What The Frame For?
A typical adult weights 60-80 kg (130-180 lb), so the frame of a bicycle has to be fairly tough if it’s not going to snap or buckle the moment the rider climbs on board. Ordinary bicycles have frames made from strong but lightweight tubular steel (literally, hollowed-out steel tubes containing nothing but air). Racing bicycles are more likely to be made from carbon composites, which are even stronger and lighter.
The frame doesn’t simply support you: its triangular shape is carefully designed to distribute your weight. Although the saddle is positioned much nearer to the back wheel, the rider leans forward to hold the handlebars. The angled bars in the frame are designed to share your weight more or less evenly between the front and back wheels. If you think about it, that’s really important. If all your weight acted over the back wheel, and you tried to pedal uphill, you’d tip backwards; similarly, if there were too much weight on the front wheel, you’d go head over heels every time you went downhill!