The 101 on Wood Science
The 101 on Wood Science
Wood describes the predominant material that is the makeup of all species of trees. While it is ultimately much more complicated it is, structurally, similar to a rigid, closed cell foam. That means it can, with sufficient energy, be crushed and absorb significant energy, just as the EPS in most bicyle helmets is designed to do. Different species of wood have somewhat different properties of course and even within the same species different samples of wood behave somewhat differently. But almost any sample of wood is capable of absorbing more energy than the types of plastics typically used in mass produced bicycle helmets.
When a force is exerted on a piece of wood the response of the wood to the force is dependent also on the piece's grain orientation. There are three types of grain in a typical tree and these are Transverse, Radial and End grain (or Perpendicular). These describe the way that wood cells “lay”. A piece of wood impacted directly on the end grain will respond differently from the same piece of wood being impacted on the radial grain. End grain tends to want to crack and radial and transverse grains tend to want to crush. Both behaviours represent the absorption of impact energy.
Some woods (mostly hardwoods) are very resilient and dense. They do not easily crush or break and thus they are more durable in mechanical applications such as impacts. Soft woods generally crush and crack more readily which means that, while they are less resilient than their hard wood cousins they provide added capacity to absorb the energy of dangerous impacts. Both Hardwoods and Softwoods are able to absorb significantly more impact energy than the typical hard plastics currently used in helmet shells (polycarbonate and ABS).
The vast varieties combined with the various grain orientations make it possible to select an idealized wood for a given impact application. More durable vs. greater energy absorption.What this means for helmets is that, when used in the shell, wood can help absorb the energy of dangerous impacts to a degree that is not currently available through helmets with plastic or composite shells. Wood typically absorbs energy best at energies somewhat higher than the high density EPS that most bicycle, skate, motorcycle and ski helmets use. This means that the wood shell provides significant protection over a greater spectrum of impact energies. Other helmet manufacturers have ambitiously developed this same performance "redundancy" by using more than one kind of foam (Kali Protectives) or using a layered shell (WRSI) but the majority of helmets on the market rely on one kind of foam to accommodate all levels of impact.