Trad archery broadheads planing

[okie archer]

I have practiced hard this summer. Trying to fine tune my set up. I have been practicing with my broadheads but they seem to shoot to the right about 5"-6" compared to field tips.
That was shooting odd feather out. I turned the nock to make the odd feather in (against riser) and that seems to bring it to left a bit. Now it's just about 3" to right compared to field tips.
I'm shooting full length gold tip arrows with 3 feathers. Total arrow weight with 125 grain tips is about 525 grains. With my draw length I'm pulling approximately 55#.
I have tried several different broadheads and they all pretty much shoot to the right.
Magnus Stinger with bleeder blade.
Magnus two blade no bleeder.
QAD 3 blade.
Muzzy 4 blade.
Not sure what to do to make broadheads shoot same as field tips.

 

[Mighty Mouse]

For a right-handed shooter, broadheads hitting right of field points indicates weak dynamic spine. Shortening the arrow, reducing front end weight, and switching to a stiffer static spine are ways to strengthen dynamic spine. Shifting the rest further away from the riser or building up the shelf would also help.

 

[okie archer]

For a right-handed shooter, broadheads hitting right of field points indicates weak dynamic spine. Shortening the arrow, reducing front end weight, and switching to a stiffer static spine are ways to strengthen dynamic spine. Shifting the rest further away from the riser or building up the shelf would also help.

Forgot to mention the arrows are 400 spine.
Of everything you mentioned I'm thinking of shifting the rest further away. It's to close to season I don't want to shorten arrows.
I gap shoot and afraid that will change to much on my point of aim.

 

[TrumpkinTheDwarf]

At what distance?
Definitely sounds like the arrows are weak. You can try lighted nocks to stiffen up the arrows a bit and try to keep your gaps. But also, a 1/4" change in arrow length is unlikely to have a massive impact on your gaps, unless you have a very short point on distance.

 

[BackofBeyond]

Like’s been said, your under spined. Shorten the arrows, take weight off the front, or add weight to the back, or a combo of all three.

 

[okie archer]

At what distance?
Definitely sounds like the arrows are weak. You can try lighted nocks to stiffen up the arrows a bit and try to keep your gaps. But also, a 1/4" change in arrow length is unlikely to have a massive impact on your gaps, unless you have a very short point on distance.

20 yards is where I practice most. Gap is about 9" @ 20 yards. Point on is approx 25 yards.

 

[JohnCushman]

Underspined

 

[TrumpkinTheDwarf]

Okay, that difference at 20 yards is tuneable. Try taking 6-8 twists out of your string, and shooting with both fletching orientations again. You're close.

*Edit to add that yes, it is counterintuitive to drop brace height when the tune is slightly weak, but just try it... I don't understand why that stiffens the dynamic spine, but it does.

 

[okie archer]

Ok so today I shot a 100 grain broadhead instead of 125 grain. Also I put a nocturnal on the back which is about 10 grains heavier.
The 100 Magnus 2 blade still shot right a bit but I shot a 100 QAD Exodus 3 blade bh and it seems to shoot same as ft.
I'm getting there I think. I appreciate all the helpful comments.

 

[R.K.]

Okay, that difference at 20 yards is tuneable. Try taking 6-8 twists out of your string, and shooting with both fletching orientations again. You're close.

*Edit to add that yes, it is counterintuitive to drop brace height when the tune is slightly weak, but just try it... I don't understand why that stiffens the dynamic spine, but it does.

It has to do with less force on the arrow. Abandon all hope, ye who read on. Let's get technical with some basic physics:

Lower brace height = longer path of string travel from same full draw = longer distance of force action on arrow.

With the same draw length, you hold the same weight, and you're holding the same potential energy, and adding the same potential energy to the arrow.

Energy imparted to the arrow is Force (called F) * distance (called D)- defined as E = F*D. So if the energy at Brace height A is the same as Energy at Brace Height B (you have the same draw weight), then the "force times distance" in both conditions are equal to each other:

Fa * Da = Fb * Db

Rearranged, you get:

Fb = Fa * Da / Db

Since Distance B is greater than Distance A, that means that Force B is going to be less than Force A. So in effect, dropping the brace height puts less force on the arrow. Less force on the arrow means it doesn't bend as intensely. And you have now tuned your bow to the arrow, instead of tuning your arrow to the bow.

