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« Last post by Tuomo on Today at 06:58:18 am »
Now we are getting to the core of bow design! And it seems that Simk knows something about bows (I knew that already...)!
Your questions:
Why the same taper? Because I didn’t want to optimize every bow model; I just wanted to compare different models while eliminating as many variables as possible. That’s why the front profile was quite normal—40 mm at the widest and 10 mm at the tips, with a realistic width taper. In computer models, you can always make the tips narrower and narrower to get more speed, but that isn’t reasonable in real life. Optimizing the taper rate has a very minor effect. When I used the same taper rate (0.008), every model had very similar stress curves, and the stresses were distributed quite evenly. So: little to no effect → no need to adjust it → fewer variables.
Your claim, “The best shooters, imho, are not achieved by tillering to even strain anyway,” is interesting—why do you think so? Let’s take the D/R design as an example. The more bending near the handle and the stiffer the limbs, the more energy is stored, right? Like the famous Möllegabet bow. But, as you said, D/R designs need more taper, which leads to lighter limbs and, in turn, a faster bow. So more taper → more evenly distributed stresses and lighter limbs.
I tried my D/R model using a 0.010 taper (more taper). It stored less energy (as expected) but had lighter limbs (also expected). However, it ended up a few fps slower due to the reduced stored energy. Most importantly, the maximum strain values were higher. With less taper (0.006), the limbs were a bit heavier, the bow was slower, and the maximum strain values were slightly higher. In VirtualBow, I get maximum speed and minimum strain when stresses are distributed as evenly as possible (within reasonable limits, of course).
When modeling, we can eliminate some variables, for example set, so the most meaningful ones (in terms of bow speed) are limb mass and stored energy. The side profile is linked to the stored energy, while the taper rate is linked to limb mass (at the cost of stored energy) and strain values. Then, you need to find the optimal solution for a specific side profile that minimizes limb mass and strain values. This solution produces the fastest bow.
In real life, we cannot “see” strain values, so it’s difficult to tiller perfectly, but we can learn a lot by using programs like VirtualBow. Of course, we also have to consider the real properties of natural materials (which we don’t really know…). My fastest bows have been "overbuilt" D/R-design, long, wide, minimally stressed.
I will publish my results tomorrow.
Recent Posts
With wood: the heavier the bow I make the more I go for even strain. The lighter the bow, the more I stress the wood and tiller. jm2c