# Plunger and tuning - stiff vs normal



## kshet26 (Dec 20, 2010)

I'm bringing up the stiff vs normal plunger tuning debate as I've always been able to see both sides as having good points. Maybe JoeT can lay it out for me? As far as I understand the thinking is:

Stiff
Tuning with a stiff (locked-up) plunger is said to remove an element that might hide incorrect arrow spine when doing initial tuning/evaluation of spine.

Normal
You're not going to be shooting with a stiff plunger, so initially setting the plunger to stiff will indicate a stiffer than normal arrow. It's also arbitrary as you'll eventually arrive at the same conclusion with further tuning methods.

Also why do we even care about plunger springs... Why don't we set plungers to be stiff all the time? What does the 'give' in a plunger correct?


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## limbwalker (Sep 26, 2003)

> What does the 'give' in a plunger correct?


The same thing the shocks in your car correct.


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## c365 (May 15, 2013)

kshet26 said:


> I'm bringing up the stiff vs normal plunger tuning debate as I've always been able to see both sides as having good points. Maybe JoeT can lay it out for me? As far as I understand the thinking is:
> 
> Stiff
> Tuning with a stiff (locked-up) plunger is said to remove an element that might hide incorrect arrow spine when doing initial tuning/evaluation of spine.
> ...


It's very simple kshet26, now I'm not saying whether it's right or wrong or anything else, only that it's worked for me just great. Maybe I'm the only one it worked well for. It's just what I did to get the best tune I've ever had. But remember I'm just a newbie. 

First center everything dead centershot with* stiff plunger*.
Shoot your bareshaft. Immediately you can tell if the shaft is too stiff or weak and your nock position by what angle it enters a good bale.
From this point, you have to adjust the bareshaft spine and/or nock point. You may have to get new shafts if the bareshaft is way off. Like sticking in the bale at 45 degs.
*Don't touch anything on your setup!* just adjust the bareshaft spine until it sticks perfectly straight into the bale. 
The problem most make at this point is they start to mess with plunger, centershot etc. to fix a wrong spine. Then they've defeated the whole purpose of this test.
*Once you've arrived at a good spine*. This is where the *stiff plunger ends*.
Put in a a medium spring.
Offset the arrow.
Shoot fletched and bareshaft, you find the arrows all land so close together.
From here on, you detail and tweak the tune, if necessary. Adjust aperture, Walkback, etc.

At least you have a well matched spine for your setup your working with instead of forever guessing is it the plunger or centershot or what.
You'll also find the tune is forgiving for *slight* quirks in your form.


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## Kendric_Hubbard (Feb 5, 2015)

Every tuning guide I have ever read has said to tune with the plunger tension set initially at the medium setting.


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## c365 (May 15, 2013)

Kendric: But then how do you know you have the right spine? all your doing is trying to adjust the setup to a perhaps unmatched spine. So, comes the questions we're all familiar with, is it the plunger? is it the centershot? something else? It's the all important spine for your particular setup that must be determined first in my mind.

But I could be all wrong!


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## Kendric_Hubbard (Feb 5, 2015)

I was advised to get my bareshafts to impact correctly without adjusting plunger tension. Also, if the centershot is off, it can be easily determined with a walk back tune


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## c365 (May 15, 2013)

Kendric_Hubbard said:


> I was advised to get my bareshafts to impact correctly without adjusting plunger tension. Also, if the centershot is off, it can be easily determined with a walk back tune


Also the plunger as well will affect centershot, so it's right back to the plaguing question, is it the plunger? is it the centershot? so why not determine the right spine first, that'll eliminate one variable as kshet26 first mentioned. Anyway, I'm not a technician or physicist, or mathematician, who could throw a bunch of numbers and diagrams at you, just someone trying to help someone sort out these archery old questions and relaying something that's worked grand for me. But it's not the only way I'm sure. If your way works good for you, great. Not trying to argue Kendric, that's fruitless, you have a way that works for you, I have mine. But, I'm always open to improving who isn't? Maybe it's like a spoked wheel, all ending up at the same place. Who knows.


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## Joe T (Apr 5, 2003)

kshet26 said:


> I'm bringing up the stiff vs normal plunger tuning debate as I've always been able to see both sides as having good points. Maybe JoeT can lay it out for me? As far as I understand the thinking is:
> 
> Also why do we even care about plunger springs... Why don't we set plungers to be stiff all the time? What does the 'give' in a plunger correct?


