Thanks to all of you who've written to me, and let me apologize to each of you who've emailed and not received a personal reply. The response to the single-bevel article has been totally overwhelming, and it would take a two-finger typist like me many weeks just to reply to them! So, here's the answer for all of you who've emailed asking precisely how I modify the 190 grain Grizzly. For all those who've asked about the L-shaped exits and fletching-rotation vs. bevel-induced rotation, I'm putting a separate post on about them.
Tip Modification:
On all Grizzly broadheads, I modify the Tanto tip to a cut on impact design. Due to some results from this year's testing with the 190 Abowyer, which will show up in the Part 6 Update, I'll be taking a critical look at what effect the type and positioning of the Tanto tip's bevel has on performance in upcoming studies. It appears there MAY be some significant difference in bone performance, depending on how and where the tip is beveled; but I won't go into the details of that here. Currently, the bevel I'm adding on the Modified Grizzly is a single-bevel that's on the reverse side of the blade face from the edge bevel; for that half of the blade. I hope to find out in the next round of test if this is the most effective, or whether a different configuration might work better.
I also alter the angle of the Tanto tip slightly, changing the angle of tip attack from the factory 40 degrees to one of 30 degrees. The ONLY reason I use 30 degrees on the Grizzly's tip is because it's the angle that 'feels right' to me as I apply it; no other reason. At this point in testing I have NO specific information indicating that 30 degrees is superior to 40 degrees. Whether or not this change has much effect remains undetermined, and is also being looked at in testing.
Narrowing the blade:
In making the Modified Grizzly, the 190's blade is narrowed at the rear, to a cut width of 1". I DO NOT narrow the entire blade taper. The purpose of the modification is to change the rate of blade taper. After grinding, the new blade profile has the same blade width AT THE REAR EDGE OF THE TANTO TIP as it did before grinding. Until I became accustomed to making the modification I found it useful to scribe a line down the face of the broadhead, to serve as a guide of for the new blade slope. The following diagram shows this modification.
This modification changes the slope of the blade, decreasing the blade's angle of attack and increasing the blade's mechanical advantage (MA). The factory 190 has a MA of 2.75. After modification the MA is 3.25. As you'll see in the new updates, this 18% increase in broadhead MA yields more than an 18% increase in average penetration; because of the mechanics of the impulse of force – and how, and to what degree, this affects penetration is explained in detail in the upcoming Updates.
Higher MA means the broadhead can do the same amount of 'work' with less force, but there's also a very important, but less-often recognized benefit. Higher broadhead MA also means the peak resistance force encountered is lower as the broadhead accomplishes any penetration task. This also means the peak resistance force is lowered, reducing the amount of 'stress' on the entire arrow system. The reduced stress makes it less likely the broadhead – or any other arrow component – is going to break or bend on hard impact.
The Edge Bevel:
The final stage is to reset the edge-bevel to 25 degrees. It's my understanding that the new production heads have had the bevel changed to as flat an angle as the grinding machine they have will do, about 27 degrees; and that would probably work very nearly as well as the 25 degree I use. However, the longer (lower angled) the bevel is the better, in terms of edge sharpness and cutting efficiency – up to the level where the hardness of the metal is no longer sufficient for the edge to resist deformation.
Here are measurements from a 190 Grizzly and the Modified Grizzly:
Grizzly 190 grain: freshly sharpened and tip modified to COI Tanto: OAL: 3.135"; Cut Width: 1.125"; Angle of blade attack: 9.0 degrees. Attack angle of Tanto tip: 30 degrees; Blade Thickness: 0.057"; MA: 2.75; Edge-bevel: 25 degrees.
Modified Grizzly (170 grains, finished weight): OAL: 3.135; Cut Width: 1.0"; Cutting angle (angle of Attack): 7.4 degrees. Attack angle of Tanto tip: 30 degrees; Blade Thickness: 0.057"; MA: 3.25; Edge-bevel: 25 degrees.
The 160 Grizzly can be modified in the same manner, and what you'll end up with is a Mini-El Grande. Before modification, the 160 has a MA of 2.58. After modification its MA is 2.75 – exactly the same as the 190 grain's.
During 2005 testing some bending problems were encountered with the 160 grain Grizzly. Modifying it offers one way of reducing this tendency to bend. How? As mentioned above, higher MA not only allows a broadhead to do the same work with less force, it also means that the broadhead is subjected to less peak resistance force during the moment of impulse as it accomplishes any individual penetration task. Less peak resistance equals less peak stress. Is the difference significant? Yes. On the 160 Grizzly the modification represents approximately a 7% reduction in applied force needed for a given task – and that means 7% less peak resistance stress. If you look back at the 2005 Updates, you'll see that I speculated that the differing MA (from the 190) might have been a factor in the bending and compression the 160 grain heads suffered.
In the newer testing I used some 160's hardened to the same level as the 190's and none bent. Whether or not this change in tempering is going to be made (or has been made) to the production 160's, I don't know. Regardless, though I'd like to see the hardness increased to R52-53, and still must consider it the likely source of the unmodified 160's bends, making this modification to the 160 Grizzly will result in a higher MA; which will mean a bit more penetration and a reduced tendency to bend.
Most would think making the blade narrower would weaken it, making it more likely to bend, but that's not the case; though it will be the case if you simply move the entire bevel in, narrowing the blade equally along its entire length. It's the changing of the angle of blade attack that helps the most; a more gradually inclined plane with a higher MA, without significantly lowering the rigidity of the tip area forward of the ferrule's fade-in.
Unlike modifying the 190, you won't have to remove nearly as much metal from the 160 gr. to make the change. You'll only need to narrow the back of the blade by about 7/100th of an inch. Considering how little metal you'll have to remove, and the fact that the factory-fresh 160's I received weigh in at 166.5 grains, I think it will come out weighing very close to 155 grains after all modifications; possibly a tad more. I haven't done a Mini-El Grade myself, and have simply worked out the 'numbers' for someone who asked me if there was a way to improve the performance of the 160's they were using.
The critical part is to make the modification correctly. Just remember that you don't want to move the entire bevel in; you want to change the slope of the blade. Narrow the blade at the rear, but leave the Tanto tip the same width.
Not only will this modification reduce the tendency for the broadhead to bend, it ups the penetration potential of your arrow. The MA gain will far exceed the tiny advantage the extra few grains of arrow mass would have made.
And, yes, applying these modifications to any of the other weights of the Grizzly will yield similar benefits, but it won't make them as strong as the modified 190 grain. Most of the lighter versions do not have as great a metal thickness as the 190, and are hardened to a slightly lower R value.
I hope this answers most of the questions everyone has asked.
Ed
Topic: Modifing The Grizzly (Read 1075 times)
