Speartip testing?

    I've pulled the trigger some 15 times on my little bay gun, with both 2 and 3 band setups, and have been trying to pay close attention to the penetration I get. I'm using a crappy JBL slip double flopper slip tip at the moment, but one of my next projects will be to build my own. All of which got me to thinking about the way a shaft penetrates. Has anyone ever seen REAL testing to compare the penetration of different shaft diameters, and pencil vs. tricut tips?


    I was thinking about setting up a little test fixture. Basically, a weighted pendulum with a few inches of shaft attached to the weight. Pull the pendulum back to a given height, and let it swing down against the target. I think that would give a pretty repeatable "input energy". Now, what to use for the target? I was thinking about using a stack of pieces cut from a scrap sheet of wallboard (drywall) because I can get all the scraps I want from the dumpster at a house under construction. It's a little softer, and hopefully more consistant in hardness, than wood.


    Things I want to test:


    1. For a given diameter (say 5/16"), which geometry penetrates better? (conical point, full tri-cut, conical with tri-cut point, or hollow ground point).


    2. Compare the penetration of a 9/32" single flopper vs. 5/16" single flopper. I don't have either one, so maybe someone could donate the front 8-12" of a wrecked shaft?


    3. What else would be good to know?


    All input is greatly appreciated. I would really hate to get this all done only to find out that I overlooked something major. I'm sure this has been done, but I can't find it anywhere.

    First it's pretty much accepted that a tri cut has better penetration, the downside of a tri cut is that it will deform more easily if you shoot it into a rock. But I'm sure you knew that. Then logic dictates that increasing the size of the projectile by any means, larger diameter shaft/floppers/basically anything that would make it less streamlined, will negatively affect penetration.


    When you first said testing 8mm vs. 7mm I thought a pendulum weight that will give about 1" penetration will be sufficient to give an accurate idea. Once you're talking about testing with floppers obviously you will need a force that will cause penetration at least past the flopper. I have a hard time imagining that you can achieve that with drywall. Of course I'm thinking of a solid piece, not just penetration through one sheet. Maybe better would be a type of Styrofoam. I used to use something for archery targets that was blue sheets of Styrofoam. Some people use them for making floating docks or maybe insulation. The sheets are about 3" thick and maybe 4x8'. Not expensive and much denser than regular white Styrofoam.


    With your type of test of course you're eliminating the variable of momentum which comes into play when the spear travels over a distance to the target. But then you're only testing for penetration based on shape.


    PS. What's a hollow ground point?

    Quote from seahunter49

    First it's pretty much accepted that a tri cut has better penetration, the downside of a tri cut is that it will deform more easily if you shoot it into a rock. But I'm sure you knew that.


    PS. What's a hollow ground point?


    Yes, I knew that a tri-cut is generally accepted to penetrate better than a conical point. But I have noticed that many people take a conical point and grind just the smallest of tri-cuts on the tip. I wondered if that falls somewhere in between, or does it perform as well as a larger tri-cut tip.


    I tip my JBL slip tip with a replaceable broadhead point. Not the blades, just the little screw on tip. When I lost it (had to cut the spectra inside a wreck) I went back to a spare tip with a regular tri-cut head and noticed a HUGE loss of penetration.


    Hollow ground means that the material between the cutting edges is removed in a concave manner, which makes the angle of the edges sharper. Cuts better, but more prone to damage and not easily sharpened. But the broadhead points are only $1.50 each, so when they get dull I just replace.


    I know the foam you're talking about. May try that instead.

    Yes, I threaded the end of the slip tip to accept the broadhead point. They are plated with something that looks suspiciously like the greenish coating on the old RA shafts. They are quite sharp right out of the box, so I don't hone them like the one in the picture, and the coating protects them pretty well. The coating does get worn off of the cutting edges eventually, and then they will rust just like an RA shaft.

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