Bow Woods

by Eric Meier

First off, a few brief disclaimers:

  • Aside from a few archery classes I took back in in college, I am neither an archer nor a bowyer.
  • The ideas presented here would apply only to simple or self bows made from a single piece of wood, and may or may not be directly applicable to other types of bows such as composite or laminated bows.

With that being said, I have access to consolidated data on a lot of different kinds of wood. While there are almost endless ways to measure wood strength (which I discuss in more detail in my article World’s Strongest Woods), when you broaden the scope out to all woods worldwide, there are three tests that seem to come up most often. Listed in order of their commonness, they are: modulus of elasticity (MOE), modulus of rupture (MOR), and crushing strength. Of the three, we will be looking more closely at the first two tests.

Cracking the Code

Archery bows present a somewhat unique challenge in finding the right requirements for the best wood. In the simplest and crudest terms possible, the wood should be able to bend, but not break.

“As a rule of thumb, the best bow materials are those that combine a high specific bending strength with a relatively low specific modulus (Hickman et al. 1947). In general, a good bow is one to which a high force can be applied under a large elastic deformation, which guarantees that a large amount of elastic energy stored during the draw is transferred effectively into kinetic energy of the arrow when the bowstring is released.”[1]Bjurhager, I., Gamstedt, E., Keunecke, D., Niemz, P. & Berglund, L. (2013). Mechanical performance of yew (Taxus baccata L.) from a longbow perspective. Holzforschung, 67(7), 763.

To summarize the above recommendations, the authors call for a wood with “high specific bending strength” (MOR) along with a “relatively low specific modulus” (MOE).

Dealing in the simplest terms:

  1. The modulus of rupture (MOR) measures how easily the wood will break—the higher the number, the harder it is to break or rupture.
  2. The modulus of elasticity (MOE) measures how easily a wood will bend—the higher the number, the more stubborn and stiff it will be.

So in terms of looking at the raw mechanical data of woods, the best bow woods tend to be those that have a low MOE and a high MOR. Stated another way, the best bow woods tend to be those that will bend easily, and not break. This is perhaps an oversimplification, but it is at least a good starting point in evaluating new woods for suitability to use as an archery bow.

Creating a “Bow Index”

Instead of displaying the absolute values of each wood species, I thought it would be an interesting pursuit to come up with a ratio that compares the two pertinent values, MOR and MOE. This would in effect emphasize those woods with the special properties of having a disproportionately high MOR and/or a low MOE.

Given the rationale and requirements, an equation can be formed to roughly assess a wood’s suitability for bow use:

(MOR/MOE) * 1000.
(The added factor of 1000 is simply to bring the number to a more manageable size and avoid dealing with tiny .00XX decimal values.)

For lack of a better term, this ratio will simply be referred to as the wood’s “bow index.” Results are listed in the table below, sorted from highest to lowest bow index.

