Flatbow vs Longbow: Up to 11 fps Faster, but Louder (Modeled)

Which Bow Fits Your Draw Length, Arrow Weight and Shooting Style?

Flatbow versus longbow comes down to four inputs: your draw length, your arrow mass in grains per pound (GPP), where you shoot, and whether the riser allows a center-cut shelf. Set them below and the tool returns a match split, a starting AMO length and a spine window.

How does the Match & Spine Calculator score work?

It is a weighted planning heuristic, not a physics solver: 30% shooting style, 30% shelf, 25% draw length (scored against the 29.0″ flatbow and 28.5″ longbow stacking onsets) and 15% arrow weight (heavier arrows shrink the fps gap). The result is clamped to 6–94% so the bar never claims certainty it does not have.

AMO length uses two rules of thumb fitted to this page’s reference bows: about 2.28 × draw length for the flatbow (64″ at 28″) and 2.43 × draw length for the longbow (68″ at 28″). The spine offset shifts your chart-matched spine by 1.5 lb per GPP away from 9 GPP, because a heavier arrow bends more at the shot. Treat every output as a starting point and confirm with a paper-tuning pass and the wood arrow spine calculator.

Every bow length on this page is an AMO length, the rating makers print on spec sheets. Bear Archery lists it beside draw weight across its traditional bows (a 62″ North Branch longbow, a 58″ Grizzly recurve), and that is how I sanity-checked my 64″ and 68″ references. Germany’s Bodnik Bows builds handmade longbows and recurves too; the site is in German with an English toggle.

Which Limb Section Strains Harder at Your Draw Length? Try the Heatmap

The D-section, at its belly apex: about 41% more peak strain than a flatbow limb at a 28-inch draw, because its neutral axis sits near the flat back. A flat rectangle strains back and belly equally. Drag the sliders below to watch both sections re-color.

How does the heatmap simulator work, and what does it leave out?

Strain: in standard beam theory, bending strain equals curvature times the distance from the neutral axis, and as long as plane sections stay plane the strain profile through the section is a straight line. I estimate curvature as κ = 2δ ÷ L², with tip deflection δ = 0.475 × (draw − 6.5″) and a working limb length L = (AMO − 12″) ÷ 2 (26″ on the flatbow, 28″ on the longbow). Everything is normalized so a 0.50″-thick flatbow at a 28″ draw reads exactly 100.

Sections: the flatbow is a 1.75″-wide rectangle. The longbow is a half-ellipse D, flat back on top, with area, moment of inertia and centroid from closed-form formulas. Rectangle torsion uses the standard series for a solid rectangle, the form tabulated in Roark’s Formulas for Stress and Strain: J = bt³[1/3 − 0.21(t/b)(1 − t⁴/12b⁴)]. The D uses Saint-Venant’s approximation, J ≈ A⁴ ÷ 40Ip, the same shortcut the AASHTO bridge-design commentary cites for solid sections; the paper also works out the exact closed-form rectangle solution if you want to check my series. It is an approximation, so treat the D-section torsion number as an estimate, not a measurement.

What it leaves out: limb taper, glass or bamboo laminations, tiller, and finite-element effects. Wood species matter too: stiffness and strength swing widely from one to the next, as the USDA Wood Handbook’s chapter on mechanical properties makes plain. For the bow physics behind the model, Bob Kooi’s Groningen PhD thesis on the mechanics of the bow and arrow is the open reference, and my version is far simpler. Shooters have also worked through torsion stiffness and why torsional stiffness shapes recurve design in their own words.

Which Bow Wins for Which Job? The 1-Minute Decision Matrix

The flatbow wins on speed, draw smoothness and spine tolerance. The D-section longbow wins on silence and forgiveness. Finger pinch and stacking onset sit close together. The sections after this table show where each row comes from, so you can check the model instead of trusting it.

How Do a Flatbow’s Rectangular Limbs and a Longbow’s D-Section Limbs Handle Stress?

A flatbow limb is a wide, thin rectangle, so its neutral axis sits at mid-thickness and back and belly fibers strain equally. A longbow’s D-section pushes the neutral axis toward the flat back, which leaves the rounded belly apex carrying the highest compressive strain in the limb.

Rectangular is the textbook answer for a bending limb: the optimal cross-section for the bending part of a bow limb is a rectangle, and almost every modern bow uses one. A flatbow shows why. With a flat back and a flat belly, the most strained wood sits the same distance from the neutral axis, so the load spreads across the whole width instead of piling up at one edge.

