Trapping Glass Laminate Bows

[canopyboy]

Bowyer question that started to bother me once I looked at the different bows at the Hill hog hunt last weekend. (Skip the big paragraphs if you want me to get to the point.)

Trapping the limbs -- doing it on wooden bows means reducing the width of the bow on the back to make a trapezoidal cross section. The idea is that wood is stronger in tension than compression. A rectangular section has a neutral axis at the mid point, putting the back and belly fibers at the same stress, even though the belly will start to fail at a lower stress. Reducing the back starts to move the neutral axis towards the belly, which increases the stress at the back fibers compared to the belly fibers. This brings those stresses into better balance relative to their respective limits.

In short, trapping the back gives you a better balanced and more efficient bow limb when the compression strength is less than the tension strength.  Now I noticed that a couple of the Hill bows were significantly trapped (back) in the same manner.

However, in a fiberglass bow, the majority of the stress is carried in the glass as it is the outer fiber and has a higher modulus of elasticity than cane or wood. Fiberglass in general actually has the opposite problem compared to wood. A highly oriented type we have researched at work has the following properties and is indicative with the trends of all glass composites. This particular type has a yield strength of 52,000 psi in tention, but 59,200 psi in compression. Looking through a bunch more samples and data that 10-15% increase in compression strength is the standard. (Carbon however, is the opposite, with tensile strength being about 50% greater than compressive.)

So theoretically, a glass lam bow should have a trapped belly if anything for improved efficiency based on engineering principles. The effect of the wood core may negate this slightly, so I could see an argument that a rectangular cross section would be close to optimum.

So my question is, why would anyone trap the back of a glass laminated bow? The only reason I can come up with is it works with wood, so people feel it must help with glass.

[monterey]

Originally posted by canopyboy:
So my question is, why would anyone trap the back of a glass laminated bow? The only reason I can come up with is it works with wood, so people feel it must help with glass.

Maybe to reduce overall draw weight without tampering with the frontal profile?

Maybe a differential trapping to alter the tiller (bend) of the limbs?

[Sixby]

I agree in theory. In answer to the why. Stability. As the back is slightly weakened the belly makes the bow limb more stable. It is the base. Therefore the limb becomes less prone to twist if the belly is stronger than the back.
In a recurve trapping is only effective if the recurve has a deep core stack and narrow limb.

Having trapped both to the belly and to the back on deep core longbows including heavy d and r bows I have found that trap to the back is an effective way to lose poundage quick with little stock removal and actually produces a more stable narrow limb. However you can remove a lot of belly glass in a belly trap and not reduce much weight. Its all a matter of what the bowyer wishes to achieve and how good his knowledge is of what he is doing.
Interesting subject and I am sure in a lot of minds controversial.
God bless you, Steve

[canopyboy]

Steve,

Stability is an argument I will buy, I'll need to think through the physics of it a little but my intuition follows your stability with a broader base idea.

BTW, I grew up in Grants Pass. Maybe next time I go home for a visit I could stop by?

-Dave

[2treks]

Dang Steve, Thats what I was gonna say! Lol

Another variable is glass thickness. Depending on what I want to do with a given design or limb, I will vary my glass thickness back and belly or taper it.
Another reason is Aero dynamics. It can reduce the wind resistance a great deal and lighten the tips.
CTT

[heartlandbowyer]

I've only trapped one bow, I was just more or less experimenting with the bow. It was a 58" mild r/d  bow 54@28, I also at the same time  took a little material of the tips. I was surprised on how much vibration trapping and tip clean up  took out of the bow. I did loose 3.5 lb of draw weight with just under 1/8 trap.

I didn't shoot the bow through a chrono before so I don't know if I gained any performance but it is definitely more shootable with lower vibration.

