WW2 Big Joe 5

First topic message reminder :

Hello
Member from this forum for some month (but experienced French crossbow builder) I decided to bring my contribution whit one of my work: The OSS BJ5 replica. For this, many thanks to Dr Brunner whit his constant help and encouragement . I bring some modifications due to the fact that I have not the wealth of the OSS research department and I had to build it whit the usual hand tool that you by in any tool general store...I am not sure to master the pictures uploading. First a picture from the original,then my version in detail.

Hi...I'm back (computer all fixed and ready to roll )

First of all...thank you for the hint on how to find your work on the forum *Owl*, I really enjoyed your research/build style. Some *very*very*very* interesting crossbows you bring to the table.

Now to catch up on the crossbow at hand.

jeep wrote:it should be possible to use other mean of propulsion, I am sure
engineers can tell us that new elastic tread can replace rubber whit
better resilience and stability like carbon or else.

I agree, but that would make it a bit more expensive to make. Perhaps first we can maximize the performance of the original - using a few clever solutions. And only then move on to space age materials. However, the frame can benefit from a bit of composite work and still remain within a budget margin...an aluminum alloy or thin stainless steel tube encased in fiberglass should make it a somewhat lighter assembly...just throwing it out there.

Well, as I see it...there are a few things with the design that can be fixed...some easily without major modifications, others... not so easy and not without rebuilding major components, but I'll stick with the easier stuff since there are a few things I'm not sure of even there.

1. Positioning of the bands.

This is the first variation that I came up with. While designing, I was aiming at reducing the amount of work required to replace the bands and also to improve the efficiency and accuracy of the crossbow.


This way I've eliminated the dead weight of the metal link that reduces the efficiency. Also I believe this way of positioning the band will improve the energy transfer since it is no longer changing the angle during the draw...possibly giving a more direct access to the bands energy storing capabilities. The idler wheel in the center of the band is something in question, I can't quite picture whether it's going to work synchronizing both sides *which I think would be an essential function for any bow* ...or if it will do the exact opposite and just throw things out of balance.

If you are wondering about the band used in this model, there are many different grades of it and it's commonly found on spear guns for fishing.

Joerg Sprave wrote:

It's all very roughly thought/drawn out, but let me know what you think guys.

Ivo

Hi ivo what a good stuff can be done whit a computer program!!! I am not smart enough ,still whit a pen !! Very interesting designe indeed can certainly pull hight tension, but like this it will supress one of the advantage of this frame : to fold it.
About the rubber: After testing different kind they find out that to realise a maximum of force in a minimum of time it wash better to use many smalls rubber rings then a massive one (don't know if they used the same material then yours:silicone or else?)There a test result paper of different type of rubber (OSS crossbow)I don't kno[w how to interpret but maybe someone know here..url=https://servimg.com/view/16369594/37][/url].

THE LBJ5
Velocity: 180fps
Accurate lethal range: 60 yards/150 yards
Extreme range: 260 yards
dart: 44g (678 grain)
rubber pull: 550#
150 rubber ring x2 = 300 rubber ring
Draw 10"
You can find those stats and a lot more in the Bible of Dr BRUNNER.:
THE OSS CROSSBOW,
ISBN-0-932572-15-4

WOAAAAAAAW!

550 pounds of power in that thing?!?! Ivo probably had seizure when he read it.
Looks very cool man. Careful - Ivo is an alien and everything he touches mutates.

Skyline

Hi ivo what a good stuff can be done whit a computer program!!! I am not
smart enough ,still whit a pen !!

No biggie. Sometimes a sketch on a napkin is more valuable than a computer model. In my opinion, it's easier to think simpler and keep ideas rational on a piece of paper than it is on the computer. On paper you just scratch it or scrap it...with a computer model you go back and redo it - adjusting everything. Thinking about it...both paper sketches and computer models have their advantages and disadvantages...I just like it because I don't have to photo/scan anything to post it...a little bit of MS Paint or Google Sketchup and we're in business.

Very interesting designe indeed can
certainly pull hight tension, but like this it will supress one of the
advantage of this frame : to fold it.

Suppress - yes, but not to a noticeable degree.

I basically pictured it like this...

First step - you pull the rubber back and over the roller on the top. This will remove the tension on the string and ultimately on the frame. Both the band and the string become easily accessible for maintenance/repair/replacement. This system will probably work with bands just fine...or even better. Can't wait to try it.



Next is another thing I pictured would speed up the process is instead of nuts, simply use a pin to fix the end of the frame in place...so remove that and the frame is free to move. Perhaps a little quick release lock for the bottom of the pin to boost it's coolness factor.



I've noticed in your frame - the front is completely removable? Something I just noticed....interesting detail.
In my model the hinges are permanently linked, but are still free to move.

