Mechanical Advantage question

[Tim Roberts]

#3059

Can the Mechanical Advantage of a BH be increased by changing it from a double bevel to a single bevel?
Just wonderin, in therory it makes sense, but was wonderin on the reality???

[L82HUNT]

#16108

I guess in theory yes. But taking a double to a single is a job. You would be removing alot of metal. Also some broadheads dont have the strength to hold up to a single bevel. The edge would roll on any major hit. All this would lower your MA. Just my thought

[David Petersen]

#16139

I’m vague on this too. Sure wish the Doc would get cured of all his ills once and for all, so he could join us daily, as he does so enthusiastically when he can. All I can say is that I have always sensed that while single-bevel would seem to be a piece of the MA pie, I can’t recall Ashby using it that way. The standard MA formula, in my mind, applies as much to 2-bevel as to 1-bevel: Long and narrow, with 3:1 optimal; quality steel of good thickness and hardness (50 as minimum); streamlined ferrule with no big projections or “bumps”; slick finish (Teflon is a bonus), etc. In sum, I think the Doc examines the advantages of single-bevel in his research separately from the advantages of high MA. I like both! Recently, an old rude “dating” saying comes to mind as an appropriate justification for “going to all this unnecessary trouble” of testing for ourselves what Dr. Ashby has tested for three decades: “Why take a sandwich to a banquet?” dave

[Tim Roberts]

#16203

Dave, you are spot on about Dr. Ed, I too am hopeing that he gets cured soon!
Ok part of the reason for askin this, shortly after a few Ashby Reports got on the internet and Trad Bow Hunters started gettin after it, a few companies started in to the marketing of single bevel BH’s.
Now unfortunately the BH that I have taken a liking to doesn’t offer the single bevel, but could maybe be talked into it, espically if it showed that it was a definate improvement.
So now thanks to Dr. Ed, we know that a single bevel BH will go through bone easier than a double beveled head, it got me to thinkin that the single bevel may, or could have something to do with the Mechanical Advantage figurin????

[stalkin4elk]

#16293

Single vs. double bevel is less an issue of MA and more an issue of rotational torque to split bones as I uderstand it with all else being equal. If MA is affected by bevel design I think it would be minimal.This is only my best guess w/o research to prove it.

[Mark Turton]

#16734

I had a brief conversation with Daniel aka Standingbear regarding single bevel with a convex grind and he suggested that it would make for a well supported cutting edge.

I would guess manufacturers harden broadheads before grinding so wonder if they would sell you some ‘blanks’, make an interesting project.

Looking forward to the good doctors return, Mark.

[Patrick]

#16849

Quoting Dr Ashyby’s “Momentum, Kinetic Energy, and Arrow Penetration (And What They Mean for the Bowhunter” study. (Page 18 ):

Dr. Ed Ashby wrote: A long and narrow single blade (2 cutting edges) broadhead will have a higher mechanical advantage than one of equal length and width, but have more blades. Also, as the profile of a broadhead’s blade(s) becomes shorter and/or wider the mechanical advantage becomes lower. Having either a convex or concave cutting edge profile, rather than a straight taper, also lowers a broadhead’s mechanical advantage.

Any abrupt rise in the countour of a broadhead results in a profile which lowers the broadhead’s mechanical advantage. This is why a very smooth and gradual fade-in of the broadhead’s ferrule into the blade is important in the broadhead design. It detracts less from a broadhead’s mechanical advantage.

By design, the bevel on a single bevel broadhead will be wider, which it turn means the “slope” of the bevel will be more gradual, which gives it a higher mechanical advantage than a double beveled broadhead (with all else being equal).

[Tim Roberts]

#16884

Ok, this morning I talked with one of the good folks at Magnus, and asked them if they would sell BH’s that hadn’t been sharpened, or if they had any thoughts on marketing a single bevel BH.
Their resopnse was this, in a nut shell, We have been doing this for 26 years and we have no problem bustin through bone as it is, and see no need to change what we are doing. Nor will we sell any BH parts, only finished product.
Now, shortly after Dr. Ed came out with the bone penatrating advantages of a single bevel head, Zwickey started marketing one.
Was this a marketing scheme to cash in on all the Dr. Ed faithful, or is there a definate advantage of how a BH will preform in the worst of situtations?
I have always been a firm believer in hoping for the best but preparing for the worst.
Just wanted to add that the BH in question is the 160 gr. 2 bladed BH.