Ok- you should really stop reading now. You've been warned.

To go deeper- spine is a measure of total shaft deflection. Static spine measures it with a set weight (1.94 lbs), at a set point (dead middle), with supports a set distance apart (28inches), and not moving. It tells you how "stiff" an arrow is, and is a good starting point to tell you how well it can hold up to a dynamic (changing forces) environment. Dynamic spine is a measure of total shaft deflection in the dynamic environment (changing forces). This is the one that really matters. It's also why there are charts for your arrow spines that vary with point weight, draw weight, arrow length, etc.- they're trying to give you a way to achieve dynamic spine with a static number. Pretty good sales pitch, really.

To relate this to you- dropping the brace height (lengthening the string) creates a less demanding dynamic environment that your arrow is better suited to.

To really understand spine, you need to understand moment of inertia calculations. But in practice: The arrow bends (deflects) because it has mass along it's length and at the tip, and none of that mass wants to move (property of inertia). When you release the arrow, that force of the string acts against the inertia of the arrow mass distributed along it's length, and acts as a compressive force. And long, skinny, compressed things bend.

Arrows of a certain size and strength will bend a certain amount per applied force, and per unit length. So more force = more deflection. Longer arrow = more deflection. And funny enough, the location of that weight effects things- the farther away the weight from the applied force, the more deflection you get- the inertia you're compressing against has more distance to act. **The thing to keep in mind is that heavier shafts are usually stronger, and have a lower deflection constant. Changing arrow shafts changes things- who knew?**

So let's say you increase the point weight. More mass wants to "not move" more intensely- there's essentially more to push against. If you add more mass at the tip, then the same "strength" shaft will experience a higher compressive force during the launch impulse. And it will deflect more.

Let's say you shorten the arrow- less unit length, and less total deflection. Try bending a 10ft piece of PVC, and then try bending just 1ft of that same pipe. How much total deflection did you get from the two? It's a lot less from the 1ft. Same basic principal.


With that all said- the arrows will spring back from bent towards straight after they are no longer experiencing the compressive force. But they have enough energy in the bend that they overshoot "straight" and end up bent the exact opposite direction. It's not 100% efficient, so the second "bend" isn't as intense as the first. BUT! This allows for a general average straightness between the two oscillating conditions, allowing you to shoot straight.

That first deflection is going to be more intense than the second, always will (Further reading: 2nd law of Thermodynamics). SO- if that first bend is too intense, then the first bend will just be too big to be swallowed by the average. Which means the general direction of travel is going to be in that direction- towards the bow riser. Underspined arrows travel in towards the bow riser. Field points have a lot of drag in the tail, and none on the front- so the fletching corrects this quick enough to not really notice. But if you add a big air catcher on the front (like a broadhead), then the fletching drag can't correct fast enough, and you'll see the changed direction of travel revealed as your broadheads not grouping with your field points- or "planing". So if you get your arrow spined correctly, then the planing doesn't occur from this (could still be misaligned broadheads not spinning true).

Bonus: This brings us to "The archer's paradox"- how can a fletched arrow be shot without fletching contact on the riser? This is where you have the arrow perfectly spined so that it bends the shaft out of contact with the riser during release, but once the nock is off the string, and the arrow rebounds to straight, and past straight. So the arrow was shaft-out to the riser, but fletching-in. Then on the rebound, it is shaft-in and fletching-out. A properly spined arrow allows for this to happen as the fletchings pass the riser- so essentially you are bending the arrow shaft out of the way of the riser for the first half, and then timing the rebound so the fletchings out of the way of the riser at the end of the shot. Damn cool to watch in slow motion.

Now go have a drink, because you earned it if you finished reading all that.

 

[Rzrbck918]

Do you guys know how much better our prehistoric ancestors would have eaten if they had simply realized ash arrows didn't have a strong enough spine?

