25 replies to this topic

[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

ok, with the data you provide I can tell you so far:
if you dry fire your crossbow, the string velocity will be 654fps. And your crossbow is transferring 81,59% of its stored energy to the arrow, assuming the string itself isn't eating up energy of course. Now, if you have a second crossbow shot with the same arrow weights, we can compare theoretical efficiency. I'm still busy including string, rail etc but ya, it is exceeding my current time and knowledge. there are some nicks in the formula of course, powerstroke, cam/wheel efficiency etc but one day.....
See, I want to compare a compound with a recurve xbow. Eliminating all the factors that swallow energy transfer, there should be a possibility to make some efficiency statements after all. I understand a recurve xbows velocity doesn't decrease the same than a compound when increasing arrow weight? I don't know, true or not? We will come close to a method now

Addendum: I've calculated expected speed with a known arrow weight, the mathematical value differs 1 footsecond from what you shot with the 484gr arrow... Nifty, you only lose 1fps in real life. Impressive

Edited by cingold, 19 June 2009 - 07:30 AM.

[Michigan Hunter]

Hey I know....how about moon listing all the crrossbows he has hanging there so I can know what they are...I can see a strykeforce:D

[rt2bowhunter]

archer's paradox  yes it does, not near as much as with a trad bow but as moon pointed out those tracks on the rail are from archer's paradox. Not many understand what archer's paradox is (beyond the arrow bends when its shot) or what it is used for. Or even how to adjust it. But the short version is if you add weight to the tip (high FOC) you will add to the paradox or flex of a given arrow on a given bow. On the other hand if you add weight to the back of the shaft the reverse will happen. Also a longer arrow will have more than if you take that same arrow and cut an inch off of it or in the case of a SF vs Ecal 2 inches. If you ever saw a slow motion video of a arrow shot you might wounder how can that snake looking thing ever hit anything. Theres some thing else that has to do with arrow flight its the nodes of the arrow. But thats something else.
At a local bow shop we took a spine tester and checked 2 doz arrows one afternoon. we found 6 that were real close. same brand they came on the UPS truck same order. I would think archer's paradox would be very hard to account for theres so many things that can change it. When looking at compounds vs recurves the paradox happens at the release on a recurve and the compound it happens later in the power stroke because of the letoff.

[Lrodshy]

My idea is to completly eliminate said paradox, so it can transfer 100% of stored energy to  shaft/bolt. I have three (3) ways to do it
however, don't know ..yet if indeed it should be eliminated , until I hear back from other experts! am not one, just have ideas
Also, how should shafts/bolt be spined with sheer force and no slamming against the rise block/arrow rest
PLease responed, as no one did in Yahoo arrowmaker group , need feed back


Sincerely, Leo Roderick Guajardo, 11...in Dallas

[See4miles]

Classic discussion of archer's paradox centers on the requirement of the arrow to flex for riser clearance. This was a necessary ingredient, and still is to a finger shooter and a vertical bow. When compound bows evolved with equipment such as release aides, the archer's paradox was not a requirement in the "classic" sense.  Side play caused by the release was almost eliminated.  Then comes the crossbow in which there is very minimal play up/down, or to either side.  
The archers paradox then evolves to "shot" spine, or the free-flexation of the arrow upon clearing the guide-rail. Spine, correct spine is still critical, just not in the classic sense.
Moon is totally correct in his observations of arrow flexation while still engaging the rail, but the spine requirements are quite different in a crossbow vs a finger-shot vertical bow and I would somewhat tend to wonder if it can be termed archer's paradox any longer?

[xbow755]

Hi Guys,
I think you've wondered off the scientific track a bit. For openers,

Archers Paradox is not a matter of vertical bow or crossbow.

It's a matter a physics and it applies to any projectile regardless of what the device is that applies "Force" and "Resistence" to that projectile.  Force in the case of a bow or a crossbow is appied to the arrow shaft by the String or Cable. Resistance is a result of the weight of the point of the arrow shaft.

It also doesn't care if there's a riser or not. The riser on a bow just so happens to support the arrow rest, which is where the Archers Paradox is taking place. We are talking about the extreme Dynamic Spine Oscillation that takes place during the launch of the projectile.  The Force and Resistance causes this oscillation at the mid point of the projectile because that is where the two opposing forces are the greatest.

The direction of the oscillation depends solely on the alignment of the stiff side of the arrows spine in relation to the positioning of the arrows nock.  If each arrows nock is not aligned in the same manner, to the stiff side of the arrows spine, then there is no consistency from arrow to arrow, in which direction these oscillations are taking place.

This is not a theory, this is the science behind dynamic spine of an arrow and how engineering physcics play a major role in arrow performance.

I hope this helps with your understanding of Archers Paradox.

Regards,

Xbow755