Trapezoid?

[pdk25]

This may be a dumb question, but what it the theory behind the trapezoid configuration used on the limbs of some bows?

For example, I have two different longbows that have essentially reverse trapezoid configurations.  My RER TD Vortex is tapered so that the belly is more narrow than the back and my Griffin is tapered so that the back is more narrow than the belly.

What characteristics do these configurations impart to the bow?  The bows are very dissimilar in other facets, so it is difficult to compare smoothness, speed, etc.. from my end.  Incidentally, I really like both bows.  I just am curious about this from a design aspect.

[Running Buck]

I was told long ago trapezoidal limbs were a way to control poundage. I know Black Widow uses this limb configuration on the longbows they make, Wes Wallace does also but not as pronounced. You might get a better answer by contacting a bowyer.

[BernieH]

In a rectangular limb cross section both halves of the limb are equally stressed although one half is in compression while the other is in tension.

In a trapped limb, the wide side is under less stress while the the narrow side is under more. This would be most useful in limbs having different materials on the back and belly of the limb or when using a material that can take more tension than compression.  In the case of using a material that is stronger in tension, you would trap the limb such that the narrow side is on the back of the limb(tension) and the wide side on the belly (compression).

[pdk25]

BernieH, Is this just for strength then?  Or does this improve other characteristics of the bow.  I seem to remember reading that Byron Ferguson designed his bows with a trapezoid taper on both back and belly.  Why would a design like this be employed?

[Bullfrog 1]

I guess it is different theories by different bowyers. I have a Dwyer that is wider on the target side. Some, like Wallace or BW are wider on the shooter side.   BILL

[Hornseeker]

There are very few hard facts/findings to back up the use of "trapping" one way or another. Some have claimed that a belly trapped bow will pop an arrow out a tad quicker than a rectangular or back trapped bow...

I'm sure Black Widow has there reasons...

[BernieH]

Its to take advantage of a materials ability to take either more tension or compression stress. An all wood bow made of a a wood that is better suited for tension would be best made with a narrow back and a wide belly. That way more of the limb material would be in tension stress and less in compression stress. What trapping does is move what's referred to (engineering jargon)as the "neutral axis" away from the center. Theoretically the "neutral axis" has zero stress.

You would want to trap a limb having a fiberglass lam in the middle so the fiberglass does its share of the "work".  If you don't the fiberglass "sees" zero stress, does no work, and actually makes the limb mass a little heavier thereby reducing its performance.

If you had a rectangular cross section limb with same thickness glass on both sides you would most likely not gain a thing by trapping.

If you had different glass thicknesses on back versus belly side, then it may make sense to trap the limb to take advantage of the thicker glass' ability to take more stress.

What I've explained about the cross section and neutral axis is engineering fact. Whether bowyers are able to take full advantage of that to produce more bow performance is a different matter.

[Andy Lupher]

fiberglass is stronger in compression then tension. you can even out the differences and extend the neutral zone by trapping with the back narrower then the belly.  I think this may be in Klopsteg and Nagler reprinted in Technical aspects of archery which was reprinted by Glenn St. Charles.or you can find it in Jack Harrisons book.

as a disclaimer... Dan  Bertilans book opposing views from different bowyers..

Maybe somebody who has built more bows can chime in???