# Bow Stacking



## SEC (Jul 13, 2007)

How does a person tell if their bow is "stacking" ?
Is it dangerous...meaning, does it stress the limbs?


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## Viper1 (Aug 21, 2003)

SEC -

You'll feel the bow begin increasing weight rapidly after a given point. The most severe case has been likened to hitting the wall on a compound (OK, slight exaggeration). 

All bows stack at some point, the trick is to get a bow that stacks AFTER you've reached full draw, then by definition, it doesn't stack - FOR YOU.

The reason for stacking are either surpassing the expansion or compression limits of the back or face of the bow (respectively) or reaching a point where you're actually pulling the limbs apart longitudinally, rather than bending them.

While it's not a good thing for the bow.,most production bows are over-built enough where it shouldn't be a problem. Still I can't see repeatedly doing it doing the bow any good and it certainly doesn't help your shooting one bit.

Viper1 out.


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## Eldermike (Mar 24, 2009)

Ok, here is my take on stacking. Not as an archery expert but a design engineer.

Stacking is an archery term that relates to spring rate curves. All bows have spring rate curves, this means that for every inch of flex the actual draw weight increase is not linear, it goes up as the flex increases. It can't be helped, all bows stack. The inside of a limb is in compression, the outside in in tension. It's the angle of compression (on the inside) or excessive tensil stress (on the outside) that will cause a limb to stop bending and break. But that point is well beyond normal bending of a well designed bow limb.


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## Bobman (Dec 18, 2004)

Eldermike said:


> Ok, here is my take on stacking. Not as an archery expert but a design engineer.
> 
> Stacking is an archery term that relates to spring rate curves. All bows have spring rate curves, this means that for every inch of flex the actual draw weight increase is not linear, it goes up as the flex increases. It can't be helped, all bows stack. The inside of a limb is in compression, the outside in in tension. It's the angle of compression (on the inside) or excessive tensil stress (on the outside) that will cause a limb to stop bending and break. But that point is well beyond normal bending of a well designed bow limb.



good post interesting way of putting it

if you have a long draw are concerned about your bow stacking use a longer bow in general they stack less


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## rattus58 (Jul 9, 2007)

So what about fiberglass or graphite or carbon strips... do they also "stack" not being "live"?

Aloha... Tom :beer:


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## Eldermike (Mar 24, 2009)

rattus58 said:


> So what about fiberglass or graphite or carbon strips... do they also "stack" not being "live"?
> 
> Aloha... Tom :beer:


Hey Tom, yes everything stacks.



In a cross section of a flat spring member (be it archery bow or truck spring) there is a line you can't see in the middle of the cross section that does not change in length when it's flexed. Above/below this line you have stress based on potential length changes. Imagine a bow limb cross section, you are looking right at the edge. The inside is being compressed, the outside is being pulled. right in the center there is a point of zero change, that's where the stress transitions from compression to tension. Also at this point of zero change you have the third component of the stress/strain curve, that is shear. So, to sum it up: All materials have the same potential to stop bending but they can have different reason. Some will stop based on compression, some on tensile, depending on which of these values is higher in the basic material.
Materials have a wide variation in tensile/compression/shear values.
But no matter what you use the forces and resultant stress occur the same way.

You can have a tensile failure, a compression failure or a shear failure but one of these will occur first if a flat beam is flexed to it's point of failure.

I have a long bow with a carbon center. I have not studied this much but I am going to guess it's there to help prevent shear failures.


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## rattus58 (Jul 9, 2007)

Thanks for the great description....

Shear is like finding the spine... only going too far.. right?

Aloha...Tom  :beer:


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## Eldermike (Mar 24, 2009)

Yep, if you go to far on a flex the shear forces will delaminate to bow, or arrow.


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## RandyHood (Jul 8, 2013)

Eldermike said:


> Yep, if you go to far on a flex the shear forces will delaminate to bow, or arrow.


