Hi, I was wondering if anyone could explain to me exactly wha treverse trapezoid limbs are?

In cross section, the limb is wider on the back, narrower on the belly.

Jeff: A reverse trap is wider on the belly and narrow on the back.

Bob

There you have it....

What's the advantage of a reverse trap? Is it used primarily on longbows? Also, what bowyers use the reverse trap?

Thanks,

Hank

Jeff isn't the brightest crayon in the box, so I'm going with Bob on this one.

My good friend Tim Meigs told me many years ago that the reverse trap was actually more efficient and had proven it to himself by making both versions the same weight and testing them.

Bob

The advantage of the geometric trapezoid limb, as I see it, is that it is very resistant to limb-twist. Also, on a self-wood bow, most limb failures occur on the belly side, therefore, there is less chance of limb breakage when the narrow side is on the back.

(A great book for any serious bow-maker is: "Archery: The Technical Side" by Hickman, Nagler, and Klopsteg." Its a bit hard to find, but it's well worth the effort.)

Some bowyers use thicker glass on the belly for the same reason: the belly compresses and the back stretches. They believe more strength in the belly makes the limbs push or work harder to move forward.

Steve Turay of Northern Mist and Jim in Oregon builds/built bows in this manner.

There are no comparisons between glass and self bows in regards to trapping and why you choose to do it.

Pearl, you are holding back. Why would a bowyer use a trapezoidal crossection on a self bow and why else on a fiberglass bow?

Tim Baker explained that a wood selfbow was limited by compression strength so he could remove some sectional area in the tension zone to balance the bows tension and compression strength; hence a narrower width on the back. The limbs were lighter so a little faster.

BW does the reverse Trap

Black Widow and Scorpion use reverse trap. Wider on the belly. I was told the bow's thrust comes from compression of the limb material so they made them wider on the belly. Also they are more stable that way.

In regards to wood species that are weaker in tension strength, virtually all the white woods, they fair much better with a trapped back and a flat, tempered belly. It evens up the tug-o-war between tension and compression. Another example, I just finished a bamboo backed osage R/D and I left the damn boo too thick off my joiner, rookie error. I trapped the boo back in order to keep more osage on the belly. Without trapping it would have been a burn barrel candidate.

Bob , OCR, and Pearl are right. A trapped limb is narrower on the back ,and wider on the belly. It is the only way I could get Black Locust to work out for me when making a bow from that wood. If reverse trap is the opposite of that, then the belly would be narrower, and the back wider. Some long bows are made with a rounded belly, so that may be an example of reverse trap.

I just realized I typed white woods weaker in tension, I meant compression. Trapping the back equalizes the weaker belly and the stronger back.

Im no glass guy, not even close. But, to me the only advantage of trapping a glass bow would be to eliminate mass weight and/or reduce draw weight without touching the sides of the bow.

Gregg at Javaman Archery regularly reverse traps his longbows and recurves.

My Liberty Chief is reverse trapped.

I've read it as "trapped to the back" or "trapped to the belly".

Glass bows can sometimes benefit from trapping the back. They are subject to the same possibility of the back Tension overcoming the belly especially in a thick cored ASL where the glass is a lesser percentage of the total stack. The result is the limb takes set. It's not the glass that takes set but the wood directly under the glass. Trapping to the back better balances the tension/compression forces just as in a self bow.

My MR Hamilton Scorpion is reverse trap.When developing the Scorpion MR made three versions of each refinement as he went along.Trap to the back,belly and no trap.The reverse trap design performed best in his testing every time.

Bernie, did MR happen to mention how much better performance was?

Isn't this also done on some weight reduction processes? I thought I might have seen a time or two.

JD Berry trapped their limbs. So did Big Horn on LB's if I recall.

Yes, it is used for weight reduction.

I understand the rationale for trapping toward the back, and think I agree with it. However the jury is still out. The J.D Berry bows I have are either parallel sided or trapped toward the belly. The Great Northern Critter Gitter I own is trapped toward the belly. These aren't weight reduced bows, they were made that way.

There's obviously not universal agreement among builders. I'd like to hear the reasoning behind trapping toward the belly on glass laminated bows from those who build bows that way.

Like a Northern Mist longbow

Orion, I built a few that way early on in my bow making. My reasoning was that any glass wood composite bow that I saw where the glass failed from fatigue, it was the glass on the back of the limb that failed, never the belly glass.

The root cause may have been a design and/or construction issue not related to the glass per se, and most, if not all, were recurves, but regardless, in that over-strained area, it was the glass on the back that failed in tension resistance, never the glass on the belly failing in compression resistance. I'm not saying it 'never happens', just recounting what I've experienced. So I figured, allowing more area on the back(making it wider) than the belly would better balance the respective abilities of the glass pieces in those locations.

I never bothered to trap a glass/wood recurve because they're just so thin relative to their width that I didn't think it would have any substantive affect.

