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How to optimize the performance of a hand-made invert roller?

Thread Status: Hello , There was no answer in this thread for more than 60 days.
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snask

snask

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Sep 25, 2020
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Actually, this was exactly what I meant. 8 bands for invert roller = 4 bands on the right side + 4 on the left side. And the energy is equal to a regular gun with 2 bands (with twice length and stretch ). This is not easy for me to digest/believe.
Well that is simply a wrong way of counting/thinking about the bands. The 2 bands on a regular gun is loops, so its "4" bands stretching if you call it 8 bands on a roller. 2 on each side, like the roller has 4 on each side.
 
Andrew the fish

Andrew the fish

Well-Known Member
Oct 17, 2010
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Eray, sounds like you could use simulator and play with numbers to optimize the performance. This is exactly what Jesse built it for.
 
Eray_spearo

Eray_spearo

Member
Oct 26, 2020
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this one here

Andrew, it is a nice one. I was literally looking for that. Just three short questions:
1. What is primary band (the one fixed on the gun with pre-tension?)?
2. What is secondary band?
3. What is Roller band per pair? Is it 3 if I have 6 in total (3 right side, 3 left side)?

Thanks for your contribution
 
Leander

Leander

Well-Known Member
Oct 17, 2017
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Andrew, it is a nice one. I was literally looking for that. Just three short questions:
1. What is primary band (the one fixed on the gun with pre-tension?)?
2. What is secondary band?
3. What is Roller band per pair? Is it 3 if I have 6 in total (3 right side, 3 left side)?

Thanks for your contribution
With that calculator, the primary and secondary invert roller band have the same calculation. The only difference is the 'double bands' switch for the primary band, which imho exists for people who cannot count to two.

So either use primary with 'double bands' set to 'no', or secondary bands, or both (for example when you have one band of different diameter or stretch).

One invert roller band has two strands, one on either side. So you would have three bands, not six.
 
snask

snask

Member
Sep 25, 2020
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Andrew, it is a nice one. I was literally looking for that. Just three short questions:
1. What is primary band (the one fixed on the gun with pre-tension?)?
2. What is secondary band?
3. What is Roller band per pair? Is it 3 if I have 6 in total (3 right side, 3 left side)?

Thanks for your contribution

1. yes!
2. the detachable bands that you stretch separately after the primary.
3. "yes" but that is the normal roller gun.
 
snask

snask

Member
Sep 25, 2020
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Your comments well appreciated mate. That was also my observation. I think bands were a little too long. I shorthened black ones by 1cm (from 20 to 19cm; stretched length: 78cm) and the red one by almost 6 cm (from 25 to 19cm). Pre-tension (for the red one) is likely to be 10-20%.
Questions:
1. What might be the power impact to use a 6,5mm x 150cm shaft considering that it is 80g lighter wrt 7mm x 150cm shaft?
2. Would you expect a "swinging" effect if 6,5mm is too thin?
3. Shall I rather try 3x 16mm rubber bands with 350% ratio to obtain a decent power with 150cm x 7mm combination?

Thanks for your feedbacks

Did you do some more tests? I find it strange that you got so little power out of it. If you are using different band setups maybe some band is slower than the others so it barely powers the shaft?
The easier adjustment would be to double up the 14mm primary bands, more power and easier to load than 3x16.
 
Z

Zbanko

New Member
Dec 12, 2020
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Hi Guys,
great discussion! exactly what i was looking for :) .
I also have thought on lots of combinations and how to start, which details to consider etc etc. Anyway some thoughts of mine:
Remember the basic machines topic form high school physics, see pic.
1607766340285

The setup right is the invert roller pulley setup. The F side would be pulling on our shaft and on the P side would be our rubbers. If we have an invert roller pulling with 4 rubber strings 14mm each we will pull with the half of the force of a regular gun pulling with 2x14mm (which means i have 4 rubber strings each 14mm), assumed the rubbers are streched equally.
But! a regular setup wont pull the shaft through the complete body with that force, lets say we have a 100cm carbon fiber pipe as body and 4 rubber strings of 14mm pull with F.