 

[R.K.]

Ok so today I shot a 100 grain broadhead instead of 125 grain. Also I put a nocturnal on the back which is about 10 grains heavier.
The 100 Magnus 2 blade still shot right a bit but I shot a 100 QAD Exodus 3 blade bh and it seems to shoot same as ft.
I'm getting there I think. I appreciate all the helpful comments.

You're still underspined. QAD has less surface area, catches less air, and will "plane" less during the initial bend of the arrow. Check my last comment if you want more details, and are OK not thinking for a couple of days.

 

[R.K.]

Do you guys know how much better our prehistoric ancestors would have eaten if they had simply realized ash arrows didn't have a strong enough spine?

They were still terrible cooks, I bet. Salt was hard to come by. No pepper. Buffalo dung fires. No amount of fiddling with arrow spine will erase the taste of buffalo crap smoke.

 

[Mighty Mouse]

It has to do with less force on the arrow. Abandon all hope, ye who read on. Let's get technical with some basic physics:

Lower brace height = longer path of string travel from same full draw = longer distance of force action on arrow.

With the same draw length, you hold the same weight, and you're holding the same potential energy, and adding the same potential energy to the arrow.

Energy imparted to the arrow is Force (called F) * distance (called D)- defined as E = F*D. So if the energy at Brace height A is the same as Energy at Brace Height B (you have the same draw weight), then the "force times distance" in both conditions are equal to each other:

Fa * Da = Fb * Db

Rearranged, you get:

Fb = Fa * Da / Db

Since Distance B is greater than Distance A, that means that Force B is going to be less than Force A. So in effect, dropping the brace height puts less force on the arrow. Less force on the arrow means it doesn't bend as intensely. And you have now tuned your bow to the arrow, instead of tuning your arrow to the bow.

Ok- you should really stop reading now. You've been warned.

To go deeper- spine is a measure of total shaft deflection. Static spine measures it with a set weight (1.94 lbs), at a set point (dead middle), with supports a set distance apart (28inches), and not moving. It tells you how "stiff" an arrow is, and is a good starting point to tell you how well it can hold up to a dynamic (changing forces) environment. Dynamic spine is a measure of total shaft deflection in the dynamic environment (changing forces). This is the one that really matters. It's also why there are charts for your arrow spines that vary with point weight, draw weight, arrow length, etc.- they're trying to give you a way to achieve dynamic spine with a static number. Pretty good sales pitch, really.

To relate this to you- dropping the brace height (lengthening the string) creates a less demanding dynamic environment that your arrow is better suited to.

To really understand spine, you need to understand moment of inertia calculations. But in practice: The arrow bends (deflects) because it has mass along it's length and at the tip, and none of that mass wants to move (property of inertia). When you release the arrow, that force of the string acts against the inertia of the arrow mass distributed along it's length, and acts as a compressive force. And long, skinny, compressed things bend.

Arrows of a certain size and strength will bend a certain amount per applied force, and per unit length. So more force = more deflection. Longer arrow = more deflection. And funny enough, the location of that weight effects things- the farther away the weight from the applied force, the more deflection you get- the inertia you're compressing against has more distance to act. **The thing to keep in mind is that heavier shafts are usually stronger, and have a lower deflection constant. Changing arrow shafts changes things- who knew?**

So let's say you increase the point weight. More mass wants to "not move" more intensely- there's essentially more to push against. If you add more mass at the tip, then the same "strength" shaft will experience a higher compressive force during the launch impulse. And it will deflect more.

Let's say you shorten the arrow- less unit length, and less total deflection. Try bending a 10ft piece of PVC, and then try bending just 1ft of that same pipe. How much total deflection did you get from the two? It's a lot less from the 1ft. Same basic principal.


With that all said- the arrows will spring back from bent towards straight after they are no longer experiencing the compressive force. But they have enough energy in the bend that they overshoot "straight" and end up bent the exact opposite direction. It's not 100% efficient, so the second "bend" isn't as intense as the first. BUT! This allows for a general average straightness between the two oscillating conditions, allowing you to shoot straight.

