Ultralight carbon fiber/epoxy road bike from Kestrel
Kestrel is the first name in carbon fiber bikes, on the strength of a lengthy list of bike-industry firsts that include the first-ever production carbon fiber bike frame (1986) and first all-carbon mountain bike frame in 1988. Recently, the company unveiled a new version of its Talon SL road bike. At $3,699, it isn't cheap, but its frame weighs in at an astonishingly low 2.4 lb, light even by carbon fiber standards.
Kestrel co-owner Preston Sandusky credits the low weight to the bike's "modular monocoque" frame, fabricated from three individually bladder-molded parts: the triangular front frame, and two two-pronged, U-shaped forks, which form the seat stay section (running from the top of the seat tube to the rear wheel dropouts) and chain stay section (from the bottom of the seat tube to the rear dropouts). Developed by former aerospace engineers at Kestrel's headquarters in Santa Cruz, Calif., U.S.A., using Pro-ENGINEER and RHINO 3-D Solid Modeling software (PTC, Needham, Mass., U.S.A.), the frame structures are fabricated in separate clamshell molds CNC-machined from PX5 tooling steel, with P20 steel inserts for frame profiles.
Parts are hand layed, using "800K" prepreg (120 g/m2 areal weight) made at a factory in China, using a factory-formulated toughened epoxy resin and T300 and T700 carbon fabric and M30S unidirectional carbon fibers from Toray Industries (Tokyo, Japan). As many as seven plies, diecut from sheets of M30S, are wrapped around silicone mandrels to form the basic assymetrical tubular profiles. Fiber orientation and number of plies vary depending on performance requirements. T300 or T700 fabric prepreg is wrapped last, to form the outer skin, lending the frame its striking surface finish. The forms are removed and tubes are positioned in the bottom mold halves. Bladders are threaded through the components and tubes are joined together by overlapping ply buildups that are incrementally stepped. The molds are closed and placed in a 4,500 lb press fitted with heated platens. The bladder consolidates the prepreg and fuses the joints as mold temperature is computer-ramped to 130°C/266°F and held for a half-hour. Demolded parts are media-blasted or manually roughed at assembly points, bonded with a two-part E332 epoxy adhesive from Permabond (Eastleigh, Hants, U.K.), and ambiently cured in an assembly jig for 24 hours. The front fork and one-piece handlebar unit are made separately, using similar methods. Final carbon content is 60 percent by volume.
The completed frame and front fork are wet sanded, coated with a clear epoxy layer and resanded, followed by decal application and a clear epoxy-based topcoat.
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