In invert roller case, we would pull the shaft with F/2 for almost the complete length of the pipe, so roughly we would load the shaft with F/2 x 100 cm energy.

1607768446317


In regular case, we would pull the shaft with F for 2/3 of the length of the pipe, so roughly we would load the shaft with F x 2/3 x 100 cm energy.

1607769069942

In a rough consideration we would be not much weaker in the invert roller setup. For an exact calculation one would have to consider the dependency of F from the stretch x ( F(x) ) and integrating the F(x) over the length x, on top of that the offset force, which Leander mentioned (pre-tensioned invertroller, no deadspace), also the friction of the pulley mechanism an so on...

To the point of recoil, its all about momentum. Since i have a shaft shooting out in one direction and the rubbers moving into the other, the momentums would almost negate each other.

1607772564200


1607772679281

(see the whip of the shaft due to the power btw :) rly bad accuracy, thats why we shall take at least a thicker shaft)

Also after i've read your conversation, i've had a moment of enlightment. Quote: For normal guns the idea of smaller diameter is that they contract faster and smoother. So this is why they almost always combine few different diameters of rubber in an invert setup ?! in order to balance the conraction ?

Anyway, i bought a handle and a invert roller kit + a carbon pipe 100cm, shaft from pathos 8mm 150cm i gues (in case its too much i have also a salvimar one 7,5mm 140cm) I will try to deploy it with either 4 rubber strings 20mm and 2 rubber strings 16m diameter or 4 of 16mm and 2 of 20mm. Or shall i go go with 6 of 20mm ? :D
I'm so excited !!

Thank you guys for the great topic.
 
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snask

snask

Member
Sep 25, 2020
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Is the 2x wishbone speed from pulley system also giving some advantage? Otherwise why not just skip the pulley and just run the cable straight to the roller? You can use half as many bands for same power. This makes it similar to normal setup but you still get the pre-stretch, full run of the barrel, easy to adjust power with bands, less recoil. But the invert roller with pulley is popular so it must be something about it?
 
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Leander

Leander

Well-Known Member
Oct 17, 2017
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A normal roller has the rubbers running around the roller, which is a lot more turning mass than with the invert's string. This force inflicted by the rotating mass you will have to counter using your wrist, so the invert would always win in terms of precision. How much difference there is exactly you could calculate, and use that to decide if it's worth it for you. I only have hands-on experience with an invert, nothing else, so I can't tell you; I made my choice after learning how recoil is one of the major influencers for the precision and by that the effective range. The old madj posts were about the same iirc but on a normal gun: less rubber gives a cleaner shot, more precise, and therefor, as long as the penetration is sufficient, better.

Of course this is not including variables like shaft whip, just the physics of moving mass.
 
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Leander

Leander

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Oct 17, 2017
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see the whip of the shaft due to the power btw :) rly bad accuracy, thats why we shall take at least a thicker shaft)
We should start exploring other alloys and compounds, at least theoretically. There are many materials pure, in alloyed and in compound form that would surpass stainless steel in weight and stiffness. Most wont have the cost efficiency, but who knows, say, a carbon tube with a tungsten core could be stiffer and heavier...? Afaik no-one has been thinking outside of the box on this one.
 
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snask

snask

Member
Sep 25, 2020
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A normal roller has the rubbers running around the roller, which is a lot more turning mass than with the invert's string. This force inflicted by the rotating mass you will have to counter using your wrist, so the invert would always win in terms of precision. How much difference there is exactly you could calculate, and use that to decide if it's worth it for you. I only have hands-on experience with an invert, nothing else, so I can't tell you; I made my choice after learning how recoil is one of the major influencers for the precision and by that the effective range. The old madj posts were about the same iirc but on a normal gun: less rubber gives a cleaner shot, more precise, and therefor, as long as the penetration is sufficient, better.

