Spine is how much an arrow bends. Too weak and it flexes too far off the string; too stiff and it never recovers. Easton publishes one chart that answers it, and the chart is a lookup, not a calculation — three measurements decide where you land on it, and that spot names a recommended spine range.
The reason it looks intimidating is the layout. It's really two tables stacked on one sheet — A/C and all-carbon shafts on top, aluminium underneath — and each has a bow-weight scale down both edges: compound poundage on the left, recurve poundage on the right. Get the three inputs right and it collapses to one row.
The three measurements
Peak bow weight, on your fingers. Not the number on the limb. A 40 lb limb pulled past its marked draw length is well over 40 lb, and pulled short of it is under. Have a shop put a bow scale on it at your draw length, at full draw. This is the single input people get wrong, and a few pounds moves you a whole band.
Arrow length, measured Easton's way. From the nock throat — where the string sits — to the end of the shaft. Not the point, not the insert. If you're between the arrow lengths the chart lists, half an inch or more rounds up: 28.25" reads as 28", 28.5" reads as 29".
Point weight. 100 grains is what the chart assumes, and it moves 3 lb of bow weight for every 25 grains either side of that — heavier points add, lighter points subtract.
The rest of the sheet is a short list of set-up variables, all of which add to or subtract from the bow weight you read the chart with. The chart is drawn for a compound rated 301–320 fps shot with a release aid, so a slower bow comes off: −10 lb up to 275 fps, −5 lb to 300, nothing to 320, then +5, +10 and +15 lb as you go past it. Shooting a compound on fingers adds 5 lb. On the recurve side the chart assumes carbon competition limbs; wood-and-glass beginner limbs take 5 lb off.
Reading the chart
Find your weight down the side, your arrow length across the top, and take the square where they meet. On the carbon table that square is a spine range, written weakest first — 640-570 means anything from .640 to .570 is in the window. On the aluminium table it's a list of sizes outright.
Actual peak bow weight down the side, arrow length across the top. Tap a square to trace a bow-weight band and arrow length.
| Bow weight | 21" | 22" | 23" | 24" | 25" | 26" | 27" | 28" | 29" | 30" | 31" | 32" | 33" | 34" |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| <20 lb | ||||||||||||||
| 21–26 lb | ||||||||||||||
| 27–31 lb | ||||||||||||||
| 32–35 lb | ||||||||||||||
| 36–39 lb | ||||||||||||||
| 40–43 lb | ||||||||||||||
| 44–47 lb | ||||||||||||||
| 48–52 lb | ||||||||||||||
| 53–57 lb | ||||||||||||||
| 58–62 lb | ||||||||||||||
| 63–67 lb | ||||||||||||||
| 68–73 lb |
Two things are worth noticing in that grid. The first is that it's diagonal: a heavier bow and a longer arrow both push you toward a stiffer shaft, so the same range appears again and again as you trade one against the other. The second is where you sit inside your band. The bands are four or five pounds wide, and the chart gives you one answer for the whole band — so 40 lb and 43 lb both read the same square, while 43 lb and 44 lb read different ones. Land within a pound of an edge and you should treat both neighbouring squares as candidates.
Picking a shaft out of the range
A range is not one arrow. Every Easton shaft whose spine falls inside it is a candidate, across a dozen models, and Easton's shaft tables give each one a size, a spine and a grains-per-inch figure.
- Shooting recurve, take the weaker end. Easton's own note on the chart: with several sizes in the recommended range, choose one on the weaker side. Easton also flags its recurve picks with an "R" on the shaft tables — those are the ones tested for a finger release.
- Then choose on weight, not stiffness. Everything in the range is the right stiffness. Grains-per-inch is what separates them — multiply by your arrow length for the bare shaft, then add point, nock, insert and vanes. Light and fast fights the wind badly; heavy and slow needs more sight elevation.
- Aluminium is its own table. If you want a fat indoor arrow, read the aluminium half rather than trying to convert a spine range into a size.
The chart is a starting point
It gets you to a range, which is where tuning starts, not where it ends. Bare-shaft or walk-back test before you commit to a full set — and if you're near a band edge, buy a few of each candidate before you buy twelve of one.
I built a version of this chart you can drive from your own numbers, including the set-up variables: the Easton shaft selector. It lists every shaft that fits your range, with the recurve picks first and bare-shaft weights at your arrow length.