Draw length ?????

[richard roop]

#149650

Question on measuring draw length;

Is there a standard way of measuring draw length ???  Back of the bow ??? ……….. To the arrow rest ???? ……….. Even with the deepest point of the grip ????  Do the different bow builders all measure the same ???

Seems like something I should know since I’m trying to set up a scale to check the marked draw weights of my bows.

This all started when I noticed that a Bear 48 Magnum marked 60@28 seemed kinda easy to pull. Arrow flight / speed is so-so with 2213s. Just for yucks & giggles I tried some 1913s ……… awesome flight and really fast.  I can understand a couple of pounds difference Bear to Pearson to Damon Howatt but 1913s out of 60#s ??? Gotta check this out.

[Stephen Graf]

#149656

Draw length is measured from the bottom of the nock to the back side (away from archer) of the arrow shelf.  I think most all bowyers stick to this standard.

Brace height can affect draw weight.  If the brace height is very low, the bow can lose several pounds of draw force.

The measured draw force can also be affected by how the bow is pulled on the scale.  If you use your hands to pull the bow on the scale, keep your hand on the grip when testing the bow.  It is common to use both hands when pulling the bow, one on the grip and one on the sight window/arrow shelf area.  Using 2 hands this way will give a false reading.

I have found that arrow spine can have a surprisingly large effect on arrow speed.  An arrow of correct spine can have a 5 or 6 fps advantage over an arrow of incorrect spine.

[richard roop]

#149659

Ok,…. Much Thanx !!!!!

One more quick question that I’ve wondered about; Is the measured draw force the same as the bow is being drawn as it is when the bow is being let down ???? Seems like it should be but then things aren’t always what they seem to be.

[Stephen Graf]

#149668

If there was no such thing as friction, then the draw-force curve of a bow being drawn from brace to full draw would be exactly the same as the draw-force curve of a bow being let down from full draw to brace.

Thankfully friction is a real thing, and in the case of a bow we call it internal friction or hysteresis.  Internal friction in the bow is friction between the fibers, molecules, and atoms that make up the limb structure that occurs as the limbs are being bent.    Since friction is a real thing, it has an effect on the draw-force curve as the bow is being drawn and let down.  When we measure the draw force as the bow is being let down, we see that it is less than it was when the bow was being drawn.

When we draw the bow, hysteresis increases the draw-force needed to pull the bow.  As the bow is being relaxed, hysteresis decreases the draw-force available to push the arrow.  Friction always acts in the opposite direction of motion.

There are a few laws of nature that are immutable.  One of them is the 1st law of thermodynamics which states that energy cannot be created or destroyed.  The astute reader will ask, what happened to the energy we put into the bow when we drew it, if it isn’t available as the bow relaxes, to push the arrow?  The answer is that the energy was converted to heat through the process of internal friction.

If you put a thermometer that was sensitive enough on the bow, you could measure its temperature rise as the bow is being drawn and let down.  You can feel this effect for yourself by bending a piece of wire.  As you repeatedly bend a piece of wire, it will get so hot that you will likely have to drop it.

And thankfully, our bows don’t get that hot 😜

[richard roop]

#149670

…………… and this seems like such a simple sport; put the pointy end of an arrow toward the target, yank it back & cut it loose.  The reality of it appears that just an arrow would take several books of Physics to cover, the bow a pile of Physics books to explain and at least a thick pamphlet of bowstring info. Add a small library on shooting form and it’s a wonder we haven’t all gone mad.

[Ralph]

#149687

I think if we overthought riding a bicycle like we do shooting a bow we’d be busting our butts on every attempt to ride.

I’ve been through the “thinking” stage and gave it up.  Nowadays I just diddle til it works and contribute it to man’s ability to make things work, go shoot my my bow and over the years have I’ve out a miss is me not any of the other billions of what if’s, molecules and ………..

Saying all that I still have gravity and friction and wind excuses…and I know some of those targets butts jump the string…

Speaking of 1916’s…Back in the day if you weren’t shooting 2117’s you didn’t know what you were doing…Now that I’m shooting 47# and 42# bows I’m simply amazed what 1916’s do….

 

Never overlook common sense ……………..nor experience……………..after 60 or so years of bowshooting and tuning I just kinda know what’s gonna work and what maybe will….Explain to you?? Doubtful…Show you one on one very likely…

[Ralph]

#149689

Another thought….Kentucky windage may not get a rocket to fly by and take pictures Pluto but it sure can put an arrow in the vitals…

[richard roop]

#149698

Yeah, I’ve told people that I started shooting back in the 20s ……………….. 2016, 2018, & 2020s were the thing.  Then the hot ticket was the 2117 & the 2219.  Somewhere in that phase Bob Jenson started using the way too light 2213s to hunt deer with. Scandalous but the guy just kept putting deer in the C.B.H. record book until we took notice.  This was back when we thought a compound shooting 235 fps (fingers / no sight) was ‘smoking’.

Good times past.

[Author]

Posts