The D-section is a compromise the stave forces on the bowyer. An English longbow is cut from a single stave into a D: the sapwood back takes tension, the rounded heartwood belly takes compression. Wide at the back, a narrow apex at the belly.

Run my reference D (1.00″ wide, 0.71″ deep, half-elliptical belly) and the neutral axis lands 0.30″ from the back and 0.41″ from the belly apex. That 1.36-to-1 split means the apex strains 36% harder than the back. It is also a thin line of wood. Fret marks and set start right there, and the heatmap above shows why.

Twist is the other half of the story. I define the Torsional Rigidity Index (TRI) as the flatbow’s Saint-Venant torsion constant divided by the longbow’s. At the reference sizes, 1.75″ × 0.50″ against 1.00″ × 0.71″, that is 0.0598 in⁴ against 0.0444 in⁴, a TRI of 1.35. Call it 35% more resistance when a nock or a finger load pulls the string off-center.

The two sections carry nearly the same bending stiffness (within 8%), so they would draw at similar weights. Same stiffness, different geometry: that is what makes the comparison fair.

Where Does Each Bow Start Stacking? Force-Draw Curves and Energy Storage

Both bows climb almost linearly until stacking starts: about 29.0 inches on a 64-inch flatbow and 28.5 inches on a 68-inch D-section longbow. Past that point the flatbow adds roughly 3.4 lb per inch and the longbow 4.2 lb per inch. Stored energy at 50 pounds: 51 ft-lb against 42.

The area under a force-draw curve is the energy the bow stores. Stacking is where the slope of that curve outruns the draw: I call it once the weight climbs faster than about 5.5% per inch, the same yardstick as my shortbow vs longbow model.

Both curves here are pinned at 50 pounds at 28 inches. The flatbow stores 1.02 ft-lb per pound of draw weight, the longbow 0.84. That gap is banked early: the flatbow climbs out of brace height faster, so much of it sits left of the 20″ edge of the chart.

One caution on the onset numbers. Half an inch separates 29.0″ from 28.5″, and that is inside the error on most archers’ measured draw length. I would not pick a bow on it.

Does Bow Length Change Finger Pinch? String Angle by AMO Length

Yes, but less than most archers assume. At a 28-inch draw the modeled string angle opens from 118 degrees on a 60-inch bow to 139 degrees on a 72-inch bow, and pinch pressure falls from about 170 to 156 N/cm². A 64-inch and a 68-inch bow sit only 3% apart.

The string angle comes from the same fitted model as the shortbow page: 104° at 52″ and 132° at 68″, extended linearly across AMO lengths. Pinch index uses the same 100 × sin 52° ÷ sin(θ/2) proxy. Archers have argued over string angle and finger pinch for years, so the feeling is well documented; the formula just puts a number on it. Limb shape does not appear in it at all.

Pressure assumes a 50-lb (222 N) draw force spread over a 1.2 cm² three-finger string contact patch at the 104° reference angle, then scales with the pinch index. Absolute values move with your finger geometry and tab or glove; the deltas between lengths are the useful part. If your fingers hurt on a 64″ bow, moving to 68″ buys you about 3%, not a cure. Fix your anchor, string height and finger placement first, or go up much more than 4″.

Does a Center-Cut Shelf Really Widen Arrow Spine Tolerance?

Yes. A center-cut shelf seats the arrow about 0.15 inches from the string plane instead of 0.75 inches off the hand. That 80% smaller offset cuts the flex needed to clear the riser and widens the spine window from plus or minus 2 lb to plus or minus 5 lb.

I call that ratio the Shelf Paradox Reduction Coefficient (SPRC): SPRC = 1 − (center-cut offset ÷ off-the-hand offset) = 1 − 0.15 ÷ 0.75 = 0.80. Those offsets are my modeled reference values for a typical grip width and shelf depth; your riser will shift them. The mechanism itself is the classic archer’s paradox: the arrow has to bend around the handle before it straightens out.

In practice, a wider spine window means you can shoot a shaft a few pounds off, or a mixed batch, and still get a clean flight from a flatbow. Off the hand, I would hold the match to ±2 lb and spend the time to paper-tune. Start with the wood arrow spine calculator or the dynamic spine and shaft flex calculator, then confirm with the paper-tune guide.

