By Larry Wise
Limb Constru onstruction ction Techni echniques ques
T
he purpose of a bow limb is to store energy and return that energy, or most of it, to the arrow. The limb contains elastic properties so that it can do this by bending during the draw stroke and unbending during the power stroke. This all sounds simple but always remember that the limb is the “action” “action” and “energy” part of the bow b ow and, in my opinion, the most critical component of the system. Please remember that humans and their expectations come to bear on the bow and arrow set. We expect to achieve “STEP TWO” of archery – you remember the simple two steps of archery: STEP ONE:LEARN TO SHOOT A TEN STEP TWO: REPEAT REPEAT STEP ONE Yep, Yep, we expect the bow to repeat its performance and, therefore, a high burden is placed on the set of bow limbs you have. (It also places a burden on your shooting form but that’s another article. In fact, it’s a complete book called Core Archery.) Building a pair of limbs that provide “repeat” performance is the challenge of every bow manufacturer as they select the materials, style, glue and building process. Beyond that, they must control this process from beginning to end to ensure longterm consistency and reliability. Failure to do so will result in returned failed limbs and, eventually, declining bow sales. Humans, by nature, want to compete. And they want to win. To do so we spend lots of time and money on the accessories but it’s the limbs that are at the heart of the system and should get more attention. I think that improvements in limb design and limb materials will provide the next big increase in archery
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accuracy and consistency. A pair of limbs that are properly matched and synchronized in their dynamic efforts makes a bow easier to aim and, therefore, easier for humans to generate a repeat performance. Understanding the basics of limb structure and construction methods will help you guide your customers to the bow that better suits their needs. I’ve presented that limb information in the following sections of this article.
TIP: The end away from the riser r iser to which the axle and cams are mounted. WORKING WORKI NG AREA: the thinner thinn er section of the limb near its middle that bends when the bow is drawn. FADE OUT: this is a wedge shaped strengthening layer that is thickest in the butt end and thins as it extends toward the middle or working area of the limb. Another fadeout may also be used in the limb tip through which the axle hole is drilled. LAMINATE: a thin layer of material, usually containing fiberglass, which is glued to other thin layers to make a complete bow limb.
COMPOUND BOW LIMB STYLES: Most of the compound bow limbs with which I am familiar fall into one of three styles: 1) Machined fiberglass blank (one piece). 2) Compression molded molded fiber glass. 3) Laminated fiberglass or fiber Most people mistake the back and front of the limb. The back is the side toward glass-carbon layers. the target while the face is toward the I’ve shot all of these and a few archer.Note archer. Note the other parts as labeled. others at one time or another. The machined one piece and the lamiLIMB NOMENCLATURE: nated continue to be the most wideFACE: the side of the limb facing ly used style. (Some compression the archer as he or she holds the bow molding techniques are patented, at full draw. The face is also called such as those Bear Archery uses to the belly or compression compression side since it make it’s tough quad limbs. That is compressed when the limb is bent. helps explain why compression BACK: the side of the limb molding is not more widespread.) toward the target when the bow is Although laminating technology held in the full draw position. The dates back to the fifteenth and sixback is also called the expansion or teenth century Turkish composite tension side since it must expand bow, I believe it will continue to when the limb is bent. make up most of the compound BUTT END: the thickest end that limb market for some time until a is mounted into the handle riser. new material or construction
3. PREHEATING PREHEATING LAYERS: LAYERS: The assembled limb layers are stacked in an oven and preheated to approximately 130* F. F. Preheating will facilitate the glue curing process. 1. LAMINATIONS LAMINATIONS:: This example example of limb laminations shows the six layers in some of Dave Barnsdale’s Barnsdale’s limbs and a completed limb. Note the two separate separate fade-outs three layers from the left. Six layers will make this limb quite stiff to provide high peak weight.
