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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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Eray_spearo

Member
Oct 26, 2020
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Hi guys,

Recently, I made a home-made invert roller by combining different parts. Eventually, I came up with a 100mm gun with following parameters:

- Carbon body
- Roisub invert roller head
- 3 x 14mm rubber band

My first combination was with 7mm 140cm shaft. However, the balance was not good as the head was floating. So I changed that with 150mm 7mm shaft. Right now, the balance is good but the performance is below my expectation. The shaft is not moving strong enough to move 5m string. The stretch ratios of rubber bands are roughly in the range of 300-340%. It seems that the effective range is not more than 3-3,5m

Do you think that it is normal? Will it help replace the existing shaft by 6,5mm 150cm? Or better to shorthen rubber band and check?

Thanks in advance for your feedback
 
Reactions: Mr. X
If an inverted roller is balanced well (balance of forces, not buoyancy), the recoil should only be very slight. So it could be overpowered massively [edit: with only shaft-whip to worry about]; at least in theory. On my MVD 82cm I got three 16mm bands and I see no reason why it wouldn't take even heavier bands. With these bands and a 6.5 120cm spear the power is enough to go through a skaros' head at the end of a double wrap.
For normal guns the idea of smaller diameter is that they contract faster and smoother, but because of the invert roller's pulley system the [edit: wishbone's speed is double the contraction-speed of the bands]. That also means the bands raw power is halved, which also hints to using thicker (or more) bands.

Instead of going for longer spears to balance the gun's buoyancy I would balance the gun's head with some small pieces of lead.

Show us some pictures of your gun! I'm curious!
 
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Reactions: Mr. X
Thanks for your quick feedback. Actually, I was referring to buoyancy. I had the chance to try out the gun only at the shore (50cm water). To check the accuracy I need to spend more time on it. By the way, the recoil was quite low.
As this has been a try out for me (I am using other spearguns actively) I used old rubber bands to find out the ideal combination. Therefore, I am planning to shorthen bands in the very first place and check if there is a significant change.

Quote: For normal guns the idea of smaller diameter is that they contract faster and smoother, but because of the invert roller's pulley system the contraction speed is doubled anyway. That also means the bands raw power is halved, which also hints to using thicker (or more) bands. End of Quote

Regarding your comment above: Does it mean that the power of a regular gun with the same rubber combination is higher than that with invert roller head, because of pulley? If yes, this explains why my invert roller is weaker wrt my wooden speargun with 2 x 14mm rubber bands.

I will take some photos in the evening and add them up. So it will be clearer what I am talking about
 
Reactions: Mr. X and Leander
Eray_spearo said:
Does it mean that the power of a regular gun with the same rubber combination is higher than that with invert roller head, because of pulley?
Click to expand...
Exactly. The total power output is force*speed, and the total output is a constant. So when the speed doubles (the wishbone travels twice the rubbers' contraction distance in the same time), the force at a given moment is halved. This is why invert rollers come with these large clusters of rubbers.
The total length of the bands, as in the length of all strands added together is the same as with a normal gun, but sliced in halve and bunched together. This, plus the fact that a pre-tensioned invert roller has no 'deadspace' (whats the correct term?) also means the formula to calculate the correct length for the rubbers is different. A few months back there was a question about the formula, so the search helps.
 
Reactions: Laliotisdim
Hello again,

Attached you can find a few photos about my new design with 3 band combination. Your comments are more than welcome. By the way, I shorthened rubber bands yesterday. The stretch ratio is likely to be in the range of 350-380% for 14mm.
 

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Reactions: Leander and Mr. X
That looks nice! Personally I would favour a shorter spear, but that's me. With a heavier spear you do need more power though. With a 150cm 7mm spear, I doubt the effective raw power of 1.5 14mm bands (three bands divided by the pulley factor) will be enough. If the bands are correctly set-up, including the pretension, and there are no other issues like a stiff roller or inflexible cord then I'd think all that would be needed is more power. Overpowering without severe penalties is one of the invert roller's strong points, use it!
 
Reactions: Mr. X
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
 
With difference in shafts my experience is only theoretical, but I'll write it anyway.

The one on my gun, a 6.5mm 120cm (for a 82cm gun), still punctures a fish at the end of a double wrap (with three 16mm bands), but it bends as hell. However, after asking around on this forum I concluded that's probably just a bad untempered spear. 'The longer the spear the easier it bends' remains true though. Let me repost a table someone posted a few months ago.



