Measuring bow draw weight



# Thread: Measuring bow draw weight

1. ## Measuring bow draw weight

OK, I guess I have too much time on my hand tonight. I was checking the draw weight with my bow scale and thought, "do I have to subtract the bow mass weight from the scale weight, I am pulling vertically down. Scale hangs from the ceiling and i get my draw weight by pulling the bow downward. So am I adding the bow weight to that and should I subtract that tare weight? yea, I did really bad in physics....lol. Thoughts?

2.

3. Originally Posted by pilotmill
\ "do I have to subtract the bow mass weight from the scale weight, I am pulling vertically down.

Nope. You just don't have to pull down as hard, because of the free assist from gravity. The scale's reading is still valid.

4.

5. F=k (x2-x1). x1 is the deflection of the spring before drawing the bow and x2 is the deflection after. Think of this as a cycle.

1) Hang the bow from the spring. Read the poundage
2) Pull the bow down to draw length. Read the poundage

Draw weight is the difference between these two measurements since that is based upon the spring deflection caused by drawing the bow only. If my reasoning is correct then you would have to subtract the bow's mass when using your particular setup. A good way to check would be to mount the scale to the floor and draw up to see if you get the same answer or an answer that differs by the bows mass.

Of course, if your scale reads zero with the bow hanging from it then you can read the weight right off the scale.

Or......

Since you are holding the bow, you, rather than the scale, is supporting the bow's mass. In this case, you would not have to subtract the bow's mass.

I think you better do the experiment I outlined above. Otherwise, this could turn into a protracted discussion with no definitive answer. It confuses me and I have a PhD in Chemical PHYSICS. (Of course after this answer USC will probably ask for it back.)

6.

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If my reasoning is correct then you would have to subtract the bow's mass when using your particular setup.
Stick to the chemical physics !. You do not have to subtract the physical weight of the bow. If the bow weighs 5# and you have a 50# draw weight bow you only need to exert an additional 45# of force downwards to get the bow to full draw. However, the reading will still, correctly, show that it took 50# of downwards force to get the bow to full draw. In the case of a scale on the floor and pulling upwards to get the reading you'll need to exert the full 50# of force, and that's exactly what the scale will read when the bow reaches full draw.

The only caveat to this is a spring bow scale laying on a table surface (i.e. a draw board) or being pulled horizontally. I believe then you may get different readings other than 50# due to possible friction in the scale units being drawn in these manners.

>>----->

8. Originally Posted by CHPro
Stick to the chemical physics !.
>>----->
Well they do call USC university of second choice. I was ready to subtract the weight of the guy pulling the bow.

9. Best way to measure draw weight is with something like Mapper where you draw the fully kitted out bow as normal.

With a vertical hanging bow from scales there's always the problem that the bow mass is spread about and not vertically beneath the suspension point. You get different readings if push/pull down on the riser or the string (whatever way you zero the scale).

Best approach is the traditional tiller, fix the riser and pull down on the string at the nocking point. If you just have a hook, hang the riser from the scale, zero the the scale and then pull down on the string.

10. Perhaps an extreme example would help. Say you attach a horizontal bow scale and draw the bow like you would if you were shooting. Say the draw weight measures 50# at 28". Now hook the bow on a vertical scale and push down enough to get to 28". Read the scale. Probably will read 50 pounds. Oh, by the way, this riser weighs 40 pounds. You would not want to subtract 40 pounds from 50 pounds and say that it has a 10-pound draw weight.

11. OK, then. While we got some physics going. If an archer is falling at terminal velocity of 120MPH (176 feet/second), and shoots an arrow at 150 feet/second, will the arrow travel backward in relation to the falling archer? This question, only a tangentially related question to the above gravity/weight question.

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I woder how the arrow manufatures designed and classed their arrow spine base on; true limbs pull weight or Bow weight + limbs pull weight.

Any takers?

