Do I need, or even WANT thickness taper on this design?

[bowhunter15]

I read a thread a while back where guys talked about what thickness taper they used in their bows. It seemed like most fell in the .002-.004 range, with more the thicker the total stack was. It also seemed important that total taper was a combination of the thickness taper and width taper, and that too much taper would allow the tips to open up early, causing stacking.

I've been playing around with the Supertiller software. The bow shape is modeled after the 58" thunderchild. Lots of deflex... some reflex also. I have straight width tapers from 1.5" at the fades to .5" at the tips, 12" handle and 14" power lam.

The first set of graphs is for NO thickness taper. The draw curve doesn't look terrible to my eye, and more importantly the string angle isn't over 90 degrees at my 28" draw. I'm not sure what to make of the stresses graph, but hopefully someone can give some feedback on that.

 
 

And here below are the graphs with a .005 thickness taper. Draw weight @ 28 for all of these models are right around 57.5#. On the energy storage curve, the blue line (.005 taper) dips a bit lower in the center. Otherwise they are equal to the zero taper. And then we see the profile graphs. If anything, the string angle is opened up a bit more, and the stresses are maybe a little more evenly distributed through the tips rather than primarily through mid limb.

 

   

The Stored Energy per Draw Weight for the zero taper is 0.841. For the .005 thickness taper it's 0.831. So the zero taper is a little more efficient at storing energy. From the software it would seem to me that zero taper would be the better choice. Are there any real world reasons I'd want to use the taper for this short bow with aggressive R/D? Such as lighter tips allowing for a little better arrow speed upon release, hand shock, etc? According to the draw force graphs, smoothness of draw should be a wash.

[Mad Max]

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[BenBow]

First thing i notice from the stress graph is the first one will hinge at the fades where as the second the stress is much smoother across the whole limb. Also 1 1/2" wide at the fades seems excessive but ive never measured a thunderchild. I'd narrow the fades to 1 1/4 or 1 1/8" then taper to 1/2".you might try adding a tip wedge on with a taper of .003" per inch.
I'm away for a week so don't have access to my main computer to try other things.

[LittleBen]

Just one thing. The back of the riser is at a position of 4-4.5", with the string drawn to -28", and braced with the string at -8".

So really you're modeling a 12" brace height (to back of riser), and a draw length of like 32-32.5".

You need to translate the whole bow -4" in the x direction.

I hope that makes sense.

[canopyboy]

I think that you make a good point about zero taper when it comes to performance (speed.) But some thickness taper will help keep the limbs more evenly sressed meaning bow will potentially live longer. And I think the big payoff is reducing weight out towards the tips -- making for a more friendly bow. That said, 0.005 seems like it might be too much taper for a bow of this design. Did you do any calcs at 0.003?

[BenBow]

Good point LittleBen. You can do that or move it back so it is +1 3/4" in front so the brace height is right then set the draw length to 26 1/4" to make a 28" amo draw length. This will give you a better fd curve as I thought those were very flat.

[bowhunter15]

Good points. I can't believe I missed the draw length discrepancy. That should make a big difference. I did model a .003 but it was so similar I didn't bother posting. I'll run some more numbers tonight with the fixed draw.

[BenBow]

Theres an awful lot to keep track of with supertiller

[bowhunter15]

Downloaded V6.6 and got translated all of the profile points so that the center is at 0, and the brace height is actually 8.125". This is with a .003 taper.

 

 

Anybody know what the other graphs represent (Gear ratio for example? Don't know what's good or bad for that graph).