Need to understand what effect (changing) the plunger spring has on the arrow.
The following graph is from the Kooi/Sparenberg paper "*On the Mechanics of the Arrow: Archer's Paradox. Journal of Engineering Mathematics, 31(4):285-306*". 









The graph indicates the calculated force the button plunger exerts on the arrow shaft (Newtons) as a function of time during the arrow power stroke. So the plunger exerts a force of around 3.5 Newtons for around 0.003 seconds in the early part of the power stroke. The direction of this force is along the plunger axis away from the bow. The plunger force is basically determined by the spring preset tension which is directly under the control of the archer (There is an additional force contribution from the acceleration of the plunger). The duration of this force is basically determined by the flex properties of the arrow (we are in the initial shaft buckling phase here) and is down to the bow/arrow system. Plunger force duration is not consciously modified by the archer.

Newtons second law of motion is force equals rate of change of momentum. We can rearrange this in terms of the shaft velocity change resulting from the force from the button plunger as:
1.....Linear Shaft Velocity = Plunger force times Force Duration divided by arrow mass
2.....Rotational Angular Velocity = Plunger force derived torque times torque duration divided by the arrow moment of inertia

Worth mentioning that both the linear and angular velocities are both (more or less) linearly related to the button spring preset value. Can now understand what effect changing the spring tension will have on the arrow.

So the effect of the button plunger on the arrow is to give the arrow a lateral sideways velocity away from the bow and a rotation around a vertical axis.

Equation 1 describes the original "Archers Paradox" problem . Using the numbers from the graph and assuming a 20 gram arrow we get from the button plunger force on the shaft a lateral velocity of 3.5*0.003/0.02 = 0.53 meters/second. So with button alone the arrow will go sideways 53 centimeters per second of flight time. The original Paradox problem was with longbow so effectively a "locked up" plunger so the sideways force on the arrow would be far greater than for the recurve and it was assumed, at the time, that the force would be applied over the whole power stroke so a further increase in the arrow lateral velocity by a factor of around 5. So a big mystery as to why the arrow flew straight and didn't have a large sideways component.

Equation 2 describes how the pressure button is used to adjust the bow tuning. It is strongly suggested that bow tuning is all about the angular velocity the arrow has as it exists the bow. Clearly by changing the button spring tension (and also the nocking point in the vertical plane) the archer can adjust the arrow's angular velocity to that which gives the best result in terms of minimizing groups. As there is only a limited adjustment of angular velocity available via the button the net arrow angular velocity from the bow set up (basically the draw weight adjustment) has to be fairly close to the required value to start with. It is this area where these "spine" rules of thumb come from; "the paper tear needs to be less than 3" or " at 20 yards the bare shaft needs to be within 3" of the fletched arrow". If we meet this rule of thumb criteria then the set up is "tuneable" the angular velocity is within range of the pressure button spring adjustment. If we can't meet the criteria then no amount of button twiddling is going to get anywhere. Either you try if possible to change the set up or else you need to change the arrow.


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## Mika Savola (Sep 2, 2008)

I wonder how people tuned their arrows before Berger button was invented...


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## Joe T (Apr 5, 2003)

Mika Savola said:


> I wonder how people tuned their arrows before Berger button was invented...


The same way people "tune" stick bows today. As best they can but nowhere near as well as you can tune a recurve. Spend enough time playing about with draw weight and arrow properties and you can get a reasonable result.


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## gjlama94 (Oct 11, 2013)

I think part of the OP's confusion is the mistaken impression that the plumber affects arrow spine, dynamic or otherwise. It does not. As JoeT explains, it helps to correct for small finger errors during release so the arrow does not fishtail, or fishtail is minimized, during arrow flight.


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## kshet26 (Dec 20, 2010)

I know that the plunger has nothing to due with the spine of the arrow (though there is a nugget on JoeT's site about the force against the plunger by the arrow may cause the arrow to flex a bit more). The plunger is all about changing the arrow's angular velocity (the result of which can _appear_ to indicate 'weak' or 'stiff' arrows).

I guess the heart of the question is this:

Does the spring _actually_ matter? Does it _actually_ contribute to dampening errors? Why don't we just put the arrow to dead centershot, make the plunger rock hard and call it day? We'd then use brace height, poundage, and modifying the arrow properties to minimize angular rotation?

The arrow is setup slightly outside of center (a 'stiffening' action), it depresses the plunger by .5mm ('weakening' the arrow). Why not just move a stiff plunger in by .5mm? Is the plunger depth the macro centershot adjustment and spring tension is the micro centershot adjustment? Can we achieve the same objective by putting the arrow further outside of center and making the plunger spring even weaker?