Of course this is not including variables like shaft whip, just the physics of moving mass.

Yes I didnt mean a regular roller with the rubber bands rolling, but invert roller with the cables/strings rolling but without the extra block /pulleys demultiplying the force. It seems better to have these thin dyneema cables rolling rather than the thick rubber. The only difference I see is that the force is half with the pulleys and the wishbone speed is doubled to the rubber. So I was wondering if this system is about accessing the double speed, otherwise I cant figure why this is used with the extra complication and double amount of rubber to offset the loss of power.

We should start exploring other alloys and compounds, at least theoretically. There are many materials pure, in alloyed and in compound form that would surpass stainless steel in weight and stiffness. Most wont have the cost efficiency, but who knows, say, a carbon tube with a tungsten core could be stiffer and heavier...? Afaik no-one has been thinking outside of the box on this one.

Carbon fiber has the stiffness you want, but the main thing about it is it's light and that's not desirable for the shaft. As you say, fill it with something heavy, or composite. It would be quite brittle as a spear tip as well I guess, so having tempered steel tips would be best combination I think. Maybe some type of ceramic composite would be hard and stiff, also not suitable as a spear tip unless its some great composite.
 
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Leander

Leander

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Oct 17, 2017
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Going directly to the rubbers without the pulley, then the rubbers won't get the needed stretch to use the full barrel. Part of the pulley advantage is that the wishbone travels twice the distance of the rubber pull.

About the lost energy... The total amount of energy put into the spear is constant for a given amount of bands, whatever type of gun. So I don't think anything is lost actually. The force is just being projected in a different way. There's a Greek spearfishing store that put some lab-test videos online of different gun types, and iirc all the spears leave the gun at about the same speed. I'll see tonight if I can find those videos.

About the spears, I was just thinking out if the box. Every aspect of the gun is being engineered to optimal efficiency, yet the spears are still the same.
 
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Z

Zbanko

New Member
Dec 12, 2020
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Yes I didnt mean a regular roller with the rubber bands rolling, but invert roller with the cables/strings rolling but without the extra block /pulleys demultiplying the force. It seems better to have these thin dyneema cables rolling rather than the thick rubber. The only difference I see is that the force is half with the pulleys and the wishbone speed is doubled to the rubber. So I was wondering if this system is about accessing the double speed, otherwise I cant figure why this is used with the extra complication and double amount of rubber to offset the loss of power.



Carbon fiber has the stiffness you want, but the main thing about it is it's light and that's not desirable for the shaft. As you say, fill it with something heavy, or composite. It would be quite brittle as a spear tip as well I guess, so having tempered steel tips would be best combination I think. Maybe some type of ceramic composite would be hard and stiff, also not suitable as a spear tip unless its some great composite.

It is actually not a matter of speed, but a matter of distance as Leander mentions. Rubber contracts x, wishbone pulls 2x. So if you would have a 6 rubber string setup without pulleys, the tips of the rubbers would stuck at your roller on the tip of the pipe, before you reach the farthest shark fin while loading.
Using pulleys, you pull the shaft through the whole length of the pipe with minimal friction and recoil due to the fact that only a 1,5-2mm dyneema pulls on the top of the pipe. This way, you can bulk up your rubbers on the bottom of the pipe to a crazy extent, see b. daniel invert roller.



 
Z

Zbanko

New Member
Dec 12, 2020
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Going directly to the rubbers without the pulley, then the rubbers won't get the needed stretch to use the full barrel. Part of the pulley advantage is that the wishbone travels twice the distance of the rubber pull.

About the lost energy... The total amount of energy put into the spear is constant for a given amount of bands, whatever type of gun. So I don't think anything is lost actually. The force is just being projected in a different way. There's a Greek spearfishing store that put some lab-test videos online of different gun types, and iirc all the spears leave the gun at about the same speed. I'll see tonight if I can find those videos.

