In a nutshell, the more moving mass there is, and the quicker it moves, the more energy is wasted. The latter depends on a number of factors, but most importantly the speed and mass of the bow's limbs and the bowstring right after the projectile is on it's way. The former is simple to plot using F/D curves (see above). Loss of energy while transferring it to the arrow.The amount of energy put into the bow while drawing it.Compound Bow Tech Review: Creation of the first 400+ fps Super BowĪccording to Baker (2000d:44) there are only two things that affect how good performance one gets from a bow:.The whole energy storage / force-draw curve is discussed in detail elsewhere on the Internet, as understanding it is especially important for modern compound bows: So, unlike what people tend to think, the draw weight of a bow (or a crossbow) does not matter much unless draw length is known, too. This means that if the poundage and other factors are kept the same, a crossbow will store 1/2 - 1/3 of the energy of the handbow. For example, a typical handbow has 30" (inch) draw length, whereas crossbows usually have 10-20" draw. The F/D curves also illustrate another important point: a shorter draw stores less energy than a longer draw, all other things being equal. This phenomenom explains why even the very heavy steel crossbows of the medieval times had relatively limited maximum range, around 400 yards, regardless of their very high draw weight and energy storage (e.g. However, this comes at the cost of reducing the increase of dry-fire speed as discussed above. So, the shorter the crossbow's draw length gets, the flatter it's F/D curve will be, meaning it's energy storage characteristics approach the optimum. This means that it's draw length has to be reduced or the bow will break. Also, as a bow is made thicker (=more powerful), it's not generally made longer in proportion. In medieval-style heavy crossbows the F/D curve is not nearly as relevant as in handbows because the final draw length is not a big issue due to mechanical cocking and release. So, essentially a modern compound bow allows storing more energy without tripping over the draw weight limitations of the shooter. With more aggressive cam configurations the energy storage could be increased further, as described on this excellent page. This bow would store more energy (14743 units) than the best case straight bow. The force-draw curve of an imaginary modern round wheel compound bow illustrate this point nicely: Or in other words, the faster the maximum draw weight is reached and the longer we stay close to it, the better. The shape of the force-draw curve is very important in handbows because the bow's maximum draw weight is limited by the human physique. In flight shooting where dry-fire speed is most important, a short bow with a very long draw is a much better choice, as it recoils faster. However, long bows tend to recoil slowly and transfer energy efficiently only to relatively heavy arrows (or bolts). If bow needs to be very powerful, keeping the final string angle small by making the bow long is a good idea. The difference is illustrated with the two (made-up) F/D curves shown below the shorter bow stores 8019 units of energy, whereas the longer bow stores 59% more energy (12741 units): Similarly, a short bow will have a very concave F/D curve and thus it stores much less energy. This means that a long bow (such as the English longbow) stores a lot of energy. Very long straight bows will have a relatively flat F/D curve: this is because the string angle does not change as much as with shorter bows. The concavity of the curve depends on the ratio between bow's length and it's draw length. This is because more energy is required to draw the bowstring back the same amount at the end of the draw than at the beginning. In real bows, however, the concavity of the curve is apparent. The bow in question would store 14000 units of energy: A theoretical best-case bow's F/D curve is a straight line and is illustrated below. In simple straight bows with no special features such as recurved tips the force-draw curve is more or less concave. For bow design purposes it's the shape of the curve that's most important the actual units or numbers are not as relevant. The area that's left below the curve approximates the energy that's stored in the bow. A force draw curve is plotted by measuring the draw weight of the bow at several points along the entire draw length. The amount of energy stored in a bow can be calculated by plotting it's force-draw or F/D curve. When the bowstring is released, this stored (potential) energy is converted into kinetic energy of the projectile (among other things).
The farther it's drawn, the more energy is stored.
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