Exercise 2: Spine Calculation and Spec Card

What you’ll do #

Work through the complete spine selection calculation for the matched set using two tools — the Rose City Archery static spine chart and Stu Miller’s dynamic spine calculator — record where they agree and where they diverge, apply the per-inch draw-length adjustment rule to produce a personalized spine target, and write a finished spec card that serves as your purchase reference for module 2.

Setup #

You need: a browser open to the Rose City Archery spine chart at rosecityarchery.com/pages/spine-weight-recurve-longbow-compound-bow-chart and to Stu Miller’s calculator at heilakka.com/stumiller. A pencil and the spec card template below. No tools required.

Starting parameters for this build: 40 lb bow, 28-inch AMO draw length (working default), 100-grain glue-on field point , 11/32-inch Port Orford cedar shaft .

Scaffold #

Step 1 — Rose City chart lookup #

Open the Rose City Archery spine chart. Locate the section for recurve/longbow and find the row for 40 lb draw weight , 100-grain point weight, at 28-inch draw length.

[TODO: Record the spine range the chart recommends for this setup. Write it as: “Chart result: –#”]

Note the chart’s note about Port Orford cedar: the spine weight is a natural property of the wood and is “virtually” fixed by shaft diameter and wood density. What does this mean for how you specify your purchase?

[TODO: In one sentence, explain what “naturally fixed spine” means for how you’ll specify the shaft when ordering — are you ordering a spine number, or something else?]

Step 2 — Stu Miller calculator #

Open heilakka.com/stumiller. Enter the following parameters:

• Bow type: Recurve (or Longbow, depending on your bow — try both if you’re unsure)
• Draw weight: 40 lb
• Draw length: 28 inches
• Point weight: 100 grains
• Arrow length: 29 inches (use this as the starting shaft length — you will cut to fit in module 2)

Run the calculator and record the output.

[TODO: Record the Stu Miller result: “Calculator result: ___# (bow type: _____)”]

Now compare the two results:

[TODO: Do the chart and calculator agree? Record both numbers and the difference in spine weight. If they disagree by more than 5#, note which one you will use as the purchase specification and why.]

Note on the Personal Form Factor (PFF) : the calculator has a PFF input that calibrates the result to your individual release style. At this stage, leave PFF at the default (1.0). Once you have arrows flying and can bareshaft test, you will calibrate PFF. For a first build, the default output is a correct starting point.

Step 3 — Per-inch draw length adjustment #

The chart and calculator both assume 28-inch draw length. Your actual draw length may differ. Use the rule: ±5 lb spine per inch above/below 28 inches.

[TODO: Measure your actual draw length, or if unmeasured, use 28 inches (the working default). Calculate your personalized spine target using the table below and record your result.]

My draw length: _____ inches My adjusted spine target: –#

[TODO: Write one sentence explaining why a longer draw length requires a weaker (higher number, in AMO convention) spine target — connect it to the dynamic spine concept from the concept page.]

Step 4 — Spec card #

Fill in every field. This card is your purchase reference. Do not leave any field blank — if you don’t know a value, use the default and note it as “default / to be confirmed.”

ARROW SPEC CARD — MATCHED SET
==============================
Date: _______________________

Shaft material:  Port Orford cedar (POC)
Shaft diameter:  11/32"

Draw weight:     40 lb (measured at ___ inches)
Draw length:     ___ inches  [ measured / default 28" — circle one ]
Point weight:    100 grains (glue-on field point)

Rose City chart spine:   ___–___#
Stu Miller spine:        ___#
Purchase spine target:   ___–___#  (use the agreed range, or note divergence)

Batch quantity to order: ___ shafts  (24 finished + ___ culls = ___ total)
Shaft length to order:   ___ inches  (cut length + ~1" for taper allowance)

Left-hand shooter:       YES
Feather wing:            LEFT-WING
Fletch profile:          Shield cut
Fletch offset:           Left offset (___ degrees)
Feather count:           3

[TODO: Fill in every line above. For the batch quantity: plan for 2–4 culls in a batch of 24 — order 26–28 shafts. For shaft length: order uncut shafts at your draw length plus 2 inches to allow for taper cuts and final trimming.]

Verification #

You’ve completed this exercise correctly when:

1. You have two recorded spine numbers (chart and calculator) that are within approximately 5–10# of each other.
2. You have a personalized spine target adjusted for your actual or default draw length.
3. Every field on the spec card is filled in.
4. You can explain — in one sentence — why a stiff arrow kicks one direction and a weak arrow kicks the other for a left-handed shooter.

Staple or tape the spec card to your bench next to your workspace-ready sticky note from Exercise 1. These two documents together are the green light to place your shaft order.