[Patrick]

#16910

Tim Roberts wrote: Ok, this morning I talked with one of the good folks at Magnus…
Their resopnse was this, in a nut shell, We have been doing this for 26 years and we have no problem bustin through bone as it is, and see no need to change what we are doing.

I believe horse and buggy makers had a very similar point of view when the first motorized vehicles came on the scene. 😆 (I really am only joking)

Tim Roberts wrote: Now, shortly after Dr. Ed came out with the bone penatrating advantages of a single bevel head, Zwickey started marketing one.
Was this a marketing scheme to cash in on all the Dr. Ed faithful, or is there a definate advantage of how a BH will preform in the worst of situtations?

There is no way to know Zwickey’s motives, beyond what they say. Unrelated to what Zwickey did or didn’t do, Dr Ashby has done exhaustive studies on this and they speak for themselves. He is the only person I know who has very rigidly used the scientific method in coming to his conclusions. To dispute his results is to disregard facts. At the same time, I’m not implying you must act on it and HAVE to use a single bevel and/or a 2 bladed broadhead, and/or an EFOC arrow. Don Thomas’ article in the Anniversary edition of TBM is a perfect compliment to that point of view. There’s alot to be said for not second guessing what’s WORKED for you, and continuing to use it. That concept is very much a part of traditional bowhunting, and why I switched to it.

[sharpster]

#20516

There are certain BHD manufacturers out there who are completely hostile towards Ed Ashby’s work/findings. That’s all I’m gonna say about it.

Ron

[Ed Zachary]

#20535

I don’t even want to bring up a theoretical
minimum required blade thickness to gain the benefit
of a single-bevel blade……………
too much retooling for some BHD manufacturers 😥

[sharpster]

#20563

Now to address the original question, the words “machanical advantage” are used to describe two very important yet distinctly different aspects of BH design. The term is used to describe the shape of the broadhead itself as Dave mentioned- Longer, narrower heads (3:1 designs) have a greater MA than shorter, wider (Delta style) heads do. The greater MA 3:1 heads require less energy to attain the same level of penetration as a wider head.

When we use the term MA in reference to the bevel geometry of the cutting edge it refers to the ability of the bevel and cutting edge in combination to efficiently slice tissue. Therefore the lower the bevel angle, the greater the MA of the cutting edge BUT, in bevel geometry there is more to it than that. We want the greatest MA we can achieve WITHOUT sacrificing the durability of the cutting edge. If we were to sharpen a double bevel BH at an extremely low angle like 10 degrees per side the MA would be excellent in theory but, the blade’s edge retention would suffer dramatically and it would dull pretty much instantly. So we willingly sacrifice a bit of MA by using higher sharpening angles in order to preserve cutting edge durability and to ensure that the blade has the ability to slice cleanly throughout the penetration cycle.

This is another case where a single bevel captures the best of both worlds. If a single bevel BHD has a 25 degree bevel on one side of the blade, and a flat, zero degree bevel on the other side, the total included bevel is 25 degrees.

A double bevel ground 12.5 degrees on each side will also have a total included bevel of 25 degrees. Both bevel types would have exactly the same MA but, the single bevel will be far more durable and will likely still shave hair off your arm after a full penetration pass through, whereas the double bevel would very likely have dulled long before even reaching the vitals of the animal.

Ron

[David Petersen]

#21232

Ron — Great to have you back discussing these compelling issues with us, thanks. In Ed’s absence, I consider you the #1 authority on what the Ashby study results mean when “realistically” translated to broadheads and lethality.

FYI, I finally scored some left-wing El Grandes in the new “seamless” 200 grain version, which last time I checked were available only in right-wing (3Rivers). I got ’em from Rick Stillman of The Feathered Shaft at the Illinois trad bow banquet. Rick says he got them from Allegheny Mtn. Archery, which shares a building with the fellow who manufactures the Grizzlies. They still are far from sharp enough to hunt with, but the finish on the bevel is a great deal smoother than the old El Grandes, almost polished as opposed to the old heads with deep grinder marks that had to be filed away before true sharpening can begin. Did they also get a 25 degree bevel this time? Your “report” on these slightly improved heads would be appreciated. I’m not going to use a file at all with these, but going right to the coarse stone on my KME knife sharpener. I have months to enjoy patiently getting a razor edge without lightening the weight 20 grains as I had to do with the old versions. Dave

