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Jeb,

That bow came out very well. Though not strictly primitive, the handle is very artfully done, and looks great. The tiller looks good too.

I have also had good experience with Home Depot red oak so long as it is well selected for grain.

I have a few questions. ..... You state at the top that there is a straight thickness taper, and elliptical width taper. Then, you show the diagram near the bottom with circular radii drawn in showing the tiller. ....If you designed in super tiller for uniform strain, either the thickness should be uniform from fades to tips, or the bend of the arms should be elliptical. The question is; Does "straight thickness taper" mean uniform thickness, or is it really thinning out as you approach the tips? .......What elastic modulus value did you use for the red oak? What stress, or strain is the bow designed for?

Dave

Thanks for answering my questions. I have an interest in how well bows "designed" for a particular strain and then built to the design come out. Your bow is a good example of how well the technique works, if straight grain, and uniform wood is selected for the project.

I have compiled a number of bend test results for Red Oak. The MOE numbers range from 2,870,000psi to 1,922,000psi for the 6 samples in my data base.Your value of 2,380,000 is in the middle of this range. My experience with "edge ring" wood (ring surfaces perpendicular to the bow back), is that it is less likely to take excessive set, but more likely to fail in tension.....Not a bad thing with tension strong wood like red oak.

I should have seen that you had thickness taper from the table. (Obvious now that you say it.) If the strain is uniform in the arms out to 6" from the tips I would have expected the radius of the bend to get tighter as you proceed out the arm up to this point. Strain is equal to 1/2 the thickness divided by the bend radius (sn=0.5T/R). Therefore, if the thickness tapers, and the strain is constant, the bend radius should shorten in proportion to the arm thickness. This would mean that the curve fit for the bend of the arms should be elliptical rather than the circular arc you show in the diagram. However, perhaps the difference between the circle and ellipse is is too small to see with such a curve fit.

I agree that having a thickness taper allows the arm to be wider out nearer to the tips while maintaining uniform strain. The "price" you pay is a slightly ellyptical tiller, but this is really a good thing in my opinion.

I am interested that you have cast the design in Pro-E. I have an excel sheet for bow design that is similar to Alan Case's Supertiller, and would be interested to see the results of your stress analysis for comparison. I do not believe that Supertiller considers shear strain within the bow arm. I think that both Alan's and my "programs" use a simple bending model. This works well if the elastic modulus values used are derived from static bending tests,as these tests usually do not separate shear from bending stress.

( P.S. I am glad to find someone else who works bow arms to 0.005" of design thickness. I usually work the arms to 0.020"(0.5mm) or so over thickness, then measure a draw curve to about 1/3 draw, re-calculate the elastic modulus (and set) of the wood to match the measurement, and then calculate the final thickness from there. This usually nails the weight to within better than 1/2 a pound. )

Dave