Crossbow Bolt Spine: Stiffness, Sorting & Alignment

Crossbow Spine in plain terms

Crossbow Spine meaning: Spine is how stiff an arrow or crossbow bolt shaft is, read as how far it bends under a set weight. Bolts in one pack should share the same spine.

Also called: arrow spine, bolt spine, spine alignment, spine-aligned, spine sorting

Spine is the stiffness of a shaft: how much it resists bending. Archery measures it as static spine, the distance a shaft bends in the middle when a standard weight hangs from it while it rests on two supports. A smaller deflection number means a stiffer shaft.

When the string drives a bolt forward, the shaft flexes for an instant. Shafts with the same spine flex the same way and leave the rail alike; shafts that differ flex differently and tend to land in different places.

For vertical-bow arrows, spine is chosen from a chart to suit the bow. Crossbow bolts are short, stiff and built for the crossbow's rated length and weight, so makers handle spine for you in two ways. Spine sorting groups shafts so each pack matches: Carbon Express rates the PileDriver's Laser Match Set at +/- .0025 inches of spine. Spine alignment turns each shaft so its stiffest side faces the same way: TenPoint spine-aligns the CenterPunch HPX to the odd-color vane.

Spine figures on reviewed bolts

Facts below come from the manufacturers' product pages or listings.

Why it matters

Spine is one of three consistency figures, with bolt straightness and weight tolerance in grain. A pack that matches on all three should group tighter, especially at 40 yards and beyond.

Mixing shafts is the usual way spine goes wrong. Bolts from different brands or models can share a length and weight and still differ in stiffness, so one may fly away from the rest.

Spine also changes with the point. A heavier head makes a shaft act weaker in flight, which is one reason to stay within the tip weight your crossbow and bolt maker list, and to check FOC after a change.

How to read it on a spec sheet

• A spine number such as .400 is a deflection in inches: smaller is stiffer.
• A spine tolerance such as +/- .0025 inches tells you how closely the shafts in a pack match each other.
• "Spine-aligned" means the maker indexed every shaft the same way; it is not a stiffness number.
• Missing spine information is common on budget packs; treat it as unknown rather than matched.
• Your crossbow manual sets length, weight and nock; follow those before comparing spine.

Common mistakes

• Mixing bolts from different brands or packs and expecting them to group together.
• Choosing crossbow bolts from a vertical-bow spine chart instead of the crossbow maker's bolt specification.
• Re-fletching a spine-aligned bolt without keeping the index vane in the same position.
• Changing to much heavier heads without re-checking where the bolts land.

See it in our reviews

• TenPoint CenterPunch HPX — rated 4.4/5
• Carbon Express PileDriver — rated 4.2/5
• Reegox Crossbow Bolts — rated 3.9/5