And, I've since changed how I shape longbows and have made the majority of glass/wood longbows trapped with narrower backs/wider bellies. The reason I changed to 'trapped to the back' is, I never had a single glass failure issue on my longbows, much prefer the look, some of the reasoning it's done that way in bows of natural material still seems relevant to me in glass/wood composites, and they 'seem' more efficient. I say 'seem' because I haven't done any scientific-type studies, just observations.

All other types of bows that I shape trapezoidal in cross section, I have always made the back narrower because the material on the back is stronger, sometimes considerably, in its tension resistance than the belly material is in compression resistance.

I do not trap the limbs 'for weight reduction' during construction. If it's a bow that benefits from trapping and I want to, I trap it. And that loss of a few pounds is figured into the stack height before the lams are ground.

From Traditional Bowyers Encyclopedia by Dan Bertalan, in a discussion with M.R. Hamilton, maker of Scorpian Longbows a few decades back.

Another feature of M.R.'s longbow design is his unique modified trapezoid limb tapering to the back. Most bowmakers taper their limbs to the belly or leave them relatively square edeged. But not M.R. “When I was experimenting with core woods, I built three identical bows at a time. I fashioned one square limbed, the second tapered to the belly, and the third trapezoidal to the back. In the marines electronics program, we had a lot of good systems for testing my bows. I discovered that the taper to the belly had the least cast, the one with square limbs was second, and the one trapezoidal to the back had the farthest.

“I shape my limb to trapezoidal tapering to the back because in physics you learn that the only force that moves something is the force from behind, a push. The force that moves a limb forward is the push from behind, the compressed belly material. By slightly tapering a limb to the back you increase the compressional portion and decrease the stretchable portion, improving limb performance. I've even shown this on recurves and compounds. The reason most people build longbows tapering to the belly is because that's the way the old English used to build self-wood bows. But with today's glues and fiberglass we can taper a limb to a trapezoid in either direction. However, my trapezoidal shape is very slight. If you shape a trapezoidal limb too much, you start developing problems with limb twist and breakage.”

From Traditional Bowyer, More Unnecessary Fun, by Jack Harrison

“Limb Profile

The back side of a selfbow not backed with any rawhide, sinew or manmade fibers is much stronger than the belly side, providing there are not latent anomalies or surface defects in the bow itself. Similarly, any FRP-laminated longbow has much more strength on the back side compared to the belly side. This is because most bow materials are much stronger in tension than they are in compression when stressed by bending. This is important to remember when designing and building bows. Knowing this, the belly should be wider than the back in order to compensate for these stress differences, belly to back. Plus, a wider belly distributes force over broader surface area of distribution for compression reducing the possibility of failure because there is more mass to handle the stress.

Many people disagree with me on this because they feel the FRP backing materials used on modern composite bows have excess tensile and compressive strengths – more than enough to compensate for any difference in stress between the back and belly of the limbs. This may seem valid if the FRPs were the only components of a bow limb. Most FRP backed bows till have cores made with natural material of wood and bamboo. Even with an FRP laminated on the back and the belly, the bow limb appears to follow similar behavior found in bows which are not backed FRPs. The cores of these bows may still fail when over-drawn, regardless of what is laminated on the outside surfaces. Limb failure tests suggest FRP-backed bows have cores affected by stress similar to those selfbow discussed above which are not backed with anything. I discovered if I laminated a thin FRP laminate of carbon on the back of a longbow limb, the stress between the back and belly balanced better with a blend of carbon and fiberglass on the opposite belly side, as opposed to any other combination of FRP used both within and on the outside surfaces of my longbow limb designs. I tried every conceivable combination between fiberglass and carbon in constructing my bow limbs, and I checked the results with a chronograph and the 838 IRD analyzer. (The 838 IRD analyzer used a transducer to measure handshock). I built hundreds of bows to compare the results, not just a dozen or so.

If there is a wider belly in a bow limb, the back must be narrower. By narrowing the back, excess mass is removed. By removing excess mass in the back of a limb, the recovery rate or elasticity will increase even more, and the bow limb will have a higher recovery rate. When designed to have a higher recovery rate, the limb will be more efficient and cast arrows more efficiently. Therefore, I taper my bow limbs to the back about 30%, which produced a trapezoidal shape. The limb thickness was also tapered from butt to the tip. In addition, my bow limbs were tapered in width from butt to tip. There three tapers reduced excess mass and increased limb elasticity(coefficient of restitution).”

Coefficient of restitution? I don't mind buying a book, but do you know what a copy of the 'Traditional Bowyer, More Unnecessary Fun' is worth?

Yeah Frank. That's what I thought.

I'm just relaying their thoughts on the matter. There's plenty I disagree with in there as well... and in Jack's book too.

'Coefficient of restitution' sounds like a government scam... like how the left is going to figure out how much each of us has to pay in 'reparations'.

JustinB No he just said it had better cast as you can see in Jeff Durnell' post.

