An efficient carbon bike frame which is not laminated, but insteadmade from load specific woven tubes. A revolutionary half-shell system that connects these tubes with strength. Establishing a production facility in Switzerland to manufacture the frame competitively. An integrated industrial manufacturing process to satisfy quality requirements. All of this could be achieved in a groundbreaking project by the Swiss bicycle manufacturer BMC Switzerland Corporation, Grenchen, Switzerland with the Impec bicycle frame.
The perfect racing machine – according to ancient wisdom – transforms maximum possible energy invested by its rider into direct propulsion without bringing its own weight to bear. The frame needs to be light and rigid and yet stable and agile at the same time. In order to bring these attributes together in an optimum ratio in the Impec bicycle frame, BMC first concentrated on the design of the frame tubes. Carbon was used as the no 1 material right from the start in the development of the Impec.
As a result, the traditional laminating process where composite mats are bonded together by hand was basically out of the question. So the Swiss engineers developed two technologies that were of vital importance for the Impec: Technology no 1 is the robot-controlled load specific weave process (LSW). At this stage of production, each individual tube of the Impec frame is made-to-measure seamlessly and with absolute precision. On the Impec, the entire frame is optimised to its specific load. Each of its tubes does a different job and therefore has a different design. The three stages of the LSW process consist of weaving, resin injection and cutting – and from these stages comes a carbon tube that only needs to be lacquered, printed and attached to other tubes to make the frame.
Weaving, injecting, cutting
The production of the Impec commences as follows: an industrial robot picks up a material carrier with the positive mould of the tube that is to be produced. It then feeds this core to a radial braiding machine which up until now has been used mainly in the production of steel cables. The data matrix is read off and the weaving process begins.
More than 100 bobbins loaded with wafer-thin threads run through the machine along sinusoidal paths to weave a seamless tube of carbon fibres around the positive core at the centre of the machine. The material density and arrangement of the fibres is determined here by the rate of advance and the mould of the positive core. The rate of advance varies according to the specific load for which the tube is configured at this point. When the carbon structure is fully woven, the tube is cut off, the material carrier retracts from the radial braider and another core takes its place on the machine.
In the next stage of production, a tube is formed from the made-to-measure carbon sleeve. To achieve the required production quality, BMC have developed the first fully automated resin injection process in the world for composite materials, according to the company. Central to the second stage of the process are the female moulds of all the tubes from which the frame of the Impec is created. In these moulds, the resin infusion of the carbon structures takes place under controlled conditions. For this process, the positive core with the previously load-specific woven carbon sleeve is fed into the corresponding mould. A special two-component resin is then injected through a mixing tube at the lower end of the mould. The workpiece is then left in the mould for as long as it takes to complete the curing process.
The third and final production stage of the load specific weave process is cutting the tubes to length. This process is also fully automated and prepared and carried out with precision by industrial robots. Step 1: the tube is separated from the mould together with its positive core. Step 2: the material carrier with the positive core is removed from the tube. Step 3: a precision saw with diamond blades cuts the tube to its final size. Step 4: the finished tube goes for final inspection and then to the paintshop.
The shell node concept
Technology no 2 is the shell node concept or SNC for short. The frame joints of the Impec each consist of two half-shells instead of one sleeve socket which are bonded with the frame tubes in the final assembly.
These shells are what is actually revolutionary about the Impec frame. SNC was possible only through a new composite compound material which can be processed by injection moulding and exploits the qualities of carbon as a material. The granules that are used consist for the most part of carbon fibres and a special two-component resin. After analysis, test and optimisation, BMC decided to manufacture these half-shells from Ems-Grivory products. The supports made of the carbon fibre reinforced semi-crystalline polyamide Grivory GC-4H and long glass fibre polyamide GVL-6H are said to provide the bicycle frame with the required bending stiffness, strength, shock-absorbance properties and weight savings to be achieved.
The direction of the carbon fibres inside the various shells were precisely defined in a mould flow analysis before the final CAD data were fed into the tool construction program. To verify the results of the analysis, the next stage involved construction of the injection moulding tools. Small batches of shells were produced which we then subjected to a series of tests. This included scanning the half-shells in a computed tomography scanner, allowing an accurate inspection of the wall thickness and check of the overall structure for possible faults.
In order to connect tubes and shells to the frame of the Impec for rigidity, the shells are first placed in a carrier system. This frame carrier is now fed into a robot workstation. The robot used is fitted with an optical monitoring system. It recognises each individual component and defines the exact quantity and position of the composite adhesive to be applied. Firmly clamped into the carrier system, the completed frame is then cured in an oven and is then ready for the final quality control which is conducted using a static test process. The open design of the shell allowed for the fact that even the last stage of production of the frame – the bonding of the shells with the frame tubes – proceeded transparently in a controlled process.
The frame is claimed to be assembled from faultless components so that it requires no finishing. All components are individually finished and inspected and only then released for final assembly.