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Thermwood

Ultra light Carbon Fibre/Epoxy Road Bike

30 April 2004

Kestrel, pioneers of the first ever-production carbon fibre bike frame in 1986, has unveiled a new version of its ultra light Talon SL road bike.

The bike sports a frame weighing an astonishingly low 2.4 lb (1.1kg), which is light even by carbon fibre standards.

Kestrel co-owner Preston Sandusky credits the low weight to the bike's ""modular monocoque"" frame, fabricated from three individually bladder-moulded 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 California, using Pro-Engineer and RHINO 3-D Solid Modelling software, the frame structures are fabricated in separate clamshell moulds CNC-machined from PX5 tooling steel, with P20 steel inserts for frame profiles.

Parts are hand layed, using ""800K"" prepreg (120 g/m2 area weight) made at a factory in China, using a factory-formulated toughened epoxy resin and T300 and T700 carbon fabric and M30S unidirectional carbon fibres from Toray Industries (Japan).

Up to seven plies, diecut from sheets of M30S, are wrapped around silicone mandrels to form the basic asymmetrical tubular profiles. Fibre 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 mould halves.

Bladders are threaded through the components and tubes are joined together by overlapping ply buildups that are incrementally stepped. The moulds are closed and placed in a 4,500 lb press fitted with heated platens. The bladder consolidates the prepreg and fuses the joints as mould temperature is computer-ramped to 130°C/266°F and held for a half-hour. Demoulded 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 re-sanded, followed by decal application and a clear epoxy-based topcoat.






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