Bow Index Listing

Bow IndexCommon NameScientific Name
13.06Iron birchBetula schmidtii
12.61Pear HawthornCrataegus calpodendron
11.90African BlackwoodDalbergia melanoxylon
11.61RowanSorbus aucuparia
11.61Lebombo ironwoodAndrostachys johnsonii
11.32VerawoodBulnesia arborea
11.27Pau FerroMachaerium spp.
11.26Pacific YewTaxus brevifolia
11.25MuningaPterocarpus angolensis
11.15Tropical black sageCordia curassavica
11.05Osage orangeMaclura pomifera
10.98River Sheoak                     Casuarina cunninghamiana
10.90GuajayviPatagonula americana
10.85ChinaberryMelia azedarach
10.85Norway mapleAcer platanoides
10.72JutahyDialium guianense
10.68PearPyrus communis
10.57MansoniaMansonia altissima
10.57Turkey OakQuercus cerris
10.57MovinguiDistemonanthus benthamianus
10.52Pau SantoZollernia paraensis
10.52MakoreTieghemella heckelii
10.50Bur OakQuercus macrocarpa
10.49River Red GumEucalyptus camaldulensis
10.47PreciosaAniba canelilla
10.47Western sheoak                     Allocasuarina fraseriana
10.44Siamese RosewoodDalbergia cochinchinensis
10.42Field mapleAcer campestre
10.40Monterey CypressCupressus macrocarpa
10.37Algarrobo BlancoProsopis alba
10.36EtimoeCopaifera salikounda
10.35ZiricoteCordia dodecandra
10.31EkkiLophira alata
10.31English WalnutJuglans regia
10.31Alligator JuniperJuniperus deppeana
10.22BrazilwoodPaubrasilia echinata
10.18Downy BirchBetula pubescens
10.08Crab appleMalus sylvestris
10.08TambootieSpirostachys africana
10.04Black MesquiteProsopis nigra
10.01CurupayAnadenanthera colubrina
10.00Eastern Red CedarJuniperus virginiana
10.00Rough-barked appleAngophora floribunda
9.98Ceylon SatinwoodChloroxylon swietenia
9.94East Indian RosewoodDalbergia latifolia
9.93Rhodesian TeakBaikiaea plurijuga
9.90African PadaukPterocarpus soyauxii
9.89Sycamore mapleAcer pseudoplatanus
9.86Peruvian WalnutJuglans neotropica
9.86Blue AshFraxinus quadrangulata
9.84EbiaraBerlinia spp.
9.82Burma PadaukPterocarpus macrocarpus
9.80Sweet CherryPrunus avium
9.76Nutmeg HickoryCarya myristiciformis
9.74Siberian ElmUlmus pumila
9.72Pau RosaBobgunnia fistuloides
9.72Pau RosaBobgunnia madagascarensis
9.69Brazilian RosewoodDalbergia nigra
9.67LemonwoodCalycophyllum candidissimum
9.63American HornbeamCarpinus caroliniana
9.62TurpentineSyncarpia glomulifera
9.62MoabiBaillonella toxisperma
9.61Rock ElmUlmus thomasii
9.58Shellbark HickoryCarya laciniosa
9.56Chakte KokSimira salvadorensis
9.55Bitternut HickoryCarya cordiformis
9.54AvodireTurraeanthus africanus
9.53European YewTaxus baccata
9.52Prosopis julifloraProsopis juliflora
9.50MalabayabasTristaniopsis decorticata
9.49AfataCordia trichotoma
9.46Black LocustRobinia pseudoacacia
9.46Nigerian pearwoodGuarea cedrata
9.46BosseGuarea spp.
9.44Horse chestnutAesculus hippocastanum
9.43SaffronheartHalfordia scleroxyloa
9.40CongotaliLetestua durissima
9.39BocoteCordia elaeagnoides
9.38Southern Silky OakGrevillea robusta
9.38SissooDalbergia sissoo
9.37African MesquiteProsopis africana
9.37QuinaMyroxylon peruiferum
9.36Oregon White OakQuercus garryana
9.36Gaboon EbonyDiospyros crassiflora
9.35Shagbark HickoryCarya ovata
9.34Oregon AshFraxinus latifolia
9.34IrokoMilicia excelsa
9.29Live OakQuercus virginiana
9.28HollyIlex opaca
