I've never worked with hackberry (doesn't grow in Europe naturally), but I think the statement "hackberry takes more set than many woods" is not quite valid. I'd like to comment on the general issue with a bow taking set. Most of my points hold true for any wood.
A bow doesn't take set by itself. The set is caused by something. It can be caused by one thing or several factors that get combined. Set basically is the manifestation of an overstressed piece of wood. Yet one wood can take set more easily than other woods. Possible reasons could be:
- Hackberry is a lightweight wood (low SG) and thus, is easily overestimated. A wood with a lower SG requires a wider limb design to deal with the stresses. Woods with a low SG (such as hackberry) tend to be made too narrow, and are thus easily overstrained, resulting in set.
- A wood can be compression weak, or tension strong (what's the difference?). Set always happens in the belly of a bow, not the back (see experiment with a bow with set sawn in half lengthwise). Therefore, the belly can become overstressed in compression weak woods more easily. A trapped back will counteract this. Heat treating the belly will also work, as this increases the compressive strength of the wood.
- A wood can require a low moisture content for optimal strength. Hickory is a good example. With a high moisture content of the wood, it tends to get set easily. While at a low MC, it is an excellent performer. Perhaps hackberry also likes a low MC. Any wood will take set more easily at a high moisture content level, but the optimum is different for each species.
- A wood can be more elastic than average. An elastic wood results in a lot of permanent bend (=set) before the wood fails. A wood with low elasticity just snaps, before taking any set. Therefore, hackberry could be a very elastic wood, that takes a lot of set before it finally fails. Jatoba, padauk, cherry and bubinga are some examples of (tropical) woods with low elasticity. These species chrysal and break, before they take a lot of set.
I hope this clarifies why a wood takes set and in which category hackberry might fall.
