Plywood Construction for Your Wood Bike
Why Try Ply?
There are so many different ways to build a bike out of wood, and each style has various methods by which it's done. There are benefits and drawbacks to each one, and in this case we'll illustrate those of building from plywood, or thin plies of wood.
The number one question about wood bike frames is generally regarding their strength - and rightfully so. Wood can be extremely strong as a frame building material, but this relies heavily on the orientation of the wood grain in the design of the bike. Put it in the wrong direction and the beautiful grain line becomes an ugly fatigue crack.
The forces that a bike frame encounters during normal usage are complex and they typically stress the wood in all different directions. For example, the head tube experiences the weight of the rider in the vertical direction, but it also sees large side to side forces from pedaling.
An easy solution to this is designing and building using high grade plywood. Each of the thin layers in plywood are oriented to alternate the grain direction, creating a material which is significantly stronger than the original wood on its own. While this is a significant strength adder, it’s still important to reference the material properties of the wood species being used in the plywood. A good source for the mechanical properties of wood is the Wood Database. You'll find this book on most woodworker's shelves and it's just the right amount of information without being overwhelming.
Whether you’re buying plywood or layering it up yourself, it’s always good to start with products constructed 100% from hardwood veneers.
Like many fellow wood bike frame builders we’re trying at every turn to reduce our impact on our surroundings. Building from a sustainable material is step one, and from there we try to use as much of it as possible in the bike. The more bikes we can product from one tree, the smaller our impact on our surroundings.
No process is perfect, but the amount of wasted material can be reduced by using layers of wood veneer, otherwise known as plywood. Veneer is produced by slicing thin layers from a log with a blade, as opposed to cutting with a saw. Imagine Grove Woodworking has a great YouTube showing the manufacturing process for veneer.
Check out the cutting machine in the video, built in Lockport, New York - just north of us here in Buffalo. By slicing layers from a log the entire piece can be used, and there is very little waste. In addition to this, using hardwood plywood means the thickness is decided by the number of layers of veneer. It requires no joining or planing, and can even be used to create the round tubes used on our Normal Bicycles.
There will always be sawdust and scrap pieces, but working with plywood or layered wood veneer can help reduce this.
When buying commercially produced plywood it's important to know what type of wood is used for the internal core layers. Often the outer layers will be high grade sanded hardwood for appearance and the core will be a softer and lower grade wood.
Fully hardwood plywood is widely available in birch and is often called baltic birch or Russian birch. When purchasing baltic birch plywood from a reputable source like Rockler Woodworking you can be sure that each layer is birch.
The same can be said for some marine specific plywood used in boat building, although the mechanical properties of these species are often lower than those of birch. Typical marine plywood is available in meranti and okoume wood species, and you can count on each layer being consistent.
Using hardwood outside of birch, meranti, or okoume will generally involve special construction, but by making your own plywood the options become wide open. Species can be mixed and different shapes become possible. For example, our Normal Bicycles wood bike tubes are constructed from layers of hardwood veneer shaped into a round tube.
Ply Design Mentality
Even if not using traditional plywood, it is beneficial to design with the mentality of using plies of wood with alternating grain direction. It’s almost impossible to design a bike frame without varying the direction of the wood grain, and in most cases the more you do it the better the frame will turn out.