Home Tournaments Calendar Weather Merchandise Sponsors

Go Back   Spearboard.com - The World's Largest Spearfishing Diving Boating Social Media Forum > Spearfishing Gear > All About Guns

All About Guns What's your weapon of choice, and why? Discuss the beloved speargun here!

Reply
 
Thread Tools Rate Thread Display Modes
Old 06-14-2019, 08:45 PM   #1
popgun pete
Registered User
 
popgun pete's Avatar
 
Join Date: Mar 2006
Posts: 2,759
Band Calculations

Here is a formula for calculating the length of the bands on any speargun, it is a trivial calculation once you understand the terminology.

BR is the Band Run length from band anchor position to rearmost shaft tab, or wishbone notch, which on a rollergun is a wraparound length taking in the top and bottom decks of the gun. The band run consists of lengths of rubber and cord or cable when the gun is fully cocked and the designer can vary the proportions of each to suit the power and cocking requirements for that gun.

CL is the cord (or cable) length and is the combined length of the wishbone and anchor cords and wishbone cables, being half the length of the loops or as they are seen in profile when the gun is cocked. The longer the length of CL then the less rubber there is in the Band Run being stretched and the rest is made up of cord.

SF the Stretch Factor is the degree of stretch required on the bands and is a number. Note we assume that the cords don’t stretch and that only the rubber length between the constrictor knots on the band ends actually stretches out. Any rubber outside the constrictor knots or wedding band rings is considered part of the cord length CL as it plays no part in spear propulsion, but it can influence wishbone lift if these ends skate along the top deck by wrapping hard against it when using soft wishbones and no in-line pull band elevators at the muzzle.

X is the desired length of the slack bands that are to be stretched, you need to allow for an additional amount of rubber that lies beyond the constrictor knots and is often ground to a taper to improve hydrodynamic flow as the band contracts at high speed through the water. Note that X is the band strand length on each side of the gun, for loop or wraparound bands you double it.

We can calculate X once we have BR from the gun layout and we then decide on an SF value that will not stretch the guts out of the bands as X = (BR - CL)/SF. Another concern is that you can cock it and how many times you expect to shoot and reload in a given session, no point is having a gun you cannot cock late in the dive as then big fish swim up and look at you, perhaps sensing that you are no longer a threat.

Stretched band length is X multiplied by the stretch factor SF. After the shot we want the bands to return to length X unless they have been arranged to have some prestretch. If they don't when all stress is removed then they will need to be binned and replaced if other parameters are to be left unchanged, however bands can take some time to recover so they need to be checked the next day.

Band Elongation is (X times SF – X)/X or X times (SF -1) and is expressed as a percentage. Stretched band length is slack length X plus band elongation (%) times X. So as you can see there is really nothing to it.
Attached Thumbnails
Click image for larger version

Name:	Rollergun terminology.jpg
Views:	105
Size:	148.5 KB
ID:	243028   Click image for larger version

Name:	Cable Rollergun.jpg
Views:	59
Size:	89.6 KB
ID:	243054  

Last edited by popgun pete; 06-16-2019 at 09:02 PM. Reason: added a cable rollergun diagram
popgun pete is online now   Reply With Quote
Old 06-14-2019, 10:20 PM   #2
R/M_Spearo
Registered User
 
Join Date: Jul 2016
Posts: 55
Re: Band Calculations

Hey Pete Thank you for this.
I still have doubt i have a roller gun and my bands need to be replaced what stretch factor would I apply as a starting point. also I'm thinking of using 16 mm standard bands not small id

Sent from my SM-N950U using Tapatalk
R/M_Spearo is offline   Reply With Quote
Old 06-14-2019, 11:30 PM   #3
popgun pete
Registered User
 
popgun pete's Avatar
 
Join Date: Mar 2006
Posts: 2,759
Re: Band Calculations

You can always make bands shorter so maybe try a stretch factor of 3.5 or 250% elongation. Often a stretch factor of 4 or 300% elongation is quoted as a starting point, but you can also play with wishbone and anchor lengths to a certain extent. Another way to gauge your effort is to pass a length of the band rubber around something that will not budge and stretch it out by grabbing both ends. If you put some white marks on the band rubber with a chinagraph pencil or mark it with chalk at two points and then have someone measure the distance between the marks while you haul on it you can see what stretch you can achieve and at what effort. Don’t use a door handle as you might break it and rip the door off the hinge.

