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How to...

...assemble and install wishbones using Spearit NTR Beads
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...assemble and install wishbones using Spearit NTR Inserts
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...assemble speargun bands
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...calculate band length:

This calculator is a work in progress. We hope to add additional functionality to it in the near future. If you experience any problems using the calculator, or have an opinion or comment about it, we would greatly appreciate your feedback to help us improve it. One recent change you may notice is how we are stating the modulus or elongation. By the old system a relaxed band had an elongation of 100% and at triple it's relaxed length it's elongation was stated as 300%. We have decided to change to the same scale that Primeline uses in it's specifications. The difference is that a relaxed band is now 0% and at triple it's relaxed length it is at 200% elongation. We hope this isn't too confusing but because we list Primeline specs on the site, we thought it best to harmonize the numbers.

Wishbone = Metal V
Spectra
Distance(D) = inch
cm
 

  Default Settings
Elongation (100 < 300)   %
Wishbone Length   Metal V
 in
Spectra
 in
Distance(D) Range   Minimum
 in
Maximum
 in
Band Length Calculator Instructions
  1. We require that users of the calculator understand the methodology behind the calculation to insure that they are able to confirm the results. If you haven't already, read the information on this page and the Calculator Disclaimer on a separate page in the Knowledge Base. We require acceptance of the Terms of Use statement at the bottom of this page to use the calculator and it will not function unless the "Agreed" button has been checked.

  2. Select the wishbone type, either Metal V or Spectra. The default length for Metal V wishbones is 6 inches and the default length for Spectra wishbones is 4.5 inches. The calculator will adjust the band length as required for the length of the selected wishbone.

  3. The default unit of measure is inches, if you prefer metric click on the button next to "cm". Note the calculator results work for any unit of length. The only difference between inches and cm results is that inches are rounded to the nearest 1/2 inch while cm rounds to the nearest whole centimeter.

  4. Enter distance D as measured from the band eye to the first notch or fin of the subject speargun. The default minimum distance(D) is 12 inches(30.5cm) and the default maximum is 60 inches(152cm).

  5. Click the CALCULATE button and the calculated nominal band length will be shown below the button.

Note: The calculator allows for adjustment to some of the default values for variables used in the calculation. Read the notes below before changing any default variable.

  • Wishbone Lengths - Defaults are the most commonly used lengths for each wishbone type.

  • Distance(D) Range - The default range should include nearly every speargun, however, the minimum and maximum can be changed.

  • Elongation - CAUTION. The default setting of 200%, approximates that of the specified bands of most speargun manufacturers. Safely changing this variable requires expert level knowledge to avoid loss of accuracy and range. Increasing the total tensile force exerted by the band set when loaded may result in equipment malfunction, damage to equipment or personal injury.
Band Length Calculation Explained

The Band Length Calculator is a handy tool but it is not a substitute for understanding the principles behind it. Users should always evaluate results to confirm that they are correct. Getting the right replacement bands for your speargun is essential. Even the best speargun can be rendered ineffective with the wrong bands. The consequences of a band set that is too weak are obvious but it is also possible to overpower a speargun. The problems of overpowering start with recoil and the inaccuracy that comes with it. The next issue is called "band whip". The combination of muzzle movement due to recoil and over acceleration can cause the spear shaft to flex when the speargun is fired. Band whip results in sharply increased drag and an actual loss of range and accuracy. At extremes, overpowering a speargun can cause malfunctions starting with jammed trigger mechanisms all the way up to permanent damage to the speargun and more importantly the spearo. The following list should be followed in order. It starts with the best way to determine the right band length and offers alternatives in order of decreasing precision.

  1. Use the manufacturer's specified band set. Many speargun manufacturer's list the band length and diameter by model on their website. If you can't find it in writing, it is worth a call to the manufacturer to ask. If your speargun is one that is widely used in the U.S.A., you may be able to find this information on our OEM Band Size Charts page in the Material information section of the Knowledge Base. This information should be used as a fall back, if you can't get the information directly from the manufacturer, as we can not guarantee its accuracy.