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## TER (Jul 5, 2003)

kshet26 said:


> I guess the heart of the question is this:
> 
> Does the spring _actually_ matter? Does it _actually_ contribute to dampening errors? Why don't we just put the arrow to dead centershot, make the plunger rock hard and call it day? We'd then use brace height, poundage, and modifying the arrow properties to minimize angular rotation?


We did do that for thousands of years. Then Vic Berger invented the Berger Button and everyone who used it got better groups.


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## dchan (Jun 29, 2004)

Making some assumptions here but if we are using the Tuning for Tens as a reference and the stiff plunger test, There is NO impact angle check in this stiff plunger test.

The stiff plunger test is only to verify if you are going to be in a range close enough to even get a possible tune.

The stiff plunger test is done with bare shafts and shot through paper. It never talks about arrow angle in the bale upon impact. Remember we keep telling you that shooting through paper is* not a tuning method* This is primarily the case here as well. All that is being accomplished is gathering of information. We can set the nock height using what we find out from the discovery shots. (nock high or low will be revealed pretty quick). The horizontal tear will tell us a few things. if the tear is 3" or less (either left or right) all it means is we can probably get the shaft to tune reasonably well using the methods we have for stiffening and weakening dynamic spine (weight manipulation and poundage) The closer we are to a bullet hole, the closer we are to being in the sweet spot. If the tear is larger than 3" your test arrows are either too weak or too stiff (depending on the tear direction, and in most cases too stiff)

Then we use the stiff plunger* WITH * fletched arrows to set a reference point for the sight and we move on to a walk back test. At this point is where you put the medium spring back in the plunger to continue.

DC


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## Viper1 (Aug 21, 2003)

Guys - 

I think once again we see how easy it is to over complicate something. 

For a new shooter, the plunger is an adjustable strike plate. The cushioning or "shock absorber effect" that John mentioned isn't going to fix the gross errors a a new shooter is going to make. In that regard, starting with a stiff plunger, set to a textbook (arrow) offset just removes a variable. 

However, a plunger set to it's mid range in tension may not be such a terrible thing for new shooters either - for the exact same reason.

There's more than one way to skin a cat, and as long as you know what you're looking either way is fine. 
It does however help if you know the difference between a weak, average and stiff plunger ...

Viper1 out.


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## Joe T (Apr 5, 2003)

kshet26 said:


> I'm bringing up the stiff vs normal plunger tuning debate as I've always been able to see both sides as having good points. Maybe JoeT can lay it out for me? As far as I understand the thinking is:
> 
> Stiff
> Tuning with a stiff (locked-up) plunger is said to remove an element that might hide incorrect arrow spine when doing initial tuning/evaluation of spine.
> ...


I didn't answer this part of the question on the assumption that once you have some understanding of the effect the plunger spring has on the arrow it becomes obvious that locking up a plunger is completely pointless for any purported reason. The "removing the plunger as a variable" suggestion goes straight out of the window when you know that stiffening the spring *increases* the effect the plunger has on the arrow.

Could be that this was a wrong assumption


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## kshet26 (Dec 20, 2010)

JoeT-

Can you describe the cause and effect between an arrow and the plunger when a shot is clean vs. a pluck? 

I'm trying to wrap my head around what exactly the plunger does to a less-than-optimally shot arrow that results in better groupings than just using the plunger as a solid strike plate. When an arrow is plucked, the shaft is pushed harder into the plunger, the plunger depresses more, then... ? Is it that when the plunger depresses more, it's absorbing some energy that would have turned into flex? So that the plunger keeps the arrow flex the same as clean shot then pushes the arrow back into proper centershot?


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## Joe T (Apr 5, 2003)

kshet26 said:


> JoeT-
> 
> Can you describe the cause and effect between an arrow and the plunger when a shot is clean vs. a pluck?
> 
> I'm trying to wrap my head around what exactly the plunger does to a less-than-optimally shot arrow that results in better groupings than just using the plunger as a solid strike plate. When an arrow is plucked, the shaft is pushed harder into the plunger, the plunger depresses more, then... ? Is it that when the plunger depresses more, it's absorbing some energy that would have turned into flex? So that the plunger keeps the arrow flex the same as clean shot then pushes the arrow back into proper centershot?