2. STACKING STACKING LAYERS: LAYERS: The different layers forming the back, interior and face are selected and stacked for gluing. More and thicker layers make the limb harder to bend so it yields more peak weight while a layer of angled fiberglass will provide tip strength to withstand twisting.
method comes along that offers better reliability, longer life and more speed through higher efficiency.
thicker to provide stable mounting to the riser while the tip end is thicker to provide tip stability and a strong mounting section for the cam axle.
1) MACH MACHIN INED ED SINGL SINGLE E PL PLY:
A rectangular rectan gular solid blank of fiberglass-reinforced fiberglass-reinforced epoxy plastic is the starting raw material for this limb style. In most cases the blank is supplied by Gordon Composites of Montrose, Colorado, and contains thousands of glass fibers running length-wise. The blank is machined on the face or belly side to create a thinner working section near the middle of the limb. The butt-end is
4. GLUE APPLICATION: APPLICATION: A steel blade is used to spread the two-part epoxy glue on both surfaces of all glue joints. This takes only a minute or two once you know what you’re doing.
2)
LAM LAMINATED:
Laminating also allows for the use of wood and other more exotic materials in one or more layers. 3) COMP COMPRE RESS SSIO ION N MOLDE MOLDED: D:
Compression molding involves the laying of fibers length wise through a forming mold while a liquid resin material is introduced. The limb is heated under high pressure for a period of time and when the limb blank is cooled its ends are shaped to make a finished product. Compression molded limbs from Bear Archery are designed with a uniform cross section so unbroken fibers can run the full length. That’s why these limbs get wider in the thinner working areas.
The laminated limb is built by gluing two to six layers of thin fiberglass-reinforced blanks. A thicker fadeout is glued between layers at the butt-end and another fadeout for axle mounting at the tip. The lamiBARNSDALE CUSTOM nated limb is stronger and allows the LAMINATED LIMBS: use of differing materials in the layers. Some layers can contain fibers I’ve seen this laminating process running at an angle to the limb a few times and actually helped length. Other limbs contain fiber build some test limbs during my fabric or glass mixed with carbon fibers to reduce the limb weight and increase the return speed.
5. ASSEMBLED LAYERS: LAYERS: After the gluing application the limb sets are stacked in groups of fourteen and the glue tools cleaned.
6. PRESS PRESS PREP: Both the top and bottom bottom press surfaces are waxed to prevent glue sticking to them. A silicon paper is also applied between the limb and press surfaces.
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PART ONE: ASSEMBL ASSEMBLY Y, PRESSING AND COOLING ORDER RAW MATERIALS – The
days with Xi Bows/Indian Industries in Evansville, Indiana (now known as Escalade Sports, parent company of Bear Archery). It was a relatively simple three-layer limb – a machined core with back and face layers glued to it. But that was twelve years ago so to get an update on the current status of limb building I called my good friend and master limb-maker Dave Barnsdale of Barnsdale Archery. Dave’s been building limbs for many years having worked for Ben Pearson and American Archery in the past. Today he builds limbs for customers who produce a wide variety of bows, including the current Pearson models. He also builds limbs for his own line of bows. I happen to have one of his Classic X bows with the shoot-through cable system system and limbs with carbon/bamboo laminates. Its one of smoothest drawing and best aiming bows I’ve ever shot. Barnsdale Archery builds custom limbs of various lengths, including 18.5 inch crossbow limbs and his standard five-layer recurved compound limb that is 13 to 19
inches long. Currently the majority of his limb business is a multilayer single cam limb that ranges from 12 to 16 inches. Dave tells tells me “This limb is 1.5-inches wide and relatively thick at the tip to eliminate limb twisting and splitting in the wheel slot. It has as many as seven layers depending on the draw weight my customer needs.”
