Theraband Gold Catapult

[Stiks]

Not sure this is the right place to ask. Mods if not please move it.

I've just bought some theraband gold and a fork to make a catapult.
I've built it with doubled 20mm strips and it certainly throws a ball bearing well.
I was wondering if anyone has tried plaiting three strips together on each side, say three 15mm strips?

I can pull it as it is easily (used to drawing a bow) and thought I can go up a bit. I could of course try 2 25mm doubled but thought it might be a little tidyer.

What do you think?

 

[Asa Samuel]

How strong is the fork? Is it an other the top attachment? If it's strong enough then it should be allright, if it's an OTT attachment you just need to make sure it's tight enough not to slip out. Most importantly are you wearing eye protection?

I point you to this guy http://www.youtube.com/user/JoergSprave he is a slingshot enthusiast and if you watch his videos you'll soon see what makes a decent slingshot and how to make a powerful band set.

 

[Stiks]

How strong is the fork? Is it an other the top attachment? If it's strong enough then it should be allright, if it's an OTT attachment you just need to make sure it's tight enough not to slip out. Most importantly are you wearing eye protection?

I point you to this guy http://www.youtube.com/user/JoergSprave he is a slingshot enthusiast and if you watch his videos you'll soon see what makes a decent slingshot and how to make a powerful band set.

Hi,
Thank you for the link I'll go and have a look in a mo.
It's a Milbro aluminium fork and has holes through which the bands are passed.
It might be a bit tight getting three 15mm bands through the holes but should be ok, just.
It's strong so that should not be an issue.

Yes I do have and wear shooting glasses, don't like the idea of a band breaking and hitting me in the eye!

Cheers!

 

[Asa Samuel]

A milbro fork is designed to be used with either square or tube rubber, where a loop is threaded through the hole then a metal bar passed through the loop to secure it in place. Not sure how well a theraband would work that way but if you're doing it like that then it must be doing all right, only problem I can see is the milbro forks are quite small and it might get a little tiring on the hand with a strong pull.

Hope this helps

 

[Stiks]

A milbro fork is designed to be used with either square or tube rubber, where a loop is threaded through the hole then a metal bar passed through the loop to secure it in place. Not sure how well a theraband would work that way but if you're doing it like that then it must be doing all right, only problem I can see is the milbro forks are quite small and it might get a little tiring on the hand with a strong pull.

Hope this helps

Hi and thanks!

Yes the forks are small'ish but I get on with it ok. I'm going to add wooden scales to it to make it more
comfortable. I got the doubled theraband through the holes no problem it might be a tad awkward to get three
through them mind. If that is the case it will be easy enough elongate and open them up a little.

My main concern/question is about plaiting the theraband in a three plait, I wonder if there might be to much friction as you pull/release
between the bands? ........ This could shorten the life of the bands?

 

[Loenja]

where did you get you theraband from??

 

[Stiks]

where did you get you theraband from??

EvilBay

 

[QDanT]

Yes the forks are small'ish but I get on with it ok. I'm going to add wooden scales to it to make it more comfortable.

Wooden scales would be great though would need work, here's my quick alternative

pushed into a handlebar grip and split a 2" lenth of brush handle to use as a wedge, pushed in with it, to keep it tight.Cloths peg keeps ball in pouch for quick use, out of pocket,pull back, pull off peg with teeth, saves having to get a ball out and fit it into pouch before use, http://www.bushcraftuk.com/forum/showthread.php?t=57442&highlight=
cheers Danny
added this on edit :evilangel:

 

[JoshS]

Danny
What rubbers are you using on your black widow? I have been buying the barnett ones and they seem to break very very quickly, right where they slip over the forks.
Josh

 

[Mojoracinguk]

The Milbro I have uses taperd Theraband gold i think it tapers from 6cm at the pouch to 8cm at the forks...this is folded to give three layers. the layers are then made to form a loop and are passed (pulled using a bent piece of wire pliers and *Ahem* lubricant) through the standard sized hole.
The wire is removed and a nice pin usen to secure the loop in place. I'll get photo's up tonight if i get time.
at full draw it takes 15Kg (from memory)
Compared to the black widow fitted with 9mm dia spear gun tubing (13kg)
To your point:
I feel the speed at which the bands conract will not increase (as it is a constant)
so adding more bands will not help increase the speed of the shot (therefore the power) unless you use very heavy shot.

If you are using light weight shot then stick with what you have. If you want to really push out the Joules then use heavy ball and put more bands on (the velocity will never exceed that of the bands contracting but if you use more bands so they can pull heavier shot nearer to their terminal velocity then you will increase the J being transfered to you targets)

good luck & keep us informed
Mojo

 

[Mojoracinguk]

Here it is in my day sack

Now on the floor...so you can fit a large section of band through the forks

Mojo

 

[QDanT]

Danny
What rubbers are you using on your black widow? I have been buying the barnett ones and they seem to break very very quickly, right where they slip over the forks.
Josh

Hi Josh in the photo you can see the bands have gone brownish in x3 places where they were folded up in the shrink wrap packet and in the shop display cabinet in the sun, because of this I managed to blag a new set (the red ones) when I bought it. I've fired a couple of hundred .375" balls and have had no problems,(apart from hitting what I aimed at !) with the original band so have not tried the red ones yet. maybe try lubricating silicon grease? vaseline ? - cheers Danny

 

[Adze]

Definitely don't ignore the advice on condom packets re vaseline and rubber... a snapped elastic to the eye is a bit worse than an apology letter from Durex

 

[JonathanD]

I know someone who put Therabands on a Milbro and one of the arms snapped under the tension. Some are weak due to the casting process used, so be careful.

