Bow efficiency and arrow weight

[forestdweller]

I've read from a few places that around 50-55# is max efficiency for most modern recurve bows that are glass backed. How does this translate to arrow speed?

If one is shooting a 30# bow or 40# bow at 10gpp and jumped up to a 50 or 55# bow with 10gpp arrows would the 50 or 55# bow cast a 10gpp arrow at a faster speed since it's more efficient?

I've also heard Byron state that he shoots a 70# longbow for the same reason, at 70# for modern glass/carbon backed longbows max efficiency is supposedly reached and gives the fastest cast.

I'm just curious because it seems like to me as this may be the case but just wanted to make sure.

[TSP]

It's theorized that kinetic energy is a reasonably good way of assessing overall efficiency.  Arrow speed X arrow weight in grains divided by 450,240 gives foot lbs. of kinetic energy.  I'll leave the plug-in math up to you.

Put simply, if you want to maximize efficiency then shoot heavier arrows.

[Ray Hammond]

a better method than KE would be momentum.  The resistance to stopping...a layman's imagery for the measurement might be-

Using a machine, throw a ping pong ball and a golf ball using the same force- at a snowbank.

Which will go deeper into the snow?

[katman]

Forestdeweller, Don't think you could generalize all recurves like that since there are several designs in recurves, standard, semi-static, static super curve for example. That is to say limb design will effect efficiency. The bowyer would usually know what his design maxes out at.

[KeganM]

As Katman said, it really comes down to the particular bow.

What happens is that at a certain weight, the stiffness of the limbs causes additional flex through the riser, which lowers the efficiency.

On the other side, you will lose efficiency to extra string mass, limb and tip mass, etc. on lighter models. This also depends on the design, which is why many bows have a "sweet spot".

They're very small differences, usually, and normally anyone shooting 45-60# will be pretty darn close to "optimal".

[forestdweller]

Originally posted by katman:
Forestdeweller, Don't think you could generalize all recurves like that since there are several designs in recurves, standard, semi-static, static super curve for example. That is to say limb design will effect efficiency. The bowyer would usually know what his design maxes out at.

Well when I said recurve I'm talking about the modern recurve used in Olympic Style archery and the likes.

[damascusdave]

Shoot as much weight as you can handle, for both bows and arrows...these days I mostly hunt with 60 pounds bows and 900 grain arrows

DDave

[robertson]

Originally posted by Ray Hammond:
a better method than KE would be momentum.  The resistance to stopping...a layman's imagery for the measurement might be-

Using a machine, throw a ping pong ball and a golf ball using the same force- at a snowbank.

Which will go deeper into the snow?

+++1
very good saying    

[Sam McMichael]

Ray, please explain something for me. Is not KE (and the resultant momentum) a function of mass and velocity? Therefore, if an arrow of a given weight is shot at the maximum achievable velocity, is that not just another way to express maximum momentum. I'm not being argumentative, I just don't know the "science" of physics, but the "English" seems to essentially be saying the result is basically the same thing.

[forestdweller]

I'm getting a lot of posts about momentum and KE but what I was asking for was is there a particular draw weight at which a bow will cast an arrow the fastest independent of arrow weight for a particular bow?

Sorry for the confusion!

[monterey]

If you mean maintaining a consistent GPP all through the various draw weights, then my answer is the heavier the bow the higher the efficiency rating.  My guess on this (and it's only a not very scientific wild ass guess) is that draw weight is going to rise at a higher rate than limb mass allowing more energy to be imparted to the arrow.

[forestdweller]

Originally posted by monterey:
If you mean maintaining a consistent GPP all through the various draw weights, then my answer is the heavier the bow the higher the efficiency rating.  My guess on this (and it's only a not very scientific wild ass guess) is that draw weight is going to rise at a higher rate than limb mass allowing more energy to be imparted to the arrow.

Thank you that's what I was asking for.

I figured this was the case since if you took a 20lb bow with a 200 grain arrow and shot with it I'd imagine if you took the same bow design and made it 50# instead with a 500 grain arrow that the 50# bow with the 10gpp arrow would be significantly faster.

[katman]

You might try this is the bowyers bench for a more technical answer. I also remember reading something like your op but don't know your answer.

 http://tradgang.com/cgi-bin/ultimatebb.cgi?ubb=forum;f=125

[Archie]

Didn't Fred Bear say that there was a weight at which increasing a bow's weight would become counterproductive, due to the increasing mass and weight of the limbs?  I thought that was around 60-65#.  

I'm paraphrasing, since I've always heard that fact second hand.

[ChuckC]

Be careful with theory and math and the like.  Whatever the answer is, I am gonna go on the line and say if you took a bow in 45# and a SAME bow in 60# and shot arrows of 10 gpp out of each, there may be a winner in terms of speed, but it will not be a huge win.  Certainly not something worth worrying about.

The difference will be in momentum gained by making a heavier arrow go the same speed as a lighter arrow.  
Chuck

[KeganM]

Originally posted by monterey:
If you mean maintaining a consistent GPP all through the various draw weights, then my answer is the heavier the bow the higher the efficiency rating.  My guess on this (and it's only a not very scientific wild ass guess) is that draw weight is going to rise at a higher rate than limb mass allowing more energy to be imparted to the arrow.

To a point, which as I said before varies between bows. When you reach a certain level of stiffness to the limbs, it becomes great enough to begin inducing flex through the riser, which lowers the efficiency.

There is a broad "sweet spot" but it depends entirely on the bow as to what the exact range is. For the most part though, most bows will reach that point around 45-60#, though.

Too heavy, and you lose efficiency to flex and vibration. Too little draw weight, and you lose efficiency to too much mass.

[monterey]

Yes, you had me thinking about riser flex when I saw it in your first post.  Not sure I can see my way to agreement with that.  The construction of the riser and the wood or other materials incorporated into the riser I think would make a significant difference in the riser flex effect.

It may not be within the scope of the OP's question, but chuckc's point about the advantage of an ever increasing arrow weight in terms of terminal penetration would be very noticeable.

[KeganM]

That's why I said it depends on the bow. The Hoyt Excel ILF riser is a good example of a riser where you can feel the flex and vibration with the heavier limbs. Most wooden recurves and longbows have short, stout risers but a thin, narrow grip or one without sufficient lamination will flex there. The rest of the riser can be made of thick steel, but a small throat will become the focal point of flex.

I've built bows from 10# up to 100# and unless your riser is significantly overbuilt, there is a point of diminishing returns. As a matter of fact, flight shooters found that the most efficient bows were in the 35# class, not the unlimited. Of course, this means little to hunters, who just need delivered energy regardless of how they get it.

[TSP]

You're making this much more complicated than it is...but I guess that's the norm for internet archery.

[forestdweller]

How am I making things complicated? We all strive for maximum performance from our equipment.

Even Byron has talked about the reasons behind him shooting a 72# bow before and in his own words "that's when his bow style reaches maximum efficiency and gives the flattest trajectory".

He shoots the classic D shaped longbow so it requires a lot more wood to increase the draw weight on them which means they reach peak efficiency at a higher draw weight than say a recurve which already has thin limbs.

Maximum effcinecy = flatter trajectory