A good visualization of what shear is trying to do to a bow is to take a few thin strips of wood and layer them. Then, bend them together. You will notice that they "slip" so that they are not bending together as a single unit. This tendency to slip is caused by the shear force in bending. If you now glue them together, they WILL bend together as a single unit, but rather than slipping, will develop "shear stress" caused by it still wanting to slip, but being prevented from doing so by the bond the glue has created.
You can see the same effect when you bend a stack of paper. the ends of the paper begin to "shear" and are no longer a square, flat end.


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## kegan (Aug 28, 2006)

Just to throw this little bit in there:

You will occasionally hear people saying a bow "really stacks up" as they try to get to full draw, and say the bow isn't right for them. Actual stack is where the bow begins to draw more than it was previously gaining per inch (usually going from about 2.5#/in to 4#/in) which is easy to see with a scale. This isn't always what people are actually experiencing, which is being overbowed. I've had a few people tell me my 66" bows started stacking for them around 27-28", which I know isn't true. It felt the same to them because they were reaching there physical limit, not the bow's:mg:


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## Chupacabras (Feb 10, 2006)

On an ILF bow if you can adjust the draw weight to start "stacking" at the end of your draw length you will end up with a nice linear draw curve that at full draw "simulates" a draw stop.


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## Destroyer (Sep 11, 2009)

Its funny how the word 'stacking' still is a negative. Once a bow would be praised for having a smooth draw and was preferable. Now days, manufactures are using more aggressive force draw curves to gain more speed and its a simply a choice for the archer, smooth or fast.


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## Hank D Thoreau (Dec 9, 2008)

Viper1 said:


> SEC -
> 
> You'll feel the bow begin increasing weight rapidly after a given point. The most severe case has been likened to hitting the wall on a compound (OK, slight exaggeration).
> 
> .


At my draw length I do feel like I am hitting a wall with some bows -- just like a compound.


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## Borderbows (Apr 4, 2009)

Eldermike said:


> Ok, here is my take on stacking. Not as an archery expert but a design engineer.
> 
> Stacking is an archery term that relates to spring rate curves. All bows have spring rate curves, this means that for every inch of flex the actual draw weight increase is not linear, it goes up as the flex increases. It can't be helped, all bows stack. The inside of a limb is in compression, the outside in in tension. It's the angle of compression (on the inside) or excessive tensil stress (on the outside) that will cause a limb to stop bending and break. But that point is well beyond normal bending of a well designed bow limb.


the concept of stack is purely a geometry thing. 
for example. the elongation percentage before failure of carbon is much less than glass but yet when put in the same profile limb with the same taper. you get the same dfc.
that is because you have the same geometric relationship as viper said about the string to limb changing from bending to stretching the limb.

there are limbs out there with a non conventional profile than dont stack at conventional draw lengths.
the reason is simple.
imagine when trying to put a wheel on your car. as you tighten the wheel studs up. your wrench got longer as things got tighter.
before you know it you would have snapped the studs.
now imagine at brace. on a recurve. the area of limb that has wrapped string is technically hiden from the pull. and as you pull it back and the geometric angles die on you. with a recurve your able to unvail more limb length to counteract the negative geometry.


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## Borderbows (Apr 4, 2009)

Destroyer said:


> Its funny how the word 'stacking' still is a negative. Once a bow would be praised for having a smooth draw and was preferable. Now days, manufactures are using more aggressive force draw curves to gain more speed and its a simply a choice for the archer, smooth or fast.


smooth means more energy. more energy means fastearrow speeds. you get a double benefit. smoother normally equals faster.
the choice is clear to us


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## Borderbows (Apr 4, 2009)

Chupacabras said:


> On an ILF bow if you can adjust the draw weight to start "stacking" at the end of your draw length you will end up with a nice linear draw curve that at full draw "simulates" a draw stop.


not all limbs can be tuned to give stack.
there has recently been posted a 66" bow with extra short limbs on a 27" riser pulled 2.4lbs between 29 and 30". it was holding 51lbs at 27".
you would be lucky to get that increase from a set of conventional long limbs at that spec. let alone some extra shorts.