9.28Sessile OakQuercus petraea
9.28ChanfutaAfzelia quanzensis
9.25Yellow GumEucalyptus leucoxylon
9.25Grey myrtleBackhousia myrtifolia
9.24HackberryCeltis occidentalis
9.21Texas EbonyEbenopsis ebano
9.19BekakAglaia lawii
9.18MachicheLonchocarpus spp.
9.17ShittimVachellia seyal
9.14BubingaGuibourtia spp.
9.14African WalnutLovoa trichilioides
9.14Japanese LarchLarix kaempferi
9.14Ceylon EbonyDiospyros ebenum
9.13English ElmUlmus procera
9.13Dutch ElmUlmus x hollandica
9.13Pink IvoryBerchemia zeyheri
9.12SapeleEntandrophragma cylindricum
9.12European HornbeamCarpinus betulus
9.12Cedar ElmUlmus crassifolia
9.12English OakQuercus robur
9.12MerbauIntsia bijuga
9.10Scarlet OakQuercus coccinea
9.09ObecheTriplochiton scleroxylon
9.06Amazon RosewoodDalbergia spruceana
9.06Santos MahoganyMyroxylon balsamum
9.06Cascara BuckthornRhamnus purshiana
9.06Macassar EbonyDiospyros celebica
9.06Argentine Osage OrangeMaclura tinctoria
9.05Tamo AshFraxinus mandshurica
9.03PrimaveraRoseodendron donnell-smithii
9.03Yucatan RosewoodDalbergia tucurensis
9.02Chico ZapoteManilkara zapota
9.02Honey LocustGleditsia triacanthos
9.02PyinmaLagerstroemia spp.
8.99Canadian ServiceberryAmelanchier canadensis
8.97Winged ElmUlmus alata
8.91UrundeuvaAstronium urundeuva
8.91UtileEntandrophragma utile
8.91AmendoimPterogyne nitens
8.89Pignut HickoryCarya glabra
8.88Afzelia xylayAfzelia xylocarpa
8.87Overcup OakQuercus lyrata
8.87MangiumAcacia mangium
8.86IdigboTerminalia ivorensis
8.85Okoume / GaboonAucoumea klaineana
8.85ItinProsopis kuntzei
8.84Black PoplarPopulus nigra
8.83GidgeeAcacia cambagei
8.82Black PalmBorassus flabellifer
8.82Wych ElmUlmus glabra
8.82ImbuyaOcotea porosa
8.82CanarywoodCentrolobium spp.
8.81Water HickoryCarya aquatica
8.81American ElmUlmus americana
8.81PersimmonDiospyros virginiana
8.80Queensland WalnutEndiandra palmerstonii
8.80LonghiChrysophyllum africanum
8.79California Black OakQuercus kelloggii
8.78OpepeNauclea diderrichii
8.76Mexican CypressCupressus lusitanica
8.74East African OliveOlea capensis
8.74Post OakQuercus stellata
8.74Pumpkin AshFraxinus profunda
8.73KotoPterygota macrocarpa
8.72Red ElmUlmus rubra
8.72Endra endraHumbertia madagascariensis
8.71Yellow BoxEucalyptus melliodora
8.70AfrormosiaPericopsis elata
8.70DogwoodCornus florida
8.69Black WalnutJuglans nigra
8.67PlumPrunus domestica
8.66American BeechFagus grandifolia
8.65Mockernut HickoryCarya tomentosa
8.65Mountain HemlockTsuga mertensiana
8.64Red MulberryMorus rubra
8.64PaulowniaPaulownia spp.
8.63Hard mapleAcer saccharum
8.63Giant ChinkapinChrysolepis chrysophylla
8.62WengeMillettia laurentii
8.62MopaneColophospermum mopane
8.62White AshFraxinus americana
8.62White MerantiShorea hypochra
8.60LatiAmphimas pterocarpoides
8.58African MahoganyKhaya senegalensis
8.58Swamp White OakQuercus bicolor
8.57Staghorn SumacRhus typhina
8.55Maritime PinePinus pinaster
8.55Virginia PinePinus virginiana
8.55White Cypress PineCallitris columellaris
8.54Swamp MahoganyEucalyptus robusta
8.54AsepokoPouteria guianensis
8.53QuebrachoSchinopsis quebracho
8.52MonkeythornSenegalia galpinii
8.52Gray BirchBetula populifolia
8.51PintobortriPouteria eugenifolia
8.49Green AshFraxinus pennsylvanica
8.48AburaMitragyna ciliata
8.47AfzeliaAfzelia spp.
