Why a Wood Bicycle Frame?
The short answer: Wood, nature's carbon fiber, has unique engineering properties that deliver superior ride quality and durability compared to man-made materials, and...it's sustainable. When the right wood is combined with an array of advanced technologies, it becomes a high performance material that will forever change your understanding of 'wood'.
Yes, But Haven't Wooden Bicycles Always Been Just Art or Novelties
Fair question. In the past, the superior properties promised by wood weren’t delivered for several reasons:
1. Past adhesives could not reliably bond metals to wood, finishes couldn't block sun damage or moisture, nor were they nearly as durable. Reliable waterproof bonds relied on glues which were difficult to use. All these issues are resolved with modern materials.
2. The lack of computer controlled woodworking machinery made the complex joinery we use simply impossible.
3. Until now it was not practical to produce a hollow frame, so in the past wooden bikes had solid, heavy frames. This instantly relegated them to art or novelty, as it would a frame of solid rather than tubular aluminum. But now, with computer-controlled machining, we have produced a hollow (monocoque) wood frame, and wood has finally delivered on its promise:
- Wood absorbs vibration better than steel, aluminum or carbon, producing a uniquely smooth ride.
- The fatigue life of wood exceeds steel or aluminum, and approaches carbon
- Wood is not susceptible to crack propagation from dents like the metals, nor is it notch-sensitive or damage-concealing like carbon.
- The Renovo wood frame has much greater impact resistance than butted metal or carbon bike frames.
- Wood is as stiff pound for pound as the metals, even as it’s better at absorbing road vibration.
- And last but not least, wood is sustainable, recyclable, biodegradable, and has a tiny carbon footprint, while the primary production of steel, aluminum, titanium and carbon fiber are incredibly polluting and unfriendly to our environment.
Monocoque is French for ‘single shell’. It’s a structural design approach where primary loads are carried by the skin or outer shell of a structure, rather than an internal framework. It was developed by Roland Aircraft in Germany in 1916 for the fuselage of their all-wood aircraft. Where the skin is strong enough to carry the structural loads, monocoque is the lightest method of construction.
Perhaps the best example of an all-wood, monocoque structure is the most successful fighter/bomber of WWII, the 400mph De Havilland Mosquito. Over 7500 were built and it saw service from Europe to the tropics from 1940 until 1963. Its monocoque fuselage was built in separate halves then bonded together down the centerline. Monocoque design is the standard for molded carbon bicycles and has been used in most high performance aircraft since WWII, as well as many current production and race cars.
As you read the following, consider the repeated and substantial stress endured by wooden diving boards. Many modern boards still have wooden cores.
Frame life is determined by the fatigue resistance of the particular frame material, that is; its ability to absorb cyclic loads, the amount of material to absorb those loads, and finally, the actual loads the frame sees; racing, abuse, accidents, etc. Wood is one of the most fatigue resistant materials on earth, simply because it used to be a tree. Trees have a structural makeup that resists constant bending and twisting from the wind for their entire life, which for some windblown trees such as the bristlecone pine, is nearly 5000 years. As a result, wood has a remarkable fatigue life that exceeds steel and aluminum and rivals carbon, so we can design our frames with adequate margins to assure a long life against normal loads, and still have a light-weight frame.
An authoritive source on wood fatigue is ANC-18, the 1951 Army, Navy joint publication titled, 'The Design of Wood Aircraft Structures', wherein they simply state 'wood is less sensitive to rapidly repeated loads than are the more crystalline structural materials (metals), resulting in a higher endurance limit in proportion to the ultimate strength."
Echoing ANC-18 are the products and testing done by the Gougeon Brothers who manufactured (among other things), 100' long wind turbine blades of epoxy bonded wood which withstood 400 million fatigue cycles over a 30 year life span, and epoxy-bonded wooden boats. They also developed the West Epoxy system and wrote the book on epoxy-bonded wooden boat construction. It is their construction technique and structural testing for Lloyds of London that made possible the beautiful 154' wooden sailing yacht, Scheherazade pictured here.
Like the Mosquito airplane pictured above, the Renovo frames are made in two halves, bonded down the centerline. The Mosquito relied on adhesive plus 30,000 screws to fasten the fuselage halves together; if they'd had epoxy, they could have eliminated the screws. Our adhesives are moisture proof and withstand the high temperatures a bike might experience left in a black van in summer in Death Valley. 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.
The Right Wood
The appearance of each bike is unique simply because grain, color and figure vary, even within the same board. Our wood selection process for each and every bike is painstakingly conducted by obsessive people. We spend ridiculous amounts of time at four different specialty lumber yards searching for and choosing the perfect lumber to achieve stunning, and much more time back at the shop deciding how to cut and match the 8 to 24 pieces of wood that make up a frame. But I confess, we all love going to the lumber yard, it's an exciting treasure hunt every time. Way better than just ordering another Reynolds tubing set...
Because bicycles endure sunshine (UV), rain, bike rack and transportation rash, as well as the occasional fall, we utilize a sophisticated proprietary coating to protect our frames from normal wear and tear as well as moisture. It's expensive, but it's the toughest, most durable finish that can be applied, period.
Heat, Cold and Moisture are Not Problems
Wood, in a properly designed structure, properly sealed and bonded, 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 control the change, but can’t prevent it.
- 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 so that in the temperature range of human activity, heat/cold has no material effect.
- Warping, Cracks. These problems most often 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.
We use metal sleeves in the head tube, upper seat tube and bottom bracket. The headtube and bottom bracket contain bearing races under load which some woods handle just fine, but others don’t. So, to avoid a mix of woods which complicates joinery and compromises appearance, we use metal inserts. The seat tube has an insert simply because it's problematic to use a seat post clamp over wood.
Before proceeding with the wooden frame concept, we made and tested wooden tubes of various species, wall thicknesses, shapes and sizes, with the goal of at least equaling the torsional stiffness, out-of-plane-bending, and weight of an Easton aluminum mountain bike down tube. We surpassed that goal, and achieved even better results against a road type steel down tube. (Had these tests been unsuccessful, we’d be doing something other than bikes now.)
We test every stick of wood that enters our shop for stiffness, moisture content and other factors that affect the quality of our frames. This accumulated database enables us to forecast ride quality and stiffness for the riding style and weight of the owner. We also test each frame we produce for stiffness, which gives feedback on our predicted stiffness and also serves as a quality check on the bonding and joinery. In addition, we proof load each head tube by loading it with 150 lbs on a steel bar at the front axle point directed towards the bottom bracket. This again is a quality check of bonding and joinery.
Wood has much greater impact resistance than carbon or butted metal tubing. Moreover, dents or scratches don’t propagate into cracks which are terminal frame damage with other materials, and unlike carbon, wood doesn’t conceal damage. We have tested the impact resistance of our wooden top tubes against both steel and aluminum butted top tubes. A 4” diameter steel tube dropped from 12” severely dents the metal tubes, but bounces off the wood, leaving just a small mark. Our polyurethane coating has ultra violet (UV) absorbers to prevent the wood from fading in sunlight, so it will retain the wood’s natural beauty for many years.
No more is required than any other painted bike, perhaps less. A scratch through the paint and epoxy 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 would eventually blister. A quick fix is clear nail polish, although a properly done repair is simple and unlikely to require a pro.