Load of inverted Roller Speargun?

[born to hunt]

Hi there

I am planning on making an inverted roller speargun, total length will be 115-120 cm and the barrel-pipe will be 100cm
i am planning on using 3 rubbers bands on each side to 6 total of 14,5 or 16mm 25-30cm each to achieve 280-350% stretch

what would be the workload for the speargun in each scenario in kg or lbs ?
and how to calculate it ? thank you so much in advance

 

[david598ball]

Hi there

I am planning on making an inverted roller speargun, total length will be 115-120 cm and the barrel-pipe will be 100cm
i am planning on using 3 rubbers bands on each side to 6 total of 14,5 or 16mm 25-30cm each to achieve 280-350% stretch

what would be the workload for the speargun in each scenario in kg or lbs ?
and how to calculate it ? thank you so much in advance

Hello,
To calculate the workload (force) of your speargun, use the formula: biglotssurvey.cc

Workload (Force) = Rubber Cross-Sectional Area × Draw Length × Stretch Percentage × Elasticity Constant

1. For rubber area (A): Calculate the cross-sectional area of one band using the formula:
   $$A = \pi r^2$$
   where r is the radius of the rubber (e.g., for 14.5 mm, r = 7.25 mm or 0.725 cm).
2. Multiply by the number of bands (6 total).
3. For draw length (L): This is the distance the rubber is stretched (e.g., from relaxed to fully drawn).
4. For stretch percentage (e.g., 300%): Convert to a decimal (300% = 3.0).
5. For elasticity constant (k): Approximate value for speargun rubber is around 1,000-1,500 N/m.

Finally, convert the result from Newtons (N) to kilograms (kg) or pounds (lbs).
Let me know if you'd like assistance with detailed calculations!

Best Regards,
David Ball

 

[born to hunt]

Hello,
To calculate the workload (force) of your speargun, use the formula: biglotssurvey.cc

Workload (Force) = Rubber Cross-Sectional Area × Draw Length × Stretch Percentage × Elasticity Constant

1. For rubber area (A): Calculate the cross-sectional area of one band using the formula:
   $$A = \pi r^2$$
   where r is the radius of the rubber (e.g., for 14.5 mm, r = 7.25 mm or 0.725 cm).
2. Multiply by the number of bands (6 total).
3. For draw length (L): This is the distance the rubber is stretched (e.g., from relaxed to fully drawn).
4. For stretch percentage (e.g., 300%): Convert to a decimal (300% = 3.0).
5. For elasticity constant (k): Approximate value for speargun rubber is around 1,000-1,500 N/m.

Finally, convert the result from Newtons (N) to kilograms (kg) or pounds (lbs).
Let me know if you'd like assistance with detailed calculations!

Best Regards,
David Ball

Hello
thank you so much for the reply,
can you explain this one more
what are the $$ for? did u use them as " " ?
can you pls give us an example of one rubber to know how to apply it more ?
thank youu

1. For rubber area (A): Calculate the cross-sectional area of one band using the formula:
   $$A = \pi r^2$$
   where r is the radius of the rubber (e.g., for 14.5 mm, r = 7.25 mm or 0.725 cm).
   
   

 

[rollerGR]

Hi there

I am planning on making an inverted roller speargun, total length will be 115-120 cm and the barrel-pipe will be 100cm
i am planning on using 3 rubbers bands on each side to 6 total of 14,5 or 16mm 25-30cm each to achieve 280-350% stretch

what would be the workload for the speargun in each scenario in kg or lbs ?
and how to calculate it ? thank you so much in advance

Nice looking gun! It's mind-blowing how rare beautiful/symmetrical guns are.

3x14,5mm pairs at ~3,5 would give around 135kg of force (135N), 3(pairs)x45kg(at ~3,5). Source is Spiller, J. (2021) iirc. It's not the exact load since every rubber is different, but that's a point of reference.

That's a bit light for an invert roller. It's very close to a 2x16mm roller. It would work well with a 7mm shaft though. If you want heavier, I'd suggest going with 1x17,5 (or anything up to 19mm) pair at ~3,2/~3,3 and 2x14,5mm ones, same stretch.

