What belongs in a saved bow setup
A bow setup record is the measured configuration of one bow at one point in time: make, model and year, actual draw weight, actual draw length, cam system and module, axle-to-axle, brace height, let-off, the accessories bolted to it, the string install date, and — above all — a chronographed arrow speed. Not the spec sheet. What your bow actually measures, today.
The reason to keep the record is that every other calculation in archery is downstream of it. A spine recommendation is meaningless without a draw weight and draw length. A sight tape generated against a catalogue speed rating will not match your pins. A wind or trajectory solve is only as good as the velocity you fed it. Save the bow once, correctly, and every tool that touches it inherits the truth instead of an assumption.
The fields, and why each one earns its place
| Field | What it decides |
|---|---|
| Make, model, year | Selects the cam system, the published specs and — critically — the correct cam-lean correction method, which differs by brand and often by model year. |
| Measured draw weight | Sets the force curve. Determines dynamic spine and velocity. Bows rarely peak at exactly the number on the limb bolt. |
| Measured draw length | The largest single lever on arrow speed, and the number archers most often get wrong about themselves. |
| Cam / module setting | Two bows of the same model at different modules are different bows for tuning purposes. |
| Axle-to-axle and brace height | Baseline measurements. When they drift, something has moved — usually the strings. |
| Let-off | Affects holding weight and, on some platforms, is adjustable and therefore worth recording. |
| Rest type (limb-driven or cable-driven) | Branches the tuning sequence. The two behave differently at the shot and are diagnosed differently. |
| Peep height, D-loop length, nock point | Anchor geometry. Reproducing a tune after a string change means reproducing these. |
| String and cable install date | Strings creep. A tune has a shelf life and this is how you know when it expired. |
| Chronographed speed with a known arrow | The one measurement that turns every downstream calculation from an estimate into a solve. |
Why the IBO number on the sticker is not your speed
An IBO speed rating is measured under one fixed set of conditions: 70 pounds of draw weight, a 30-inch draw length, and a 350-grain arrow. Almost nobody shoots that. Every deviation costs velocity, and the deviations stack: a shorter draw length, a lighter draw weight and a heavier hunting arrow will each pull you further below the sticker, in that order of severity.
There is also a bow-specific offset that has nothing to do with your setup — real bows measured on a real chronograph rarely hit their published rating even under IBO conditions. Our velocity model uses a per-bow offset series rather than assuming the spec sheet, and the honest confidence bands are documented on our Methods page: roughly ±5 fps for a bow we have chronograph data on, versus ±15 fps for one we do not.
The conclusion is unglamorous and worth stating plainly: a measured chronograph number beats any prediction, including ours. If you have access to a chronograph, or a phone and our acoustic chronograph, take the reading and save it against the bow. Prediction is what you use until you have a measurement.
Draw length is the number to get right first
Draw length is where most saved setups go wrong, because archers record what they were sold rather than what they draw. AMO draw length is measured from the throat of the nock to the pivot point of the grip plus 1.75 inches — a definition, not an opinion — and the module on your cam is a claim about that number, not proof of it. If your bow has been re-moduled, if your release or D-loop length has changed, or if you bought the bow second hand, verify it rather than inherit it.
Getting it wrong propagates everywhere. Draw length changes your effective draw weight on most cam systems, which changes dynamic spine, which changes the arrow the recommender hands you. It changes velocity, which changes every pin gap on a sight tape. It is one measurement that corrupts three tools.
A bow is not a static object
Strings and cables stretch and settle, particularly in the first few hundred shots on a new set. As they creep, axle-to-axle grows, brace height shrinks, cam timing walks out of synchronisation and peep rotation moves. Nothing announces itself; the first symptom is usually a group that has quietly widened at long range while everything felt normal.
That is why the install date belongs in the record next to the baseline ATA and brace measurements. Re-measure occasionally and compare against what you saved. A drift in those numbers is your cue to re-check timing and re-run a paper tune before you conclude the arrows are at fault.
What a saved bow unlocks elsewhere
Once a bow exists in your library, the spine recommender can solve against your real draw weight and length instead of a category; the sight tape engine can build from your measured velocity and the day's air density instead of a spec-sheet estimate; the wind and trajectory tools inherit the same velocity; and each Range Log session can carry a snapshot of the setup as it was on that date. That last one matters more than it sounds — without it, a season of range data is a pile of numbers with no idea which bow produced them.
Bow setups stay in your browser's local storage on the free tier. Anonymous by default, sign-in optional; the full contract is on our privacy and data page.
Common questions
What should I record about my bow setup?
Make, model and year; measured draw weight and draw length; cam module; axle-to-axle and brace height; let-off; rest type; peep height, D-loop length and nock point; string install date; and a chronographed arrow speed with a known arrow weight.
Why is my bow slower than its IBO rating?
IBO is measured at 70 pounds, a 30-inch draw and a 350-grain arrow. Any shorter draw, lighter poundage or heavier arrow costs speed, and the losses stack. Most real bows also fall somewhat short of their published rating even under IBO conditions.
How is AMO draw length defined?
From the throat of the nock to the pivot point of the grip, plus 1.75 inches. Your cam module claims a draw length; verify it rather than assume it, especially on a second-hand bow or after a module change.
How often should I re-measure axle-to-axle and brace height?
Often enough to catch string creep before it costs you a group — certainly after breaking in a new string set, and any time long-range groups widen without an obvious cause. A drift in those baselines is a signal to re-check cam timing.
Do I need a chronograph reading, or is the estimate good enough?
An estimate is good enough to pick a starting arrow. It is not good enough for a sight tape you intend to trust past 50 yards. A measured number removes the largest uncertainty in every trajectory calculation you will run.
Can I save more than one bow?
Yes, and you should keep them separate rather than editing one record as you change bows. A target rig and a hunting rig have different specs, different arrows and different tunes, and merging them loses the history that makes the log useful.
My Bows is part of The Forge by Sparrow Expeditions. Velocity modelling, offsets and confidence bands are documented on our Methods page. Built by archnerds, for archnerds.