Dry Fired My Bow

[Mark Smeltzer]

Makes sense but in this case those limb tips (recurves) were pretty narrow, I suspect that the string could have slammed hard enough to slide off to the side and out of the groove in the underlay. That to me would explain why/how that bottom limb got to be more than a 90 deg angle from the handle. I'm thinking the string didnt stretch that much.

Mark

[Marc St Louis]

I would have to disagree with Pat.  A stretchy string is easier on a bow if dry fired.  I have, unfortunately, had the misfortune of having a few dry fires, mostly due to arrow nock failures and I was using FF strings on most.  In every case the FF strings destroyed the bows while some bows with the dacron string survived.

[PatM]

What was the type of destruction? Separation of growth rings/lams, broken in half ,nock failure etc? This is very interesting.

[tallpine]

Makes sense to me Marc. A stretchy string would absorb a lot of energy before the limb tips hit home. a good analogy would be bungee jumping, think about jumping off of a bridge with a non stretching rope. I bet that bow was smoking fast which also made it less likely  to survive a dry fire.     

[Mark Smeltzer]

 I made the last 6" very narrow on that one hoping to lighten the tips and get more speed out of it. It worked, it was very fast and no handshock but in doing so I think the tips may not have been as stable as some other recurves I've made.
I really do think that when the string slammed that hard it slipped past the grooves in the underlay and allowed the limbs to over travel as bad as they did.
My opinion is in this case it would not have mattered either way. I'll try it again but modify the string grooves to be deeper and more open maybe.

[PatM]

It would have to miss the grooves by twisting than actually "slam past them".

[Mark Smeltzer]

Yea, thats what I'm thinking. The string wasn't hitting right in the bottom of the groove and the dry fire just was too much. Weather it was twisting due to instability or just misalignment, thats what I think is why it failed in that way. The limbs on that were only 1 1/8 at the widest part.  It was a bit of an experiment just going for all out speed.
Live and learn, I'll try that one again.

[Marc St Louis]

What was the type of destruction? Separation of growth rings/lams, broken in half ,nock failure etc? This is very interesting.

One of the FF failures was exactly like Mark's Yew bow.  A couple of others had the string snap the overlays and travel down into the limb several inches.  Nasty stuff

[JW_Halverson]

Of course they do. That's why arm or even wrist slap is so evident.
 A stretchy string may absorb vibration but it also allows the creation of more vibration/limb movement after the dry-fire.
 A non stretch string doesn't allow the limbs any room to move.

Ok, I see how you have arrived at your conjecture.  My conjecture would arise from thinking of this as someone bungee jumping.  You hit the end of the lenght of elastic rope long before you stop moving, and the energy of deceleration (actually reverse acceleration) is slowly released.  Make the same jump with a sisal rope, which has elasticity, albeit significantly less, and the reverse acceleration happens over a much faster period.  Ouch.  Now make that rope out of numerous strands of FF, zero stretch.  Worse yet.  The shock of immediate stopping has to be absorbed somewhere? 

Is there any high speed photography of how a bow limb behaves when it hits home under FF vs Dacron?

[PatM]

I can definitely see the nock scenario failure where it is more of the splitting action but less so the limb failure.
 The string definitely stretches with Dacron but the allowed limb movement is much greater. With a no stretch string, the limb just doesn't have anywhere to go.
 The shock can still be absorbed by a no stretch string without it actually stretching, no?
 If you use a fastflite bungee cord with a Kettlebell as the weight and it doesn't break the cord on the drop, that energy was still absorbed.

[JW_Halverson]

Yup, kinda like a long drop with a short rope around the neck is absorbed.  What does that shocked energy do to the neck/limb?

[PatM]

You're comparing vastly different materials and scenarios. For all intents and purposes, the neck is the weak link in part of the rope.
 A well made bow has no "neck".
 

[LivingElemental]

Every time I hear that cracking sound, it breaks my heart.