Is a Flatbow Faster? Chronograph-Style FPS, Kinetic Energy and Momentum

Yes, by 7 to 11 fps at 50 pounds. A 400-grain arrow leaves a modeled flatbow at 191 fps against 180 fps from a longbow, worth 3.6 ft-lb of extra kinetic energy. The lead narrows to 7 fps and 2.9 ft-lb by 600 grains.

The table holds draw weight at 50 pounds and moves arrow mass from 8 to 12 GPP: roughly a light target arrow up to a heavy hunting build. Kinetic energy is grains × fps² ÷ 450,240. Momentum is grains × fps ÷ 225,218, in slug·ft/s.

Read the KE and momentum columns, not just fps. The flatbow’s kinetic-energy lead is 3.6 ft-lb at 8 GPP and 2.9 ft-lb at 12 GPP. Its momentum lead is 6.1% at 8 GPP and 4.6% at 12 GPP. Push arrow mass toward what deep penetration on big game wants and the two bows land much closer together on target.

Where does the speed come from? Not from wasting less. The flatbow banks 21% more stored energy (51 vs 42 ft-lb at 50 lb). Divide delivered kinetic energy by stored energy and the longbow converts a bigger share in this model, about 69–74% against 64–67% for the flatbow. More energy in, not a better conversion rate. I would rather show you that than sell flatbows as magic, and Kooi’s finite-difference study of the modern working-recurve bow lands in a similar place: materials that store more energy per unit of mass buy more performance than design tweaks.

My take: spend the edge on arrow weight, not on bragging rights. In this model a flatbow shooting a 465-grain arrow leaves at about 180 fps, the same speed as a longbow shooting 400 grains, and carries roughly 16% more kinetic energy and momentum. Same flight, harder hit.

To see how much of the speed gap comes from arrow mass alone, my arrow weight vs speed breakdown and the arrow speed and performance calculator let you swap grains and watch fps and energy move.

Which Is Quieter, and Which Kicks Less? Noise and Hand Shock

The longbow is quieter and the flatbow kicks less. The model puts flatbows at 65 to 72 dB, mostly limb slap and string buzz, against 58 to 64 dB for a longbow. On hand shock, the flatbow scores 8.4 out of 10, a D-section longbow 6.4, a reflex-deflex one 8.1.

Treat both ranges as modeled; limb mass, string material and where you put the microphone will all move them. The direction holds regardless: flatbows shed energy as noise, D-section longbows shed it as hand vibration. For a still-hunter, that noise row outweighs every fps in this article.

Which Bow Fits Timber, a Stand, a 3D Course or a Cold Sit? The Field Matrix

For most field setups the flatbow is the more adaptable bow: a 64-inch AMO length needs about 69 inches of vertical arc against about 74 for a 68-inch longbow, and the center-cut shelf tolerates awkward, canted draws. The longbow wins when silence or authenticity comes first.

Read down the last two columns and one pattern jumps out. The flatbow takes the rows where you shoot fast from an awkward position; the longbow takes the rows where you get one quiet chance. Neither owns the stand.

What Do Traditional Archers Say? Community Data Synthesis

My editorial read of traditional archery forum discussion: flatbows get credit for smooth, forgiving draws and spine tolerance, while D-section longbows get credit for silence and feel. The scores below turn that read into a 1–10 scale for five traits so you can compare them at a glance.

*Longbow hand-shock splits by profile: 6.4 for a traditional D-section, 8.1 for a reflex-deflex build. The radar chart plots the 7.25 midpoint.

How were these scores built, and how solid are they?

The geometry is plain trigonometry: five axes spaced exactly 72 degrees apart around one center, each running from 0 at the center to 10 at the outer ring. Every score is plotted at (value ÷ 10) × outer-radius on its own axis, and the coordinates were computed, not hand-drawn.

The numbers are an editorial synthesis: recurring themes in traditional-archery forum discussion, turned into one 1–10 scale so five traits can share a chart. They are not a poll and they are not instrument readings. The acoustic and speed rows elsewhere on the page are model outputs.

Frequently Asked Questions

Before you order either, confirm your real draw length with the draw length calculator and the bow size calculator, then paper-tune the setup and run your final arrow through the Kinetic Energy & Momentum Calculator. If your own results disagree with anything above, tell me through the contact page or over on the ArcheryEra subreddit: which bow would you pick, and what did it do at the range? New builds land in your inbox if you sign up for the newsletter at the bottom of the page.