We’re pleased to share the following reader contribution authored by Richard Craddock of Filament Bikes.
Filament Bikes is a company that makes custom carbon fibre bicycle frames in the UK and exports around the world. I recently took on a couple of tandem commissions and wanted to share the process with the wider tandem community. I’ll describe some of the technical aspects of designing and making the frames.
I took on a road tandem commission which needed to be lightweight and built for disc brakes. The riders wanted a design that could be used for long road rides around Oregon. The frame was made using custom round tubes which offered excellent stiffness thanks to the laminate design and a fibre type that has a very high strength and modulus. They’re also quite a large diameter at 2 inches.
The first part that I needed to make was the pilot’s bottom bracket shell to use with an eccentric bottom bracket insert. The internal mould tool was made from stainless steel and the shell is made by wrapping sheets of carbon prepreg around it until the desired wall thickness and laminate design is achieve. Once the lay-up is complete the epoxy is cured under heat and pressure in the oven. I used heat shrink tape in this case. Here’s the bottom bracket shell being made:
The construction method I use is known as tube-to-tube, where the frame is made from separate tubes which are shaped to fit together, held in a fixture, then joined with structural adhesive. This method allows the use of any sizing, geometry and tube diameters without the need for fixed mould tools for each size. Here’s the frame held together with the adhesive:
Once the adhesive is cured the frame can be taken out of the fixture and the wrapping process is carried out. Each tube junction needs to be wrapped with many layers of prepreg carbon fibre at specific angles to reinforce the joint. Here’s the frame with the completed wraps before is gets cured under heat and pressure:
Here’s a closer look at a wrapped junction. There are lots of layers formed from individual pieces of prepreg carbon sheet laid down here.
The completed frame weighed 2.4kg / 5.3lb and was fitted with through-axles, a titanium threaded stoker BB shell, and four bottle cage mounts. It will be built with a right side timing belt.
TT Aero Tandem:
For this project, the brief from the client was for an aerodynamic frame to be used exclusively in competitive time trials. The UK has a long history of time trial racing and it’s still a thriving scene. The team are a husband and wife who aimed to set new National Records for mixed teams at the standard distances of 10, 25 and 50 miles.
The design called for narrow section aerofoil shaped tubes and for the frame to be stiff enough to handle the power produced by the team. For this reason I chose to use a second down tube.
The tubes were made by a company in the USA which specialises in carbon fibre masts and spars for sailing boats. We worked out the optimal laminate and wall thickness to use with a suitable aerofoil section, and they also made some short sections to slide over the main tubes to be used as seat posts.
Anvil Bikeworks of Colorado made the fixture parts required for tandem construction. Here’s the frame in the Anvil fixture:
I used a separate mount for the stoker stem because it could not be attached in the usual way due to the aero seat posts. After the wrapping process the frame looked like this:
The final steps were to make the seat posts and have the frame painted. Here it is finished:
The team rode fantastically well and managed to set new UK national records at the 10 and 25 mile distances.
The tubular carbon construction method is particularly useful for tandem frames because it can efficiently cover the many variables of individual custom sizing for the both the pilot and stoker. For the investment in tooling and design I’m now happy to be a tandem builder and I plan to make many more tandems in the future alongside the ‘half bikes’!