BUILDING STEPS FOR THE LAMINATED LIMB: So what does it take to make a laminated compound limb? I spent some time discussing this process with Dave and will outline it here so you can understan unde rstand d this most important part of the compound system. As I said earlier, the limb pair you have on your bow must work together to provide a consistent thrust to each and every arrow you shoot and, since I’ve used a lot of Dave’s limbs over the years, I know that they are closely matched and among the very best available. Here’s a step-by-step look at making a laminated limb:
7. PRESSING: PRESSING: The glue press press top is closed on the limb,alignment is checked and about 110 pounds of pressure pressure is applied. The pres sure and 180 degreeF heat accelerates the glue curing process.
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layers of fiberglass material must be ordered from Gordon Composites. These materials are E-Glass, S-Glass, Core Tuff and shaped rectangular unidirectional blanks. Our example has six layers as follows: BACK: BACK: .030” S-Glass S-Glass WEFT: WEFT: .030” multidir multidirectio ectional nal layer CORE TUFF:.020” TUFF:.020” to .070” unidirectional unidirectional layer SHAPED SHAPED BLANK: BLANK: .410” E-glass E-glass blank/thinner middle CORE TUFF:.020” TUFF:.020” to .070”: unidirectional unidirectional layer FACE:.050” FACE:.050” E-Glass LAMINATIONS: Gordon’s Core Tuff layers are used to make the limb produce a given stiffness to match a desired draw weight range. The thicker the Core Tuff layers then the stiffer the limb and the more draw weight it will yield yield when bending bending during during the draw stroke. Dave says, “I get the ready-touse shaped blanks, CNC machined at Gordon Composites, made to my specs. The specs depend on the length of the limb because the longer limbs have a longer fadeout on the butt end.” “It takes about eight weeks to receive all of these materials in their ready-to-use form” Dave informed
8. EDGE CUT: After the limbs have been removed from the press and have cooled their edges are sanded sanded and shaped. Safe operation of the machines and dust collection are essentials at Barnsdale Archery.
me. That puts a burden on planning ahead. Two types of glass composites are used for limb manufacturing. SGlass is a higher performance version of E-Glass where the S-glass fibers have a higher modulus (they vibrate and recover at a higher frequency) and greater strength. Matthews is using S-Glass in its new 1-1/8” limb on the Drenaline to reduce overall limb mass but still provide the strength needed. LAYUP & STACK: A set of raw materials for a limb must be assembled according to the above list. The six layers are selected, grouped in order from back to face and stacked. Dave prefers to build a hundred limbs at a time and, since he has fourteen limb presses, he stacks fourteen limb-sets together. These limb-sets are then preheated to facilitate the gluing process to come. GLUE PREPARATION: The twopart epoxy glue is mixed using a dispensing machine and preheated. PREHEAT PRESSES: The limb presses are preheated to about 130 degrees. GLUE APPLICATION: The glue is then spread onto each surface that is to be glued. In our six-layer example that would be ten surfaces – two surfaces for each of five joints. Using a steel blade, this takes about one minute per limb-set (When you know what you’re doing). Fourteen minutes later a set of fourteen limbs is prepared for the gluing press. The fourteen limb-sets are set aside and the tools cleaned. PRESS PREPARATION: The limb presses are opened and the surfaces waxed to prevent sticking. A nonstick silicon paper is also applied to the flat top surface and the shaped (the back and face are not parallel in a limb) bottom b ottom surface. INSERT LIMB IN PRESS: A limbset is inserted into the press and the press is closed onto it. The layer alignment must be checked to be sure that they are directly on top of each other as the pressure can cause some slippage between layers. PRESSURIZATION: The top press-surface is now forced down onto the bottom using about 110 psi
of air pressure. HEATING: The press surfaces are
heated to about 185 degrees and held at that temperature for 30 minutes. This accelerates the chemical reaction that “cures” the epoxy. epoxy. The presses must be COOLING: allowed to cool to about 130 degrees before the limbs can be removed. During this time the next set of fourteen limb sets can have glue applied to them. REMOVAL: The limbs are
removed and allowed to cool to room temperature. At this point the limbs are ready to be machined to customer specs. REPEAT: Continue this process until 100 limbs have been assembled, pressed and cooled.
