Why do so many broadheads have straight edges?

[UCBerzerkeley]

I have little experience in this department, only having used magnus stingers in my short career.

But if you take a look at basically any cutting instruments you see curves - knives, axes, a variety of swords (yes, some are straight). Curves creates a shearing motion, so that the edge slices through the target as the weight of the blade drives it deeper. It is the principle reason a kukuri or katana make such effective slicers.

Seeing as not all broadheads exhibit this curvature, I must be wrong. Can someone please set me straight? (haha...get it...straight...straight-edge...nevermind)

[George D. Stout]

Well,  it may well be that it is much cheaper to tool up for a straight edge, and that the broadhead edge is rarely over 2 to 2 1/2 inches long.  Some heads like the Simmons and the Howard Hill are curved.

[VAFarmer]

Keep in mind that this type of cutting is a little different than most cuts made with a curved blade.   You are comparing a slicing style of cut(with the Kat) versus a stabbing type of cut.  does this make a difference?   Who knows.
Most other "stabbing" blades are straight.
I would venture to say Mr Stout is getting close when he speculates that cost is a factor.  Not to mention sharpening.   MUCH easier to sharpen a straight blade than a curved, if you ask me.

If anything were to be more effective, it would seem like something that would create less drag.

farmer

[Gary Logsdon]

Bear Razorheads have a convex curved edge.  The Simmons heads are curved too, but concave.

[freefeet]

I'd put my money on the sharpening thing.

I wouldn't buy a curved bh simply because it would be a pain to get them sharp in comparison to a flat edge.  So if i were to design or manufacture one that would be the first priority that i would presume my customers would want.

As a chef i can tell you why large knifes have curved edges.  Because you can do more with them on a chopping board.  The only straight edge knives i own are carving knife (that never touches a chopping board) and bread knife, which has a very specific role and serves no other function.  All other knives are curved for use with the chopping board so i can roll from one end of the blade to the other cutting a bit at a time instead of struggling to cut all at once.  A broadhead, whether curved or not, has to cut all at once so i can't see that curved will make any difference.

I can see some bh makers going for the curved trad look of a knapped flint head as a style thing and people buying them as such.  But the steeper you make the angle of the cutting edge the less sharp the cutting edge is.

[Jason R. Wesbrock]

My guess would also be that it comes down to cost of production. Ironically, I've found slightly convex blades much easier to sharpen than straight ones.

[Ragnarok Forge]

I suspect it is both production cost and simple physics.  A straight edge in a thrusting weapon cuts and penetrates far more effectively than a curved edge.  A 3 to 1 length to width ration with straigh edges will provide maximum penetration.

[UCBerzerkeley]

Originally posted by Clay Walker:
A straight edge in a thrusting weapon cuts and penetrates far more effectively than a curved edge.  A 3 to 1 length to width ration with straigh edges will provide maximum penetration.

You say that pretty convincingly. In a double pass through, a large amount of slicing goes on. Basically all the damage happens after the initial penetration. That is what you want, yah? TO do as much damage as possible? Curved blades slice better.

[GingivitisKahn]

The curved knives you are speaking of have one curved cutting edge for slicing. We are talking about a two (or more) edged object where both (or all) edges must cut as efficiently as they can.  Comparing a multi-edged broadhead to a single-edged knife is not a valid comparison.

When you slice a piece of meat with your knife, you tend to start with the back end of the blade and draw the knife towards you over the meat.  A broadhead must penetrate and clear the way for all of the broadhead and the arrow shaft behind it.  I doubt you could easily take your curved knife and push it and the handle and your arm through the meat.

There are others here that have experimented extensively with different sorts of broadheads and their penetration qualities.  You might want to have a look at what they say on the topic.

[DuffyRP]

You are comparing apples to oranges.  The broadheads do not hit edge first they hit point first.  If you take a straight edge broadhead and push it point down into something. as it enters the hole gets wider.  notice the shearing action you mentioned.  just like a gillitine. Ironically the more curve you put on an edge the more you decrease the shearing angle and increase surface area upon impact both of which lead to a loss in penatration.  You have to weigh all the factors that is why a 3 to 1 penatrates so well.

[lt-m-grow]

Originally posted by UCBerzerkeley:
 

Originally posted by Clay Walker:
A straight edge in a thrusting weapon cuts and penetrates far more effectively than a curved edge.  A 3 to 1 length to width ration with straigh edges will provide maximum penetration.

You say that pretty convincingly. In a double pass through, a large amount of slicing goes on. Basically all the damage happens after the initial penetration. That is what you want, yah? TO do as much damage as possible? Curved blades slice better. [/b]

So how did the 3 to 1 ratio get dubbed "perfect" and who said so?  

No intention to hi-jack UCB's thread.  This seems pertinent to his comment here.

[JimB]

Howard Hill and others decided that 3 to 1 was ideal.Dr Ashby thoroughly tested all ratios and found that 3 to 1 out penetrated the others.It is about a much gentler angle of entry having less resistance.He also found that less resistance to the broadhead meant less frequency of damage to the broadhead and all the component parts of the arrow behind it.This also helped penetration.

It is all in the Ashby reports.

I have no idea if curved vs straight makes any difference.I can't see it if it does.It seems like thousands of animals are taken with both.

I'm guessing that how sharp the edge is and possibly how structurally sound the tip is,would far outweigh this issue.

