How do u calculate IBO Speed?

[cassowary]

Suppose you have a 50 lb bow. You shoot 500 grains arrows through a chronometer and get a average reading of 160 feet per second. How do you convert that to IBO speed which requires you to shoot an arrow weighing 5 grains per lb of draw-weight?

I have figured this out but am not sure if my math is correct.

Now the formula for calculating the kinetic energy of the arrow (or anything else) is :

1/2 x mass x velocity x velocity.

Using this, I set up the equation:

1/2 x M1 x V1 xV1 = 1/2 x M2 x V2 x V2

where:

M1 = 5 grains/lb x 50 lbs = 250 grains
V1 = IBO speed
M2 = 500 grains
V2 = 160 ft/s (measured from chronometer)

V1 (IBO Speed) works out to be 226 ft/s

Is my calculation correct? Does anybody know?

[Pete W]

You can't convert it.
 To find the IBO speed you need to chrono the bow to IBO specs and then it will probably break., 70# 30" 350 gr arrow.
 IBO is for compounds. Lots of them break too.

[Cupcake]

Cassowary,
Your calculation is correct in an idea case but does not work in the real world.  Also, to get IBO speed one must measure with IBO standards as stated.

I looked at some data on a couple of different bows that were measured with different weight arrows and was intrigued by the kinetic energy results, which is what your equation provides.

The data showed a heavier arrow had a bit more speed than the equation predicts.  I think it is because the heavier arrow is on the string for a bit longer so it gets a bit more time to accelerate.  Much like a dragster that runs a slower time often has a higher terminal speed.
Kevin

[cassowary]

Dear Cupcake,

My equation is based on the assumption that both arrows have no bearing on the efficiency of the bow which is not the case in the real world.

For some reason, the efficiency is slightly lower when you use a lighter arrow. Conversely, the heavier arrow gives the bow higher efficiency.

As you said, the heavier arrow has a bit more speed than the equation predicts.

I don't think we need to use an arrow which is exactly 5 grains per lb of draw weight to find out the IBO speed if we can factor in the efficiency of the bow with different weight arrows.

All I need is a rough idea. Do u have a link to the data you spoke of?

Your explanation sounds reasonable. Since lighter arrows give higher velocity, it leaves the bow string earlier before more energy could be imparted on it. But is the difference 10%? 20? 30?

[cassowary]

Dear 6 point,

Why find out IBO speed? Well, firstly I am a curious person. Secondly, I want to know how much faster compounds are to our traditional bows.

[wingnut]

Man I can't believe the number of rude people that showed up on this thread. Fellows if you feel the need to snipe at someone, call me and I'll give you a website that allows that.  Here it is about respect.  When someone asks a question we try to answer it, not critisize the question.

Anyway you can calc the speed to within a few feet by figuring the KE of the first arrow and reverse calculating the Speed of the second.

Your bow delivers 28.4 ft lbs of KE with the 500 gr arrow and will deliver 226 ft per sec with the 250 gr.  Of course this is in a perfect world.  Five grains is much too light for a trad bow and it will give up a bunch in sound and vibration.  Also the bow may not take the punishment.

Hope this helps.

Mike

[Eric Krewson]

I noticed right away that any posted IBO speed was really an advertising ploy to make a wheel bow look like a barn burner. If I recall correctly the bow is drawn to 30" and shot with a very light arrow. This criteria is not realistic for a traditional bow as few of us draw 30" or shoot an arrow weight of 5gr per pound pull.

[sar]

I'm curious too.  You can always try what I'm gonna do.  Next time I get the chance, I'm going to shoot some of my arrows through my wife's compound bows.There's a guy on a pig hunting site who shoots heavy arrows from a compound.  I think he shoots 585 from 70 ish if memory serves.  Not "heavy" per our standards, but he gets in the 280s with his set up.

[cassowary]

Thanks for the info, guys.

[Otto]

"I really don't mean to be rude but I would like to know why I should care anything about IBO?"