PART TWO: TWO : MACHINING AND SANDING Each edge of each limb must be belt-sandEDGE
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PREPARATION:
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ed to remove the excess glue that squeezes from the glue joints. SIDE CUT: One edge of each limb must be cut using a diamondhead cutting machine. When one edge is cut the limb is moved to a second position in the cutting fixture and a new limb is inserted into the first position. The cutting machine then cuts the second edge on the first limb and the first edge on the second limb. Repeat this process until all 100 limbs are completed. WHEE L SLOT AND HOLE: HOLE : A hole near the end of the wheel slot is drilled and a ½ inch wheel slot is cut into the tip-end of each limb using a double-bladed machine tool. The slot edges are given a finish cut using a second machine. AXLE HOLE: The axle hole is drilled through the tip end. The tip end is then sanded round relative to this axle hole. BUTT CUT: The butt end is sawcut to length relative to the axle hole position. LIMB BOLT SLOT: The slot for the limb bolt is cut into the proper butt-end position. SURFACE SANDING: All sur-
faces are belt-sanded using 100 grit papers. FIT CHECK: Each limb is fitted into the customer’s limb pocket sample. Only enough extra space is allowed to accommodate the finish yet to be applied by the customer at his facility. surf aces are BUFFING: All surfaces buffed with a flap buffing wheel to remove all rough edges. DEFLECTION CHECK: Each
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PART THREE: BARNSDA BARNSDALE LE CUSTOM LIMB FINISH Dave does the following to his custom made limbs: PAINT: Aircraft or auto urethane paint is applied to each limb. It is either clear or colored. WET SAND: The finish is wet sanded using 600 grit papers. SILKSCREEN: Any logo or other decoration is applied using a threecolor silkscreen process. TOOLING CHECK: The limb is again cleared through the tooling to be sure all openings meet specifications. CLEAR COAT: Three coats of clear finish are applied to each limb and wet sanded with 1500 grit paper.
9. SLOT SLOT CUT: One of the main steps in limb making is cutting the wheel wheel slot. The process is two-step, first the slot is rough cut then a second machine does the smoother, finish cut.
10. TIP SHAPING: SHAPING: Once the axle axle hole is drilled it is used as a reference to shape the limb tips. The process ends with sanding.
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limb is inserted into a deflectiontesting unit to determine its “spine” or strength of bend. This deflection rating value is recorded on each limb and the limbs stacked. Any limb that is too heavy will be sanded to lower its deflection value into the usable range. SHIPPING: The limbs are then prepared for shipping to the customer. The customer then decorates the limb with his own logo and applies the desired finishes.
11. BUFFING: BUFFING: The shaped limb is buffed on a flap wheel buffer.The smoothed limb is now ready to receive any paint or other coating.