Last edited by popgun pete; 06-17-2019 at 04:14 PM.
popgun pete is online now   Reply With Quote
Old 06-15-2019, 08:51 AM   #4
jfjf
.
 
jfjf's Avatar
 
Join Date: Oct 2003
Location: Palm Bch County
Posts: 10,643
Re: Band Calculations

Thanks, I think it would be helpful to more clearly define exactly what SF stretch factor is.

For example, if a single piece of latex is 12 inches long in a relaxed state and it is stretch to 24 inches, what is the SF? Is it 100% or 200%?

It would be helpful to fill in the table below:

Length SF
12
24
36
jfjf is offline   Reply With Quote
Old 06-15-2019, 09:23 AM   #5
popgun pete
Registered User
 
popgun pete's Avatar
 
Join Date: Mar 2006
Posts: 2,759
Re: Band Calculations

"Stretched band length is X multiplied by the stretch factor SF".
Length in the stretched state is original length plus the elongation as a percentage of the original length, i.e. X + X times Elongation %
12, not stretched.
24, SF is 2.0, Elongation is 100%, 12 + 12 times 1.0 = 24
36, SF is 3.0, Elongation is 200%, 12 + 12 times 2.0 = 36
popgun pete is online now   Reply With Quote
Old 06-16-2019, 04:18 PM   #6
popgun pete
Registered User
 
popgun pete's Avatar
 
Join Date: Mar 2006
Posts: 2,759
Re: Band Calculations

As a matter of interest here is a band gun where the Band Run BR is all rubber, the Mordem "Boreal". This gun uses a continuous loop band and a sliding spear pusher running in a track, no expense has been spared!
Attached Thumbnails
Click image for larger version

Name:	MORDEM  Boreal comp.jpg
Views:	82
Size:	139.4 KB
ID:	243050   Click image for larger version

Name:	MORDEM  Boreal muzzle.jpg
Views:	71
Size:	64.0 KB
ID:	243053  

Last edited by popgun pete; 06-16-2019 at 05:56 PM.
popgun pete is online now   Reply With Quote
Old 06-17-2019, 10:16 AM   #7
Mana'squal
Registered User
 
Mana'squal's Avatar
 
Join Date: Jul 2016
Location: North East US
Posts: 289
Re: Band Calculations

Quote:
Originally Posted by popgun pete View Post
As a matter of interest here is a band gun where the Band Run BR is all rubber, the Mordem "Boreal". This gun uses a continuous loop band and a sliding spear pusher running in a track, no expense has been spared!
Thats some goofy shit right there. Is there any advantage to all that friction?
Mana'squal is offline   Reply With Quote
Old 06-17-2019, 02:58 PM   #8
popgun pete
Registered User
 
popgun pete's Avatar
 
Join Date: Mar 2006
Posts: 2,759
Re: Band Calculations

Quote:
Originally Posted by Mana'squal View Post
Thats some goofy shit right there. Is there any advantage to all that friction?
No, it is just being different and the “Boreal” gun certainly achieved that in spades. A lot of hydrodynamic drag as that double handle “wishbone” unit ploughed through the water during the shot. There do not seem to be many around and they must have been expensive in their day. Note the butt pad cum tethering rings in the rear end of the gun. The black grip handle was also used in the Mordem "Saturno" pneumatic gun and sits a bit low in terms of the center of grip pressure wrt the shooting axis.

However with a fat circular cross-section timber barrel and all that metal hardware at muzzle and butt this in-line band pull gun would be a stable shooter.
Attached Thumbnails
Click image for larger version

Name:	saturno models.jpg
Views:	30
Size:	48.9 KB
ID:	243097   Click image for larger version

Name:	MORDEM-Saturno 4.jpg
Views:	28
Size:	64.8 KB
ID:	243098  

Last edited by popgun pete; 06-19-2019 at 02:30 AM. Reason: added Saturno gun image circa 1969 models
popgun pete is online now   Reply With Quote
Old 06-21-2019, 12:20 AM   #9
Diving Gecko
Shooter & Shooter
 
