Another question...physics of arrows n bows

[Joe2Crow]

ummm, go shoot your bows, fellas

[**DONOTDELETE**]

Originally posted by Mike Mecredy:
The shooting/tuning part is simple. But when it comes to design there's actually a lot to it, so much that could make two bows that look similar, shoot as different as night and day.

Amen brutha.... Amen....
   

As has been stated the way I understand it the longer the power stroke on the same bow ie. shorter brace height, gives more time to accelerate the arrow. According to my chrony, not much velocity gained. At my hunting distances not enough to matter.

The reason for that is typically the optimum brace height on any bow is when the tension on the string is greatest.....

by running your brace down to 6" you may get a longer power stroke, but the tension on the string is not as tight as it is at 7" and it doesn't transfer the the same amount of energy into the shaft......  

Are we splitting hairs?  You bet we are.    

but that higher preload on the string that stops the forward motion of the limbs mitigates that extra inch of power stroke.   Same thing goes for lengthening your brace beyond the sweet spot, only you are loosing both preload & power stroke....

Clear as mud eh?     :p

[katman]

Certainly agree with you Kirk as we have had this discussion before. Set bow at best brace height ie. maximum tension which yields quietest and least vibration, and leave it alone. Use other variables to tune.  

[Mike Mecredy]

Also, there are some of us that actually like to discuss such things, we get a lot from each other, especially those of us that sell bows. Reading things might make a little light bulb come on, we try it, it improves what we already build, and it transfers on to those that buy from us.

[moebow]

OK,  Look at the force draw curve Kirkll posted earlier.  There if you look at the brace height entry, it starts at 9".  Now look at the LITTLE triangle formed between 9 inches and 10 inches.  That triangle is the energy difference between  a brace height of 9 to 10 inches.  See how small it is??

Now look at the 28" draw length and the 29" draw length.  See how much larger the area below the curve is?

My point is that with trad equipment, draw length has a MUCH greater effect on the energy available to produce a force than a similar change in brace height.  One inch vs. one inch.

Now we get into individual shooter characteristics.  It is self defeating to TRY to artificially increase your draw length. Your draw length is your draw length!  That leads to all kinds of form problems and loss of accuracy.  But with trad equipment, a brace height change is nearly negligible for velocity change. Since the energy change is so small at brace height compared to the full draw end.

Arne

[McDave]

Arne, if I'm reading Kirk's chart correctly, the brace height is 7 1/2". This is measured from the string to the inside of the grip.  Draw starts at 9", with zero force, because draw is measured from the string to (approx) the back of the bow, a difference in this case of 1 1/2".  The energy stored in the little triangle between 9 and 10" is the energy stored in the first inch of the draw, which is different from the effect on total stored energy of increasing or decreasing brace height by 1".  To bend the limbs by the same amount they would be bent by increasing the brace height by 1" would require much more than 1" of draw.  So we're really talking apples and oranges here.  Increasing the brace height by 1" would require a whole new force draw curve, starting with zero force at 10", and the effects would be measured over the whole length of the curve.

[moebow]

That's not how I'm interpreting it but you may be right.  Still, the area under the curve at OR NEAR brace height is very small compared to the area under the curve at 28/29".  My point still is that the brace height adjustment has a very small effect on the force applied to the arrow compared to a full draw length change.

Arne

[**DONOTDELETE**]

McDave,.... You are correct. If we changed the brace height on the same bow i used as an example on the DFC chart from 7.25" to 6.25" The weight measurement in the first inch of draw would be lower, and would change the whole chart a bit. The amazing part, or the part that makes you scratch your head is that you would see a lower velocity coming out of the bow..... Granted... its a rather small amount. maybe 2-3 FPS. but you can feel the buzz in the riser caused by the vibration....That vibration is lost energy that stayed in the limbs.


(Btw... that chart should have read 7.25" on top for actual brace height instead of 7.5")

This one is a real hard one to wrap your head around, but sometimes just watching the difference in slow motion what that string & limb is doing after the arrow leaves the bow can be an eye opener.

check out some of these video clips. i've got a bunch of different bows on photo bucket that i filmed..... i'll have to post that link later ... photo bucket is down right now.

[Gray Buffalo]

Some of you over think a fun sport' This is suppose to be simple and fun.

[**DONOTDELETE**]

Well Grey Buffalo.... if somebody isn't interested in the physics of how these bows work, they would all be the same and no improvements would ever be made...  To a lot of us... this IS fun....

Here are some interesting slow motion videos you might enjoy.

 

[Homebru]

ChuckC,
Let's go beyond the bow and look at the arrow.

You thought about the gun barrel length and the unburned powder and gas temp and such.  Let's think about the arrow.

The arrow bends at the release of the string.  To some extent, it will continue to bend to a point (in time) at which it starts to "un-bend".  It can "un-bend" three different ways (if we only consider one dimension - lets say "out" from the sight window).

Scenario 1:  the arrow never completely "un-bends" at the time it leaves the string.  This will probably be an "inefficient" scenario.

Scenario 2:  the arrow completely "un-bends" and then starts to bend "the other way" before leaving the string.  Again, an "inefficient" scenario.