8.46MadroneArbutus menziesii
8.45MangkonoXanthostemon verdugonianus
8.45CocoboloDalbergia retusa
8.45Brown EbonyLibidibia paraguariensis
8.44Mediterranean CypressCupressus sempervirens
8.44BrownheartVouacapoua americana
8.43Grey BoxEucalyptus moluccana
8.43Suriname IronwoodBocoa prouacensis
8.42Andaman PadaukPterocarpus dalbergioides
8.42White OakQuercus alba
8.41European AshFraxinus excelsior
8.40Amourette (snakewood sapwood)Brosimum guianense
8.40SnakewoodBrosimum guianense
8.40BoxwoodBuxus sempervirens
8.40MgurureCombretum schumannii
8.39KoaAcacia koa
8.39BloodwoodBrosimum rubescens
8.39TamarindTamarindus indica
8.39London plane FSPlatanus x acerifolia
8.39London plane QSPlatanus x acerifolia
8.37BeliJulbernardia pellegriniana
8.36Chestnut OakQuercus prinus
8.36Monkey potLecythis zabucajo
8.35LyptusEucalyptus urograndis
8.34Black wattleAcacia mearnsii
8.34Panga PangaMillettia stuhlmannii
8.33BulletwoodManilkara bidentata
8.32TineoWeinmannia trichosperma
8.32Cape HollyIlex mitis
8.32Monkey PuzzleAraucaria araucana
8.31Black OakQuercus velutina
8.31European alderAlnus glutinosa
8.30MonkeypodSamanea saman
8.30Sweet ChestnutCastanea sativa
8.29Eastern HophornbeamOstrya virginiana
8.29River BirchBetula nigra
8.28Shumard OakQuercus shumardii
8.27Scots Pine (R)Pinus sylvestris
8.27Huon PineLagarostrobos franklinii
8.26Gum ArabicVachellia nilotica
8.26Yellow BirchBetula alleghaniensis
8.26ParicaSchizolobium amazonicum
8.26Black CherryPrunus serotina
8.25Horsetail CasuarinaCasuarina equisetifolia
8.25Smooth-barked appleAngophora costata
8.24Willow OakQuercus phellos
8.23Slash PinePinus elliottii
8.23Sand PinePinus clausa
8.23LimbaTerminalia superba
8.22SneezewoodPtaeroxylon obliquum
8.21Black mapleAcer nigrum
8.21GarapaApuleia leiocarpa
8.21Queensland kauriAgathis robusta
8.20JatobaHymenaea courbaril
8.19MutenyeGuibourtia arnoldiana
8.19Black sirisAlbizia odoratissima
8.19Silver BirchBetula pendula
8.17LancewoodOxandra lanceolata
8.17Red mapleAcer rubrum
8.17Water OakQuercus nigra
8.17Red OakQuercus rubra
8.16Crack WillowSalix fragilis
8.14PericopsisPericopsis mooniana
8.14Indian LaurelTerminalia elliptica
8.14Southern Red OakQuercus falcata
8.13Grey IronbarkEucalyptus paniculata
8.13Northern White CedarThuja occidentalis
8.12Parana PineAraucaria angustifolia
8.12Cedar of LebanonCedrus libani
8.10NarraPterocarpus indicus
8.09Pin OakQuercus palustris
8.08Mora (glued)Mora excelsa
8.08MoraMora gonggrijpii
8.07Lemon-Scented GumCorymbia citriodora
8.06RaminGonystylus spp.
8.06WamaraSwartzia benthamiana
8.06NargustaTerminalia amazonia
8.05OhiaMetrosideros collina
8.04TanoakNotholithocarpus densiflorus
8.04SassafrasSassafras albidum
8.03Southern RedcedarJuniperus silicicola
8.03Honduran MahoganySwietenia macrophylla
8.02Angelim vermelhoDinizia excelsa
8.02IpeHandroanthus serratifolius
8.01PurpleheartPeltogyne spp.
8.00Black TupeloNyssa sylvatica
8.00Southern MagnoliaMagnolia grandiflora
7.99European silver firAbies alba
7.99Laurel OakQuercus laurifolia
7.99Cuban MahoganySwietenia mahogani
7.98HububaliLoxopterygium sagotii
7.97Blackheart SassafrasAtherosperma moschatum
7.97African JuniperJuniperus procera
7.94Cherrybark OakQuercus pagoda
7.93KaneelhartLicaria canella
