[Jim H.]

First of I know many people thinks speed is silly but I want to get past 500fps with at least 54grain balls (30 fpe which should be possible with a fast system) and have been looking for info on flight shooting and compounding but I can not find a lot of info...

As most people know we are losing efficiency with light ammo and fast speed.
Terotically so should the lighter ammo have the same energy as the slow heavy one but the bands can't keep up.

So too get high velocities we need too increase the efficiency with delicate taped bands, cutting down on weight and at the same time still sacrifice energy with unreasonably light ammo...

But with compounding we can "gear up" the speed of the bands, sure we loss some efficency as no gearbox has zero loses.
A typical compound bow will have a gear ratio of ~1:2 wich means simplified that a 70 pound bow does really have 140lb limbs and those limbs will only need to move half the speed as a recurve and on that they use cams to manipulat the force draw curve too put into more energy into the system.

So now is my question how do we do it and has anyone experience in this?

If we have a very wide fork would we not get a compounding effect like the rüdiger koltze crossbow? https://patents.google.com/patent/US5720268

Other systems that I know have been used.
Levers
Pulleys
Vacuum (not really a compound principle but it should have a optimal force draw curve)

I have got over 300fps with a recurve bow at 85fpe, but I don't think I will get any faster without some kind of compound system.

Any ideas is greatly appreciated!

 

[jazz]

The bands that I have:

- if cut as double 2cm wide each, straight cut (8 cm wide in total)

- it would be some 9.335 kg of power

- 15 cm active

- 5 stretch ratio

- 75 cm total length

- pouch weight 1 g

- would propel the 3.5 ball at 267 f/s

If I extend the total draw to 150 cm (again 5 stretch ratio but now active band 30 cm), and under same width of bands and power would propel the 3.5 g ball at 377 ft/s.

Now, as you know yourself, some tapering would help push the velocity even further, but I think that the rate of taper will be crucial (too much taper for a given weight wont work).

Other than that, I have seen that people heat the rubbers just before shooting and this all together should go over 500 ft/s - if what I saw is true, of course.

Probably there are some other .parameters that could help but I can not think of them for the moment.

cheers,

jazz

 

[Winnie]

I don't think you'll get it with the elsasicity of the latex alone. Years ago someone (porbably Jorg) did something with springs but I'm not certain that there was an improvement.

- Just thinking that if the bands are strong enough to activate a secondary mechanism they might be too strong to work efficiently with the projectile (increased latex will often not increase velocity - in fact it will often decrease it).

I'm curious to see what you come up with.

 

[Winnie]

I've mentioned several times in the last several years an experiment I did long ago where I took a one inch straight cut band set and dissasembled it and then discarded one of the bands. The remaining band was split down the middle resulting in two tapered bands. The new bandset (half the original rubber) was reasembled and clocked and the velocity of the same size ball was higher than the straight cut set.

Increasing the rubber does not necessarily corelate with increased velocity. It's a trap we are all inclined to fall into.

Now you know why I'm curious about your idea.

 

[Pebble Shooter]

The US-based company Shoottech developed efficient bow and a crossbow designs to shoot steel ball bearings:

http://www.shoottech.com/Page/Web/AR480

420 fps with a 110 grain (about 7 grams) steel ball is pretty impressive.

Here is a review of the crossbow version:

A review of the steel ball shooting compound bow:

Unfortunately, rubber elongation and retraction velocities have physical limitations, which imply that increasing projectile mass is the only way of achieving a higher power output. Compounding with pulleys like on a compound bow does not yield the results one might expect either.

In a nutshell, it looks like Shoottech solved the problem of what is technically a dry-fire (no projectile) when using light steel ball ammo: the "raptor" bow does this with strategically placed rubber inserts, with very good results, but also a fair degree of mechanical complexity.

For the rest of us, choosing the right quality and thickness of rubber in conjunction with tapering to calibrate our bands or tubes for a specific steel ammo size and weight is the only way to optimize projectile velocity - and ideally shooting when the weather is warmer (or indoors).

 

[Ibojoe]

Welcome to the forum

 

[Jim H.]

Thanks for all sugestions!

I will cut a bandset and set up my crony and see what I can get with a regular setup.

I use natural latex with 0.6mm thickness, I think I go with a tape ratio of 2:1 to test with.

Unfortunately, rubber elongation and retraction velocities have physical limitations, which imply that increasing projectile mass is the only way of achieving a higher power output. Compounding with pulleys like on a compound bow does not yield the results one might expect either.

What kind of results could one expect with pulleys and rubber? The compound system would not now what the power source come from?