Scenario 3:  The arrow completely "un-bends" or unfurls to be perfectly straight at the exact moment that the arrow leaves the string.  This is, in my own speculation, going to be the MOST efficient scenario.

This is just one small bit of arrow tuning that could effect the overall "efficiency" of your system.

Now, all above being said by everyone, you could create the most efficient bow / arrow combination but find that it is so sensitive to changes in release that it's impossible to hit the "sweet spot" without a machine.  You then have to cut some corners on efficiency to improve overall shootability.

Interesting question(s).  Would be fun to sit around a table and discuss over a beverage or two.

homebru

[**DONOTDELETE**]

Homebru,

Scenario 2 & 3 are not going to happen because the arrow doesn't quit flexing until it comes off the string.... as long as the force is applied to the end of the shaft its going to stay in paradox....

watch this video... its got some real slow motion showing two different arrow spines coming out of the same bow.

Another thing to take a close look at is how the most force put on the arrow is in the last 2" of the power stroke.

   

[Homebru]

Originally posted by Kirkll:
Homebru,

Scenario 2 & 3 are not going to happen because the arrow doesn't quit flexing until it comes off the string.... as long as the force is applied to the end of the shaft its going to stay in paradox....

Not saying your incorrect or that you've done anything wrong......always room for lots of armchair speculation on all of this.

The point I was trying to make is that there will be a "most efficient point" in the paradox of the arrow.  The Goldilocks principle.  Not too stiff, not too soft.  Just right
homebru

[ChuckC]

In the video, at the very end, is the slow motion of the recovery of the bow limbs,  Does that string look as if it is going in circles instead of just back and forth ?

Watching each of the arrows, during that last two inches prior to release from the string... it makes the arrow jump sidewise, most so with the heavier spined arrows.  At first I thought it (arrow nock end) was bouncing off the riser.

What is up with that ?  Why ?  Maybe nocks too tight ?  Maybe that string "spin" I mentioned ?

Interesting video there, thanks
ChuckC

[**DONOTDELETE**]

I don't think you would see that jump sideways as much with a fingers release because the string is rolling off your finger tips instead of coming straight off the caliper release jaws at a slightly different angle.

I saw a hooter shooter release one time that pushed the string off the end of a steel rod that matched a fingers release a little closer than the caliper release does.... Maybe i should look into that closer and see what the difference shows.

I saw a slow motion video done with a shooting machine years ago showing the paradox with a compound and a longbow.  With the compound there was a minimal, if any inward bend at the initial release, even less than you get with yours.  However, with the longbow there was considerable inward bend at release.  I assume that is because the bow is not center shot and the arrow resisting acceleration, it has no choice but to bend inward.  A simple test, take a fishing rod set the tip off to the left and then push it straight forward, it will bend to the right every time.  I was told that initial inward bend is because of the fingers.  The Hooter Shooter proved it was because of the outside of center arrow angle.  More center shot adds to arrow spine forgiveness.   I wonder if having an arrow outside of center adds something like a stable launch point where the arrow positively gets its flex going.  Recurves for many years were all outside of center.  Dick Robertson told me that it helps with longbows to have something for the arrow to fly around.  Could that be because of the dynamic differences from wood arrows to carbon arrow?

[JMR]

This is way more interesting than the average bare shaft tuning posts so keep it coming.

[**DONOTDELETE**]

Hey Pavan.... You brought up some good points on the cut to center issue. What i need to do now is set my ILF rig up in the hooter shooter and shoot some film at true center shot with different spine arrows.....

You mentioned the "Spine" being more forgiving as you get closer to center shot. This i agree with. you can use your strike plate thickness to fine tune the arrow flight... But lets throw something else out there just for speculation fun....

Lets take a bow that is cut an 1/8" past center and tune it with the stiffest shafts we can....  
Now take the same bow, and tune some weaker spine arrows by shimming the strike plate out to accommodate more paradox flex....

 Would a weaker spine arrow that goes into more paradox be more forgiving to a less than perfect release using fingers than the stiffer shaft closer to center?

This question could stimulate some interesting theories..... What do you think guys?

[McDave]

 Would a weaker spine arrow that goes into more paradox be more forgiving to a less than perfect release using fingers than the stiffer shaft closer to center?

This question could stimulate some interesting theories..... What do you think guys?

If you agree that a bow cut closer to center will be less sensitive to spine variations than a bow not cut as close to center, then I would have to think that it would also be less sensitive to release errors.  Just my guess, of course.

Of course a lot of variable come into play when any bow is hand held.  Also, shooting a bow that is far out from center, even, though I have had very good successes with such a bow, does rely on something different from the arrow that a center shot bow can avoid.  In theory the center shot bow should win on all counts.  Providing I am taking care to control my release, I can stack arrows tighter with more deliberate procedures with a center shot bow.   When things start going off, a whole series of variables comes into play, so I would guess that different aspects would show either a better or worse reaction through the bow limbs and/or the arrow, again depending what went off.  It would be very interesting to see in slow motion what happens when I have my more typical screw up with either type of bow.   My own conclusion from the difference from a cedar arrow to a carbon arrow is that the carbon arrow, in my hands, seems to have a narrower happy window than a cedar arrows.  What I do not know is, if it is me, the arrows or the bows.