25 replies to this topic

[cingold]

I address this question to the experienced vertical/crossbow shooters:
Does an arrow shot with a crossbow show archer's paradox too, and if how much? I'm working with Koltze's physical equation (based on Dr  Buncsak's work) that could be useful comparing crossbow efficiency, do I have to include energy loss due to archer's paradox or is this only valid for longer bow arrows? If there's no significant loss in energy due to "spine flex" we could have a solution for an empiric comparison soon

Tnx, Chris

[Hunter2]

cingold said:

I address this question to the experienced vertical/crossbow shooters:
Does an arrow shot with a crossbow show archer's paradox too, and if how much? I'm working with Koltze's physical equation (based on Dr  Buncsak's work) that could be useful comparing crossbow efficiency, do I have to include energy loss due to archer's paradox or is this only valid for longer bow arrows? If there's no significant loss in energy due to "spine flex" we could have a solution for an empiric comparison soon

Tnx, Chris

Since its not being shot around a riser, will it have the same effect on a crossbow going down a rail? The arrows are shorter and generally more rigid and flex from a high speed I saw was so little will it make a difference? I know more questions, but honestly havent given it any thought other than reading post. In efficiency how would you take in account the friction loss on the rail, their must be some or you wouldnt need rail lube? To much for me more than those thoughts for now. Good luck with your findings, and post to see how they relate, I guess. Hunter2:)

[cingold]

Hunter2 said:

Since its not being shot around a riser, will it have the same effect on a crossbow going down a rail? The arrows are shorter and generally more rigid and flex from a high speed I saw was so little will it make a difference? I know more questions, but honestly havent given it any thought other than reading post. In efficiency how would you take in account the friction loss on the rail, their must be some or you wouldnt need rail lube? To much for me more than those thoughts for now. Good luck with your findings, and post to see how they relate, I guess. Hunter2:)

Ok, I see I must eliminate some confusing things...
The goal is a mathematic formula based upon physics that allows me true comparison of two different crossbows, let's say strykeforce and excal. Not something that is usually done because it's two different systems, poundage, draw length, different minimum arrow weights / lenghts, one has silencers mounted etc etc etc, difficult to get comparable data (just follow the thread about testing of bows).
Now, obviously if I want to compare crossbows the mechanic is the same, an arrow propelled by a string over a rail. Don't bother with energy loss due to friction, this is part of the equation. The only factor I'm concerned about is archer's paradox. Comparing bows with that method, energy loss because an arrow is physically affected by archer's paradox must be included, thus my question. If we agree that crossbow arrows are stiff sticks and not affected I can use the formula without a detour, otherwise it's going to be a bit more complicated.
I will work together with some well known professionals on that matter, let's see if they can come up with one comparable number at the end. No more confusing comparisons but one figure based on empiric data, period.

[Hunter2]

but like I said in that super slow motion camera a crossbow arrow vs a vertical bow arrows flight is totally different when it comes to the bending. After seeing the show I wodered how they ever flew straight or did not break. Its almost unbelievable how much they warp and bend etc. Hunter2:)

[Moon]

with crossbows. If you will notice, there is wear on crossbow barrel deck grooves in certain areas where the anodized finish is worn down to the bare aluminum. This must be caused by shaft flexing of th earrow as it leaves the barrel. My point is that the wear is not down the complete length of the deck.

NOW...................this flex absolutely changes from bow to bow and arrow to arrow shaft material and stiffness. I contend that arrow shaft contact with the barrel is minimal (when released) EXCEPT in the areas of outward bend of the shaft which could be the very rear nock area of the shaft as it has the hell knocked out of it by the hit of the bowstring.

I shoot carbons exclusively due to my reasoning (right or wrong :-) that aluminums, although fine for hunting arrows, flex much more than the stiffer carbons, especially in higher draw weight crossbows. Even though crossbow arrows are normally a short 20 ro 22 inches, they are subject to extreme forces of crossbow draw weights, upwards to 385 lbs.

Then there are the point and insert weight variables:D Have fun guys!  My old brain has a tendancy to work these formulas out this way.............if a certain arrow combination all hit in the dot at 50 to 70 yards............it's all good:p

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•  Mini Moon nock.jpg   50.05K   95 downloads

Edited by Moon, 17 June 2009 - 09:17 AM.