Far too big a topic for a forum. For my musings work through the tuning section at http://www.tap46home.plus.com/mechanics
/

For a one line answer you need to change your viewpoint. Say you shoot a perfect arrow from a perfect setup and the arrow, as it must do by definition, hits the cross at the center of the 10. You shoot a second arrow but this time you pluck the string so the bow is going to behave slightly differently. There is no way the bow can somehow change how it works to correct for (each of your 100 different) mistakes, your arrow is going to miss the cross and hit some distance away. What tuning does is minimize the consequences of your mistake i.e. reduce the distance the arrow hits from the cross. Tuning is about minimizing group sizes. You don't get a sequence of Robin Hoods what you get is a reduction in the arrow spread resulting from all the variations in how you shoot the bow. In order to understand tuning you have to forget about the bow and start thinking about the arrow flight.

When you shoot a bow the string torques the arrow. The button is used to balance the overall torque on the arrow to give you the optimum launch angular velocity to minimize the group size. When you change your shot you change how the limbs/string torque the arrow. The effect of the button largely remains the same. (The plunger doesn't actively do anything, it's function is to adjust the arrow launch angular momentum to the most forgiving value). The arrow leaves the bow with a different angular velocity. What you do when tuning is to adjust the bow to give you the launch angular velocity which is most forgiving to variation.


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## kshet26 (Dec 20, 2010)

Joe T said:


> When you shoot a bow the string torques the arrow. The button is used to balance the overall torque on the arrow to give you the optimum launch angular velocity to minimize the group size. When you change your shot you change how the limbs/string torque the arrow. The effect of the button largely remains the same. (The plunger doesn't actively do anything, it's function is to adjust the arrow launch angular momentum to the most forgiving value). The arrow leaves the bow with a different angular velocity. What you do when tuning is to adjust the bow to give you the launch angular velocity which is most forgiving to variation.


Ah! So let me see if I have this correct: 

Let's assume that the nock of an arrow shot from a 'clean release' detaches at 0 units measured laterally. The nock of an arrow shot from a plucked release detaches at -1 units measured laterally. The goal when tuning the plunger is to get plunger to move the front of the arrow back into a position that minimizes the angular velocity (aka leave the bow straight). So roughly for this example, the front of the arrow might move -1 units laterally to 'line everything back up' so that the arrow still leaves the bow traveling straight, just -1 units measured laterally. In a perfect world this might translate into hitting the cross on a perfect shot and hitting -1 units laterally of the cross on a pluck.


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## Joe T (Apr 5, 2003)

To understand tuning you need to forget about anything to do with the specific behaviour of the arrow on the bow. Just regard the arrow as a black box with the arrow coming out of it with some physical properties. The main physical property related to tuning is the arrow rotational speed and direction of rotation (it's angular velocity). The archer//bow combination will have an average angular velocity (AV). The launch AV will vary from shot to shot because of the archer shot to shot variation. Objective with tuning is to find the best average launch angular velocity.

Why? - because what changes the arrow flight direction is the arrow angular velocity. Just as spinning a golf ball will change it's flight direction (called hook/slice) spinning an arrow will change its flight direction (called stiff/weak). Arrow hit scatter is cause by variations in the arrow launch angular velocity resulting from variations in how the arrow is shot by the archer.

The secret is that the amount of of arrow scatter you get (group size) varies with the average arrow launch angular velocity. If you plot a graph of group size against average launch angular velocity you get something "U" shaped. There is a specific launch angular velocity which will give you the minimum group size. When archers group tune they are trying to find this optimum arrow launch angular velocity. Where the bow comes into it is by changing bow parameters you change this average launch angular velocity.

If you put an arrow on a table and flick it with your finger near the point the arrow will spin at some speed. If you flick the arrow harder the arrow will spin faster. This is how you adjust the arrow launch angular velocity with the pressure button. The more you wind up the button spring increasing the force with which the button "flicks" the arrow the more launch angular velocity you will put into the arrow (in the stiff direction). Changing just about anything to do with the bow or the archer will change the launch angular velocity (i.e. the tuning). Some none obvious examples are changing the bow stabilizer system or the archer's bow shoulder alignment.

Once you understand how tuning works you will be able to understand how the various tuning methods work (and usefully also be able to tell the the difference between good tuning methods and bad or rubbish tuning methods).


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## b0w_bender (Apr 30, 2006)

OK I'll weigh in on this and JoeT and the rest of you can tell me where you think I may be in error. Here is a photo of the famous Beiter slow motion video illustrating archers paradox. I've highlighted where the cushion plunger is and drew a line from the front to the back of the arrow. This is the moment as the string roles around the finger tips and the string starts it's forward push. That amount of bend in the arrow is what we are looking for when choosing the correct spine. Now you can see that the arrow is going to press against the cushion plunger as it moves forward and rebounds. SO that cushion plunger helps to manage that force and helps to moderate the variances in arrow flex. So lets go back to what john was saying on the first response which bump do you feel more the one you hit while riding on a hay wagon or the one you hit in a Cadillac? Surely the car with the shock absorbers has a smoother ride.

