Building Hollow Wood Bicycle Frames - Teaching an Old Material New Tricks


The use of wood technology in bicycle construction dates back to 1817 and the invention of the bicycle by Baron Karl von Drais. His “Draisine” was built with the best technology of the time; frame and wheels held together with nails and pegs. Since then, wood bikes were manufactured off and on worldwide through the late 1940's, but virtually none have been hollow nor were they engineered for performance.

Renovo introduced modern technology to wooden bike frames in 2007, 190 years after the first wooden bike. We originated the process of laminating hardwoods and using a computer-controlled (CNC) router to precisely mill our frames in hollow halves, which are later bonded together.

Our use of wood was not an aesthetic choice, and Renovo's founders are not woodworkers of any sort. They chose wood for its outstanding engineering properties. Our background is composite aircraft design, engineering and manufacturing and our tools are CAD/CAM, or Computer Aided Design and Computer Aided Machining. This plus a detailed engineering analysis of potential frame materials led us to wood. With state-of-the-art adhesives, finishes and manufacturing techniques we have produced high performance, lightweight wood bicycles so durable and beautiful they have become treasured heirlooms. 

It takes many steps to make our frames, and at each one we take steps to ensure quality.



 

 

The Right Boards- The appearance of each bike is unique because grain, color and figure vary, even within the same board. We spend ridiculous amounts of time at four different specialty lumber yards choosing the perfect lumber, and more time back at the shop deciding how to cut and match the 16 to 40 pieces of wood in a frame. But we confess we love going to the lumber yard, it's an exciting treasure hunt every time. It’s way more fun than just ordering another steel tube set or sheet of carbon fiber.


Planking and Milling- 
Once we’ve selected our lumber, it must be cut to precise dimensions for our laminating process, whereby we join together multiple layers of wood into a single plank to achieve the desired stiffness, weight and aesthetics. These laminate planks then make one or more trips to the CNC router as we mill frame parts, bond them together and mill them again and seal hollow structures before joining the frame sub-assemblies into a complete frame. Each frame component’s trip to the CNC represents hours of time spent by our engineers planning the sizing, wall thicknesses and production methods. (Table saw closeup)


Bonding- 
We use four different epoxy formulations from two suppliers to meet our requirements of withstanding high temperatures, resisting moisture and bonding complex joinery. All hollow frame components are sealed inside prior to bonding. We have bikes that endure extreme weather in Alaska, Norway, Belize, Singapore, Dubai and the Pacific Northwest with never a failure from these conditions. To ensure consistent and good bond quality, we follow industry standard practice: meticulous surface preparation, good process control and good training—pretty much the same set of criteria to ensure good bonds on carbon bikes or good welds on metal bikes. All bonding is done in a climate-controlled room and the frames are oven cured to achieve the highest possible bond strength. 

Metal and Carbon Fiber- We use metal for sleeves in the head tube, upper seat tube and bottom bracket, as well as our house-designed dropouts. These alloy fittings allow us to use industry-standard parts. All bonded aluminum is abrasive blasted, acid etched and alodined for maximum bonding strength. We also use carbon fiber to increase the strength of our head tube bonding system, and to strategically reduce frame weight in certain applications.


Shaping- 
Our CNC mill does the heavy lifting, making the rough shapes of our frames from straight pieces of lumber. That said, the beauty of our frames is made possible by our craftspeople, who painstakingly blend the sharp edges cut by machine into the flowing lines that mark our frames as unmistakably hand-finished. Every frame is hand-sanded for hours, smoothing and refining machine work into the flowing lines envisioned by our designers.


Finishing- 
Our finish coating is a multi-layer affair, with separate sealers and waterproof polyurethane formulated for outdoor use on wooden structures. We have found it to be the toughest, most waterproof clear coat available. No other coating compares to our finish for blocking Ultraviolet damage and moisture.  

 

 

 

 

 

 

 

 

 

Surviving Real-world Conditions

In a  properly designed structure, properly sealed and bonded, wood is unaffected by hot, wet or cold outdoor environments; modern wooden boats live in the water, wooden aircraft and wooden propellers fly through it. 

  • Moisture content- Wood “seeks” moisture equilibrium with its environment, so its content varies by season, ranging from about 8% to 14%, which has no structural or cosmetic consequence to the frame. The finish on the outside and sealer on the inside minimize the change.
  • Dimensional change- Moisture causes expansion/contraction of the wood across the grain, but only in proportion to its thickness, so for the majority of the Renovo frame, the change is only a few thousandths of an inch over a 6 month period, a non-issue. There is negligible dimensional change lengthwise with the grain.
  • Heat/cold effects- Wood gains strength as it gets colder, and loses strength with heat, but the Renovo frame is designed for temperatures in the range of human activity, so  heat or cold has no material effect.
  • Warping, Cracks- These problems occur in wood from improper drying before machining, or for example in the case of the twisted studs in the walls, using non-kiln dried wood. Look at fine quality furniture hundreds of years old, you won’t find cracks. All wood used in the Renovo is kiln-dried before we buy it, but we check and adjust the moisture content to make sure it’s in the correct range if necessary.

 

 


Maintenance- 
No more is required than any other painted bike, perhaps less. A scratch through the paint and sealer could be a cosmetic problem if left unsealed, much like a steel frame—if the scratch got wet enough, often enough, the finish around the scratch might eventually blister. A quick fix is clear nail polish, although a more complete repair is simple and unlikely to require a pro.




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