Invert roller

[eduard]

Hi guys, I'm currently designing a 100cm wooden invert roller speargun (teak) and I need some guidance on shaping the belly/underside of the stock.
Since the rubber band carriage (muleto) and lower bands run along the bottom, what is the best cross-section geometry for the belly?
My plan is to keep a completely flat, central track (about 3.5cm - 4cm wide) from butt to muzzle, with 45-degree lateral bevels sloping up to the cuttlefish side profile. Is this the standard approach, or should the bottom be shaped differently to avoid hydraulic drag and keep the carriage aligned? Any photos, diagrams, or advice would be greatly appreciated

 

[musubi]

Even though I use invert rollers exclusively, I can't really provide too much useful input. I find that my bands (6 bands or 3 pairs) lay better with a wider bottom. I'd estimate at least 3 inches wide, including the taper. Wider if you can manage that without the gun looking off. I use pipe guns primarily, but using my invert setup on regular pipe vs cuttled, it's better with the cuttled shape. Plus with carbon guns, the cuttle provides much more buoyancy and ballasting capabilities. For a wood gun, I'm guessing it won't matter much.

I was recently designing a cuttle attachment to my carbon pipe and this is the profile I landed on (at the largest section):

This was done mainly for buoyancy, but I knew I wouldn't have issues with the band laying fine. The wider shape also helps with recoil, but recoil is rarely an issue with inverts anyway.

 

[eduard]

Thank you so much for sharing this Your input is pure gold and clears up my doubts completely. I really appreciate your help and all this great information. I wish you the best of luck with your projects and great catches ahead. I'll definitely let you know and share photos when I finish building my speargun

 

[eduard]

Hey guys, I want to create this design for the roller muzzle area at the front of the speargun. Does anyone know where to find this, or have any ideas/suggestions? Thanks a lot for the help

 

[rollerGR]

That seems to be just a pair of rollers and 2x stainless steel axis, one for the rollers to rotate on and the other acting as a stopper. Theoretically you can just take a 6mm or 6.5 or 6.75 or 7mm speargun shaft and cut two pieces then hammer them on the muzzle where you want them, and glue them in place. No need for screws
Make sure the rollers have a small leeway to rotate on the axis, so for a 6mm axis their hole will be 6.1mm and so on

 

[rollerGR]

also the stopper axis could be thinner
and there's another DIY stopper alternative that is easier, see pic
drill two holes and put two dyneema loops in, acting as the stopper for the wishbone knots

 

[eduard]

Thanks a lot for your input, it's very helpful! What I actually meant was the specific curves, angles, and shaping of the wood itself in the muzzle area. I’ve marked it with arrows in the photo to show what I mean