Limb chamfer

[JRHunter]

I am building my 2nd bow and have been looking at alot of bows since I built the first one. I am building a R/D longbow. I have seen alot of bows that have a chamfer on the edge of the limbs tapering from front to back. Some bows have no chamfer only radius edges, some with small chamfer and some with larger chamfer. Why the chamfer, is this used to reduce draw weight and achieve targeted weight, or is there some advantage for the chamfer? Just learning and woould like to know why the chamfer.
Thanks

[strungstick]

I'm assuming that by Chamfer you mean "trapping" the limb.  Trapping is removing more material either on the back or belly side limb edge which leaves a taper of sorts one direction or the other.  This leaves a narrower back or belly side of the limb depending on which direction the limb is trapped.
When you start talking about limb trapping and whether to trap to the belly or back you will get all kinds of answers as to which is more advantages and some will tell you neither.  Some say it adds to limb stability, some will say it reduces mass and increases speed, and some will tell you it's done just to bring a bow into the desired draw weight.
My opinion is that limb trapping, by itself, doesn't change the bows performance a great deal but it can be a design feature that has a cumulative affect depending on what your goal is.  
That's prabably as clear as mud, but do an internet search on trapping a bow limb and see how many items pop up with just as many opinions on what, if anything, it does for performance.

[Glunt]

I've trapped a few to make weight, but haven't built enough similar bows to see what the effects are.  "Trapping" comes from trapezoid.  It does look cool, but takes some practice to keep the lines nice and straight.  As strungstick said, you will hear alot of theories on belly vs back, what angle, and how it effects performance.

[kennym]

I asked a pretty well respected bowyer about trapping and he said...


Pretty much what these guys did!

Looks cool, but couldn't see any performance diff!

[kentowl]

On wood-backed bows, one purpose of trapping is to keep a stronger wood from overpowering the weaker wood. If you put a really strong tension wood like hickory on black cherry, for example, the cherry will give up in compression (forming chrysals) long before the hickory is strained. In this case you've got much more strength in your hickory back than necessary.

You need to know that most of the work is done by the belly or back surface, since they are farthest from the neutral line in between (not much happens to the wood on the neutral line - balanced between stretch and compression). When the bow bends, the back surface stretches, and the belly surface compresses. The thickness of the limb, the amount of bend, and the relative strengths of the belly and back wood determine how much stretch and compression takes place -the characteristic of each wood determines whether it will break under that much load.

One way to equalize the strengths is to narrow the stronger surface - that's done by "trapping" or chamfering the edges so the working surface is narrower. The side benefit is that you've removed wood that wasn't doing you any good - gaining efficiency.

[JRHunter]

Thanks for the input. I understand much better. I did a search on chamfer and came up with nothing. I did not know it was called trapping. I also searched trapping and read most of the threads. Interesting concept. There is alot more to it than initially meets the eye. I understand the need for it based on the different types of wood. I am looking at it to reduce some weight.
JR

[Gundog68]

I once trapped a standard longbow (internature viper deluxe). Its weight goes from 42# down to 38#. The performance (fps) was the same with the same  arrow and less vibration.

[fujimo]

gundog did you trap the belly or back- to reduce weight?

[fujimo]

ttt

[wookiewebster]

Can anyone post an image of a bow(s) with this feature?  I am curious about what it looks like.

[Pat B]

It is camphered from back to belly or vice versa and not from front to back! d;^)
  In selfbows and other wood bow constructions trapping is used to releave tension strength of the bows back if it has a compression weak wood belly or has a backing of bamboo that can overpower the belly.

[Swissbow]

The theory is, when trapping towards the back, that you need less glass on the back than on the belly, since glass can take more stress in tension than in compression. Therefore you can build the limbs lighter in mass and get the same draw weight. Less mass means more speed.
I have two bows with the same profile and the same length, one trapped towards the back and the other the opposite way. The one that is trapped towards the back has like 5lbs less draw weight but they shoot about the same speed. Now i haven't used exactly the same wood on both bows and the stack isn't the same, so I don't know if we can compare like that.
And if it's not true at least it looks kind a cool.

By the way the limbs of Black Widow bows are trapped this way.

----------
Andy

[2treks]

This is a JD Berry Bow that I just refinished,so the pics are handy and he has about the most severe trapp that I can think of.
This bow is trapped to the belly.As you look at this bows belly,you can see all of the core laminations going down the sides. Or an easier way would be to say that the belly is narrower than the back.
I trap the back of the bow,but not as heavy as this. Mostly toward the tips.


 

[Gundog68]

Originally posted by fujimo:
gundog did you trap the belly or back- to reduce weight?

I trapped the back.