About the spears, I was just thinking out if the box. Every aspect of the gun is being engineered to optimal efficiency, yet the spears are still the same.
I think carbon fiber spear would not go far in the water. Fluids apply a friction force on the material moving in them proportional to the viscosity of the fluid and the velocity of the material moving. Meaning the faster the shaft is, the stronger the fluid would brake our shaft, i.e. the less distance the fuild would need to stop the shaft moving.
The terminal velocity would be arround the same for shafts of diffrent materials due to the shape etc., but you can load a massive shaft with more energy than you could load a less massive shaft.

Im also not sure about the stiffness of a hypothetical 8mm carbon shaft...

But i'm sure that you could strengthen the stiffness of the shafts through further hardening processes. Nevertheless i cannot imagine that it would make any sense commercially, since such processes would make the shaft ridiculously expensive. But it would definitely be worth a try for hobby experimenters :)
 
Leander

Leander

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Oct 17, 2017
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but you can load a massive shaft with more energy than you could load a less massive shaft.
The best spear would be the one with the highest average density while keeping the stiffness and elasticity in a practical range. The example was just given to indicate that there's just no research being done here. We see the craziest cannons being invented, but they're still using the same old steel spear. I gave the example of a carbon tube with a tungsten core only because that would be a mix between a stiff outer, with a very dense inner. It wasn't meant as a practical example :)
 
Last edited:
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snask

snask

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Sep 25, 2020
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Going directly to the rubbers without the pulley, then the rubbers won't get the needed stretch to use the full barrel. Part of the pulley advantage is that the wishbone travels twice the distance of the rubber pull.

About the lost energy... The total amount of energy put into the spear is constant for a given amount of bands, whatever type of gun. So I don't think anything is lost actually. The force is just being projected in a different way. There's a Greek spearfishing store that put some lab-test videos online of different gun types, and iirc all the spears leave the gun at about the same speed. I'll see tonight if I can find those videos.

About the spears, I was just thinking out if the box. Every aspect of the gun is being engineered to optimal efficiency, yet the spears are still the same.
Of course! I see now, this is where the "fusion" setup steps in, with rubber on the top aswell so you stretch the top cable to the last Shark Fin. It seems the invert demultiplier is more popular though, more potential i guess. Did anyone figure out any loss of the extra pulleys/shackles in friction etc, maybe it's negligible. Someone mentioned the complexity and maintenance cost of such system. Traditional setup seems to be most popular still.
 
Last edited:
marco15499

marco15499

Laguneros Spearfishing
Apr 4, 2011
353
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Your gun is amazing, Zbanko! Congrats!
 
Z

Zbanko

New Member
Dec 12, 2020
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Your gun is amazing, Zbanko! Congrats!
Hi Marco, if you are referring to the videos, it’s not my gun :) i wish it was...
It is an invert roller designed by Basil Daniel, one of the strongest (maybe the actual strongest?!) spearguns out there. I have referred to it just to give an example how you can bulk up the rubbers in an invert roller setup without worrying about recoil and losing much precision.

My invert roller will be modest compared to b. daniels :) . I have a trigger which could take 375 kg of load, so i guess it will be ok with 6x20mm rubber strings. Btw. @Leander @Eray_spearo @snask do you habe any table or info on very roughly how much load caused by different rubber diameters stretched 300-400% ?
 
Eray_spearo

Eray_spearo

Member
Oct 26, 2020
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Did you do some more tests? I find it strange that you got so little power out of it. If you are using different band setups maybe some band is slower than the others so it barely powers the shaft?
The easier adjustment would be to double up the 14mm primary bands, more power and easier to load than 3x16.
Unfortunately, I was not able to run further tests as waters got cold here and I dont need a high range gun for this period. So I will make new tests in June of July when snapper will be my target fish again :) However, I am quit happy about the discussions here. Accordingly, I will shape my approach.
 
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