Straightness versus stiffness.
The processes used to make aluminum tubes automatically produced arrows with consistent weight and spine stiffness. You didn't have to hand select aluminum arrows to achieve these objectives--they were built in. No one ever talked about spine stiffness tolerances or weight tolerances in the days of aluminum because it was always good.
Carbon arrows are made differently. Not only is stiffness and weight more variable in carbon than in aluminum construction, these aspects have a much greater effect on group size and accuracy than the one thing everyone does talk about: straightness. Here's why you should consider more than just shaft straightness when buying your next dozen carbon arrows.
Straightness is the measure of how much the arrow deviates from perfect as you turn it. Commonly, straightness is measured as +/- .003-inch or +/- .005-inch, etc. That means that an arrow will deviate by as much as .003-inch or .005-inch in either direction from perfectly straight as you turn it and still be accepted.
Conversely, you measure spine stiffness by hanging a weight from the arrow and measuring how far the arrow bends. Spine tolerance is a measurement of how much the stiffness measurement changes within a dozen arrows. Finally, SAS (spine around the shaft) is the measurement of how much the spine stiffness changes as you turn the arrow. If the arrow were stiffer on one side than another, a dial indicator attached to the weight would go up and down as you turned the arrow.
When you shoot arrows with poor stiffness tolerance (either within a dozen shafts or around a single shaft), you have problems. Each arrow flexes differently when you shoot it and therefore, your dozen shafts will hit the target at different spots. When you place a broadhead on these arrows, the problem gets worse and the groups wider.
Stiffness tolerance is more important than straightness tolerance in determining the impact point of your carbon arrows. When ranked, the most important characteristic of a dozen carbon arrows is its stiffness tolerance. The second most important is its weight tolerance (how closely the arrows weigh to the same amount) and the third most important characteristic is straightness. All qualities matter, but some matter more than others.
In other words, if you are buying aluminum shafts, low-grade arrows with a straightness tolerance of +/- .005-inch are nearly as accurate at typical hunting distances as those with super low tolerances of +/- .001-inch that cost more. However, when you get into carbon arrows, you have to be more careful. You can't just buy the bargains and enjoy consistent accuracy. In truth, you end up getting what you pay for in carbon. When you pay more, you generally get better arrows.
The only way to hold tight tolerances in carbon arrows is to hand select these arrows based on the chosen tolerance range. When intense hand labor is required, cost goes up.
How Bad Can It Be?
I spoke to Ted Palomaki, an engineer at Easton, about the subject of stiffness tolerance. Not surprisingly, Ted has tested nearly every carbon arrow on the market. He bought them off the shelf, the same way you and I do. I asked him how bad the worst dozen was. It was bad. Ted told me that at Easton they hold the stiffness of a dozen arrows within roughly +/- .005-inch deflection of the target stiffness. The worst dozen arrows he tested had a stiffness range that was .090-inch wide. That translates into a range of three full stiffness sizes at most companies. Can you imagine having a quiver full of arrows that span three stiffness sizes? That can't be good.
This column is not an ad for Easton, so I need to be very clear in stating that other companies also make high quality carbon shafts. For example, I have tested shafts from PSE, Carbon Tech, Carbon Express, Beman, Gold Tip and Blackhawk and their top grades were all excellent. I'm sure there are some that I haven't tested that are also very good.
However, there are companies that make arrows that are way below par. Also, it is hard to know what goes into a dozen low-grade shafts even at some of the bigger arrow companies. You need good arrows if you are going to be consistent. Unfortunately, it is hard to tell them apart. We need a way to test arrows in the pro shop to judge them.
I used to make all my own fishing rods. If you've ever done that you know that you have to roll the rod blank on a table top as you apply pressure on the center to find the stiffest axis, or "spine" as it is called. That way you can properly locate the line guides to produce a stable rod. Unfortunately, the amount of spine difference needed to throw an arrow off target is so small that most people can't feel it when they roll the arrow on the table. Additionally, the difference from one shaft to the next is also just as important. A more sophisticated test is necessary.
You can test the stiffness of a dozen arrows by suspending a weight at the center of a 28-inch span of shaft (the industry standard is 1.94 pounds or 880 grams) and carefully measure how far the weight sags. Easton actually uses this measurement in its shaft sizes (for example, a size 300 shaft flexes .300-inch). Other companies use different conventions to specify size.
Presumably, as you turn the shaft, you will notice the indicator changing. But it is not a very accurate test when you turn the arrow by hand. All of this may seem a bit much. You just want to shoot a whitetail at 20 to 30 yards. But even at that distance, stiffness tolerance plays a role in your success. It is worth worrying about.
I hope that someday stiffness tolerance will become a more important issue in the minds of archery hunters. Everything else will follow: dealers will start to do the kinds of tests that separate the good from the bad and makers will respond by improving their shafts.
In the meantime, the best thing you can do is ask the pro shop attendant for the stiffness tolerances of the carbon arrows they sell. If they don't know it, ask them to call customer service at the arrow company to find out. You might even get a dial indicator and try a few tests at the local pro shop. If enough archery hunters start making informed buying decisions, quality will improve.