Primitive Archer

Willie: I do not have a good explanlation for these findings. The only thing I could think of is that hickory maybe has more strectch than boo and thus does not "stress" the belly lamiante like bamboo does. However this idea does not fit into virtual-bow-logic as there is no stretch parameter. My "stretch" must somehow be connected to MOE and here the differences betwween boo and hickory are not so big....yes boo is a little stiffer.

Other - maybe related - question: Why do we have different stress on back and belly if we bend a piece of wood?

I think the main reason is that different wood and grass species have different rheological properties, and therefore their viscoelastic behavior differs. For example, hickory is well known to perform excellently at very low humidity but is generally sluggish at high humidity. Its intrinsic properties vary with conditions (both external and internal). Additionally, viscoelastic material properties are time-dependent. A well-known example of this is that with a fast release we obtain a higher arrow speed than with a long anchor at full draw. Thus, the main reason lies in the intrinsic material properties, which we do not know well enough and which are difficult to model accurately. In practice, we must rely on measured data. For example, the measured difference between hickory and bamboo backing is both interesting and important.

The second question "Why do we have different stress on back and belly if we bend a piece of wood?". In theory, a homogeneous, symmetric bending beam should have identical absolute values of tensile and compressive stresses within the elastic range. However, in a bending bow there is also an axial force (a force in the direction of the bowstring), which affects the stress distribution by introducing additional compression force. As a result, the compressive strain is higher than the tensile strain, and this effect is also modeled in VirtualBow. In reality, there are additional material and geometrical factors that influence the stress distribution, each contributing its own effect, but which are difficult to model.

Several questions about the programs. Do they allow for some set at given stresses? Does increased set affect efficiency or FDC? If so, by how much? Are there any allowances made for working part of the limb length? Efficiency is lost in working limb length and increases as the working limb shortens, even though strain figures might go up.

No, it is not possible to define set as we understand it. However, there is a “damping” parameter, but it only describes some general energy losses in a bow. Set relates to material properties, and it will be included in the future, so you cannot currently take set into account. Of course, you can interpret strain values: if the compression strain is high, the bow will take set.

Regarding the working part of the limb – yes, of course. You simply define how the bow tapers (just like in my Straight Bow example) and simulate it. I am not sure that efficiency is lost with a longer working limb length; I do not believe it is the determining parameter. Efficiency relates mainly to moving limb mass and hysteresis, and working limb length affects these factors.

Nah, unfortunately it doesn't account for set. Alan Case's spreadhseet allows a bow to be 'pre-stressed' to account for set.

If One happens to have bend test data, then One can predict how much set the bow will take at a given stress level. That's what I've done with my D/R bows above, and anticipate they'll take between 3/4 and 1" set once shot in.

Alan Case’s SuperTiller program (in Excel) does not take set into account. The “pre-stress” feature is purely for pre-stressing, similar to Perry-reflexing a bow. For example, in VirtualBow a reflexed bow has zero stress when unbraced, whereas in SuperTiller you can introduce pre-stress so that the bow has a certain amount of stress even when not braced. As a result, at full draw this kind of bow is more highly stressed than a bow with no pre-stress. It is a nice feature, and hopefully we will have it in VirtualBow in the future as well.

Compression strain values are very useful when predicting set. Wood will take a permanent set when the strain value exceeds about 0.4–0.6% in compression, depending on the wood species. In tension, wood can tolerate on average about 0.9–1.0% strain, but the limit is much lower in compression. Of course, there is an important difference: if the maximum strain is exceeded in tension, the bow breaks, whereas if the bow takes set, it is not yet broken. I am not sure how much compression strain is required to cause chrysals, and I am sure it varies greatly depending on the wood species. However, if you want to estimate set in VirtualBow, I would use 0.4–0.6% compression strain as a guiding range – beyond that, the bow will take set.

Kinkier than a garden hose wrapped around a lawnmower.

That tiller looks good considering the limbs each leave the handle at a different angle.

Awfully pretty work!  Just enough character to be interesting too.

Nah, unfortunately it doesn't account for set. Alan Case's spreadhseet allows a bow to be 'pre-stressed' to account for set.

If One happens to have bend test data, then One can predict how much set the bow will take at a given stress level. That's what I've done with my D/R bows above, and anticipate they'll take between 3/4 and 1" set once shot in.

Several questions about the programs. Do they allow for some set at given stresses? Does increased set affect efficiency or FDC? If so, by how much? Are there any allowances made for working part of the limb length? Efficiency is lost in working limb length and increases as the working limb shortens, even though strain figures might go up.