This article is from the Misc Bicycles FAQs, by various authors.
Frame design is a truly controversial subject on which every
framebuilder and rider has a different idea of how to design a frame. I
don't want to start a war on what the best frame design is, but I'll
simply relate how Dave designs his frames and the processes he goes
Most of the frames he designs are done with a CAD/Frame design frame,
but one of the frame he had drawn to half-scale on a drawing table. He
starts with a line to define the axle height which will vary depending
on the size of the wheel, but most are 700c for road bikes, 26" for
mountain bikes. Next he determines the bottom bracket drop, or how far
below the axle line the center of the bottom bracket will be. The
farther below, the more stable the bike will be, since the center of
gravity is being lowered. The bottom bracket cannot go too low since
there has to be room for pedal and crank clearance. The drop on my bike
was fairly large (I don't remember the number) to make for a stable ride
for my dad. Most mountain bikes are higher for bottom bracket clearance
and most road bikes are a little lower.
Once the bottom bracket is positioned, the seat tube angle is determined
and the seat tube drawn in. Mine was 73 degree seat tube, which is
about average for a road bike. The next step was to determine the seat
tube height which is determined by leg length. For my dad, the seat
tube height was 52.5cm. I don't recall if he was measuring center to
top or center to center (BB to top tube). The difference between top
and center of top tube can be as much as _" (about 20mm) with large
diameter top tubes. With the seat tube drawn in, the top tube is drawn
next. My dad's top tube would be parallel to the ground, where some top
tubes can be above or below parallel, depending on the bike - time trial
will have a downward sloping top tube from seat tube to head tube,
mountain bikes and smaller bikes will have an upward sloping top tube.
This sloping is important because the angle for the mitering needs to be
known. The top tube length is next and it is drawn in. The angle of
the head tube becomes a function of the size of the bike and of the rake
of the fork. Actually these are closely intertwined. Since this is a
smaller bike, the head tube angle is shallower at 71.5 degrees to allow
the front wheel to be moved out so that there is clearance for pedal,
toe clip, and fenders. How Dave actually determined this and what
numbers he used I'm not sure. A head tube and down tube can now be
drawn in. The rake of the fork is determined as the perpendicular
distance from the center of the axle to the axis of the head tube.
I'll double check this method and order and include some numbers next
time so that this might make more sense.