POWER BUFF: The last step is
how far the limb bends before failpower buffing much like the auto ing. body shop does to your car. Rubbing All of these tests help manufaccompound and polish are used to turers build a reliable limb but nothgive the limb its great final appear- ing takes the place of putting them ance. on bows and sending them out to consumers to do their thing. That’s TESTING PROCESS: when you find out if limbs can take Manufacturers test their limbs the punishment of being run over by before putting them into produc- an ATV, dropped from a treestand or tion. The reason is obvious; they (this is a big one) left in a car trunk don’t want limb failure in the cus- for five, hot summer days. As a dealtomers’ hands. Limbs that come er you could probably add ten more back to the factory are no fun for to this list – you’ve seen lots of good manufacturers because they use up examples of creative ways customers any profit made on that bow sale and abuse their bows. also create a negative reputation that may be impossible to overcome. SPINE TESTING I remember some years ago that AND LIMB LIMB MA MATCH TCHING: ING: Xi/Indian Xi/Ind ian Industrie Indus triess tested teste d their So how does a pair of limbs get twenty-layer laminated limbs by selected to be on a given bow? The placing them in a dr y-firing machine process involves inserting the butt for 4000 to 5000 cycles at seventy end in a fixture and then bending b ending the pounds of draw weight. They did this limb. One of two methods is then at the Archery Trade Show for all to used to gauge the limb spine: see. I got several pair of those wellRecording the load-force needed tested limbs and shot them on my to bend the limb a fixed distance. target bows with good results – I figRecording the distance the limb ured they were just getting limber bends under a and ready to use. They were incredi- fixed load. bly strong. Both are Most companies do some kind done using arbiof dry-fire test as well as actual trary measuring shooting with a shooting machine. units that are They may also do a bend-test to find convenient for
the purpose and can be easily used to assign limbs to a particular draw weight weigh t requirement require ment for a given give n model bow. bow. Dave uses the second method to rate his limbs. His bending fixture holds the limb in place, a fixed weight is attached through the axle holes and the deflection distance is recorded using a number value between one and one hundred fifty. The higher numbers indicate more limb-bend and a weaker limb. The first method, recording the loadforce amount, assigns lower number values to the weaker limb. For most of Dave’s customers he says “the number system approximates about one-half pound of draw weight for each number unit on my scale.” Keep in mind that there are a lot of variables to take into account when bending a limb on a bow and this ½ to 1 ratio doesn’t always hold true. Limb length, mounting angle on the bow and cam design all affect the draw weight that is generated on a given bow model. So, to get a good
12. BEND TEST: TEST: Dave bends each each limb in a testing device device using a fixed weight force.The amount it bends is recorded on the buttend of the limb so it can be matched with another to make a working pair. Circle 195 on Response Card
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peak weight may be different because the limbs may bend a different distance – more bend distance (tip deflection) and the peak weight will increase, increase, less tip deflection and the peak weight will decrease. match to a customer’s bow Dave number making them as closely When dealers deale rs or consumers consu mers reports “ I have to send the customer matched as possible. For example, order new limbs or cams for a given several sample pairs for testing. my Pro Tec has a set of four limb bow the manufacturer wants to Based on the resulting experimental sticks that are all rated at #46. This know the spine value of the limb peak weight values I know what bow with the Wheel & ½ cams on will being replaced or the peak weight spine number will yield a given peak generate a maximum of 50 pounds desired. Then they must consult weight for that bow model.” model.” but with the Cam & ½ cams they information charts on limb spine On his own line of custom bows only make about 40 pounds. values and/or cam match ups in Dave prefers to place a slightly weakDifferent shaped cams bend the order to get the right limb or cam for er limb on the top. That means he limbs a different amount and, there- the bow in question. To get the places, for example, example, a #98 on the bot- fore, yield different peak weights. match up correct all the variables tom and a #98+ on the top so that the Limbs marked #56 (Hoyt uses the must be taken into consideration. top bends a pinch more than the first method for bend testing) seem CHECKING TIP bottom limb. For custom twin-cam to generate about 60 # peak weight finger-shooter bows he would place with the Cam & ½ setup on my Pro DEFLECTION AT YOUR SHOP: a #98 on the bottom and a #99 on top Tec. With a different cam the limbs I check static and dynamic tip to get the best shooting results. may generate a different peak deflection on all of my own bows Other companies may assign limb weight. This means that