So now the question is why use a stiff plunger? 
when you use a stiff plunger, we know we are attempting to measure how much does the arrow bend while we eliminate the moderating affect of the cushion plunger. All we are attempting to do is to ID are these arrows flexing in the amount that will allow us to tune them? If they are then the cushion plunger can do it's job which is to moderate the inconsistent flex one experiences from shot to shot (human factor).


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## Joe T (Apr 5, 2003)

b0w_bender said:


> That amount of bend in the arrow is what we are looking for when choosing the correct spine.


Arrow selection ("correct spine") is based on the flex rate of the arrow not how much it bends. How much the arrow bends affects the tuning. These are different things and get confused because they use the same terminology.

Simple example:- If we add weight to the nock of an arrow does it stiffen or weaken the arrow? Correct answer is either Yes or No. Both are correct. It depends what definition of "stiff/weak" you are using. Adding weight to the nock lowers the flex rate of the arrow so in terms of flex rate (arrow stiffness) it weakens in the arrow. On the other hand adding weight to the arrow nock reduces the initial bend amplitude which reduces the string torque on the arrow which changes the arrow launch angular velocity in the "stiff" direction (as in the arrow swings to the left for an RH archer).


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## kshet26 (Dec 20, 2010)

Joe T - I agree that that's the best way of thinking of it and I get the meta concept of unified tuning. I'm just curious if this is one of those 'many ways to skin a cat' things. In the same way that we have other 'rules of thumb' that are often fuzzy and seem quasi-arbitrary (like tiller - some say set it positive, some set it to 0, centershot - some say a little outside of center, some say center etc). I know I'm being ridiculous, but I like understanding things more than just saying that 'it's what we do'.

Let's take some info from the Archery Australia tuning guide. In it they state that a plunger will typically be depressed by an elite archer only .5mm and that the plunger only contacts the arrow for 20-30mm of the shaft during the forward push. My theory is that the .5mm value is actually the distance the plunger is setup outside of centershot/optimal launch angle (the geometry seems to work out). 

The arrow force against the plunger then enters equilibrium with the plunger force against the arrow. Let's say this force is equal to 200 grams. It seems like you could also just setup your plunger to remove that .5mm and set the plunger tension to not start depressing until the force against it is greater than, say, 250 grams (to account for the increase in force against the plunger due to moving the shaft in .5mm and the reduction of time of the force of the plunger acting on the arrow).

In the "Overly-Complicated Kshet26 Tuning Method of Pointless Tuning" or OCKTMPT(tm):

(note that this ignores nock height, brace height, ect for the sake of this conversation)

- Set up centershot to exact center (the barreling on barreled shafts needs to be compensated for)
- Lock up plunger
- Shoot bareshafts
- Shoot fletched
- If bareshafts and fletched arrows group laterally centered on the target proceed, if not adjust arrow or limb strength until they do
- Get pressure sensor and attach it to the plunger
- Shoot a bunch of fletched arrows and record forces against plunger
- Set plunger spring tension to recorded mode value force (this means the plunger isn't doing anything but acting as strike plate on 'normal' shots and doing a little work on 'abnormal' shots)
- ??
- Podium finishes!

I guess the whole point of the thread is about removing 'levers' when doing the angular velocity push-pull. How much can we simplify and still achieve elite performance (the irony about simplifying and this thread is not lost on me).


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## b0w_bender (Apr 30, 2006)

Joe T said:


> Arrow selection ("correct spine") is based on the flex rate of the arrow not how much it bends. How much the arrow bends affects the tuning. These are different things and get confused because they use the same terminology.


Hmm in my nomenclature I've always viewed an arrow with correct spine as having an arrow that will bend the correct amount when I shoot it. True I can manipulate the dynamic spine with multiple variables but at some point the variables that you can reasonably manipulate don't fall within acceptable parameters in those cases the arrow is not the correct spine.


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## b0w_bender (Apr 30, 2006)

kshet26 said:


> I guess the whole point of the thread is about removing 'levers' when doing the angular velocity push-pull. How much can we simplify and still achieve elite performance (the irony about simplifying and this thread is not lost on me).


That right there is really funny!
JoeT has a way of pointing out just how much is actually going on in a very reductionist way, and as sick as it may seem I really enjoy it.


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