Why trap? Are there different ways to trap? What about Decrowning?

[willie]

A question raised in another thread made me think a bit about trapping.

what are the pros and cons?

are there different ways to do it?

how effective have you found it with self bows? and lam bows?

My experience is limited to high crowned natural back staves, but I do have some theories about things I have not tried.

My experience with sapling staves indicates any kind if imperfection at all along the peak of the crown can be fatal. should I try to decrown next?

Edit title to include decrowning

[bassman]

Not to many cons if you don't do it to the extremes which I have done ,and broke the back of the bow. You can use a file ,and eye ball it, or scribe a line on the back of the bow ,and follow the line. It is very effective with Black locust bows. Most bow woods are stronger in tension than compression thus belly heat treat ,and trap, but it is a balancing act.

[Pat B]

Trapping gives the bow limb a trapezoidal cross section favoring either the back or the belly. It can work especially well with selfbows and wood backed bows by increasing or decreasing stress on the limb by favoring the back or belly. Most woods are stronger in tension than in compression and by trapping in favor of the belly(wider) you can increase it's compression strength or at least add to it's durability.

[willie]

Not to many cons if you don't do it to the extremes which I have done

Bassman, did you file off the top of a small diameter stave (decrown), or slope the sides of a natural back, maybe from a larger diameter tree?

[bassman]

Do not touch the back of the bow. File from the sides,and if it is a high crowned stave it takes very little filing. With a flatter stave a little more trap. Black Locust I have trapped as much as 3/16's of an inch with a flatter back. If their is a crown 1/8. I Just broke  an Elm bow by taking to much off the back. It had one weak spot about 14 inches down on the top limb. Elm is strong ,and I was reducing weight from the sides, and got carried away.

[bownarra]

Saplings are trapping taken to an extreme and yes de-crowning any iffy saplings is a very good idea. Just make parallel lines on the back and you have essentially 'chased the ring' leaving a stronger back than if you left it alone.
Trapping is best utilized on 'normal' split staves.
As mentioned virtually all wood is stronger in tension.
You are trying to match the woods abilities to resist compression or its ability to resist tension. Like a childrens see-saw works best when you have two children of similar weights.
Doing my test of making a bow, pulling it far enough to induce some set then cutting in half along the neutral plane will really opern your eyes on this one. Do this test with bows that are made from the same wood. Trap one and leave the other normal. Once cut in half you will notice a drastic difference in what shape the bellies are.....
Best to do the tests yourself and come to your own conclusions with this one

[Tommy D]

Saplings are trapping taken to an extreme and yes de-crowning any iffy saplings is a very good idea. Just make parallel lines on the back and you have essentially 'chased the ring' leaving a stronger back than if you left it alone

Can you elaborate on what you mean by parallel lines? Do you mean if you take the crown off correctly you should  see the lines of the growth rings running the length of the back of the  limb?

[willie]

If I remember correctly from TBB, ring lines running parallel down the crowned back were observed in primitive bows. This should follow the grain well if there is no spiraling of the grain in the stave.

I have always been a bit curious why decrowning is not seen more often. Having a flatter but  somewhat wider back than a half round crown, would spread the tension stress evenly, and avoid concentrations at the peak of the crown.

[simson]

I'm no fan of decrowning, I always found other things to match a high crowned stave. When the crown is homogeneous, I prefer a HLD (hollow limb design).
Trapping is good explained above by Pat and others. We should have in mind that the most change of woodcells is on the belly side. In other words the springiness of bow is done by the recoverable compression of the belly cells. On the back the cells are of course stressed also, but the stretch is minimal. The cross section of an ELB is a kind of trapping and only a few woods can handle that high stressed belly.
just my thoughts ...

[Selfbowman]

Pat and I are on the same page on this one! Arvin

[willie]

Pat and I are on the same page on this one! Arvin

Hey Arvin, Pat B, Simson, Bassman and all,

Pats explanation is right on, It is the most commonly understood meaning of the term "trap".
 I was hoping to enlarge the discussion some and include decrowning and other methods of making the back narrower.

With the primitive bows as an example, how come decrowning is not done more?  People just haven't tried it yet?,  or are there some things to learn if you want to do it successfully? Please share you experience if you have used this to work small diameter staves.

[Pat B]

I've only decrowned one or 2 bows in many years of making wood bows. I never found a reason to do so and I initially learned a pristine back was a must for a selfbow and it's hard to break old habits. I know that decrowning is an option but it isn't something I consider when planning a build.

[bradsmith2010]

the reason its not done more,decrowning, is its usually not needed,, theoriticly it would do as you say, but the real results not giving much advantage if any,, and probably takes alot of skill to do on most bows,, unless back is pristine,,no knots etc,,
I think you would be ok doing it if you gonna rawhide back or sinew back,, but not gonna see a signifigant increase in performance or durability ,,,thats why you dont see it done much,, yes we have tried it,, not much we have not tried,, or talked someone else into trying,,
  you should definitely try it to make your own conclusion,,

[Selfbowman]

I use very clean wide staves for the most part. But I will make the back as flat as possible yet not go thru the back growth ring. That is why I prefer 1/8 thick or more for my back ring. I am a flat bow guy.
But if you have round back staves  they will make successful bows. Usually take a bit more set though.
Arvin

[DC]

I've never decrowned a stave. Chicken mostly. It just seems like asking for trouble. I have lost a couple of OS bows that popped a splinter at a pin right on the crown.They may have survived if I had decrowned them but I really don't think they would have survived the decrowning. For wood like Yew that can take ring violations then I can see it being a viable option.