manufac- using a carpenter’s square. You can pairs in a different manner accord- turers don’t make a “70 pound limb”; do the same with any bow at your ing to their experience. they make limbs of different spine shop. The procedure is simple: On my Hoyt bows I have noticed value that generate 70 pound for a Place the long side of the square that the limbs have the same bend given cam. So, if you switch cams the along the flat back surface of the limb’s butt end. Measure the perpendicular distance from this surface to the axle Get Answers Fr ee at using the shorter side of the square. Arr c h er A ery y Tal alk k .co m The resulting measurement should be between 2 and 4 inches for most bows and is the static tip deflec•New AT AT Auction with no listing fees. tion. If you hold the bow at full draw in •Over 20 million hits per month. a draw rig (which is as simple as a 2x4 •Worlds largest archery website. board with a winch at one end) then •Sponsored by over 90 you can can also also measu measure re the the dynami dynamic c tip tip manufacturers. deflection in the same way. These New •Talk •T alk to fellow archers, pros, AUCTION A UCTION!! measurements typically fall into the and bowhunters worldwide. with Fre Free e Listing! range of 4 to 6 inches. In almost all cases I find the two Message Me ssage Board i ncludes: tip deflections to be within .1 inch of •General Archery Discussion each other. On rare occasions I have •Bowhunting Discussion found as much as .25 inch difference •Manufacturer’s Product in deflections but that’s been years Announcements ago. Those mismatched limbs didn’t •Traditional Archery group well for me and I replaced •Live Chat @ archerychat.com n e them. Today, however, manufactur Auction Listings Free Auction Free s l e ers are paying close attention to test i Archery Articles Free Archery Free N ing and matching limbs so this probFree Archery Reviews g Free Archery e lem is very rare. r Free Classifieds Free Classifieds G Through all the shoot testing y Free Picture Free Picture Galleries b and deflection measuring I agree o t with Dave about having the slightly o h archerytalk.com P stiffer limb on the bottom. My best © archeryhistory.com grouping bows have all had that trait archerytalkauctions.com so now I look for that in my bows.
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CUSTOM LIMB: After painting and three-color silk-screening the limb is clear coated. coated. Wet sanding brings the limb surface to its glossy final finish. These limbs contain carbon/bamboo laminations from Carl Lekavich that have high efficiency through quicker recovery on the power stroke.
EXOTIC LIMB MATERIALS: For the past fifteen years much has been publicized about the use of carbon in some of the laminate layers. The purpose of using carbon is to make the limb lighter in mass weight so that the limb has less of its own weight to move when it unbends from its full draw position. During the power stroke about 25 percent of the stored energy available is used to overcome friction and move the wheels, string, cables and limbs - the other 75 percent goes to moving the arrow. Therefore, if the limb itself has less mass weight then it will require less of the stored energy to move itself and more energy is available for the arrow. Also, if the material in the limb has a faster recovery recovery speed then even more energy is available to power the arrow. So, limb materials should have lighter weight and fast return properties. The most recent limbs that Barnsdale Archery is making contain laminate layers of carbon and bamboo to meet both objectives of light weigh and fast return. In my experience, they shoot super smooth.
being done by some really good bowyers. Paying close attention to the materials and the method used in limb making can help you better understand the materials in limbs and how to better guide customers to the various price levels of bows. Check these web sites for more information: BarnsdaleArchery.com GordonComposites.com Keep well, shoot straight. Larry Editor's Note: Larry Wise's first
book, “Tuning Your Compound Bow,” has been updated with a new chapter on hybrid cams plus other new information. His latest, "Core Archery" details correct form in a step-by-step format, defines back tension and how to execute it, and presents a plan for the high performance mental game. Get either though his web site www.la www.larrywi rrywise. se.com, com, or by phone phone at (717) 436-9168. They are also available from Target Communications, 7626 W. W. Donges Bay Bay Road, Mequon, Mequon, WI 53079. Larry Wise is available to conduct one-day and two-day shooting CONCLUSION: schools at your facility. This is a great I want to thank Dave for giving way to jump start start a league league program program,, to to me so much of his time while I was grow participation among your cusputting this article together. He also tomer base in competitive archery did most of the pictures. Learning and to help position your pro shop as more about the limb making process the place where people learn to shoot helps me understand what I’m see- better. better. Contact Contact Larry Wise at the ing when I look at all the bows on the address or phone number above for market as well as the custom work more details.
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