7.93AndirobaCarapa spp.
7.92RubberwoodHevea brasiliensis
7.92PecanCarya illinoinensis
7.92MyrtleUmbellularia californica
7.91GreenheartChlorocardium rodiei
7.90TeakTectona grandis
7.88Black AshFraxinus nigra
7.87Radiata PinePinus radiata
7.87PheasantwoodSenna siamea
7.86MessmateEucalyptus obliqua
7.86Patula PinePinus patula
7.85Swamp Chestnut OakQuercus michauxii
7.84CumaruDipteryx odorata
7.84Red PalmCocos nucifera
7.82Yellow CedarCupressus nootkatensis
7.81Peroba RosaAspidosperma polyneuron
7.81Silver mapleAcer saccharinum
7.80Tiete RosewoodGuibourtia hymenaeifolia
7.79Sweet BirchBetula lenta
7.78Limber PinePinus flexilis
7.78Tasmanian MyrtleLophozonia cunninghamii
7.77Northern CatalpaCatalpa speciosa
7.76Spanish CedarCedrela odorata
7.76Australian Red CedarToona ciliata
7.74PaldaoDracontomelon dao
7.74Paper BirchBetula papyrifera
7.73Red BloodwoodCorymbia gummifera
7.72Black WillowSalix nigra
7.71Water TupeloNyssa aquatica
7.70European BeechFagus sylvatica
7.69Incense CedarCalocedrus decurrens
7.67MangoMangifera indica
7.65Brazilian Pau RosaAniba rosaeodora
7.64Caribbean PinePinus caribaea
7.63BoxelderAcer negundo
7.63European LarchLarix decidua
7.62SweetgumLiquidambar styraciflua
7.62Rose GumEucalyptus grandis
7.62Grey alderAlnus incana
7.59Hormigo NegroPlatymiscium dimorphandrum
7.58West African albiziaAlbizia ferruginea
7.58AnigrePouteria altissima
7.57Yellow MerantiShorea spp.
7.56BelahCasuarina cristata
7.56ThuyaTetraclinis articulata
7.55Pitch PinePinus rigida
7.54New Guinea WalnutDracontomelon mangiferum
7.54Pacific mapleAglaia cucullata
7.54KataloxSwartzia cubensis
7.54EveussKlainedoxa gabonensis
7.53OvangkolGuibourtia ehie
7.53SourwoodOxydendrum arboreum
7.50Jeffrey PinePinus jeffreyi
7.50ZebrawoodMicroberlinia brazzavillensis
7.49Shortleaf PinePinus echinata
7.48Queensland MapleFlindersia brayleyana
7.48Table Mountain PinePinus pungens
7.48LebbeckAlbizia lebbeck
7.47Port Orford CedarChamaecyparis lawsoniana
7.42Eastern HemlockTsuga canadensis
7.39CoffeetreeGymnocladus dioicus
7.38Bigleaf mapleAcer macrophyllum
7.38DegluptaEucalyptus deglupta
7.36Yellow silverballiAniba hypoglauca
7.36Northern Silky OakCardwellia sublimis
7.36CypressTaxodium distichum
7.34JarrahEucalyptus marginata
7.34RedwoodSequoia sempervirens
7.34Mexican alderAlnus jorullensis
7.33Jack PinePinus banksiana
7.33Spruce PinePinus glabra
7.32Longleaf PinePinus palustris
7.31TimboranaPseudopiptadenia suaveolens
7.31Atlantic White CedarChamaecyparis thyoides
7.31White PoplarPopulus alba
7.30New Zealand kauriAgathis australis
7.30Dark Red MerantiShorea spp.
7.29Common LimeTilia x europaea
7.29KeruingDipterocarpus spp.
7.29Ponderosa PinePinus ponderosa
7.27Alder-leaf BirchBetula alnoides
7.26AromataClathrotropis macrocarpa
7.25AraracangaAspidosperma megalocarpon
7.25Rose sheoakAllocasuarina torulosa
7.25White WillowSalix alba
7.24Lignum VitaeGuaiacum officinale
7.22Hoop PineAraucaria cunninghamii
7.22BalauShorea spp.
7.20AngeliqueDicorynia guianensis
7.20East Indian KauriAgathis dammara
7.18NyatohPalaquium spp. Payena spp.
7.18Blue GumEucalyptus globulus
7.18EspaveAnacardium excelsum
7.18CheesewoodAlstonia congensis
7.17Spotted GumCorymbia maculata
7.16Alaska Paper BirchBetula neoalaskana
