What a saved arrow build actually is
A saved arrow build is the complete component-level specification of one arrow — shaft, cut length, insert, point, nock, fletching — together with every number that follows from it: total weight, balance point, FOC, kinetic energy, momentum, ballistic coefficient and dynamic spine. It is a record of a physical object, not a wish list. If you cannot rebuild the arrow exactly from the record, the record is incomplete.
Archers keep build libraries because arrow behaviour is a property of the whole stack, not of any one part. Swap a 100-grain point for a 125 and you have not merely added 25 grains; you have moved the balance point forward, raised FOC, softened the arrow's dynamic spine, lowered launch velocity and changed the sight tape. Six months later, standing at the bench trying to remember which of two dozens flew better, "the heavier ones" is not a specification. The list below is.
The fields that define a build
| Field | Why it belongs in the record |
|---|---|
| Shaft brand, model and spine | Sets the static stiffness and the inside diameter class, which decides what components will even fit. |
| GPI (grains per inch) | Turns cut length into shaft mass. Two shafts of the same spine can differ meaningfully in GPI. |
| Cut length | The single biggest lever on dynamic spine after point weight, and it is irreversible. |
| Insert, outsert or HIT collar | Front mass and the interface that has to survive impact. Fit is ID-specific. |
| Point or broadhead weight | Drives FOC, dynamic spine and total mass all at once. |
| Nock type and weight | Rear mass. A lighted nock is a real weight change, not a cosmetic one, and it shifts FOC down. |
| Vanes: model, count, offset or helical | Steering authority, drag, and rear weight. Decides whether a fixed blade will behave. |
| Wrap | Small rear mass most people forget to log, and it changes FOC in the wrong direction. |
| Straightness and weight tolerance | What the manufacturer guarantees about consistency across the dozen. |
| The bow it was built for | A build is only correct relative to a draw weight, draw length and cam. Divorced from the bow, the spine call is meaningless. |
The derived numbers, and where they come from
Everything below is calculated from the fields above. None of it is a matter of opinion.
Total weight is the sum of the components: shaft mass (GPI × cut length) plus insert, point, nock, vanes and wrap. Balance point is measured, not guessed — balance the finished arrow on an edge and measure from the throat of the nock. FOC then follows from the Easton ATA standard:
FOC % = ((balance point − L/2) / L) × 100, where L is the full arrow length from nock throat to the end of the shaft.
Dynamic spine is the one to be careful with. The number printed on the shaft is a static spine: a deflection measured on a bench under a standard load. It says nothing about how that shaft behaves when your specific bow's force curve accelerates it with your specific point weight hanging off the front. Dynamic spine is an estimate of that, and estimates get you to a starting shaft — they do not replace a paper tear, a bareshaft or a broadhead group. Treat the number as a hypothesis and go and test it.
Change one thing, move several numbers
This table is the reason a build library beats a notebook. Nothing here moves alone.
| Change | FOC | Dynamic spine | Total weight | Speed |
|---|---|---|---|---|
| Heavier point or insert | Up | Weaker | Up | Down |
| Longer cut length | Down | Weaker | Up | Down |
| Shorter cut length | Up | Stiffer | Down | Up |
| Heavier nock, wrap or vanes | Down | Slightly stiffer | Up | Down |
| Higher-GPI shaft, same spine | Down | Roughly unchanged | Up | Down |
| More draw weight on the bow | Unchanged | Effectively weaker | Unchanged | Up |
Read the second row twice. Cutting a shaft is the only entry on that list you cannot undo, which is why the Builder clamps suggested cut lengths to a floor for rest and broadhead clearance and flags any override rather than silently accepting it.
Version builds, do not overwrite them
The useful discipline is to save a new build every time you change a component, rather than editing the old one. A library of eight builds where you can see exactly what changed between five and six is worth far more than one build that has quietly drifted across a season. The comparison is the value; the record on its own is only bookkeeping.
Two habits worth adopting. Name builds for what distinguishes them — "4mm 300, 26.5", 125gr, 4-fletch" beats "hunting arrow v2". And record the date, because strings creep and bows change, and a build that grouped in March against a fresh string is not automatically the same build in October.
Where your builds live
The Forge is anonymous by default. Builds stay in your own browser's local storage on the free tier — no account, no email, nothing sent to us. Signing in is opt-in and currently unlocks cross-device sync for Range Log sessions; cross-device build sync is queued for a later release rather than shipped, and we would rather say so than imply otherwise. The full contract is on our privacy and data page.
Common questions
What information do I need to save an arrow build?
Shaft model and spine, GPI, cut length, insert or collar, point weight, nock, vanes and wrap, plus the bow it was built for. From those, total weight, balance point, FOC, energy, momentum and dynamic spine are all calculated rather than entered.
How is FOC calculated?
FOC % = ((balance point − L/2) / L) × 100, where L is the full arrow length from the nock throat to the end of the shaft and the balance point is measured from the same origin. That is the Easton ATA standard definition.
Does adding point weight really change my spine?
It changes your dynamic spine, which is what matters in flight. The static number stamped on the shaft does not change, but a heavier point makes the shaft behave weaker under the same bow, which is why a point-weight swap often needs a re-tune.
Should I save a build before or after I shoot it?
Both. Save the spec before you cut anything so the plan is on record, then update it with the measured balance point and any tuning notes after the arrow exists. A build with a measured balance point is worth far more than one with a calculated estimate.
Why keep old builds I no longer shoot?
Because the comparison is the whole point. Knowing that the 26.5-inch build grouped better than the 27.5-inch build at the same point weight is the kind of finding you only get from having both on file.
Are my saved builds visible to anyone else?
No. On the free tier they live in your browser's local storage and never reach us. Sign-in and community data contribution are separate opt-ins, both reversible.
My Arrows is part of The Forge by Sparrow Expeditions. The formulas behind every derived number are documented on our Methods page. Built by archnerds, for archnerds.