We are almost ready to start live testing of a groundbreaking, new carbon 29″/700c rims. The basic parameters such as a fully aerodynamic 35mm deep and 27mm wide airfoil rim shape are not actually the main attraction, special as they are. The big deal with these rims is the way they are manufactured.
Traditional carbon rim manufacturing method is by and large identical across all manufacturers: mold is made, carbon fiber cloth is cut into pieces, pieces of carbon fiber cloth are laid up inside the mold (or over a solid core), the core material is inserted (bladder, foam), mold is closed and the rim is baked. There are some variations and different solutions for core removal, tire bed molding, etc. but in every case a carbon rim is made of many individual fabric patches laid up by hand. By hand.
Now, we are all brought up to be nice consumers and respond positively to “hand made” description in a product’s origin. In many cases this is a good thing as a trained artisan or maker can spot the material or component variation and either reject it, or work around it and still deliver a fantastic finished product. However having to lay up carbon fiber layer by layer in order to maximize the material properties is the main thing that is holding the composites industry back from being able to displace metals from even more applications. In bicycle component manufacture this laying up is done by hand while in aerospace and large industrial applications, layup is often done by automated systems, including robots.
Automation of layup is beneficial from the point of view of being able to scale up production, and more importantly in aerospace, to avoid variation that is introduced by human hands. Every part must be identical and every part must meet the very stringent safety and performance requirements. You do not want one wing on an airplane to be 5% stiffer or heavier than the other wing, for example.
So, machines are good if you want the ability to scale up production, and if you want to make very accurate composite parts. Besides fabric/cloth layup there are two more composites manufacturing methods that use unbroken carbon fiber and that can be automated and involve machines, not hands to make lightweight parts: hose weaving as used by Time and by BMC for the Impec frame, and filament winding which is mainly used for pressure vessel or long beam production.
The video below shows an example of filament winding of a high pressure submarine sphere.
Both hose weaving and filament winding offer tangible advantages over hand layup when it comes to accuracy of manufacture, and process repeatability. Of course hose weaving will never deliver a composite part that is as stiff as what is achievable by hand layup simply because weaving causes crimping of the fibers. Filament winding on the other hand has no performance disadvantages at all. In fact filament winding is theoretically the best way to utilize carbon fiber in manufacture of composite parts – the fibers are not broken or crimped at all for the entire length of the fiber – from the start to the finish it is the same length of carbon fiber. The only problem with filament winding is that it is not suitable for manufacture of complex parts. Until now.
Using a novel process and winding geometry our manufacturing partner has managed to produce filament wound carbon rims for us. They are machine made using a proprietary filemant winding machine and process, utilizing a single length of 12k carbon fiber tow, from start to the finish. There are no carbon fiber patches, no cuts, no overlaps, just one continuous filament bundle. The practical benefits are a stronger and lighter rim than what is possible to achieve using hand layup. Since the carbon fiber bundle is never cut, the loads and strains are effectively borne by the entire rim, without any discontinuity or load concentrations.
The rims are not yet perfect and are not yet at our target weight. They pass all the machine safety testing however and we are confident that through careful selection of resins (proprietary resin formulated specifically for this process) and carbon fiber we will be able to meet our weight target of 375g per rim while exceeding all the vertical and torsional stiffness and strength targets.