The topic of handle shaping and tapers comes up a lot especially when dealing with broken bows, so I figured I'd talk a bit on the basics of PVC bow handles and bow tapers.
Like +Bill Anderson
mentioned earlier, a circle is one of the strongest shapes. It is equally strong at all points. A round PVC pipe is therefore strong in all directions. This means it takes the same amount of force to flex it backwards, forwards, and sideways.
Flattening or squishing the pipe causes the strength of the pipe to change. The pipe becomes weaker along the area that is made thinner and stronger in the area that is made thicker. The thinner area is weaker or less stiff than a round pipe. The thicker area is stronger or more stiff than a round pipe.
Basically when you taper a pipe, you make it weaker front to back but make it stronger from side to side. This is why tapered bows are more stable than bows that are fully round, especially when recurves are involved.
Shaping the handle has the same effect. It makes the bow stronger from front to back while making it weaker from side to side. The trick is shaping the handle enough to stiffen the handle and drive stress away from it, while keeping it thick enough to keep from flexing sideways and folding over.
To understand why you would want to shape the pipe, it helps to understand what the taper does for a bow. A PVC pipe is uniform from end to end. Because it is round, it is equally strong in all directions. String up a piece of pipe and it will primarily bend through the middle and straighten out at it reaches the string nocks.
This bow is highly stressed near the center. In a bow like this, most of the work is being done by the handle section with the limbs doing very little work at all. The result is a bow with a high draw weight that is also very slow and inefficient. The bow cannot store as much energy and the force-draw curve is usually quite steep near the end. The outer limbs also do no work and slow the bow down. The hand shock on a bow like this is fierce.
The biggest problem with a bow like this is the amount of stress in the center. The higher the stress, the heavier the draw weight and the higher the chances of a bow breaking. A bow like this is likely to fold in the center or snap a little out from the handle. The main break points are where energy feeds back, not directly on the most stressed areas. The bow could also snap right in the middle of the high-stress area if the stress is too high (bow too short, temperature fluctuations, etc). Ideally, the center of a bow should be its strongest point. In this case, the tips of the bow are its strongest points.
Tapering a PVC bow from center to both limb tips distributes the stress across both limbs. Basically the main areas of stress are near the mid-limbs. The very center is actually under very little stress and the load is distributed along the bow. This results in a fairly efficient bow that draws less weight than if it were fully round. The arrow pass is also slightly wider than a round handle, but the bow overall is rock solid and stable. Bows like this also bend slightly in the middle and can have some handshock to them. The way they bend is similar to an English or Welsh longbow.
Tapering a PVC bow at the handle marks and leaving the center round creates an interesting problem. First off, it creates an area of stress in the center of the bow. There is also less taper overall and more stress in general, leading to higher draw weights and slightly faster speeds. Normally something like this would be a good thing, but in the case of a PVC bow with nothing in the handle, this creates a weak spot near the center. Chances of the bow snapping clean in the middle is unlikely, but a bow like this is more likely to snap right above or below the center of the handle.
The reason why this causes problems is because the round handle acts like a non-tapered bow. Because the taper starts beyond the handle, stress builds up in the center due to the missing or broken taper. In a lot of ways it's like taking an English longbow and then removing material from the very center of the handle. You're weakening it there and making it more likely to break. The center of the bow should be its strongest point but in this case the strongest points are where the taper starts, resulting in a weaker handle.
A bow that is tapered at the handle marks and then shaped so that the handle is slightly taller compensates for the missing taper. Because the handle is pressed, the taper is allowed to continue into the handle. This results in a bow that has the benefits of a round handled bow but without the extra strain in the handle. A bow like this generally is under even less strain in the handle and flexes less in the handle. Handshock is reduced.
The downside to shaping the handle is that you gain extra handle rigidity at the cost of stability. A shaped handle is going to be less stable than a round or flattened handle. The trick is shaping the handle enough to continue the taper and allow for less arrow deflection while at the same time keeping it thick enough to prevent the bow from twisting or flexing sideways. The more aggressive your taper, the more stability you end up with.
An aggressive taper is one that starts out shorter. So a non-aggressive taper would be like starting out at 1 inch for ¾ inch pipe. An aggressive taper would be starting out at ¾ inches or less for a ¾ inch pipe.
If you really like a round handle, the best way to do it is to start out with a more aggressive taper, or start the taper in the center of the bow. Once the handle is puffed back out to round, the taper is not interrupted or broken because the center of the bow is still its strongest point.
It took the sacrifice of a lot of perfectly good PVC bows to build up all this info, so I hope you guys can put it to good use!