These days,I gravitate toward the straight edge because I can sharpen them easier and have been putting a big emphasis on getting and keeping them sharper than I ever have.

[hickstick]

I'm pretty sure in the Ashby testing straight edges tested better that concave or convex on penetration.  

   

In a double pass through, a large amount of slicing goes on. Basically all the damage happens after the initial penetration. That is what you want, yah? TO do as much damage as possible? Curved blades slice better.

by 'double pass through' I'm assuming you mean complete penetration?  and yes I'd rate it 1) getting complete penetration first 2) doing the most amount of damage to Pulmonary and Circulatory systems as possible (but not in any way negate #1)...

right now I'm shooting the orignal woodsmans. and I like what I've seen so far.

[Bill Carlsen]

I find the when using a sharpening jig like the KME or the Razoredge system a concave bh is much easier and faster to get sharp than a straight edge, esp. if the edge has been well dinged up. I just found some old Pearson Switchblaedes and I had them shaving  sharp in about 2 minutes with the KME or the Razoredge. On the other hand I find that three blade heads like Snuffers, Razorcaps and Woodsmen are the easiest of all to get shaving sharp and you don't need a jig to do it....qnd they put down game mostly in sight.

[UCBerzerkeley]

Originally posted by GingivitisKahn:
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There are others here that have experimented extensively with different sorts of broadheads and their penetration qualities.  You might want to have a look at what they say on the topic.

The ashby reports are what got me started thinking bout this in the first place. i'm not doubting the 3:1, history has proven it works. but the curves I was unsure about, specially since i've read that ashby has used the nanook (from ABS) on some very big game (nanook is curved, single bevel).

i threw this out as food for thought, i don't claim to be an expert or even particularly knowledgeable. So thank you for all the well thought out responses! i feel smarter already

although, if knives are not a good example for curved blades, i dont think the guillotine for straight blades is either. i might be totally daft, but it would seem that penetrating curves blades mimic the draw motion of a curved knife on meat just fine

i admit, i feel this is a pressing matter in an age when deer, boar and elk have all began to acquire impenetrable body army    

[Soilarch]

I'll throw my two cents in:


The curvature (or lack) is not the secret.

The angles between the line of motion (path) and cutting edge ARE.

Kukari and Samurai Swords are curved because when you add in our human anatomy they allow the angle between the path of the blade and the cutting edge to be lower. (Our wrists our only so flexible.) Hence, better slicing.

3:1 heads have a lower angle between the path and the cutting edge than say a 2:1...Hench, better slicing.

Sure, with a convex broadhead you have a lower angle towards the outside of the head.  But you also have a higher angle towards the inside of the head.   On a straight edged broadhead you keep the happy medium from the tip to the outside.

Six of one, half a dozen of the others.

[Ragnarok Forge]

As noted above both Kukri and Samurai swords allow a better slashing cut but are not good thrusting impliments.  As archers we are not making slashing cuts.  We are making thrusting cuts with out arrows.  

If you look at any of the swords made for penetrating armor they have diamond shaped tips with straight edges at the tip of the blade and then a radius which transitions into the full width of the blade.

the 3 to 1 width the length ration was discovered thousands of years ago and used in both spear heads, knife ti, and sword tip designs.  All of these weapons need to penetrate on a hard thrust.  Check out a Roman Gladius some time.  The tips are really close to 3 to 1.  Recent research has verified what has been known for a long time.  Physics also verifies the testing without having to perform it. Any time you make something narrower at the same thickness of material it will penetrate further.  Less friction means more penetration.  Newtons laws and all that.

3 to 1 is a compromise between adequate width and reasonable length of broadhead.  why not 4 to 1 or 6 to 1.  We would all look prettys silly shooting knives on the end of our arrows.

Oh and just for reference, I am a civil engineer by trade and a blacksmith by hobby.

[freefeet]

Originally posted by Clay Walker:
why not 4 to 1 or 6 to 1.  We would all look prettys silly shooting knives on the end of our arrows.

More length = more drag!  Shorter length = less dynamic sharpness or weaker edge.

[Bill Carlsen]

i would like to add that when all is considered I believe that even Dr. Ashby's findings are that one of the biggest factors in penetration is that the arrow must be flying perfectly which translates into precise bow/arrow tuning. Even if you follow all of Ashby's findings for yourself and do not properly tune your equipment you might be in for disappointment whether you shoot convex, concave, straight, single bevel, double bevel or multiblade heads. Too many people have shot too many big game animals before Ashby came on the scene, simply because they were shooting equipment that was matched and tuned properly. Pass thus were not uncommon in the 50's when the ave. bow weight was 45# and most everyone was using four blade Razorheads.

[Butch Speer]

I've got to agree with Bill. When it comes to Ashby's testing, most everyone goes all dreamy eyed. People look at the end result & not much else. It's like looking at the destination but not the journey.

The 3:1 thing. I don't have enough experience with them to make a statement. The native Americans & earlier people killed a few animals with arrows. Check out some flint heads sometime. Definitely not 3:1. People usually refer to Howard Hill when it comes to 3:1. The man killed a lot of animals. So did Fred Bear. His heads weren't 3:1. I believe that what these two & many others proved is that it' shot placement  & sharpness that kills.
As far as using physics instead of testing. I might be wrong(sure wouldn't be the first time) but, I believe a bumble bee can't fly.
This is all just my opinion.