“Stacking” is what we call it when the draw weight hits a point during the draw back where it starts to increase very rapidly. Responding to a question about stacking by John Payne, I came up with this blurb about how it seems to work with inswingers.  At least, how it seems to work with Firefly.  With catapults, everything is a situation.


 

There are two types of stacking.  The usual archery type that has to do with the angle between the string and the limb at full draw, and another type that has to do with the spring saying “Oh! now I’ve hit a wall. My fibers are getting stubborn.  I’m not going to budge so easily anymore.”  (Not bad for a talking spring, huh?)

The first type is easy to understand.  Imagine an ordinary limb on a hand bow at full draw.  The more you pull it back the closer the limb and string get to forming a straight line.  In other words a straight line pull.  That’s like pulling on an anchored chain.  Not move so much.  Here’s a rough idea of how this type of stacking works on an inswinger.

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The second type of stacking, the springs “hitting the wall” phenomenon, is only something you can judge by how much the draw weight goes up and at what point that happens during the draw back.  With an inswinger I don’t think the first stacking issue (the archery one) starts to take effect until we get beyond 100 degrees.   Therefore:  If your stacking is happening at 100 degrees or less then you are probably hearing the spring talking to you.  This sort of thing is more likely with a short fat spring, rather than a taller, skinny one.  It’s certainly something to be aware of if the draw weight skyrockets all of a sudden.  Back off.  Play with it.  At some point you will have to say, “That’s enough for this framework I’ve built.  I will not ever draw it back any further than this.”  Whatever that draw length happens to be.  That is what I did with Firefly.  Fortunately by the time I made the decision  to enforce a ban on further draw back, she was performing at the level we see today.  I would be pretty nervous about going any further.  Too many bends and breakages for one lifetime.

In her current configuration, Firefly clearly shows the spring type stacking.  That is why I’m keeping her at 45 degrees of limb rotation.  There are a lot of advantages here.  If it’s done right, little things like:  faster cocking times, more compact machines, and hopefully, the highest velocity Dura bolts possible out of authentic geometry.   With catapults, everything’s a situation. What a grand hobby! It’s all just tickety-boo when you find the right combo.

….or so you think, at least for a while….

 

One Response to “Two types of stacking defined”


  1. Charles W. Fink says:

    Wonderful explanation, thank you. Working on the final stages of my inswinger and I had questions about ‘stacking’. Searched the terminology with no luck, knew I’d seen it in your archives. This is the most comprehensive page I could hope to find and I got lucky and zeroed right in on it. Just finished the trigger and winch, so I can start experimenting with torsion pressures and fine tuning. It all seems to be going well and I realize its because I’m ” standing on the shoulders of giants” that went ahead of me and blazed the trail. Now its time to recognize weak links before structural failures.

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