  2. If you have the original bands that came with the speargun you can measure them. Measuring the rubber diameter is relatively straight forward however measuring and translating the length, not so much. The convention for stating band length is the overall length of the rubber portion. This can result in inaccuracy when translating the nominal length from one band maker to another. This is because a small amount of the overall length is inactive and that small amount varies depending on the method of termination used to attach the wishbone. The extreme case is a manufacturer that uses rings to constrict the rubber. The long taper required to install the ring is inactive but is still usually included when stating the nominal band length. This can result in a variance of up to a couple inches of active rubber from one manufacturer to another with the same nominal length. If we had our way, the convention for stating band length would be the length of rubber between the constriction points. This is what matters, and matching this dimension is the best way to keep performance consistent. Until the industry comes around however, we do have a chart that will help you make adjustments for different termination types. See Band Length Defined

  3. Because most spearguns made over the last century are simple rugged devices, there are a lot of old spearguns still in use that have outlived their manufacturer. If you buy a second handed 30 year old gun and the band does not turn to powder in your hands, chances are it is not original and if the speargun has had a few owners before you, chances are pretty good that the previous owner had no more clue than you do of the correct band size. So even if the existing band is in decent shape it is a good idea to confirm that the size is correct. If you can't find any legacy info online and the manufacturer is out of business, try crowd sourcing the information by posting a question on an online spearfishing forum. Chances are that someone else out there has the same speargun and knows the specs on the original band set. Just be sure to double check the size with the calculator to confirm that it is in the ball park

  4. Finally, if all else fails our band length calculator will help you estimate a band length that should suffice. For help determining the rubber tubing diameter refer to the Band Power page in the Material Information section of the Knowledge Base.
The Simple Math of Calculating Band Length

If you failed algebra or you just don't care about the math, you can skip this section for now but find someone who can check the results against the formula below before using the results.

Estimating the right band length is simple enough if you know two things:

  • The distance between the point where the band is attached to the speargun and the point where the band engages the spear shaft. Most spearguns use circular bands, like the bands we make, that are attached to the speargun by passing through a hole, called a "Band Eye", in the muzzle end of the speargun. Depending on the manufacturer, the band engages the spear shaft by seating the wishbone in a notch cut into the shaft or by placing the wishbone over a fin or pin that protrudes from the spear shaft. Most spearguns have multiple notches or fins. Purists may prefer to size each band, for a multi band speargun, for each specific fin or notch. There is nothing wrong with doing this, but most multiband spearguns come with multiple bands of the same length, so when calculating the length of those identical bands, you need to pick a fin or notch to measure from. We suggest using the closest notch or fin to the band eye.

  • The other variable you need to know, is how much the band will be stretched when loaded to the closest fin or pin. We will call this the elongation and it is expressed in terms of the percentage of elongation from the bands relaxed length (0%) to it's loaded length. We have found that the most common elongation with stock bands is 200% to the first notch. This is not set in stone but unless you know differently 200% is a good place to start. The rubber we sell is specified by the manufacturer to elongate up to 750% before it breaks but for speargun bands a range of 100% to 300% should be considered the minimum and maximum.

When you know those two variables you can plug them into the following formula to calculate the band length:

D = the distance between the band eye and the closest fin or notch

E = the elongation (note: 200%=2)

L = the band length

L = 2D/(E+1)

Example: For a speargun with a distance of 30 inches between the band eye and the first notch and a target of 300% elongation:

L=(2 x 30)/(2+1)=60/3=20, so a 20 inch band will do the trick.

Band Length Calculator Terms of Use Statement
By checking the "Agreed" box below this statement, I affirm that I have read and understand all instructional material and warnings and that I agree to all terms and conditions contained in the Band Length Calculator Instructions (above) and the Calculator Disclaimer located on a separate page in the spearitco.com Knowledge Base.
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...select the best wishbone for your speargun
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...tie terminal knots in Spectra® braid
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...tie the constrictor knot
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