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## CFGuy (Sep 14, 2012)

kegan said:


> Just to throw this little bit in there:
> 
> You will occasionally hear people saying a bow "really stacks up" as they try to get to full draw, and say the bow isn't right for them. Actual stack is where the bow begins to draw more than it was previously gaining per inch (usually going from about 2.5#/in to 4#/in) which is easy to see with a scale. This isn't always what people are actually experiencing, which is being overbowed. I've had a few people tell me my 66" bows started stacking for them around 27-28", which I know isn't true. It felt the same to them because they were reaching there physical limit, not the bow's:mg:


Your 66" bow "stacking" at 27" is laughable. I draw 31" and it doesn't even stack then. Definitely important to be able to recognize the difference but I would assume that comes with time or education.


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## Destroyer (Sep 11, 2009)

Borderbows said:


> smooth means more energy


You mean that 'smooth' is a result of less wasted energy therefore it transfers more energy to the arrow and you get faster arrow speeds? 



Borderbows said:


> smoother normally equals faster.


Not what I have found.


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## Destroyer (Sep 11, 2009)

CFGuy said:


> Your 66" bow "stacking" at 27" is laughable..


Not all to do with bows overall length, limb length has a bigger effect and the same with quality of the limb and its design. Put decent longs on a 17" riser and get a 62" bow that won't stack. Bow length has more effect on string angle for a clean release than stacking.


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## CFGuy (Sep 14, 2012)

Destroyer said:


> Not all to do with bows overall length, limb length has a bigger effect and the same with quality of the limb and its design. Put decent longs on a 17" riser and get a 62" bow that won't stack. Bow length has more effect on string angle for a clean release than stacking.


Interesting, didn't know that. Specifically for Kegan's Omega though, it definitely doesn't stack anywhere near 27".


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## Borderbows (Apr 4, 2009)

pulling a bow is like climbing a hill. there are steep sided peak. like knife edges and those with rounded tops.
what your looking to do is get high volumes of rock under your feet. stack is like a steep sided knife edge peak. where there is little rock under you. you have achived your 4000 feet. but your standing on very little volume.
take a rounded hill of 4000 feet. you had all the climbing to do up front. but the cruz to the summit is an easy stroll. its flat on the top
the reason is you did the climb upfront and you now have an easy stroll. this gives you masssive volumes of rock under your feet.

speed also comes from chassis mass. a big v8 engine wont do well at pulling a train. but will make a go kart fly.

so when looking for bow performanc. you need low limb tip mass and high stored energy per pound of draw weight.
values of over 0.95 pounds of energy per pound of draw force would be classed as good at 28".
limb tip mass is fun to measure but an example is 205 grams for a long ilf glass limb is about average. while 150grams is possible for the same spec carbon limb.
the bow mentioned above. had an se/pdf value of close to 1 at 28. so high energy. and a total mass of 140 grams.
it was pulling about 2.1lbs per inch at 28". for a 53-54lbs bow at 28.
205fps with 8.9gpp finger shot with some string silencers. at 27.25".
smooth. light and fast


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## Destroyer (Sep 11, 2009)

CFGuy said:


> Specifically for Kegan's Omega though, it definitely doesn't stack anywhere near 27".


It might not be stacking, you really need to check with a scale to find out.

Some ppl can think a bow is stacking even when its not. I use to think the same thing with some of my bows but it wasn't stacking that was the issue, I just wasn't strong enough...


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## Borderbows (Apr 4, 2009)

stack for me has two meanings. one is its pulling more than the average rate. eg. 8" brace height 28" draw equals 20" power stroke.
40lbs /20" = 2lbs per inch. anything over that at full draw and its stacking.

the other definition. is anything where the dfc graph is concaved and not convex.
this can very clearly be seen on a smoothness graph. this one is the least felt be the archer as its earler in the draw. but its very definate.

this link has a lot of data. spring constant graphs. smoothness graphs. stored energy graphs.http://tradtalk.com/forums/showthread.php?t=40667


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## CFGuy (Sep 14, 2012)

Destroyer said:


> It might not be stacking, you really need to check with a scale to find out.
> 
> Some ppl can think a bow is stacking even when its not. I use to think the same thing with some of my bows but it wasn't stacking that was the issue, I just wasn't strong enough...


Kegan's bows definitely don't stack at short draws, overbowing is often the issue as he mentioned.


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