So whats actually the short answer and man could get TP reading all this.LOL

Heck, Cat, if there were short answers there wouldn't be a leatherwall. 8^)

Here's my short answer...

On most of my bows, I'm going to stick with a trapezoid shape that makes the back narrower than the belly. Call it trapped, or reverse trapped, or whatever flats your boat. I just call it trapped. It makes the most sense with single or combinations of materials, is the most often purported to offer benefits, and seems to offer slightly better performance and aestetics, in my experience.

But I haven't done any serious scientific testing, so I don't expect anyone to agree with me just because I said so, so.... like most anything related to bow design and construction... I say... try it/test it yourself. Buy em and try em, or make a few of each and find out for yourself. Yep, that's my 'short' answer.

Don't ask. Do.

Well I guess this about clears it up.....

Eggs-actly.

Well, Jeff you haven't changed my mind. Probably cause I agreed with you from the start. 8^)

Jason Kendall traps the limbs of his Kanati to the back also, and you'd be hard pressed to find a better performer.

Trapezious contradendo

Reverse trapping is best used with woods that are especially stronger in tension than compression, elm and hickory for example. Limbs are a touch lighter but the main benefit is reduced string follow, the back no longer overpowering the belly. Most woods are stronger in tension than compression, but bows only break on their backs, so trapping can be dangerous, especially for less tension-strong woods, like black cherry, for example, which can even benefit from trapping the belly. 

I thought a trapped back was narrow on the back, wider on the belly so that a particular wood wouldn’t overpower the belly. So wouldn’t a reverse trap be wider on the back?

I just finished this BBO and I was calling it a trapped back. So should I be calling it a trapped belly??

Mr. Tim Baker, or others. I am by coincidence, building a Bamboo backed Black Cherry bow from lumber off my property. Should I narrow the back? Or what profile should I make the limbs. I just glued it up yesterday. So great timing.

Tim, should I use this wider limb taper for the Black Cherry.

Cool looking bow Jay.

Thanks Randog, it’s hard to show the trap in a photo

My Mahantango Red Tail Classic is reverse trapped. Jim told me he got the idea from Steve Turray. Something about limbs being lighter/faster with the narrower back.

Yes, Jim traps the back of his Red Tail and his Kestrel d\r.

I think you are going to find very little difference in a glass bows performance trapping the length of the limbs in either direction. Trapping the limbs tips down 6" will reduce mass weight and help you.

Most of your bows performance is negated with the limbs design itself. The geometry , or limb shape,the taper rates used in the lams, and type of wedges and location of the working portion of the limb, are all huge factors for how a limb stores energy, and the resulting string angle at full draw....

But.... Storing energy, and transfering that energy to the shaft are two different things....the key elements to a good transfer to the arrow shaft are limb travel, limb tip mass weight, and enough preload , or string tension at brace height to stop those limbs dead.... THAT is the most important part of a bows efficiency..... If everything is balanced properly, and the preload is right. No more hand shock...no more noise...and much better overall performance is the result.

Note* I do not build all wood bows or self bows.But I think these trapping techniques could be used effectively depending on wood types used. Kirk

I have a question. Would trapping the back of all Bamboo longbow limbs be different from other woods used in the limb construction? Also would trapping the belly side be more effective for weight reduction and speed?

It is way to technical for me to figure out & takes too much testing. It is trying to balance the tension & compression of the back and belly. Not only wood species but the thickness you have made back & belly laminations. Trapping,wood species,lamination thickness,and glass thickness can all be used as all come into play. Achieving the perfect balance may best be left to folks that have the budget & equipment for testing or following tried & true plans. >>>----> Ken

In bow making jargon a trapped bow limb is narrower on the back than the belly. The reverse of that, and call it what ever you want is counter productive with wood limb bows since the tension side is most always stronger than the compression side, but their are some compression woods out their.

2 bears thanks for the reply. I think I best leave things as they are.

Sent my Black Widow SAV Ironwood to Widow to have it refinished and previous owners name removed, and requested weight reduction... When I received it back they said they trapezoided the limbs to reduce the weight... The limbs are narrower on the back than the belly.... That was Black Windows inturpretition of trapezoid...

Seems as most the longbows being trapped in the 90s were trapped with a narrower Belly.J.D Berry's,Great Northern,Frederick longbows etc.I think Robertson Stykbows started trapping the oppisite way back then

Then saw more start trapping the other way to the back.Widow for one and a few others

Thier is not much trapping to be done on a recurve.Thin limb.On a narrow deep core recurve you can get more material off with the trapezoid.

They can look nice and and sleek.I Had 2 Berry's with all Yew limbs that were trapped alot more than most you will see.He then rounded off the trap.

That made for a handsome looking limb when viewed from the belly or the side in profile.Most all trappezoid limbs are squared off.

to me it feels like a narrowed back on a glass bow potentially leaves the core wood unsupported on the back edges. Depending on the grain that seems like a bit of a risk.

If you have already registered, please

sign in now

For new registrations

Click Here