[Swissbow]

I'm a bit confused, because if I check the data sheet of Gordon UL glass I can see that it's approx. 1.3 - 1.5 times stronger in tensile strength than in compression strength.
I agree that trapping to the back takes out a lot of vibration and the limbs stay stable, while when trapping to the belly one has to remove a lot of material to drop the weight.
I have never build identical bows one trapped to the belly and one to the back, so I can't tell which one is more effective. To me it seems that trapping to the back gives a better overall performance. But as Sixby said this is a very controversial topic and there are probably as much opinions as there are bowyers.

----------
Andy

[mcbowguy]

Jack Harrisons book talks about this topic in great detail.  He believes (as do I) that trapping the back for stability is best.  He discusses what he calls the transition zone or nuetral plane within the limb. That is the area in the limb core between the back and belly. He says part of the core stretches (backs side) and part is under compression (belly side). Core material cannot do both at the same time, so as I understand it, there is an area in the limb that does neither, hence nuetral. When the bow is braced this zone makes up say the majority of the middle part of the core. When the bow is drawn back, that zone gets much smaller and shrinks to the middle of the core. The limb uses more of the core fibers as more are needed to be stretched or compressed. His idea is that because the belly is being compressed, it is under more strain than the back, so the back needs less material. Trap away that material to reduce mass and increase speed. Seems to make sense, however, there are great bowyer (Hill and Great Northern to name a two) that do the opposite.

[MoeM]

 Seems to make sense, however, there are great bowyer (Hill and Great Northern to name a two) that do the opposite.

A usuall LB has no big problems with stability as a recurve has so reasons for the trapping are different ones.
Another point for me is that using thinner glass on belly can even reduce same weight as trapping would do while using the better stress-than-compression abilities of the glass AND keeping better stability!

[canopyboy]

Originally posted by Swissbow:
I'm a bit confused, because if I check the data sheet of Gordon UL glass I can see that it's approx. 1.3 - 1.5 times stronger in tensile strength than in compression strength.

I'll have to check out their data sheets.  I was just looking at all the glass composites we work with or test here at work and talking to the structures phds.  It may be that certain layups and matrix materials (resins) can switch things around.

[canopyboy]

Yup, you're right Andy.  Their spec sheets not only list tensile as higher, but their numbers in ksi are quite a bit higher than the stuff we have been working with. I'm guessing the prepreg laminates are just in a different world than stuff that gets hand laid or vacuum molded.

Still this discussion has been enlightening, thanks for the input!

[Dan Bonner]

I agree with Sixby. Trap the back if you are way overweight. You can cut weight quick and it tends keep the limb stable. I think if you are trying to maximize performance there is a strength vs mass advantage to be had from trapping towards the belly. You can remove mass in the moving portion of the limb without losing much draw weight thus maximizing cast. And to Heartlandbowyer's point. I have found that reducing the tips as much as possible in mass without sacrificing stability will make a huge improvement in vibration level. It sux though when you reduce them beyond stability. That's when a 64" bow becomes a 62" bow and a retailer job ensues.

Bonner

[Dan Bonner]

I hate autocorrect. That should have said re-tiller job in the last sentence.

[Sixby]

I've trapped both way and done it with lots of bows; I actually argued against Jack Harrison until I found out that he was right. I read the book and thought, LOL he doesn'tknow what he is talking about. Then a bow I had trapped to the belly developed a limb problem and I started doing some real testing. I have never since lost stability in a trapped limb.

There was even a time when I trapped the first third of the limb to the back and the rest of the limb to the belly LOL.

You name it and I done that;. Sounds like Bro Dan been there too.

Chuck, You are spot on IMHO.

David, Coffees on bro. Stop in any time


God bless you all, Steve

[shade mt]

i don't have the experience some of you have. But i have tried both way's on a number of bows. I have pretty much settled on the fact that by gluing in a tad overweight and reducing the back to bring in into weight, and slimming down my tips as little as possible i achieve better performance with less shock..as far as the psi tension and compression figures i haven't a clue but iv'e found it most certainly works on my R/D bows.