[Bert]

#21306

Good news, Dave, on the new Grizzly’s- we’ve all been waiting for a left-wing. Looking forward to our sharpening guru’s report. Usually, in the field, I carry a diamond stick(use water for lube) but recently at the old True Value, I ran across a ‘points file’- and if you know what that is you most probably have a touch of gray. Very small with a flattened handle, very fine teeth of- tungsten!
Hmmm?- worked fine on resharpening an aluminum Svea bow-type saw with Danish steel teeth- haven’t used it on a broadhead yet, but it looks like it may have some possibilities. Any comments or observations, Ron?
And Dave, we await your post on ‘What’s in your pack?’ on the Campfire forum!

[sapcut]

#21606

The new 200 Grizzlys are extremely easy to sharpen.
In literally 13 minutes I had both edges on one broadhead ridiculously sticky, shaving sharp. I have never gotten anything that sharp before. I lost a total of 2 grains.

I used a dull 7 inch flat file and a ceramic stick.

Richie

[Ed Zachary]

#21618

Sapcut wrote: I used a dull 7 inch flat file and a ceramic stick. Richie

The dull file, is that by choice or by circumstance? 😮

[sapcut]

#21751

It is just a file I have had a while now. It works well because it does not take off more metal than I want or faster than I want. Sometimes I will use a flat file backwards so it doesn’t take off too much.

Then I use the ceramic rod to take off even less metal but just enough. After that I like to strop it using cardboard. I am not an expert in sharp metal edges but I’m telling you I can’t imagine getting anything sharper. The new factory 25 degree bevel is key.

[sharpster]

#22232

In order to preserve the original topic, I’ll add this one point…Single bevel broadheads rotate due to the huge difference in surface area between the beveled side and the unbeveled side. In other words the bevel needs to be wide, and the wider the better.

To achieve the greatest rotational benefit of a single bevel grind, the blade blank must be sufficiently thick to result in a big, wide 25 degree bevel, ideally for the entire length of the blade. Grinding a 25 degree bevel on a 0.20 thick blade won’t produce a very wide bevel and the resulting difference in surface area of the two bevels will be minor, so the rotational benefit gained by single beveling a blade this thin would be minimal.

Ed Zachary said he didn’t want to bring up a theoretical minimum blade thickness so I’ll stick my neck out… I’ve single beveled blades as thin as 0.37 (the older Tusker Concords). The resulting bevel contrast was adequate but the structural integrity of the head wasn’t great at 3:1. I had to shorten the broadhead by adding a big tanto tip in order to stiffen the heads up. I’d say 0.50 is pretty much minimum, and I’ve yet to see a blade that was too thick.

Dave, The only new grizzlies I’ve seen are the 3 sample heads that Shawn sent me last year and since I think they’re from one of the very first runs, I haven’t touched them so I can’t say how much easier the new ones are to sharpen. I’ll be getting some in the next month or so and report back. Let us know your thoughts after you sharpen a couple.

Bert, yea I’ve got a touch of gray…. a very light touch mind you, but a touch none the less. I remember when I was a kid my Dad had a small inexpensive file that I swear would have cut through a diamond. It’s long lost now and I’ve never seen another like it. I love very aggressive files and extremely coarse stones for quickly establishing a bevel and burr. Believe it or not, I don’t particularly enjoy sharpening. I view it as a necessary evil and I want to get it done quickly. I have to regularly preach that files, and x-coarse sharpening stones are our friends. So many people want to jump right to fine grit stones too soon, and all that does is prolong the task and/or polish dull edges! Get the blade very sharp with your file or coarsest stone, then refine and polish the edge with the finer grits.

Ron

[David Petersen]

#22353

Bert — OK, I just posted on the pack thread. 😈

Sharpster — I’ll start sharpening the new El Grandes as soon as I get another project off my worktable. Meanwhile, the Tanto tip on these heads seems almost too much of a good thing. Your thots there? dave

[Hiram]

#22965

And they (single bevels on opposite sides) Torque. Bones are busted rather than cut. Busting them by nature of the reverse pressure mechanism causes bone to be disrupted, rather than merely penetrated.

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