There isn't any harm in making up drawings, but in the end when you get to the point where you want a real design for your build, it is very a very good idea to actually have some parts on hand. You don't need everything, but the wheels, and inflated tires of the type you intend to use. If you have a target BB height in mind the size of tire you have will make some slight difference, some people run 32mm and other 50mm. Headset, at least a very good idea of the saddle and seat post. The fork is a really important part if you are buying it. Of course pros know all this stuff, but I am always building something I don't already have so I need to get the parts first. As a beginer it is normal to want to work on the frame part and not sink already scarce money into project parts. But it really helps to have the parts on hand. I like really long stays, pretty much as much as I can get out of a given part. You might be surprised how different the OAL of various parts can be.
For wheel clearances I am somewhat influenced by the fork I am using. The front fork will have a certain clearance and it helps if the rear layout looks as though it belongs. The front fork on a touring bike can also dictate to some extent what brakes will work best, and that may have some minor effect on how I space the rear end.
The way you want to attach your fenders will also affect stuff. I custom make bamboo fenders and have clean attachments for them and that requires planing also. Part of what creates custom look and function is to start with the end in site. Start with the parts, and then build back from them. (I also work back from the bags to the racks and the racks to the bike, though it isn' particularly a wheel thing). Otherwise it will look like the whole thing was shoehorned together like something from the LBS.
Last edited by NoReg; 07-20-10 at 04:28 PM.