[Hunter2]

When you get tight groups of 1.5 inch or what ever or less it makes no matter what happens before it gets there. It is pretty wild to watch arrow flight though. Hunter2

[cingold]

Hunter2 said:

When you get tight groups of 1.5 inch or what ever or less it makes no matter what happens before it gets there. It is pretty wild to watch arrow flight though. Hunter2

Well, that's not the message I try to get out. It matters to me what's happening before, during and after the shot. And at the end I hope to have one double digit number which can be used to rate a crossbow and comparing it to another model. If I'd tell you your Barnett has a rating of 85 while my GT Curve reaches 94, it would add some spice to a selection process. But if you shoot groups of 1,5" over 50yd while I shoot 4" over 20yd? We can for sure eliminate the xbow then, not? Proofing efficiency of the bow, not the shooter is my goal. Other than that I'm just bored and need to do something useless again...

[Hunter2]

I love numbers thats how I choose my bow with graphs ratings to my needs and numbers. Its a deep subject but like a car the more linkage etc you have they are less effient, thus why you see front wheel drive, plus its cheaper to make. A recurve should have the more efficency since its one string vs the compound. Mine might be less effient but with the speed and accuracy I am getting as a hunter it is interesting, but proofs in the pudding as they say. Alot of folks shoot and shoot, I practice just to hunt. To many other irons in the fire. Keep it up I anxious to see how I may be able to use the info for any needs. Hunter2:)

[Moon]

slowest, heaviest, lightest, ugliest, "purdiest", most accurate, least accurate, most reliable and least reliable.  Good luck Cingold on you quest to develope a rating for crossbows.

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•  TB 010.jpg   171.12K   11 downloads
•  TB 009.jpg   164.45K   11 downloads

[StoredEnergy]

There are informal standards for archery efficiency discussed back in 1999 at the AMO show. IIRC it was the ASTM standards committee but no concensus was achieved and no formal definitions were accepted by the committee.  

That being said there are many ways to look at the system of energy storage and compare it to the output (accepted KE equation). You will have to lump in several loss coefficients to use a simple input / output function. The KE/stored is the classic efficiency of the entire system.
In order to measure the force draw curve for any given bow you will need some form of load cell and way of incremental measurement.

The Cam/limb system only store a percentage of maximum available work (peak weight of the bow times the powerstroke). I refer to this as the cam storage effectiveness or cam efficiency. A radical hatchet cam may store 85% or more of the max potential work where a simple wheel will by definition store 70% assuming maximum left-off.

The simplist way of measuring the efficiency would be to define a function of KE output / total potential work. This would be KE / peakweight *powerstroke (in feet).

Now I sound like I am rambling so I will stop but I would be glad to discuss any archery specific engineering topic in a PM.

[xbowguy]

cingold

Yes there is a paradox with the crossbow just like vertical bow.  It is usually less than that of a vertical bow.  The forces at the moment of launch are very great as mentioned by Moon.  As he has stated, a lot of Hortons for example show sign of arrow shaft flex along not only the arrow shaft itself but also the flight groove.

Another way you may wish to analyze this is to shoot through some paper just in front of the crossbow and at various distances.  Yep!  Paper tuning can tell you allot about the shaft recovery.  Most crossbow arrows will show you what vertical guys would call stiff.  Remove the fletching and the tear in the paper will show you allot.  In a vertical system your somewhat counting on the shaft size and material to make the arrow recover as fast as it can.  In a shorter stiffer crossbow arrow it’s more about the fletching and broadhead profile not trying to over steer each other yet offer enough rudder to correct the flight.

Paper tune some of yours arrows at various yardages.  What might be great at one particular distance will surprise you at the others.  Indoor vertical archers will specifically tune their set up for that one distance.  At various yardages some concessions will need to be made.

[SPECIALIZED]

Just out of my own curiosity I have done just what was described about the paper tuning.
I set up the paper about 4 and 8  feet from the crossbow in front of the Spyderweb target.

Shot several different brand shafts all with foc , no fletching with 100 and 150 gr target points.

Very Much to my surprise ,they all made perfect bullet holes in the paper,if only my vertical bows paper tune so well and easy? .