7.15Burmese BlackwoodDalbergia cultrata
7.15MarblewoodZygia racemosa
7.15Loblolly PinePinus taeda
7.13White SerayaParashorea spp.
7.12Quaking AspenPopulus tremuloides
7.10Red alderAlnus rubra
7.10Khasi PinePinus kesiya
7.08QuipoCavanillesia platanifolia
7.08Douglas-FirPseudotsuga menziesii
7.07TamarackLarix larcina
7.07Goncalo AlvesAstronium graveolens
7.05Siam balsaAlstonia spatulata
7.05Red ashAlphitonia excelsa
7.04SycamorePlatanus occidentalis
7.04GuanacasteEnterolobium cyclocarpum
7.03Raspberry jamAcacia acuminata
7.03BulokeAllocasuarina luehmannii
7.02PartridgewoodAndira inermis
7.01Lodgepole PinePinus contorta
7.00DoiAlphitonia zizyphoides
6.99California red firAbies magnifica
6.99American ChestnutCastanea dentata
6.99Australian blackwoodAcacia melanoxylon
6.98Heavy hopeaHopea iriana
6.98YarranAcacia homalophylla
6.97YellowheartEuxylophora paraensis
6.96CamphorCinnamomum camphora
6.95Western LarchLarix occidentalis
6.94CandlenutAleurites moluccanus
6.94Eastern White PinePinus strobus
6.93Western HemlockTsuga heterophylla
6.89Mountain AshEucalyptus regnans
6.89Sugar PinePinus lambertiana
6.86ButternutJuglans cinerea
6.84Red MangroveRhizophora mangle
6.84Pinyon PinePinus edulis
6.84RengasGluta spp.
6.81Tree of HeavenAilanthus altissima
6.81Norfolk Island PineAraucaria heterophylla
6.79Light Red MerantiShorea contorta
6.77BataiFalcataria moluccana
6.76CucumbertreeMagnolia acuminata
6.76Western Red CedarThuja plicata
6.75Red PinePinus resinosa
6.74TzalamLysiloma latisiliquum
6.71BoonareeAlectryon oleifolius
6.71Rock sheoakAllocasuarina huegeliana
6.71BeefwoodGrevillea striata
6.70SalmwoodCordia alliodora
6.70Indian Silver GreywoodTerminalia bialata
6.70CerejeiraAmburana cearensis
6.69Black CottonwoodPopulus trichocarpa
6.69Pin CherryPrunus pensylvanica
6.66Noble firAbies procera
6.66Ocote PinePinus oocarpa
6.65SweetbayMagnolia virginiana
6.64Western White PinePinus monticola
6.64Black SprucePicea mariana
6.63Pond PinePinus serotina
6.59Indian pulaiAlstonia scholaris
6.58Engelmann SprucePicea engelmannii
6.57White SprucePicea glauca
6.56JelutongDyera costulata
6.55Andean alderAlnus acuminata
6.53White firAbies concolor
6.52TatajubaBagassa guianensis
6.49Norway SprucePicea abies
6.48Coracao de negroSwartzia panacoco
6.47Sumatran PinePinus merkusii
6.41Yellow buckeyeAesculus flava
6.39Yellow poplarLiriodendron tulipifera
6.36Bigtooth AspenPopulus grandidentata
6.36European AspenPopulus tremula
6.36Subalpine firAbies lasiocarpa
6.35Balsam firAbies balsamea
6.34Sitka SprucePicea sitchensis
6.27Hard milkwoodAlstonia spectabilis
6.27TornilloCedrelinga cateniformis
6.25KarriEucalyptus diversicolor
6.24Broad-leaved appleAngophora subvelutina
6.20Eastern CottonwoodPopulus deltoides
6.18Balsam PoplarPopulus balsamifera
6.18Manil montagneMoronobea coccinea
6.17Nepalese alderAlnus nepalensis
6.14Red SprucePicea rubens
6.14Black IronwoodKrugiodendron ferreum
6.12Fijian kauriAgathis macrophylla
6.09Pacific silver firAbies amabilis
6.05Pink ashAlphitonia petriei
5.99ChechenMetopium brownei
5.96BasswoodTilia americana
5.96Austrian PinePinus nigra
5.90KempasKoompassia malaccensis
5.71Grand firAbies grandis
5.28BalsaOchroma pyramidale
4.76SugiCryptomeria japonica