Well 6 Point, whether you meant to be rude or not, you are.  If you have no interest in the discusion of the thread then why bother posting?

My suggestion is take a look in the mirror and ask yourself if you're proud of what you post on this website.  I think you already know the answer.

[DarkeGreen]

Cassowary,

Don't forget the IBO speeds posted for compounds are with clean arrows and bow strings. Neither the popular single cam 320 fps bow or the other companies 340 fps bow will hit these speed marks with a hunting setup. the peep, knock, silencers, etc all steel 3 to 5 fps each. The posted bow speeds are with unfletched arrow that weigh 5gr per lbs including the field tip.

Well tuned single cams will shoot in the 285 fps range setup real world. You're going to lose as much as 10fps for each inch of draw weight under 30" but a good tech can keep a 28" draw in the 270 fsp range with a few tricks.

Also, these bows are warranted for shooting with 5gr per pound of draw weight and will handle arrow that light. I don't know too many people doing it though. Normal hunting weight arrow range from 340 gr to 440gr, for all draw weights, and lenghts.  

As an added note, I haven't found a deer yet that will stop an arrow that lite traveling that fast either.

There are plenty of sites you can search that will show you how fast glass longbows and curves shoot. Again, with hunting weight arrows and bow setups I think you'll find most in the 165 fps to 175 fps range. This is using arrows in the 480 to 540 gr range and these may or may not exit. I base that on a bunch of reviews and others accounts as well as 3 does shot with no pass thrus (I'm new to traditional too) but the arrow sticking out both sides.

I'm not sure why you're asking but a fair answer to your question is 100 fps and both kill deer equally well.  

[JImmyDee]

Your arithmetic is right: I get 226.3 fps.

 

Originally posted by cassowary:
My equation is based on the assumption that both arrows have no bearing on the efficiency of the bow which is not the case in the real world.

For some reason, the efficiency is slightly lower when you use a lighter arrow. Conversely, the heavier arrow gives the bow higher efficiency.  

In a way, you've answered your own question...

The AMO speed standard called for arrows of fixed weight pulled to specifid draw length and specific draw force.  If the bow can't be adjusted for draw force, they supply a formula which provides "corrections to conform to standard conditions."

IBO is different in that the weight of the arrow to be used is specified in terms of maximum draw force -- and is so low that one risks damaging traditional gear.

They had to specify fixed arrow weight in order to avoid exactly what you're seeing: all else held constant, heavier arrows increase bow efficiency.

[Cupcake]

Cassowary,
I found test data on a bunch of different bows at:
peteward.com
Cupcake

[Papa Bull]

Originally posted by Pete W:
You can't convert it.
 To find the IBO speed you need to chrono the bow to IBO specs and then it will probably break., 70# 30" 350 gr arrow.
 IBO is for compounds. Lots of them break too.

I agree with Pete.  IBO speed is something that can only be measured.  It can't be calculated.  The purpose of IBO speed rating wasn't initially to sell bows and I still don't consider it to be that.  The compound classes are extremely competitive and speed is critically important to them.  Well, actually, it is for us, too, but they're very well aware of how important speed is while a lot of us aren't.  At any rate, IBO has an 80 pound draw weight restriction and also requires a minimum of 5 grains per pound or 280 fps, your choice.  Knowing how fast a bow can be tweaked to shoot within IBO regulations is one important consideration competitors weigh heavily.

But as useful as the IBO rating is for a compound bow, I feel it's equally useless for traditional equipment.  Good luck finding an arrow that will be spined stiff enough to shoot from our bows at 5 grains per pound from a 70 pound bow with a 30 inch draw.

[Van/TX]

cassowary, there is no conversion.  You have to use the criteria established.

   

Why find out IBO speed? Well, firstly I am a curious person. Secondly, I want to know how much faster compounds are to our traditional bows.  

