Mechanical Broadheads and Quartering Shot Angles - October 2008
October 28, 2010
Question: I have always used fixed blade broadheads, but I am considering mechanical heads. My friends tell me that quartering shots may be a problem. What is your opinion? -- Jeff Dimon, Ashaway, RI
MECHANICAL BROADHEADS AND QUARTERING SHOT ANGLES
Some people call it pole-vaulting; another appropriate term is deflection. There is the contention, in some circles, that when you make a quartering hit with a conventional mechanical broadheads that open from the front rearward, that you run the risk that one of the blades will open before the others and push the tip of the arrow away from the animal. It is as if the arrow is pole-vaulting, using the single blade that opens first as its pole.
MY PERSONAL EXPERIENCES
I have used mechanical heads of all types and sizes since 1995. I have probably shot somewhere around 180 big game animals with mechanical broadheads including bears, mule deer, elk, caribou and whitetails (mostly whitetails-of which probably 75-percent were does). I have never had a deflection. In fact, I am nothing short of satisfied with the effectiveness of these heads other than the fact that some of the early models didn't penetrate very well.
To see if deflection is a bigger problem than I thought, I called a few broadhead industry representatives to get their viewpoints.
Bryan Wohlfeil produces the Aftershock broadhead. It is a mechanical head with blades that open from the front backward. The blades are completely enclosed in the ferrule and swing open only after the tip of the head has penetrated at least one-inch into the animal. The way it is designed, the broadhead eliminates any tendency to skip, skid or pole-vault on impact. This was an important consideration when Bryan designed the head. (Please note that the Aftershock is not legal in every state, so check your local regulations before trying them).
"I personally had a problem with a conventional mechanical head deflecting on a sharp quartering shot," Bryan said. "That was the inspiration for the Aftershock head. If you look at an animal closely that is in a sharply quartering angle you will see that the gaps between the ribs grow smaller and smaller.
"When a standard mechanical broadhead hits this nearly solid, angled wall of ribs, the blade closest to the animal opens first. Not only does this tend to push the head away from the animal, but also that side of the arrow is immediately slowed by contact between the blade and the rib. The result is that the arrow can turn toward the animal losing a lot of its forward momentum. It is like trying to drive a nail crooked. You can't make it go into the wood. When the forward momentum of the arrow is no longer acting down the centerline of the arrow and straight into the animal, your penetration is going to suffer badly.""
I also called Bob Mizek, Director of Engineering at New Archery Products. Bob told me that the problem of deflection does exist in certain situations but it is not confined only to mechanical broadheads. It can also occur with fixed-blade heads when the conditions are right. "The only thing we can say conclusively is that when an arrow is under-spined, the performance on impact can be bad enough that pole-vaulting is something to be concerned about.
"If the animal is turned less than 30 degrees from broadside, any broadhead will do fine," Bob added. "But when you get to where the angle is 45 degrees or more, you have the possibility for problems. We get a lot of calls in from customers with broadhead performance problems. Almost all of these come down to tuning and most of the tuning problems come down to the archer shooting an under-spined arrow. When you combine an under-spined arrow that is not properly tuned and is yawing back and forth with a 45-degree (or higher), quartering angle shot you can see some very strange stuff take place. It is more likely with a mechanical head but it also occurs with a fixed-blade head.
"The tendency to deflect or to turn sharply into the animal can be so severe in some cases that it would literally snap an aluminum arrow on impact. But with the carbon arrows you don't see that kind of failure, only very poor penetration. It is worse on short shots than on longer shots because on short shots the arrow doesn't have as much time to stabilize and point straight at the animal.
"Also, the closer the ends of the blades are (for mechanical heads that open from the front backward) to the tip of the broadhead the more likely they are to deflect on a sharply angled impact."
Bruce Barrie designed the Rocky Mountain Snyper and has consulted on the design of the Rage Broadhead. Both heads open from the rear forward to reduce or eliminate the possibility of pole-vaulting. Bruce said that he has not personally encountered the pole-vaulting issue while hunting but he received enough feedback from customers who were using conventional mechanical heads that he decided to design the Snyper,
"I've had some strange deflections even with fixed-blade heads," said Bruce. "This seems to be the most common when shooting light arrows. I even shot a broadside cow elk in the lungs with a fixed-blade head and got a deflection off one of the ribs causing the arrow to exit near her back leg. People naturally have a tendency to blame the broadhead when something happens that they can't explain. Because mechanical heads are controversial to begin with, they may often unfairly become the scapegoat."
The amount of momentum carried by an arrow determines how well it will continue to move forward when forces act on it from different sides. For example, an arrow with a high degree of momentum (the physical equation for momentum is mass times velocity) won't be affected as much when a single mechanical broadhead blade opens before the others.
Again, you will see significantly less problems when you shoot a heavy arrow. It is also more difficult to turn an arrow that is heavy and long. A long heavy arrow has a greater rotational inertia that opposes turning. It is the same reason that a long heavy stabilizer is the best choice to keep your bow from turning during the shot.
Maybe one of the reasons I have never seen a pole-vaulting problem is because I shoot a heavy (530 grains), long (29.5 inches) arrow that is not as likely to be affected by the force of one short blade opening. I say short blades because I have settled on mechanical heads with short blades that don't have the leverage to move or turn the arrow much.
If you insist on shooting short, lightweight arrows, that are possibly under-spined, you may experience deflection on shots where the animal is angled 45 degrees, or more. Personally, I would rather see everyone buy some heavier arrows before they start blaming their broadheads for poor performance.