A Closer Look at MOE and Wood Anatomy

Regardless of one’s feelings about the results of the bow index list above, it is almost universally recognized that modulus of elasticity (MOE) is a very important measurement for wood bows. After all, the very act of bending an archery bow has direct bearing on this measurement. But unlike wood hardness, which has been shown to have a very strong and predictable relation to wood density[2]Wiemann, M. C. (2007). Estimating Janka hardness from specific gravity for tropical and temperate species (Vol. 643). US Department of Agriculture, Forest Service, Forest Products Laboratory., MOE has shown much more of a variation with relation to a wood’s density. In short, there appears to be some woods that are heavy, where you’d expect them to be commensurately stiffer as well, but that’s not always the case. There appears to be at least one other factor in play in determining MOE.

Enter microfibrils.

Microfibrils are tiny strands found within the cell walls of wood. Usually they run parallel with the wood grain, but that’s not always the case, and they can sometimes run at varying degrees off of parallel, which is called the microfibril angle (MFA). (See the notes in red lettering pertaining to the central S2 layer in the reference image below.)

Note this is not to be confused with interlocked or spiral grain found in some wood species, as microfibrils are much smaller—it’s possible to have a wood with straight grain but angled microfibrils.

It appears that the microfibril angle (MFA) is the hidden variable that accounts for (most) of the remaining variation in predicting MOE, since the value is so hard to predict if relying on wood density alone. In studying species of Eucalyptus, it was found that “MFA alone accounted for 87 percent of the variation in MOE, while density alone accounted for 81 percent. Together, MFA and density (as Density/MFA) accounted for 92 percent of the variation in MOE.”[3]Yang, J. L., & Evans, R. (2003). Prediction of MOE of eucalypt wood from microfibril angle and density. Holz als Roh-und Werkstoff, 61(6), 449-452. So an increased off-axis microfibril angle contributes to a decrease in MOE. (Specifically, the microfibril angle in the larger central S2 layer pictured above.[4]Donaldson, L. (2008). Microfibril angle: measurement, variation and relationships–a review. IAWA Journal, 29(4), 345-386.) But perhaps equally important, though easy to overlook, the same study found that “MFA had little independent influence on MOR.”[5]Yang, J. L., & Evans, R. (2003). Prediction of MOE of eucalypt wood from microfibril angle and density. Holz als Roh-und Werkstoff, 61(6), 449-452.

In practical terms, the microfibril angle acts as a special variable that allows otherwise dense and strong woods to have a disproportionately low MOE with no effect on the MOR. An increased density would also mean an increase in both MOE and MOR, which would have little to no effect on a wood’s bow index. But woods with a high MFA could have the peculiar combination of a high MOR and a low MOE, exactly what we’re hunting for.

Are you an aspiring wood nerd?

The poster, Worldwide Woods, Ranked by Hardness, should be required reading for anyone enrolled in the school of wood nerdery. I have amassed over 500 wood species on a single poster, arranged into eight major geographic regions, with each wood sorted and ranked according to its Janka hardness. Each wood has been meticulously documented and photographed, listed with its Janka hardness value (in lbf) and geographic and global hardness rankings. Consider this: the venerable Red Oak (Quercus rubra) sits at only #33 in North America and #278 worldwide for hardness! Aspiring wood nerds be advised: your syllabus may be calling for Worldwide Woods as part of your next assignment!

References

References
1 Bjurhager, I., Gamstedt, E., Keunecke, D., Niemz, P. & Berglund, L. (2013). Mechanical performance of yew (Taxus baccata L.) from a longbow perspective. Holzforschung, 67(7), 763.
2 Wiemann, M. C. (2007). Estimating Janka hardness from specific gravity for tropical and temperate species (Vol. 643). US Department of Agriculture, Forest Service, Forest Products Laboratory.
3, 5 Yang, J. L., & Evans, R. (2003). Prediction of MOE of eucalypt wood from microfibril angle and density. Holz als Roh-und Werkstoff, 61(6), 449-452.
4 Donaldson, L. (2008). Microfibril angle: measurement, variation and relationships–a review. IAWA Journal, 29(4), 345-386.
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Dave Gerty

Eric, thanks for a large helping of food for thought. I have been chewing on this off and on for years. As I search the web for more information, I find a lot of sites referring back here to Eric’s Bow Index. Eric, you made your mark. I am not an archer either, I was a materials engineer by trade, so this is interesting, but I’ll never build a bow. The bow index, 1000*(MOR/MOE) is 10 times the strain to failure as expressed in percentages. In other words, MOR/MOE is the strain to failure. So the index is a mechanical… Read more »

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Dave Gerty

There is a typo in my spreadsheet in the fling speed numbers. The SI unit for density was inadvertently left in place when I tried to replaced it the imperial pounds per cubic foot. All of the fling speed numbers are wrong. For the heavier woods, the error is small, but for the high and low density materials the errors are proportionally larger. The correct numbers are: Wood Species . . self-fling speed v Yew . . . . . . . . . 141 Osage orange .. 133 Brazil wood . . .. 142 Madagascar Rosewood . . .… Read more »

Jackon

Ive made a small recurve bow that is twisted spring steel rods as the core and pvc backing and i used high copper or brass alloy wire to wrap it with. So pvc =, horn. Spring steel rods= core. Wire = sinew. It shoots very fast and power ful and is hard to string alone even tho its only 18 inches or sometime. 9560 steel can bend 90 degrees and spring back with out being deformed. That would be the ultimate bow material i believe.