Is Arrow Spine Selection Broken?

Spine charts, software dynamic-spine calculators, and the like have always pushed this idea of an optimal spine for a particular setup.

By setup, this usually meant a draw weight, arrow length, and point weight combination. "Your ideal spine is 300. A 250 will be too stiff, and a 340 will be too weak." That story made a lot of sense on the bows the charts were built for, and it still makes sense for recurve archers and finger shooters.

For older compound bows, it went something like this:

• I want my arrow rest set to the proper centershot, or close to it.
• I want to keep any cam lean to a minimum and cam timing reasonable.
• I want my arrow to shoot bullet holes.

Due to the adjustment mechanisms available, one of the only ways to accomplish all three was to tweak the dynamic spine reaction of the arrow itself until it reacted how we needed it to out of that specific bow. That bow had an optimal spine.

From an arrow manufacturer's perspective, there is also a safety aspect. They don't want folks shooting a crazy light-spined arrow out of their 80lb hunting bow.

A couple of notes before we go. This article is specifically about modern compound bows with tuning methods besides moving the rest. Recurves, traditional bows, and older compound bows that can't be tuned to a wide range of arrows are out of scope. And: lower spine number means stiffer shaft, so a 200 is stiffer than a 340.

The matrix we lean on: 34 builds, four nominal spines (200, 250, 300, 340), two shaft families (Easton 5.0 and FMJ Max), and a range of internal weight behind the same 100gr point. Every build paper-tuned from a fixed centershot on a Hoyt AX3 33, all groups shot indoors out of the Easton Precision Shooting Machine at 70 yards. No rest adjustment was made for any build.

This article doesn't introduce new tests. Every claim here comes from the front-of-center matrix. For the full matrix, the per-build tuning protocol, the measured static spine table, and the regression framework, see the Front-of-Center Testing Overview and Front-of-Center Analysis Overview pages.

Modern compound bows generally include one or both of two families of adjustment. Both let the bow be tuned to the arrow, not the other way around.

• Per-limb deflection adjustment (XTS is the obvious example). The top and bottom limb deflection can be set independently so dynamic timing and vertical nock travel are matched to the arrow, instead of forcing the arrow to compensate for a fixed deflection mismatch.
• Lateral cam position adjustment (yokes, cam shims, axle / cam-lean controls). These move the cam between the limbs so the arrow leaves the bow square to a sensible centershot, instead of forcing the arrow's dynamic spine to compensate for a fixed cam position.

A bow with one or both of those, plus a rest that can stay at a sensible centershot, can be tuned to a much wider range of arrows than a static spine chart's narrow window implies.

Every build in the front-of-center matrix shot bullet holes through paper from a fixed sensible centershot, after the bow was tuned for that build with its standard tuning workflow. No rest position adjustment was made for any build. The set of arrows that covered:

• Four nominal spines: 200, 250, 300, 340.
• Two shaft families: Easton 5.0 and FMJ Max.
• A wide range of internal weight stacks behind the same 100gr point.
• A front-of-center range of roughly 10% to 25%.

Every single build performed reasonably well, both in terms of groups and in terms of resilience to simulated shooter error. Some better, some worse, sure. But none were ridiculously bad.

That's direct evidence that, on this modern compound, a much wider range of spine-and-front-weights can be brought to a bullet hole than a static spine chart's narrow window implies. The point isn't "charts are wrong." It's that a tunable modern bow can be tuned to a much wider variation of spines than a chart assumes.

With every build bullet-holing, the next question is whether any spines grouped better than others once we control for the other things that change with spine. We ran the same regression as the Front-of-Center Test Results Overview, this time leaning on the static-spine row with front-of-center already accounted for.

Stepping 100 nominal spine units stiffer (for example, a 300 shaft to a 200) predicted about 0.9 inches tighter mean radius on the tuned-bow broadhead group at 70 yards. As a sanity check, we also ran the regression with a hand-measured static spine instead of the manufacturer label, and the direction was the same.

We did not see an interior sweet spot. The trend looked smooth across the spines we tested. No peak, plateau, or reversal that the data could pin down inside the 200 to 340 range.

Based on this testing, within this slice of spine on this bow, stiffer always performed better. The implication is to go as stiff as possible while still hitting our total weight, speed, and front-of-center goals.

We should also feel reasonably free to go less stiff than a spine chart suggests if we want to be as light as possible. The bow tuned to every build in our range, and the regression said stiffer was slightly better with front-of-center accounted for, not dramatically better.

A few caveats:

• This article is explicitly about modern compound bows with modern tuning systems. It isn't advice for recurves, traditional bows, or older compound bows that can't be tuned to a wide range of arrows. Spine on those platforms is a tighter constraint.
• This test did not establish that "spine charts are wrong." It established that the chart was not a detectable predictor of arrow performance inside this matrix on a bow that could be tuned to every build.
• We tested four spines (200 to 340) and two shaft families. We did not test ultra-stiff or ultra-weak shafts outside that range.
• One bow, one draw weight, one draw length. A different bow class may tune to a different range of spines.
• Each build was scored on one 6-arrow group per condition. With six arrows, any single build's mean radius should be taken with a grain of salt.