 

[born to hunt]

Nice looking gun! It's mind-blowing how rare beautiful/symmetrical guns are.

3x14,5mm pairs at ~3,5 would give around 135kg of force (135N), 3(pairs)x45kg(at ~3,5). Source is Spiller, J. (2021) iirc. It's not the exact load since every rubber is different, but that's a point of reference.

That's a bit light for an invert roller. It's very close to a 2x16mm roller. It would work well with a 7mm shaft though. If you want heavier, I'd suggest going with 1x17,5 (or anything up to 19mm) pair at ~3,2/~3,3 and 2x14,5mm ones, same stretch.

Thank you, i still added more changes to it , and the bottom is still a bit too pointy so going to work on that too !

oh thank you so much, very important information,
i thought about 1 pair of 18 mm + (2 pairs of 16 or 14,5) or (1 pair of 16 and 1 of 14,5)
which one do you think would be more optimal giving that the size of the tube will be 100 cm and the shaft is around 140 cm of 8 mm


by any chance do you know the diameter of double roller mechanism ?

 

[musubi]

Hi there

I am planning on making an inverted roller speargun, total length will be 115-120 cm and the barrel-pipe will be 100cm
i am planning on using 3 rubbers bands on each side to 6 total of 14,5 or 16mm 25-30cm each to achieve 280-350% stretch

what would be the workload for the speargun in each scenario in kg or lbs ?
and how to calculate it ? thank you so much in advance
View attachment 60831View attachment 60832

The best is to manually measure it, with a scale. There's just too many variables to account for, from differences in band brands and composition to where people actually tie the wishbone, etc. It's how I dialed in my gun and determined how much more or less powerful I want it.

Also, what matters is how the load changes as you draw back each set of bands. The load ramps as you pull back. Ideally, you need to calculate Work: Force x Distance. Then determine your resultant velocity from that. I plotted mine through CAD since it's easy to integrate through that. Once you have the velocity, you can compare that value to other setups. That will give you a good starting point for power adjustment.

If you're planning on running an 8mm shaft on a 100 cm gun, you'll need more power than 3 pairs of 14.5 mm bands maxed out in stretch to propel that nicely. I would recommend working with 16 mm bands. Just be aware of overpowering any setup as that will cause poor shaft flight.

 

[rollerGR]

oh thank you so much, very important information,
i thought about 1 pair of 18 mm + (2 pairs of 16 or 14,5) or (1 pair of 16 and 1 of 14,5)
which one do you think would be more optimal giving that the size of the tube will be 100 cm and the shaft is around 140 cm of 8 mm


by any chance do you know the diameter of double roller mechanism ?

For an 8mm shaft (heavy) go with 1x18mm + 2x16mm. It will be almost as strong as a double roller with an 8mm shaft, where 2x16mm bands are traditionally used. You could even go with a 7.5mm shaft. I've seen rollers with this setup using 7mm shafts with zero problem and crazy fast shots (shaft itself is heavy enough to maintain enough momentum and surpass 5m).

Most invert rollers tend to be underpowered.

No idea about the roller system, but there are some 3D designs of Meandros' pin roller mechanism if you can get anything from there.

 

[rollerGR]

Then determine your resultant velocity from that.

It's probably best to measure the energy (J) like in Spiller (2021) because a conventional gun will have the same or higher muzzle velocity than a roller or invert, but the shot will be nowhere as strong due to significant differences in stored energy. Energy per X propulsion distance would most likely be ideal.

 

[rollerGR]

Best image I could find, you could probably try to recreate it and get the dimensions this way.

 

[born to hunt]

For an 8mm shaft (heavy) go with 1x18mm + 2x16mm. It will be almost as strong as a double roller with an 8mm shaft, where 2x16mm bands are traditionally used. You could even go with a 7.5mm shaft. I've seen rollers with this setup using 7mm shafts with zero problem and crazy fast shots (shaft itself is heavy enough to maintain enough momentum and surpass 5m).

Most invert rollers tend to be underpowered.

