It's relatively simple, at least to get your basic design on paper.
1) You need to know the parameters of the kind of frame you want. You can find these by looking at the numbers for non-extreme versions of the kind of frame you want to make. What is typical head tube and seat tube angles, and what is the BB drop. Wheel size, real with inflated actual tires. Fork with offset.
2) Seat tube - Assume you have determined the typical ride on your wheel size is say 73 degrees, 73 degree, and 80mm drop. You presumably have ridden a bike before and have one that feels like a good fit, and you have been doing recent fit work. So you should for starters know what seat height and tube angle are. Now you buy the actual seat post, and seat you want to use, and you look at those and determine whether the best position for them, the seat centered on the post with adjustment room, in a structurally beneficial position, will fit over your seat post angle of your fit bike at the same angle. In other words, if the bike riding style and your fit numbers both yell 73 degree, you still want to match that up sensibly with a seat post (what offset) to be sure that the seat ass contact point relative to your existing fit, is actually a 73 degree seat tube angle in the real world. Where your ass hits the seat relative to the BB is the real issue, how you get there depends a little on the parts. This seat/bb relationship thing is bio-mechanical to some extent, does not vary all that much from bike to bike, except if it is a bike type with super laid bad cruiser style, or say time trial style. I could ride a 72-73 tube depending on seat, for just about any normal riding I do. But your philosophy might vary.
3 - Head tube- So once you have you seat tube on the diagram you need your front end. So in your fit , and prior bike experience you should have some idea in mind of a reach number. Again, whatever it is needs to be massaged for the actual parts you will be using: the real bars, a stem proportional to your frame - we aren't going to use a stem to make the frame right; taking into account the head tube angle, and how that affects fit, etc... So once you have that position worked out, you should know where the top end of you head tube needs to be. The lower end will depend on the fork you will be using, and the stack height of your headset, real wheel size. Again, if you are using parts, these need to be real numbers, based on parts you have on hand. This will work out far better than guessing.
4 - Stand over - OK, so you know your HT, your ST. You know your BB because it is a function of ST position, wheel size and BB drop. Next you need to know what a reasonable stand over is, and you get that from your fit work, previous bikes owned, etc.... This will determine the rear end of the top tube, and the front end was established with the HT work, and joinery offsets.
5 - You connect the downtube by drawing it centered on the BB and a workable amount above the base of the HT, and that depends on type of joinery. The lugs will define it, or weld size, or fillet size.
6 - Rear Triangle - You now have the front triangle. The rear triangle comes from absorbing the CS lenght of the average type of bike you are building, that will determine how far back you place the rear wheel relative to the BB. Do your research. I try not to get too carried away with extremism here, I don't think the average rider need to have the rear wheel rubbing up against the BB.
The Seat stay position is determined by your rear wheel position, the drops actually used, and in hand, and the type of joinery you are using at the seat tube.
You're done. Except you never are. You need to keep going around the design spiral, working the design as a whole, and integrating real numbers as more parts become available.
You might say, well I don't know x or y number, offset, parameter. Well design is all about knowing where everything goes, putting numbers on it, and having your own opinion about it. You just need to do the work. One thing you can do is scale nice side views of existing bikes in your interest segment and draw them, and keep files, over time you will learn what kind of stuff goes where. Do this with your existing rides, some day they may be gone, but you will still possibly want to know the numbers. These don't need to be beautiful drawings, just so long as they locate the basic parameters.
I'm sure BikeCad, is a useful tool, but I like doing drawings in 2d CAD with parts files that I have accumulated that are real parts. And I own the results and don't pay any fees. For one thing, when you do that, you need to draw out these parts (mostly dead simple), and learn more about the parts and how they go together, than if you use pre-digested stuff. I would probably use BC if I was producing proposals for clients.
Last edited by NoReg; 03-17-11 at 11:16 AM.