Recent Posts1
Bows / Re: Common mistakes in deflex/reflex bows?
« Last post by Tuomo on Today at 02:50:31 am »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.