Adding weight to the spear by increasing the diameter doesn't work completely as expected, as the increase in diameter also gives an equal increase in water-resistance. But increasing the length while keeping the diameter constant makes it more flexible, and therefore more prone to shaft-whip and being bent by impact or fish.

My expectations for a 6.5mm spear vs a 7mm, is that the 6.5mm launches faster and has a little more range, if the bands are kept the same. The impact force of both would be equal right in front of the muzzle as both spears would now carry the same amount of energy; but at distance the 6.5mm would win, unless you also increase the bands to counter the added weight of the 7mm. A longer spear of the same diameter would have more range than a shorter one, if the bands are adapted to the difference in weight, but in that case, also go for a longer barrel to make optimal use of the longer spear. I would expect a thinner shaft to have a greater penetration than a thick one. Like if someone throws a <1gram sewing needle at you *really* fast, it would go right through you, while if a train would hit you extremely slowly (with the same actual force as the needle) it would just push you.

The amount of overhang is another variable. From my theoretical pov I would say the shorter the better as all overhang is 'wasted space' and doesn't add anything beneficial. But people tweaked with this for many years and most have found the 40cm barrel-spear difference optimal. However, for a large part people do stuff not because of knowledge or experience, but 'because it is done this way'.

In the end it all depends on what you want to hunt and where.
Fast moving targets, thin shafts.
Resistance to bending from impact or fish and raw power at close range, thick shafts.
Range, long shafts (and barrel).
Maneuverability, short shafts.
 
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I really liked those diagrams. However, I am not familiar with how to interprete the values for "resistance to shaft whip" and "Bending likelihood". Is there an optimum level or range for that? My target fish for that gun is snapper with long-range shots. For close-range targets in dirty water I am planning to use a shorter gun with thicker band offering a shorter range but high impact. Using a reactive band rather than a progressive one will help me additionally. So this gun is meant for fast moving spooky targets.
Checking your comments, 6,5mm shaft seems to be a better option (less weight, higher penetration, less friction in water, greater range). My main concern is that the shaft will not move straight if overpowered due to bending effect. I dont have real experience and personal observation in this.
I have only few options on hand (6,5mm x 150cm, 7mm x 140cm, 7mm x 150cm). My trial with 7x 140 didnt run well in terms of bouyancy. Of course, I can add some weight on the head however I rather prefer to do it without. Therefore, other options look more attractive

waiting for your comments

BR
 
Reactions: Leander
That diagram isn't made by me. It was posted here on the forum a few months ago by someone who did a lot of lab tests on shafts. Dig around the forum, perhaps a google-image-search to find it. I saved it to my phone as I do with all stuff interesting, but that of course separated it from the accompanying forum post. You could interprete the values relational to each other and use it as a guide, but I would recommend looking up the original post.

What exactly do you consider to be long range? My post above was as mentioned mostly theoretical, so you'll have to do some experimenting. With the shafts you have and the option to perhaps add temporarily some more bands you can make quite a lot of combinations. As for balancing the guns buoyancy during tests, you could just add a fishing sinker with some tape as a temporary solution just to see what works.

In theory the best shaft for long range fast moving targets would be long and thin. But a thin shaft is also fragile and too much overhang gives the spear a relatively short acceleration track. I can imagine a too big overhang (how much is too much?) could also make the tip sag a little, leading to low shots, but again, theoretical.
 
Reactions: Eray_spearo
In the end, I have to make numerous experiments Thanks for helping out
 
Reactions: Leander
You always have to when you build something yourself. Theory can only provide so much. But that's part of the fun!

Still I think that if what you try to achieve is a gun that shoots like a long range gun, that you won't get there simply by making the shaft longer and longer. It has to be in balance with the launcher's track and bands. One thing you can change without doubt is to add more or thicker bands for more power input. The way the invert roller works just asks for it.

As for the shafts, on a day where the sea is calm set up some target, like a bottle with some string to a weight and do some shots. Don't even need to have a line attached to the spear for that as the bottle won't run off with your spear (at least, normally they don't.. )
 
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Leander said:
If an inverted roller is balanced well (balance of forces, not buoyancy), the recoil should only be very slight. So it could be overpowered massively without shaft-whip; at least in theory.
Click to expand...
Leander, I don't get this part. How reduced recoil translate into reduced shaft whip? Also, later in the same post of yours, I don't understand what you said about contraction speed.
 
