This video looks at the different grades of steel used in the frame design of a Brompton bicycle, typically steel tube and steel sheet. Designers consider several criteria when choosing materials for a folding bicycle, including strength, weight and durability. Due to the manufacturing process, Brompton does not use aluminium for its frames as it has to be welded and not brazed. The fatigue life and cost of steel and aluminium are also compared. The film also looks at the materials used for other parts of the bicycle, such as tyres, handle bars and pedals. Titanium is used in some models. This is lighter but significantly increases the unit cost. The film explains how a new part was designed using CAD and then produced by rapid prototyping.