Building a Bike

Sometimes a normal off the shelf bike won't do it for you. You can always build a bike exactly how you want from a bare frame. This is a good idea if you have access to lots of cheap parts, or have lots of spare parts that you want to use. If you want to buy everything brand new, however, then things will get pretty expensive pretty quickly. Anyway, if you're trying to cobble a bike together out of used parts or build your dream bike there are a few little details you need to know:

Frame size and type - Obviously you should get a frame that fits you perfectly. The type of frame will also make a big difference in how the bike rides. Road bike frames make fast, agile bikes. They also make terrible off road bikes because the bottom brackets are low to the ground, and you will strike your chainrings on every little tree branch in your way. Generally, build a frame that suits your intended purpose rather than trying to adapt everything.

Wheel size - You'll need wheels to match your frame, so pick a set that is at least close to the original size. 27" and 700c wheels are almost always interchangeable. If your frame was built for 700c wheels you might not be able to stuff a larger 27" set in the frame, but the opposite works since 700c is about 8mm smaller in diameter. You cannot, however, make a drastic change like putting mountain bike wheels on a road frame. Even if they physically fit the frame, you will never get the brakes to reach the rim.

Crankset and Bottom Bracket - These parts go hand in hand with each other. If you're using old cranksets with square tapered bottom brackets, you need to know the bottom bracket shell width (check the frame first, it could be 68mm or 73), spindle length (determined by the crankset), and whether the shell is English, French, or Italian threaded. Modern high end frames use press fit bottom brackets, and many are a proprietary size so keep track of this.

Drive train - How many gears do you want? 1? 3? 27? Your back wheel will play a big part. If it is threaded for a freewheel, you can have up to 7 gears in the back. If you have the right cassette hub then you can have even more speeds. Old cassette hubs have shorter freehub bodies, so you might still be limited to 7 gears with those. If you want a fixed gear bike, then you need a rear wheel with a track hub. These have 2 sets of threading for a track cog and a reverse threaded lockring. I have also made fixed gear bikes with a normal freewheel hub by securing the track cog with a bottom bracket lockring and some threadlocker (same threading), but I would only do this if you have a brake. Otherwise if this arrangement loosens up you're screwed.

If you're using multiple gears, then you also need to choose your shifters and derailleurs. If you want indexed shifting, there are a few little details about which ones are compatible with what. Friction shifters are compatible with any derailleur and gear cluster, but indexed ones are picky. You need to match the number of sprockets in the back to the number of speeds in the shifter. You also need derailleurs that are compatible with the shifters.

Brakes - Generally you should use the type of brakes that the frame was built to use, but there are some exceptions. If you have cantilever studs then you can pick either V-brakes or cantilevers (just make sure you get the right brake levers to match). On mountain bikes with disc brakes you can sometimes change disc size with an adapter bracket. For road bikes you can choose between sidepull, centerpull, and dual pivot brakes (single pivot sidepulls are weaker than the other two types, but centerpulls need cable hanger brackets and dual pivots don't fit old frames without modification).

I guess the best thing I can tell you is to do your research. There aren't a whole lot of parts you need to buy to build up a bike frame from scratch, but there are a lot of little details that can set you back (like rear dropout spacing, cable routing, seatpost and handlebar stem sizes, etc.). Good luck, and most especially have fun!