Frame Manufacturing Unit

The Raw Materials

The most important part of the bicycle is the diamond-shaped frame, which links the components together in the proper geometric configuration. The frame provides strength and rigidity to the bicycle and largely determines the handling of the bicycle. The frame consists of the front and rear triangles, the front really forming more of a quadrilateral of four tubes: the top, seat, down, and head tubes. The rear triangle consists of the chainstays, seatstays, and rear wheel dropouts. Attached to the head tube at the front of the frame are the fork and steering tube.

For much of the bicycle's history the frame was constructed of heavy, but strong, steel and alloy steel. Frame material was continually improved to increase strength, rigidity, lightness, and durability. The 1970s ushered in a new generation of more versatile alloy steels which could be welded mechanically, thereby increasing the availability of light and inexpensive frames. In the following decade lightweight aluminum frames became the popular choice. The strongest metals, however, are steel and titanium with life-expectancy spanning decades, while aluminum may fatigue within three to five years.

Advances in technology by the 1990s led to the use of even lighter and stronger frames made of composites of structural fibers such as carbon. Composite materials, unlike metals, are anisotropic; that is, they are strongest along the axis of the fibers. Thus, composites can be shaped into single-piece frames, providing strength where needed.

The components, such as wheels, derailleurs, brakes, and chains, are usually made of stainless steel. These components are generally made elsewhere and purchased by the bicycle manufacturer.

The Manufacturing Process

Seamless frame tubes are constructed from solid blocks of steel that are pierced and "drawn" into tubes through several stages. These are usually superior to seamed tubes, which are made by drawing flat steel strip stock, wrapping it into a tube, and welding it together along the length of the tube. Seamless tubes may then be further manipulated to increase their strength and decrease their weight by butting, or altering the thickness of the tube walls. Butting involves increasing the thickness of the walls at the joints, or ends of the tube, where the most stress is delivered, and thinning the walls at the center of the tube, where there is relatively little stress. Butted tubing also improves the resiliency of the frame. Butted tubes may be single-butted, with one end thicker; double-butted, with both ends thicker than the center; triple-butted, with different thicknesses at either end; and quad-butted, similar to a triple, but with the center thinning towards the middle. Constant thickness tubes, however, are also appropriate for certain bikes.

The tubes are assembled into a frame by hand-brazing or welding by machine, the former being a more labor-intensive process and therefore more expensive. Composites may be joined with strong glue or plastic binders. The components are generally manufactured by machine and may be attached to the frame by hand or machine. Final adjustments are made by skilled bicycle builders.

Assembling the Frame

Tailoring the tubes

• 1 The metal is annealed, or softened by heating, and hollowed out to form "hollows," or "blooms." These are heated again, pickled in acid to remove scale, and lubricated.
• 2 The hollows are measured, cut, and precision mitered to the appropriate dimensions. Frame sizes for adult bicycles generally run from 19-25 inches (48-63 cm) from the top of the seat post tube to the middle of the crank hanger.
• 3 Next, the hollows are fitted over a mandrel, or rod, attached to a draw bench. To achieve the right gauge, the hollows pass through dies which stretch them into thinner and longer tubes, a process called cold drawing.
• 4 The tubes may be shaped and tapered into a variety of designs and lengths. The taper-gauge fork blades may have to pass through more than a dozen operations to achieve the correct strength, weight, and resilience.

Brazing, welding, and gluing

• 5 Tubes can be joined into a frame either by hand or machine. Frames may be brazed, welded, or glued, with or without lugs, which are the metal sleeves joining two or more tubes at a joint. Brazing is essentially welding at a temperature of about 1600°F (871°C) or lower. Gas burners are arranged evenly around the lugs which are heated, forming a white flux that melts and cleans the surface, preparing it for brazing. The brazing filler is generally brass (copper-zinc alloy) or silver, which melt at lower temperatures than the tubes being joined. The filler is applied and as it melts, it flows around the joint, sealing it.

Aligning and cleaning

• 6 The assembled frames are placed into jigs and checked for proper alignment. Adjustments are made while the frame is still hot and malleable
• 7 The excess flux and brazing metals are cleaned off by pickling in acid solutions and by washing and grinding the brazing until it is smooth.
• 8 After the metals have cooled, further precision alignments are made.

Finishing

• 9 The frames are painted, not only to create a more finished appearance, but also to protect the frame. The frame is first primed with an undercoat and then painted with a colored enamel. Paint may be applied by hand-spraying or by passing the frames through automatic electrostatic spraying rooms. The negatively charged frames attract the positively charged paint spray as the frames rotate for full coverage. Finally, transfers and lacquer are applied to the frame. Chrome plating may also be used instead of paint on components such as the fork blades.