Join Date: Nov 2014
Posts: 741
Re: Band Calculations

Of course Pete is right and the math is, too.
But one of the first things I noticed once I had to start cutting my own bands is how the spearo community in general has kinda decided that stretch factor and percentage wise stretch is one and the same. So, often when people talk about, say, 300% stretch they actually mean a factor of 3 which as Pete has explained is really only 200%.
Perhaps that's why he made this thread, to try to "educate" people.
Diving Gecko is offline   Reply With Quote
Old 06-21-2019, 04:28 AM   #10
pbguy420
Registered User
 
Join Date: Mar 2016
Posts: 40
Re: Band Calculations

Hats off to you Pete! I’ve never heard of anyone using the term stretch factor but I think speaking this way could alleviate a lot of confusion.

While I was learning I found it very frustrating that some people call a band at rest at 100% or 0%. Hopefully this clears up some of that confusion and people pick up a standard that makes sense universally


Sent from my iPhone using Tapatalk
pbguy420 is offline   Reply With Quote
Old 06-21-2019, 04:32 AM   #11
Diving Gecko
Shooter & Shooter
 
Join Date: Nov 2014
Posts: 741
Re: Band Calculations

Quote:
Originally Posted by pbguy420 View Post
Hats off to you Pete! I’ve never heard of anyone using the term stretch factor but I think speaking this way could alleviate a lot of confusion.

While I was learning I found it very frustrating that some people call a band at rest at 100% or 0%. Hopefully this clears up some of that confusion and people pick up a standard that makes sense universally


Sent from my iPhone using Tapatalk
Sure, it would be cool if everyone used the same lingo. I have heard stretch factor used quite a bit and I honestly think it is the most intuitive. Loads of people from all walks of life don't have a 100% handle on percentages...
Diving Gecko is offline   Reply With Quote
Old 06-21-2019, 04:40 AM   #12
pbguy420
Registered User
 
Join Date: Mar 2016
Posts: 40
Re: Band Calculations

I feel like the reason “400% stretch” at SF of 4 became what I would consider the norm (at least where I live) is that the first number tells you stretch factor and less thinking is involved.

At least this clears it up for new folks. I believe at least wong and pathos guns speak in “ proper” elongation percentages and I remember being very confused when one of the two websites said 250% stretch was optimal for their guns


Sent from my iPhone using Tapatalk
pbguy420 is offline   Reply With Quote
Old 06-21-2019, 04:44 AM   #13
Diving Gecko
Shooter & Shooter
 
Join Date: Nov 2014
Posts: 741
Re: Band Calculations

Quote:
Originally Posted by pbguy420 View Post
I feel like the reason “400% stretch” at SF of 4 became what I would consider the norm (at least where I live) is that the first number tells you stretch factor and less thinking is involved.

At least this clears it up for new folks. I believe at least wong and pathos guns speak in “ proper” elongation percentages and I remember being very confused when one of the two websites said 250% stretch was optimal for their guns


Sent from my iPhone using Tapatalk
I believe Primeline uses proper percentage speak, too. So, either we should all just learn from Pete's math lesson or try to increasingly switch to stretch factor.
I like the "the first number tells you stretch factor and less thinking is involved" explanation; 4 gives the factor and well, the two zeroes can just be thrown overboard.

Last edited by Diving Gecko; 06-21-2019 at 05:52 AM.
Diving Gecko is offline   Reply With Quote
Old 06-21-2019, 09:30 AM   #14
popgun pete
Registered User
 
popgun pete's Avatar
 
Join Date: Mar 2006
Posts: 2,759
Re: Band Calculations

Band rubber needs to be used in the operating zone where it does not yield and becomes permanently elongated as the bands are required to repeatedly deliver the same performance on each shot. For that you need to use an elongation that they can recover from, although bands always stretch out a small amount with repeated use they eventually recover if used conservatively.

Primeline have a specifications page here: https://primelineindustries.com/spec...-57e9ca32-2028.

Better to use more bands rather than stretch the guts out of them, but some bands are better than others as rubber batches can vary.
popgun pete is online now   Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 08:01 PM.


The World's Largest Spearfishing Diving Social Media Forum Powered by vBulletin® Version 3.8.1
Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Copyright ©2002 - 2014 Spearboard.com