[Michigan Hunter]

I suggest you rite a letter to Norb Malaney of Bow Hunting World Magazine
Not kidding here... I have written Him letters on cam Synchronization
and he wrote back and published my tunning method in Bow hunters magazine.
He loves this kind of stuff and is a fantastic guy. He is an older fellow
also has couple Engineering degrees and  Physics degree or something.
He loves answering questions correctly about stuff like this.  
I usually make sure I get the rite info from the rite source if it is really important to me.

Just my .02;)

[Urban Legend]

How quickly an arrow recovers will also determine how efficient down range it is.  And how quickly and arrow recovers can be directly related to the way the energy is transferred to an arrow, or the draw force curve. (See Norm on that:)).  Two bows shooting the same speed out of the gate can have significant differences in down range speed do to arrow oscillation and drag because of the way the energy is transferred to the arrow.  I don't know what kind of test you are trying to run, but there can be an infinite amount of variables. Good Luck.

[cingold]

thanks for your replies guys, lot's of work waiting for me though. I might take a chance with Norb, good idea.

Jeff, what I play around with has a short history. David Wilkins started a thread on another forum about about increasing arrow weights, KE and speed decrease with increasing arrow weight. I have read about recurve crossbows gaining KE while not losing the same ammount of speed like compounds when a heavier arrow is shot, vice versa etc... And then of course our domestic thread about testing bows and what to consider etc. I have limited knowledge of physics and maths and started reading about the work of Koltze, the inventor of the 1000lbs twin crossbow (not Twinbow!!) I posted recently. His theories allow me certain calculations, upon which I will try to work out more detailed results. At the end (lot's of wasted time, sleepless nights) I can calculate every crossbows physical behaviour, the ammount of energy effectively passed to the arrow, the ammount of energy retained etc.
Hopefully I can help David mathematically backing his theory.

[Urban Legend]

OK.  If you want.  But in the end, dead is dead.

[cingold]

Urban Legend said:

OK.  If you want.  But in the end, dead is dead.

true... no doubt about that   but I can tell you the deers exact birth place and date, and its social security number...

[Urban Legend]

cingold said:

true... no doubt about that   but I can tell you the deers exact birth place and date, and its social security number...

:D:D:D:D:D

[OLDFLYER]

cingold

Quote;

"David Wilkins started a thread on another forum about about increasing arrow weights, KE and speed decrease with increasing arrow weight."

That is totally wrong. Take any given bow where the only thing that is changed is the weight of the arrow. The heavier the arrow, the MORE the KE and of course, the slower the speed.

Did some tests a few years back.
350 gr arrow - 299 FPS - 69.5 ft/lbs
394 gr arrow - 285 FPS - 71.1 ft/lbs
432 gr arrow - 275 FPS - 72.6 ft/lbs
484 gr arrow - 261 FPS - 73.3 ft/lbs
550 gr arrow - 249 FPS - 75.7 ft/lbs
752 gr arrow - 217 FPS - 78.6 ft/lbs

[cingold]

OLDFLYER said:

cingold

Quote;

"David Wilkins started a thread on another forum about about increasing arrow weights, KE and speed decrease with increasing arrow weight."

That is totally wrong. Take any given bow where the only thing that is changed is the weight of the arrow. The heavier the arrow, the MORE the KE and of course, the slower the speed.

Did some tests a few years back.
350 gr arrow - 299 FPS - 69.5 ft/lbs
394 gr arrow - 285 FPS - 71.1 ft/lbs
432 gr arrow - 275 FPS - 72.6 ft/lbs
484 gr arrow - 261 FPS - 73.3 ft/lbs
550 gr arrow - 249 FPS - 75.7 ft/lbs
752 gr arrow - 217 FPS - 78.6 ft/lbs

You got me wrong, he started a thread because there are some weird statements out there, I didn't quote him. Sorry, it sounds strange after reading my own post, english isn't my first and preferred language... I think there's a comma missing after KE
Must actually read what he was posting word for word. I'll pm you the link to the thread he posted. I just want to play around with data. Did you shoot this speeds with the same xbow? I try something quickly...

Edited by cingold, 19 June 2009 - 07:06 AM.