Renovo in Portland in the 2010's used a clam shell design and most other builders since then have adopted it, thinking it was "state of the art" in building wood bike frames. I'm going to explain why this building method is weak and prone to failures.
When I bought the company in 2019, I realized right away that the clam-shell was a serious shortcoming in design, structure, execution and length of service. It made sense for someone who was trying to automate the build process, making it somewhat like making parts from metal. But it didn't make sense for wood machining. Wood is nothing like metal. It has a grain, meaning it has strength in one direction, but not in the other direction. Wood along its length is very strong, stronger than steel sometimes. But along its width, it is weak. Expose that weak side of wood in a curved part and the part becomes compromised. The solution is to bend the wood around the curves, using the long grain strength. Sawing or routing through the long grain to make a curve produces a weak curve. Those weak areas end up being at the seat tube, bottom bracket and head tube areas. This is exactly where the most strength is needed from woods long grain.
After I bought Renovo, I started receiving frames to repair. This was curious since the former owner said clearly on promotional videos that they had never had a frame failure. The failures were all at the seat tube junction to the top tube, at the bottom bracket, or less often at the head tube. Also, some failed on the clam shell joint along the center of the tubes. All of this was repairable, and the repairs used bent wood at those joints for strength. Better, more capable epoxies for joining dis-similar materials were employed as well. The knowledge of wood working, and hands on experience fixing the clam shell builds led to a new style of building wood bike frames that is better looking at the joints, and stronger at the corners where the critical components of seat, crank and fork reside.
Speaking of joints, the old Renovo use Finger Joints and advertised that they were like joints in fine furniture. Here is another exaggeration possible due to lack of knowledge of woodworking. Or it was simply marketing. But finger joints are not used in fine furniture making. They are used in the cheapest engineered wood dimensional lumber to make hidden joists, trusses and beams. They are not usually shown off for visual effect. And carve through a finger joint to make a curved part and all kinds of weird glue line and interlocking joint shapes are exposed. In my opinion, those could be very unattractive, but from a marketing standpoint, the old Renovo leaned into it like it was a desirable feature.
One of the big problems of the old Renovo clam shell design besides the weak end grain parts and unsightly finger joints was that wood is poorly suited to CNC machining. Metal parts are perfect for metal machining because they retain the precision expected after cutting. They don’t have a grain that pulls the part into a new and unwanted shape. Wood does have grain, and expands and contracts, which can lead to a change in shape. But more so, it harbors tension which can be released during CNC machining, or afterwards, changing the shape of the part. This becomes a problem mainly when trying to put the two sides of clam shell together. Parts that have moved won’t align well, and can result in thin and weak joints when they are later surfaced to get a smooth exterior surface. I have re-bonded several frames that split on the clam shell joint where they didn’t have enough bonding surface to begin with. As precise as CNC machining is, it can’t compensate for wood movement. Only the craftsman can.
Another issue with the clam shell design is that the bond between the two halves has to be made exactly in the middle of the tubes, which is unsightly, but more importantly is usually the thinnest part of the tube. There is very little bonding surface available, and it is often made even smaller by poorly aligned CNC cut parts.
Having explained what is wrong with the clam shell (if you want some improvements on the method, see the end of this discussion), I need to review my build method since 2019 and how it is better than a clam shell. Note that there is little difference in the seat stays and chain stays, though both have been updated. Both methods have light hollow tubes made primarily of wood. Both use sleeves of aluminum or carbon fiber where they interface with any modern bike components…at the seat post, bottom bracket and head tube. The superficial difference between a clam shell and my bent wood core build is that the clam shell is made from two main layers and my bent wood core is a total of 3 layers. Instead of a bisected frame, the core has a single layer of bentwood that occupies the middle 1.25”, 32mm of the frame and creates the space for hollow tubes. There is no center line joint. The picture at the head of this blog shows that middle bentwood core that is 1.25" thick.
The middle bent wood core contains the carbon fiber or aluminum sleeves at the seat post, bottom bracket and head tube. This hollow, solid bentwood core with the sleeves bonded, and stays attached, is decked on both sides to create the full main triangle of the frame. It is the structure or skeleton of the frame, and is responsible for the elasticity of the frame. The sides of the hollow tubes are not bent, but sawn to shape, then hollowed. These sides provide the bulk of the stiffness of the frame. Together, these three layers (mid bent wood core decked on both sides), create a complex box beam shape which can also be called a monocoque. In this way, it is still the same look as the original clam shell, just stronger, and better looking with no exposed end grain or finger joints.
For clam shell builders, I can suggest three improvements that make that build method still effective. Since Renovo popularized this method in the 2010’s, I am updating it for the 2020’s…
1. Use strong bentwood components at the curves at the seat post, bottom bracket, and head tube areas to minimize exposing end grain when machining.
2. The center joint area should be made with more thickness than the rest of the tube, so that it can have more surface area for bonding, and act as a stiffener for the tube. Deliberately make the part about 1.5mm (.065) (1/16”) too thick on the outside of that joint (taper to about 6 mm (¼”) to either side of the joint, and 2.5mm (.100) (3/32”) thicker on the inside of the tube. After joining the 2 clam shell parts, sand back to the desired tube shape. The extra outside thickness will give you a chance to use misaligned parts due to wood movement. If there is no wood movement, it will be easy to remove during fine sanding.
3. If machining on a CNC router, do a coarse cut, then set aside in a climate controlled room to allow tension to release, and any uneven moisture to equilibrate. Then (a week or two later), do the final cut on the CNC machine.