffic jams, accidents, getting lost, constant military checkpoints along with what ever local festivals maybe blocking the road (some times for hours) you begin to get an idea that even traveling a couple of hundred miles in a day can be an achievement.
Roads are also places of foot travel for both people and animals. They are places where people of the village congregate in the evening to visit, or during the day to sell you things while your stopping at all these speed bumps. One dark and stormy night there was an entire class of students walking down the side of the road not more than a foot or so from speeding trucks, I didn't see them until I almost hit them myself. Roads in Central America and Mexico are not just for cars, they are for everybody at all times! And paved roads are a luxury no human or animal is going to pass on when the alternative is trying to walk through the dense underbrush of a jungle. Roads and driving down here needs to be approached with a lot of respect, your not only sharing them with other drivers, your sharing them with all the local life, both animal and human.