 

[Mojoracinguk]

A milbro fork is designed to be used with either square or tube rubber, where a loop is threaded through the hole then a metal bar passed through the loop to secure it in place.

I'll counter that!!!
The Early Milbro Had the square elastic, but it was held in place by a split collar made of two wooden halfs....no pins in sight.

If you really are interested i'll take some pics.

Mojo

 

[lucifer93]

You should try the blue, black or silver thera band

 

[Stiks]

The Milbro I have uses taperd Theraband gold i think it tapers from 6cm at the pouch to 8cm at the forks...this is folded to give three layers. the layers are then made to form a loop and are passed (pulled using a bent piece of wire pliers and *Ahem* lubricant) through the standard sized hole.
The wire is removed and a nice pin usen to secure the loop in place. I'll get photo's up tonight if i get time.
at full draw it takes 15Kg (from memory)
Compared to the black widow fitted with 9mm dia spear gun tubing (13kg)
To your point:
I feel the speed at which the bands conract will not increase (as it is a constant)
so adding more bands will not help increase the speed of the shot (therefore the power) unless you use very heavy shot.

If you are using light weight shot then stick with what you have. If you want to really push out the Joules then use heavy ball and put more bands on (the velocity will never exceed that of the bands contracting but if you use more bands so they can pull heavier shot nearer to their terminal velocity then you will increase the J being transfered to you targets)

good luck & keep us informed
Mojo

Hi,
Thank you all for your input. I might have a go at cutting to that size and folding as you have. It looks nice and neat. What were the lengths you used? I guess that will depend some on your draw length to your anchor point.
I'm a tad confused regarding your elastic contraction theory. I'm an archer and I know that if the bow draw weight is increased, draw length and arrow weight kept the same
you will get greater velocity on your arrow and greater impact force.
I can't remember my "hooks Law" (not sure if that is more related to springs mind you) but surely if draw weight is increased by greater band width then that extra energy is transfered into the shot. Ok maybe less of that extra energy is held and transfered to the target with lighter shot but the velocity must be greater surely?
In archery the length of draw is an important factor, the longer the arrow is on the string more engery is imparted to the arrow.
I guess then you have to decide whether speed is more useful than transfered energy on impact. I bought some 9mm steel ball to try but will get some lead ball at some point cus I know it will give me more clout on impact, I'm just a tad concerned about leaving lumps of lead lying on the fields.

Obviously elastic bands will behave differently, I'd be interested in more tech info if you don't mind.

 

[Mojoracinguk]

ok....so if you had a single strand of elastic band and you pull it to a draw of 12" it will (with no mass attached to it) contract at a given speed or rate...this rate of contraction will never change for the particular composition of elastic (if all other variables such as temperature stay constant) you can then take a second peice of elastic and place them together...now pull them to the same draw length and release. The rate of contraction will be the same as for the single length.....although it will have twice the 'working force' so could pull a larger mass and impart more energy to it.
I wait to be corrected (as it's been a while since my days of study)

now the tricky bit.....you can plot a chart. Rate of contraction v.s mass....... as given a particular rate of contraction the band (held fast at one end) will contract at different velocities given different masses are attached to its free end.
no mass on the end and it will contract as if free (this is it's maximum velocity) therefore any mass you attach will now make it sloow down....the more mass you attach to the end of it the slower the velocity.....so it's the velocity of the shot is the end result

In essence if you add another band to the equation....you have increased the potenial energy of the system greatly and will see higher velocity gains if using heavier shot. although it will you will not increase the maximum velocity of the system. you can increase the velocity of a given mass as long as it is in the range (on the chart) that your system of elastic fails to pull at maximum velocity
the upshot i see is that the heavier shot you use the more benifit you see from adding an extra band....if you use light shot...you see very marginal if any increase in net energy imparted to the shot even though your catapult or system has increased greatly in potential energy...any use?

 

[saurian]

I would not use anything thicker than 25mm single bands on a Milbro they are too thin. (and not that accurate) try something like this or there is one called a Dankung panther which is good

 

[saurian]

ok....so if you had a single strand of elastic band and you pull it to a draw of 12" it will (with no mass attached to it) contract at a given speed or rate...this rate of contraction will never change for the particular composition of elastic (if all other variables such as temperature stay constant) you can then take a second peice of elastic and place them together...now pull them to the same draw length and release. The rate of contraction will be the same as for the single length.....although it will have twice the 'working force' so could pull a larger mass and impart more energy to it.
I wait to be corrected (as it's been a while since my days of study)

now the tricky bit.....you can plot a chart. Rate of contraction v.s mass....... as given a particular rate of contraction the band (held fast at one end) will contract at different velocities given different masses are attached to its free end.
no mass on the end and it will contract as if free (this is it's maximum velocity) therefore any mass you attach will now make it sloow down....the more mass you attach to the end of it the slower the velocity.....so it's the velocity of the shot is the end result

In essence if you add another band to the equation....you have increased the potenial energy of the system greatly and will see higher velocity gains if using heavier shot. although it will you will not increase the maximum velocity of the system. you can increase the velocity of a given mass as long as it is in the range (on the chart) that your system of elastic fails to pull at maximum velocity
the upshot i see is that the heavier shot you use the more benifit you see from adding an extra band....if you use light shot...you see very marginal if any increase in net energy imparted to the shot even though your catapult or system has increased greatly in potential energy...any use?

making a band thicker will not make the projectile go faster but will enable you to use a larger projectile and therefore transfer more energy, there is a very useful band selector gizmo here http://slingshotchannel.com/band_calc.html