That first deflection is going to be more intense than the second, always will (Further reading: 2nd law of Thermodynamics). SO- if that first bend is too intense, then the first bend will just be too big to be swallowed by the average. Which means the general direction of travel is going to be in that direction- towards the bow riser. Underspined arrows travel in towards the bow riser. Field points have a lot of drag in the tail, and none on the front- so the fletching corrects this quick enough to not really notice. But if you add a big air catcher on the front (like a broadhead), then the fletching drag can't correct fast enough, and you'll see the changed direction of travel revealed as your broadheads not grouping with your field points- or "planing". So if you get your arrow spined correctly, then the planing doesn't occur from this (could still be misaligned broadheads not spinning true).

Bonus: This brings us to "The archer's paradox"- how can a fletched arrow be shot without fletching contact on the riser? This is where you have the arrow perfectly spined so that it bends the shaft out of contact with the riser during release, but once the nock is off the string, and the arrow rebounds to straight, and past straight. So the arrow was shaft-out to the riser, but fletching-in. Then on the rebound, it is shaft-in and fletching-out. A properly spined arrow allows for this to happen as the fletchings pass the riser- so essentially you are bending the arrow shaft out of the way of the riser for the first half, and then timing the rebound so the fletchings out of the way of the riser at the end of the shot. Damn cool to watch in slow motion.

Now go have a drink, because you earned it if you finished reading all that.

Good post. I love discussing the physics of archery. I would suggest one minor revision (that's mostly pedantic and doesn't really affect your broader point about tuning via brace height adjustment): peak draw weight and energy typically do change (slightly) when you alter brace height.

Twisting the string to increase brace height induces more limb deflection and increases draw weight. From an energy perspective, this draw weight increase is counteracted by the decreased "power stroke" (distance the string is drawn from brace to full draw), which causes a decrease in the bow's potential energy (which can be calculated as ∫ F•D dD from brace to full draw and visualized as the area under the draw-force curve) and, consequently, a decrease in the arrow's kinetic energy. To summarize using your nomenclature: BH ↑ = F ↑, D ↓, E ↓ The inverse of these relationships is also true; thus, untwisting the string to decrease BH will decrease peak draw weight, which stiffens the arrow's dynamic spine.

Page 10 of the good old Easton tuning guide has a good explanation:

For recurve bows, another way of altering arrow spine is with the brace height. By increasing or decreasing the distance from the bowstring to the pivot point of the grip, the dynamic spine of the arrow can be made slightly weaker or stiffer. Increasing brace height will make the arrow shoot weaker, and decreasing brace height will make the arrow shoot stiffer.​

​

Brace height affects arrow spine by increasing or decreasing the amount of energy delivered to the arrow at the moment of release. Raising the brace height (shortening the bowstring) compresses the limbs, increasing stress (prestress or preload) in the limb material. The more preloading of the limbs, the greater the actual bow poundage at full draw. The reverse is true when lowering brace height. A lower brace height (lengthening the bowstring) reduces the prestress in the limbs and reduces bow weight at full draw. ​

​

However, raising brace height produces some small loss in arrow velocity as the slight increase in draw weight does not equally compensate for the reduction in the bow's “power stroke.” When the power stroke is reduced, the amount of time the arrow stays on the bowstring is also reduced, in turn, decreasing the length of time the arrow has to absorb the bow's energy.​

 

[TheJason]

 

[okie archer]

It has to do with less force on the arrow. Abandon all hope, ye who read on. Let's get technical with some basic physics:

Lower brace height = longer path of string travel from same full draw = longer distance of force action on arrow.

With the same draw length, you hold the same weight, and you're holding the same potential energy, and adding the same potential energy to the arrow.