No idea about the roller system, but there are some 3D designs of Meandros' pin roller mechanism if you can get anything from there.

Thank you so much for the valuable advice, yeah i'll start by double 18 mm and double 16 and i'll determine what should the last one be

thing is, can't find 7mm shafts here, it's either 6 mm that are cheap or 6,5 that' are over priced, or i can make 8 for cheap and as many as i want since there is 8 mm stainless steel rods for good price, so i can work with them

i'm still on the look for 7mm which will be ideal for me

 

[Adammoore]

how it went whats the final power you manager to get would love to see the results

 

[rollerGR]

For an 8mm shaft (heavy) go with 1x18mm + 2x16mm. It will be almost as strong as a double roller with an 8mm shaft, where 2x16mm bands are traditionally used. You could even go with a 7.5mm shaft. I've seen rollers with this setup using 7mm shafts with zero problem and crazy fast shots (shaft itself is heavy enough to maintain enough momentum and surpass 5m).

Looking back, nowadays I'd recommend 4x17,5 or 1x19+3x17,5 for an 8mm shaft

 

[Sorcerer]

While 100cm is still quite short, higher diameter shafts are stiffer. Less whip and bending, especially if you wanna use like 140cm+ shaft.

Nowadays I'd eference myself to personal table I try to not overpower guns.
6.5 x 115-135cm single 16mm
7.0-7.5 x 125-150cm double 14/16mm lenient
7.5 x 150-160cm double 14mm(350+)/16mm (320+)
8.0 is probably a bit heavy so only double roller or inverted.

Than you can convert it to double/invert power levels.

Try to match shaft diameters based on what species you are for.
Sometimes less complicated is better, less failure points, less tangles, easier to dial in, less money spent on rubbers and cutting them in, time testing.

My long 7.5mm x190cm RA shafts bent like crazy on 140cm double roller. Unusable even with 14mm, and with low preload not enough power. Impossible to dial them in.
I think I got cured from double rollers, my friends too -_-

Ulu for this reason does not recomend 120cm+ double roller guns from his experiences aswell.

 

[rollerGR]

While 100cm is still quite short, higher diameter shafts are stiffer. Less whip and bending, especially if you wanna use like 140cm+ shaft.

Nowadays I'd eference myself to personal table I try to not overpower guns.
6.5 x 115-135cm single 16mm
7.0-7.5 x 125-150cm double 14/16mm lenient
7.5 x 150-160cm double 14mm(350+)/16mm (320+)
8.0 is probably a bit heavy so only double roller or inverted.

Than you can convert it to double/invert power levels.

Try to match shaft diameters based on what species you are for.
Sometimes less complicated is better, less failure points, less tangles, easier to dial in, less money spent on rubbers and cutting them in, time testing.

My long 7.5mm x190cm RA shafts bent like crazy on 140cm double roller. Unusable even with 14mm, and with low preload not enough power. Impossible to dial them in.
I think I got cured from double rollers, my friends too -_-

Ulu for this reason does not recomend 120cm+ double roller guns from his experiences aswell.

For reference some setups we use here
6.5 with 2x14,5 or 2x16 classic
6.5 with 3x16 invert (mvd)
6.5 with 17,5 single roller
6,75 with 3x17,5 invert
6,75 with 3x16 classic (yes i've seen this, on video too)
7 with 3x14,5 classic
7 with 3x16 classic
7,2 with 2x16 double roller (this one still shoots slightly low due to inherent roller muzzle geometry issues)
7,2 with 1x20+2x18 invert (close to 2x16 double roller)
7,5 with 4x17,5 invert
7,5 with 1x19 + 3x17,5 invert

