Best rubber´s lenght for speargun
This is a normal question when we are changing our rubbers. Maybe we don´t remember the size, maybe we are trying a new brand or changing into rubbers by meter and dyneema wishbone.
Most of the times my customers asked me about a rubbers for their speargun. A 75cm, 85cm, 90cm… But it´s a wrong question because it depends on the brand and model and even the shaft.
Some brands call 75cm a speargun which is bigger than 85cm in other ones. And some shafts have the shark fins closer to the muzzle than other ones. So won´t be the same rubber.
To choose the right rubber there are only a few important things.
The distance from where the rubbers are located in the muzzle (parallel rubbers) or where they pass through (circular rubbers) to the shark fin (where the wishbone fits in the shaft) Must be careful because not all the shafts are the same, some of them have it closer to the muzzle or a bit far away.
Wishbone´s length. It´s important. As bigger is the wishbone we will use a bit shorter rubber. That´s why it´s so important using short wishbones.
So to calculate our rubber´s length we must know:
- Distance from where the rubbers are located in the muzzle (parallel rubbers) or where they pass through (circular rubbers) to the shark fin (where the wishbone fits in the shaft)
- Size of the wishbone.
- The range of stretching. That will depend on the type and diameter. Even our own likes. Some rubbers works better at 300% and other ones at 380%. Stretching a rubber too much won´t give us more power. It will brake our rubbers in less time and we will have more recoil and less accuracy. It´s rubber is designed to offer some properties and stretching it too much won´t give us something that they don´t were made to. Usually thinner rubbers (14-16mm) have more stretching (320%-380%) 17-18mm rubbers used to be from 320% to 350% and the bigger ones will be around 300%
So we need to do an easy calculation.
We are going to see an example. We have an 85cm speargun. And the length from the muzzle to the first shark fin is 82cm. To the second one is 90cm. The wishbone is 2cm (short one) We are going to use a 17,5mm rubber. And we are looking for a 300% in the first shark fin.
82cm (total length) – 2cm ( wishbone) = 80cm
Total length/Percentage = 80/3 = 26.6cm
So we would need a 26-27cm rubber. This is for a parallel rubber. If it´s circular we will need the double (both sides) so we would need a 52-54cm.
And if we load it in the second shark fin we would have: (90-2)/26.6cm = 3.3
So the stretching in the second rubber would be the 330%
Now we are going to see the most used in normal spearguns in a 350%. To have an idea.
75cm parallel: 20cm
75cm Circular: 40-42cm
85cm retarded trigger or 90cm normal trigger in parallel rubbers: 22-23cm
85cm retarded trigger or 90cm normal trigger in circular rubbers: 44-46cm
95cm retarded trigger or 100cm normal trigger in parallel rubbers: 25-26cm
95cm retarded trigger or 100cm normal trigger in circular rubbers: 50-52cm
But we see that it´s very easy know the perfect length for our spearguns.