Energy imparted to the arrow is Force (called F) * distance (called D)- defined as E = F*D. So if the energy at Brace height A is the same as Energy at Brace Height B (you have the same draw weight), then the "force times distance" in both conditions are equal to each other:

Fa * Da = Fb * Db

Rearranged, you get:

Fb = Fa * Da / Db

Since Distance B is greater than Distance A, that means that Force B is going to be less than Force A. So in effect, dropping the brace height puts less force on the arrow. Less force on the arrow means it doesn't bend as intensely. And you have now tuned your bow to the arrow, instead of tuning your arrow to the bow.

Ok- you should really stop reading now. You've been warned.

To go deeper- spine is a measure of total shaft deflection. Static spine measures it with a set weight (1.94 lbs), at a set point (dead middle), with supports a set distance apart (28inches), and not moving. It tells you how "stiff" an arrow is, and is a good starting point to tell you how well it can hold up to a dynamic (changing forces) environment. Dynamic spine is a measure of total shaft deflection in the dynamic environment (changing forces). This is the one that really matters. It's also why there are charts for your arrow spines that vary with point weight, draw weight, arrow length, etc.- they're trying to give you a way to achieve dynamic spine with a static number. Pretty good sales pitch, really.

To relate this to you- dropping the brace height (lengthening the string) creates a less demanding dynamic environment that your arrow is better suited to.

To really understand spine, you need to understand moment of inertia calculations. But in practice: The arrow bends (deflects) because it has mass along it's length and at the tip, and none of that mass wants to move (property of inertia). When you release the arrow, that force of the string acts against the inertia of the arrow mass distributed along it's length, and acts as a compressive force. And long, skinny, compressed things bend.

Arrows of a certain size and strength will bend a certain amount per applied force, and per unit length. So more force = more deflection. Longer arrow = more deflection. And funny enough, the location of that weight effects things- the farther away the weight from the applied force, the more deflection you get- the inertia you're compressing against has more distance to act. **The thing to keep in mind is that heavier shafts are usually stronger, and have a lower deflection constant. Changing arrow shafts changes things- who knew?**

So let's say you increase the point weight. More mass wants to "not move" more intensely- there's essentially more to push against. If you add more mass at the tip, then the same "strength" shaft will experience a higher compressive force during the launch impulse. And it will deflect more.

Let's say you shorten the arrow- less unit length, and less total deflection. Try bending a 10ft piece of PVC, and then try bending just 1ft of that same pipe. How much total deflection did you get from the two? It's a lot less from the 1ft. Same basic principal.


With that all said- the arrows will spring back from bent towards straight after they are no longer experiencing the compressive force. But they have enough energy in the bend that they overshoot "straight" and end up bent the exact opposite direction. It's not 100% efficient, so the second "bend" isn't as intense as the first. BUT! This allows for a general average straightness between the two oscillating conditions, allowing you to shoot straight.

That first deflection is going to be more intense than the second, always will (Further reading: 2nd law of Thermodynamics). SO- if that first bend is too intense, then the first bend will just be too big to be swallowed by the average. Which means the general direction of travel is going to be in that direction- towards the bow riser. Underspined arrows travel in towards the bow riser. Field points have a lot of drag in the tail, and none on the front- so the fletching corrects this quick enough to not really notice. But if you add a big air catcher on the front (like a broadhead), then the fletching drag can't correct fast enough, and you'll see the changed direction of travel revealed as your broadheads not grouping with your field points- or "planing". So if you get your arrow spined correctly, then the planing doesn't occur from this (could still be misaligned broadheads not spinning true).

Bonus: This brings us to "The archer's paradox"- how can a fletched arrow be shot without fletching contact on the riser? This is where you have the arrow perfectly spined so that it bends the shaft out of contact with the riser during release, but once the nock is off the string, and the arrow rebounds to straight, and past straight. So the arrow was shaft-out to the riser, but fletching-in. Then on the rebound, it is shaft-in and fletching-out. A properly spined arrow allows for this to happen as the fletchings pass the riser- so essentially you are bending the arrow shaft out of the way of the riser for the first half, and then timing the rebound so the fletchings out of the way of the riser at the end of the shot. Damn cool to watch in slow motion.

Now go have a drink, because you earned it if you finished reading all that.

Dude I just want to kill a deer.

Just kidding, thank you for your comments.