I think it comes down to recoil and hunting preference and style of hunting. all of these setups have amazing accuracy and range
~3 elongation, so this differs to eg 2x14,5 at ~3,5 which in practice are closer to 2x16 at ~3
heavy recoil will destroy accuracy and shaft energy, but the shaft bending isn't an issue imo. if we eliminate recoil and jerk then the shaft will still hit dead henter with good penetration, hence why rollers and invert rollers have incredible power with a lower load
to see what i mean, reference bow arrows that bend like crazy during flight yet still hit dead center with great speed, or an alemanni double roller posted here some years ago where the shaft was bending a lot but the owner reported good accuracy despite that.
edit: archer's paradox actually shows how shaft/arrow flexibility can improve accuracy. so it seems like it's not a problem at all, if anything it's necessary in powerful setups

btw 7,2-7,5 setup examples are all with 125+ guns and some Neptunus have additional booster bands that increase the load even more even though without significant benefit
there are a few other BW examples like kraken or carl's with 130/140 double HEPA rollers (nearly a quad roller)

7mm and above i would say is safe to use up to 160cm easily without any issue. a problem with thick shafts is that they have greater profile drag so they lose energy quicker than a thinner longer shaft of equivalent mass

 

[Sorcerer]

Some setups you put on example are so off it's beyond me, but some can slip by if it's not a carbon/alu gun due to extra mas of wood gun.
I can't imagine 3x standard bands on light gun. It's just stops adding much and you have huge diminishing returns.
Besides it's better to stretch bands more than add 1 more rubber and less stretch on light guns. The 3rd rubber will add a ton of recoil with little to not benefit when I was playing with the tecnicaspearguns calculator. (Talking about light guns only)

Back than, my simplest light gun would almost break my wrist at 6.25 x 140(?) and 2x14mm 380% and accuracy way all over the place. I'd get a fish, yes but no... It wasn't dialed.

The archers paradox is very well known, but I believe you can't apply it to more viscus environment because the initial bent is gonna affect the accuracy greatly, different shaft materials and much higher drag scenario.

I have one video on my IG where my double roller 7.5x190 shaft on 140cm would fly sideways out of the muzzle, it's clean as day. The shaft trajectory was couple degrees off even before shaft would entirely exit the muzzle.
Issue was even when I put like 150g lead into the head lol

2 identical guns balanced carbon guns, on ermes muzzle handle and soriatec type barrel. 2 new RA shafts.

 

[rollerGR]

Some setups you put on example are so off it's beyond me, but some can slip by if it's not a carbon/alu gun due to extra mas of wood gun.
I can't imagine 3x standard bands on light gun. It's just stops adding much and you have huge diminishing returns.
Besides it's better to stretch bands more than add 1 more rubber and less stretch on light guns. The 3rd rubber will add a ton of recoil with little to not benefit when I was playing with the tecnicaspearguns calculator. (Talking about light guns only)

Back than, my simplest light gun would almost break my wrist at 6.25 x 140(?) and 2x14mm 380% and accuracy way all over the place. I'd get a fish, yes but no... It wasn't dialed.

The archers paradox is very well known, but I believe you can't apply it to more viscus environment because the initial bent is gonna affect the accuracy greatly, different shaft materials and much higher drag scenario.

I have one video on my IG where my double roller 7.5x190 shaft on 140cm would fly sideways out of the muzzle, it's clean as day. The shaft trajectory was couple degrees off even before shaft would entirely exit the muzzle.
Issue was even when I put like 150g lead into the head lol

2 identical guns balanced carbon guns, on ermes muzzle handle and soriatec type barrel. 2 new RA shafts.

exactly that's the key, these are never for light guns. it's either wood or heavy duty ballasted carbon
even with two thick bands the recoil is uncomfortable on a light pipe gun even if it has additional mass (or regular light carbon barrels)

regarding the analysis/theory, i have had similar experiences with rollers as well - but not invert rollers even with greater loads. actually even light rollers (like a 95 with 1x16 and 6,5 shaft) would have bad accuracy. the stoppers slam on the arches and this will lift the entire muzzle at a leverage-heavy distance from the handle resulting in hitting the shaft during flight. there's also the issue of the bands going over the rollers instead of stopping before and with such large material diameter this will life the shaft as well. unfortunately it's not a recoil issue and for now there's no way to fix it. it's similar to having recoil despite not having any recoil. even without muzzle lift there would still be tail lift
whereas in classic or invert guns we only have the initial shaft lift upon release (visible in slow mo) but then the shaft restores its trajectory on the guide. with classic setups there is additional heavy jerk causing vibrations/bending which will negatively affect energy

