Shaft Flex
Daryl Hrdlicka
Jeffers Petroglyphs Historic Site
February 5, 2003
The flex of the shaft (how much it bends under pressure) is the most important factor of a dart. A crooked or improperly fletched dart with the right flex will still fly straight, but a dart that is too rigid or too flexible won't fly right at all.
The flex, or spine, is determined by measuring how many pounds of force are needed to make the shaft flex. A lower rating means it is more flexible; a higher rating means it is stiffer. When the spine is matched to the propulsion force, it flies better. An arrow with a 50 lb. spine is designed to be used with a bow with a 50 lb. draw. A dart should have a 5 to 10 lb. spine, because that's how hard a person throws.
You can only check the flex of shafts that are completely dry. Green shafts will be very flexible, just because there's so much moisture in them. You can't get an accurate reading until the moisture is out and your shaft is ready to be used.
The best way to determine the flex of a shaft is to get out the bathroom scale. Put one end (the thicker end, if it's tapered) on the scale and stand it upright. Put a finger on the top of the shaft and press down. When it begins to flex, look at the scale. That's it.
After you've done a few, you can just trust your judgment. Put the dart on the ground instead of the scale and push down. If it flexes with very little pressure, it's probably too limber (too little spine). If it takes a lot of force, it's probably too stiff (too much spine).
If you throw with a lot of force, you use should use stiffer darts; if you throw more casually, use more flexible ones. But be careful -- too little spine will cause the dart to nose-dive when you throw it, with the point dropping down almost immediately. A dart with too much spine will put the point up and fly almost vertically. The reason for this can be explained through physics, but to be honest I don't really understand it myself. It works. That's what matters.
If your shaft has too much or too little spine, it may be possible to fix it.
- If the dart has too much spine (is too rigid), you can try to correct it by reducing the diameter of the shaft, either along the entire length or by tapering it smoothly towards the nock end. You can do this by sanding, shaving, or planing. In addition, you may be able to correct too much spine by adding a fairly long foreshaft, depending on how far off it is. This changes the balance point and FOC, however, and doesn't always work.
- If the dart has too little spine (too flexible), you can correct it by cutting off the narrow end of the shaft. Do this an inch at a time -- take some off, then re-test the spine. If you're using a tapered shaft, almost all of the flex will be in the bottom 1/3 (by the fletching) anyway, so it usually doesn't take too much trimming to adjust it.
If you can't get the spine close enough to the 5 to 10 lb. range, it's better to throw the shaft out and start over. It's a hard decision to make, but it will save you a lot of time and effort working on a bad dart.
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