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Load of inverted Roller Speargun?

born to hunt

born to hunt

New Member
Mar 19, 2025
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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
1742619091484.jpeg1742619097170.jpeg
 
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
 
Reactions: born to hunt
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).

 
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.
 
Reactions: born to hunt
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 ?
 
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.
 
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