archer's paradox is more about the projectile's ability to maintain energy even upon deformation (otherwise accuracy would be bad), not about accuracy in spearguns (since we can't aim the same way like with bows)

i saw your vid, from 2024 if i got the right one. that's the classic issue we have on all rollerguns (had it thrice myself on 3 different guns, both single and hepa)
one DIY solution for better accuracy I have found for this without having to convert the gun into an invert roller, is to convert it into a fusion 2 or fusion 3 but it requires thicker rubbers

 

[Sorcerer]

The double roller muzzle I think has so many issue as you say from my experience too ton of rubber hiting the muzzle at 1.5m lever with no stabilization is insane force.
We have tried to make it work hard but jezus, even if I spend 100 days a year in the water, testing is annoying as f#@#

With decent preload I had also very high recoil on the hand with it, was reported by my buddies too compared to triple par 16mm inverted. The difference was insane, granted it wasn't the same shaft size or same power band ratio obviously but a lot of my guys are going back to standard guns nowadays me included. I might try the 140cm in inverted 3x2x16mm soon if I get rubbers, if it won't work I'm cutting the barrel to 120 and go inverted with shorter shaft. If this doesn't work?
Gonna convert to 2x16mm and screw this.

The amount of missed wahoos I went through with the double roller blue water idea cost me at least 3k in revenue.

I think the general population is too sold on the words
Light and carbon fiber

It has its uses, but jezus,.some thing should not be blindly followed.
Same shit happened around bicycles manufacturing around 2013-2015 when I retired due to bad knee as an XC bike racer, I'd not upgrade my frame to carbon.
The weight benefits almost noone and the risks are tremendous because hairline cracks are easy to spot while doing maintance but carbon when fails it can have life alternating consequences.

Going back to archers paradox, I think in water medium, the bending, flexing and whipping might be much worse than we think due to cavitation. The turbulence and cavitation drag to my understanding is no joke.

I wish we have done more high FPS camera testing next to the muzzle for better footage as I've seen some frames where cavitation would be enormous on the leaward side of the bending shaft on the exit.
Maybe if we get bored I'll get it done... I doubt though.

 

[Bill McIntyre]

A lot of guys I know have given up on rollers and gone back to standard guns. And wood. You can drill holes it to add loading assist tabs, switch to a different reel that has a different hole pattern, etc.

 

[rollerGR]

rollers have the tendency of the long bands vibrating against the gun's body after the shot which can feel like recoil and is about as annoying. so this could definitely be a factor

regarding rollers, rob allen has said that most of younger gen tend to buy rollers. i think it's a compromise between upgrading from a beginner gun without going for an expensive wooden one and with lower recoil since most are monorollers
bad accuracy does not affect them as much as they usually go reef hunting and shooting at small distances, but have better penetration

as for the water medium, shaft deformation would add to the drag, although shaft mass would be the main issue here. the lighter the shaft the higher the speed the higher the drag, and additional deformation would add some extra drag although not much
so it definitely does have an effect, just not a very big one as was suggested in the past few decades (turns out it's sharp acceleration causing jerk and recoil that affects both energy and accuracy mainly)
with rollers/invert rollers the shaft has generally lower maximum velocity so that helps with kinetic energy too

for turbulence and cavitation, that's an interesting one. not sure about cavitation but i have seen the bubbles too in slow mo. it looks they come from the rubbers though, there is a slow mo vid from worthington (for his study) with a double band and it seems to come entirely from the rubbers. not sure if this could affect the shaft through contact, but it's not unlikely at all. so rollers and invert rollers would save up more energy from this as well

 

[rollerGR]

A lot of guys I know have given up on rollers and gone back to standard guns. And wood. You can drill holes it to add loading assist tabs, switch to a different reel that has a different hole pattern, etc.

rollers are virtually nonexistent here in greece too. italians do seem to use them a lot more though, but then again mostly for close shots