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konradlau
04-12-2010, 02:18 PM
I have been researching modern archery for some three years now and have yet to find a discussion on the effects of hysteresis on the efficiency of limbs on compound bows.

Whenever I am at an archery range, I am always surprised by how little time is given for the bow to recover before the next shot is fired.

From past life in the elastomer industry, I learned the effects of repeated flexing of materials (supposedly elastic) and the resulting internal heat build up dramatically changing the material’s rebound properties (hysteresis). Those tests are based counting flexural cycles to failure at a known flex cycle time and percentage of bend measured in degrees of a standard sized flat sample of material.
Failures are always related to internal heat build up.

If the material in question is given time for heat dissipation, the tested material invariably withstands many more flex cycles before less energy is required to flex and ultimate failure occurs.

I would think that modern bow limbs would exhibit the same types of heat build up and losses of efficiency (i.e. the difference between the energy required to draw the bow and the energy released during the firing sequence) during rapid shooting sessions.

Nearly all of the bow tests I have read have graphs showing draw force curves and many reports actually show the efficiency of the tested “system” (bow). Invariably, there is no data related to limb temperature.

I would like to see testing reports comparing cold limb, warm limb and hot limb efficiencies.
It seems to me this would be an excellent application for thermal imaging photography. I would guess a perfect limb design would show heat stresses evenly distributed along the entire limb length.

Some of my questions would include:
Does limb temperature affect group size?
How much arrow velocity is lost (or gained) by varying temperatures of the limb tested?
Is a quad limb system better at heat dissipation than a solid limb?
Is a “thin” limb better at heat dissipation than a standard limb?
Is a pre-loaded limb better at spreading stresses than a straight limb?
Does it take longer for a limb with concentrated heat stress longer to cool than one whose stress is spread more evenly?
How much time is required for any given limb to return to “cold normal”?

After all, a bowhunter most often has a cold bow in his hand and rarely has the opportunity for multiple shots.

…or am just thinking about all of this too much?

K

copterdoc
04-12-2010, 05:57 PM
…or am just thinking about all of this too much?
yes!

Hysteresis is something that can be measured, but it doesn't seem to change very much if a bow is fired at a high rate, vs a lower rate.

I personally do not believe that hysteresis accounts for a very high percentage of the wasted energy released during the shot.

I think the overwhelming majority of energy loss, is due to the limbs having to move so much "stuff" that doesn't leave with the arrow. The string, cables, cams and limbs, all possess mass, that must be moved. High speed video shows how much kinetic energy remains in the bow following the shot.

As archers, we feel and hear it as vibration following the shot. If we use a heavier arrow, the ratio of arrow-to-allthatotherstuff's mass, is improved and the efficiency of the bow is increased. As a result, less energy is left over to make everything "bounce around" and the bow will be quieter as well.

DeepRiverBowman
04-13-2010, 09:38 AM
Those are some good questions that could possibly make for an interesting thesis and post-grad/doctorate work, but my suspicion is that the hysteresis effects are minimal (at best) based on the target groups of the best archers in world. Looking at their indoor targets, it seems as if there is only one hole in the center of each bulls-eye. And while I've seen a few limbs de-lam and some that cracked after being dry fired, I can't say I've ever seen a limb fracture as a result of being shot too rapidly and not having time to cool down. That would be sort of an annealing process which I don't believe happens or limbs would become increasingly brittle over time. My 1961 Kodiak Magnum, my 1962 Staghorn, and my 1981 Laser Magnum would all be in evidence against brittleness of the limbs.

mike

Spiker
04-13-2010, 10:58 AM
I would think (scary) that to build up heat in a bow limb would require rapid fire far beyond what we can reload and shoot at. Unless you are just flinging arrows and dont care where they go. Sounds like a dry-fire waiting to happen.
Again - just my thoughts...

konradlau
04-14-2010, 05:45 PM
The failures during testing of the materials I was referencing were caused by high frequency oscillations produced by testing machinery.

I never considered a human could produce a failure by hand.

My question is mainly, can a human induce heat build up and does it affect a bow’s performance?