Woops, I used the wrong term. Apparently shaft-whip has to do with the compression and bending of a shaft due to the sudden acceleration. I completely overlooked that effect! (and misused it's naming a few times...)

I meant the effects of the recoil on the gun, hand and wrist, and subsequently also the spear. The more stable the platform, the more stable the spear.

With the contraction speed I meant that because the way a pulley works, the speed of the wishbone going forward is twice that of the speed at which the bands contract.
 
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Reactions: Andrew the fish
Dear Leander,

Let me ask you a question. Let's assume that we have 2 bands(springs). One of them is 0,2m and stretched to 0,6m. The second one is 0,4m and stretched to 1,2m. What is F1 wrt to F2 if k factor is the same?

I think it is F1 = F2/2. What is your opinion?

BR
 
I don't completely understand your question.

How does the energy of the first band relate to the second? The second band contains twice the energy. I don't think it will make a spear go [much] faster in water, but it would certainly accelerate a heavier spear to the same speed, which would then fly further.

---

This thread that you started really got me wanting to find out everything about it. Like the pulley system of an invert roller doubles the speed of the wishbone if it were shot in a frictionless environment. But how does this translate to a spear being fired in water?
I am planning to build a normal slingshot, based on a pulley system, but there the projectile would be flying in air, with much less friction, so the pulley effect would be much more noticeable.

Today is a day without meter high waves, finally!... But now I'm just sitting here, reading articles on this topic.
 
Actually, you already answered my question. So not the stretch ratio but stretch distance is important. Therefore, the energy kept by a shorter band is half of the longer band with the same stretch ratio. This is explaning why the force on the shaft is less than that of a regular one.

By the above assumption, lets make a comparison between two guns. One is a regular gun with 2 x 14mm bands (stretch from 40 cm to 120cm); the other one is invert roller with 8 x 14mm bands (stretch from 20 to 60cm). In this case, the power of gun1 will be equal to the power of gun2. Correct?

It is hard for me to digest. 8 bands vs 2 bands. Or am I making a mistake somewhere

P.S: I kept the same stretch ratio (300%) for both cases
 
*It can be that my brain is fried; today was exhausting, almost no sleep and many hours in the water chasing ghosts...*

A pulley changes the output so that for a given band-movement you get twice that movement for the wishbone, in the same time. So double distance and double speed, with half the force. So the output is only the same if we measure it for the full wishbone-movement. But we won't bring a 200cm invert roller gun instead of 100cm normal gun, we keep both at 100cm. So to use the full amount of energy from the bands, we shorten the bands by the same factor; we cut them in half and put the halves parallel to each other. So when people say that an invert roller with 4 strands of rubber has 4 bands, that's not correct. It would actually be 2 invert-roller-bands (count just one side), which gives energy equal to one normal band of the same total length and the same stretch.

So if by 8 bands, you mean 4 per side, then it would indeed contain the same energy.

Now I do wonder how other forces play with this, like the recoil of a normal gun vs the limited recoil of an invert roller...
 
Eray_spearo said:
It is hard for me to digest. 8 bands vs 2 bands. Or am I making a mistake somewhere

P.S: I kept the same stretch ratio (300%) for both cases
Click to expand...
Unless you mean 8 bands per side, it amounts to the same amount of rubber and that is your power source. At least that's the simple way I see it. If you add more rubber in diameter or length you can access more power. The dynamics of these pulley system confuses me though. I thought the point of roller was to get a virtually longer more powerful gun by the longer bandspan, and wishbone have travel all the way to the muzzle.
 
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. The combination I prepared is with 3 bands on each side. I will try that out. Tough, my expectation is that the power will be higher than my regular gun ( 2 x 14mm band). Perhaps, recoil is damping a certain energy and this is creating a difference.

Is there anybody who had the chance to compare a regular gun vs invert roller in that respect? An observation in the field will help us understand the real mechanism behind. Last weekend, I tried invert roller with 2 bands on both sides (3rd one fell apart). My 150cm x 7mm shaft hit a rock from a distance of 3-4m and the front part heavily damaged. So I assume that the real range would be much higher than that.

Kindly asking for your experiences in the water. Come on guys, time to get more involved
 
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