When you use 10 to 12 grains of arrow weight per pound of draw weigh @ 28" you will quickly see how a compound is not that much faster than a fast recurve.  The compound shines with light arrows....Van

[cassowary]

Thanks cupcake. I will check that site. Meanwhile, I found out a site that gave bow speeds for Turkish bows. Seems like traditional Asian bows can compete with modern compounds. See link:

  http://www.atarn.org/islamic/akarpowicz/turkish_bow_tests.htm  

From this data, I would put the IBO speed for the bows tested to be about 250 feet per second. That's pretty good and is comparable to compounds.

Compouds typcially have an advertised speed in excess of 300 feet per second but as someone said, that is based on unrealistic conditions.

There is a discussion on Mongolian bows in this thread:

 http://tradgang.com/noncgi/ultimatebb.php?ubb=get_topic;f=31;t=000451

[RamiusEng]

I think any test that has a design that is as flawed and unrealistic as the IBO test, as described here, is completely useless.  Makes me wonder who these IBO yahoo's are and why does anyone follow their specs.  They obviously don't know how a bow works.

Just the g.p.p. of the arrow would destroy many traditional bows if shot too many times.  And what, you would need over 20 shots to get anything statistically meaningful.  Who in their right mind is going to, basically, dry fire a custom traditional bow 20+ times in a row?  That is like poking holes in the Mono Lisa with an ice pick, to see how strong the canvas is.

We should get some of the guys here together and come up with a new testing standard for Traiditonal equipment.  While we are at it we can reddefine other terms like string length to bow length conversions, brace heights, etc.

[O.L. Adcock]

Wingnuts calculations will be close plus or minus the efficiency change which will vary from bow to bow. Drop the 226 to 220 and be very close. 5gr/lb@30" for IBO or 9gr/lb@30" for AMO, don't worry about the poundage. A 40# bow at 9gpp will shoot just as fast as a 60# at 9gpp, all else equal. You can run 9gr/lb @28" and still find lots of comparision data between Blacky, Pete and other sources on the net.

You can run the raw data knowing just 1 measured speed. The way bows work, if you double arrow weight, you'll lose 25% of your velocity and vice versa give or take a few % efficiency. You'll gain or lose 3-4fps for every inch of draw up or down keeping the gpp the same. Compounds work exactly the same way, no bow "excels" with light or heavy arrows. If one's faster then another at 5 gpp, it will still be so a 9 or 20.

Cassowary, Yep the Turkish bows are capable of good storage but you can't find data from anyone that's not romantically involved with them nor can you find any documented proof of their superiority in distance records.   ...O.L.

[doctorbrady]

It is a curious thing to see how the newer trad bows compare in effeciency to the newer compounds.  I think many of the more effecient trad bows have surpassed the compounds of a decade ago.
It's equally curious to see how the compound manufacturers have traded the old AMO speeds for IBO speeds in their advertising.  This is a recent change over the past several years designed, no doubt, to make the compounds look even faster.  If you switch back to AMO speeds my bet is that most of today's bows aren't much faster than the bows advertised 10 years ago with AMO ratings...but they sound faster!
Anyway, enough ranting.  I like fast bows as much as the next guy.  I just prefer 'em without wheels and gadgets.

[O.L. Adcock]

Doc, you are absolutly correct. The most "efficient" bows are trad bows but that's a poor gauge as the most efficient isn't neccessarily the "fastest", 100% of nothing is still nothing! The better recurves and longbows do out perform compounds of just a few years ago. The latest greatest compounds are in the 230 AMO range which is about 20-30% over most recurves but compounds also store 20-30% more horsepower so their "efficiency" is no better.

Ray, "Who in their right mind is going to, basically, dry fire a custom traditional bow 20+ times in a row?"  The answer is I WILL! Doesn't mean I'm in my right mind though.    Just got back from watching a lot of bows being shot in the 2-4gpp range. The only one I saw broken was a selfbow and it had survived 5-6 dry fires due to broken nocks before it finally let go. Didn't see any glass bows break being shot but I have in the past.....O.L.