I am using the limbs pull weight to select my arrow spine. For your information; I found differences between vertical and horizontal masurements, 4 pounds as the difference.

13. Originally Posted by 22big
I woder how the arrow manufatures designed and classed their arrow spine base on; true limbs pull weight or Bow weight + limbs pull weight.
If you start the scale at -0-, there is no bow weight to consider. It's all just pull against the scale spring sans the bow's weight. If you could let go to allow the bow's mass to add to pull, it "could" register, but obviously, that won't work.

Test: How much does the bow weigh when you fully support it on the scale? That's the weight the bow's mass exerts when you fully support it at any position of draw.

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Seriously guys, the bow weight does NOT get added if you are pulling down on the scale.

Zero the scale with nothing on it, hang the bow on the scale via the string center. Two things happen. You get the weight of the bow and you also see exactly how much draw length you get at that weight. It's not like there isn't some flex happening in the limbs.

Don't over think it. Just do it.

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Originally Posted by CHPro

The only caveat to this is a spring bow scale laying on a table surface (i.e. a draw board) or being pulled horizontally. I believe then you may get different readings other than 50# due to possible friction in the scale units being drawn in these manners.

>>----->
and of course if the bow weighs more than the draw weight.

16. Generally, not just in archery one of the main sources of error is the unjustified assumption - often there is not even any awareness that an assumption is being made.

In this case the assumption being made is that the increase in draw length due to the weight of the bow is identical to the increase in draw length if the equivalent weight were applied by the archer's hand on the bow grip.

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Just curious tie a three pound dumbell on your bow and see if it reads different. When you measure your draw weight it is the weight where your cam breaks not full draw.

18. wow

19. Originally Posted by whiz-Oz
Seriously guys, the bow weight does NOT get added if you are pulling down on the scale.

Zero the scale with nothing on it, hang the bow on the scale via the string center. Two things happen. You get the weight of the bow and you also see exactly how much draw length you get at that weight. It's not like there isn't some flex happening in the limbs.

Don't over think it. Just do it.
Exactly, I don't know how something that simple can become so complicated.

20. There is a difference between when you pull the bow pointed up, and pointed down.

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Originally Posted by Joe T
Generally, not just in archery one of the main sources of error is the unjustified assumption - often there is not even any awareness that an assumption is being made.

In this case the assumption being made is that the increase in draw length due to the weight of the bow is identical to the increase in draw length if the equivalent weight were applied by the archer's hand on the bow grip.
I'd think it is close enough, since the weight of the riser does deflect the limbs, and its additive. Granted the force vectors are not 100% identical to a horizontal pull, where you have torque from gravity in addition to spring force, but the question is are they close enough for measuring draw weight with reasonable accuracy?

But, you are the experienced physics guy. What are the experimental differences you have recorded so we can get an idea of whether the difference is significant, between the recorded draw weight of a bare bow suspended by its string, vs. a bare bow drawn horizontally? What about other kinds of rigs, including FITA recurve with stabs, and compounds?

22. I am guessing not many archers here own a digital hand held bow scale.....

23. Imagine you have a 40lb pair of limbs.

Now imagine your riser weighs 2lbs and your limbs weigh 40lbs each.

I hope that clears up the air slightly.

24. If you zero the (pull-down) scale with nothing on it, then hang the bow by the riser and pull down on the string to draw length, the measured force will be draw weight PLUS the weight of the bow.
If you zero the (pull-down) scale with nothing on it, then hang the bow by the string and pull down on the riser to draw length, the measured force will be the draw weight (the weight of the string being considered negligible).

In the second case, the force that you put into the handle to get to draw length is the actual draw weight MINUS the weight of the bow, because that weight is helping you in that situation. When you hold the bow in your hands and draw it, the weight of the bow is no longer assisting you and you have to supply the full force of drawing the bow.

So, bottom line, don't overthink it. Zero the scale. Hang the bow by the string. Pull down to draw length. Read draw weight from the scale.

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