---

About the rubber: After testing different kind they find out that to
realise a maximum of force in a minimum of time it wash better to use
many smalls rubber rings then a massive one (don't know if they used the
same material then yours:silicone or else?)There a test result paper of
different type of rubber (OSS crossbow)I don't know how to interpret
but maybe someone know here.

Interesting table. Some of the items are easy to understand abbreviations, while a few others are quite the puzzles.

Scale:

Good

Fair(OK)

N.G.(Not Good)

Are probably the abbreviations we see through out the table:

G=Good

F=Fair

NG=Not Good

Hyst. Loss

Mod.

Pers. Set.

Tear Resist.

Abras. Resist.

30 to 50 Cycles #70 at 360%

Type Hitch

Thickness Size

Letter Date

Life

"Hyst. Loss" probably means "Hysteresis Loss"

This is what I came up with after a bit of searching:

Spoiler:

Mod. - No idea what that is....hopefully some one will stop by to enlighten us.

Pers. Set. - Probably the amount of "permanent" stretch(set) the rubber takes during use.

Abras. Resist. - Abrasion Resistance

Tear Resist. - Tear Resistance

30 to 50 Cycles #70
at 360% - I would bet that this is the description of testing conditions -30 to 50 Cycles...load 70 pounds...stretching to 360%. Just my guess.

Type Hitch - I'll get back to it *

Thickness Size - Dimensions of the rubber being tested.

Letter date - Probably the date when report was received.

Life - Life is good. Too bad we will never know how long subjects 1 through 26 (& #32) lived.

So that's my take on it...and now the puzzle.

Type Hitch

I first though it was a type of knot, but then put the names in the search. Here's what came up for Mentzer:

Spoiler:

As I understood it, Hitch described in the table is a "hitch" to or by which the bands where attached. Guessing...Like a trailer attaches to a truck.

Not much info on the PIN and Clover Leaf...just too many random things came up in the search...so that's another thing I'm wondering about and hope some one can come in with a fresh mind and try to clear this up.

---

I guess I'm going to start building and see what happens in the process...think this triangular frame platform will work very well with the lever(ballista)crossbow concept. Everything should balance pretty well and it can be rubber powered...hmmm...I think I just got a glimpse at what I'll be doing after I finish the recurve I'm building.


Good Day,

Ivo

PS:

Looks very cool man. Careful - Ivo is an alien and everything he touches mutates.

Yep...that was once a banana.

IVO - TL;DR -- Actually I did read the post it was good.

Ivo wrote: Basically saying that the rubber "lags" upon return once the load is removed. This guy did a little video on it.

I Liked your comment on Hysteresis I'll have to try to rememeber that as a way to quickly explain it. I have a bad habit of forgeting that the 4 years of Engineering classes added random things like that to my vocabulary and so I'll throw them into conversations thinking that everyone should know what Hysteresis is then draw a blank trying to think of a simple way to explain it. Well Hysteresis wouldn't be too bad trying to explain Moment of Inertia on the spot was bad, I fumbled initial explination so got derailed.



And ctrl V (paste function) rocks. Didn't have to try to muddle through spelling Hysteresis a bunch of times. Given my usuall attempts at spelling each time would have been different.

Thank you Ivo today I learn a lot more about rubber, now the checking paper should be more clear for all. I want to do my own test about small rubber rings because i plan to use them for a lever crossbow. The Bj5 spanning system is very efficient (like a crannequin) but heavy and complicate. There is another OSS crossbow that a would like to duplicate it is the final one produced ,taking all the best feature already studied by North west University this is the" Willam Tell" model (Dr brunne, OSS crossbow)

william tell

As you can see they use the stock for loading and instead of a ratchet bar there is a winch whit a cable :Lighter,simple,compact It also can shoot stock folded like a pistol crossbow.

Interesting.

So they decided to step away from the triangular frame concept and simply did two posts linked by a cable that has a sight bead on it.

I hope they got rid of the steel "H" connector, because in my opinion that was the biggest issue with the other crossbow.

I like the folding stock that doubles as a ratchet arm - simple, but to the point. You can probably re-purpose an ordinary automotive two way ratchet to simplify the build a little. Still however, I'm wondering about the system they used, because a ratchet is pretty damn loud.



Crossbow in the video - АК-77Б (Custom made by one of Saint-Peterburg)

Ivo

PS: I'm also wondering about the trigger mechanism. I've seen it on the
French forum and overall the chain-link/dog-leg triggers are known for their ease of release if made properly, but I didn't notice any information on your triggers performance. It
obviously holds the weight, but what is the trigger pull weight?

It is not a ratchet bar but it is spanned whit a cable and a winch drum,it was better,lighter simpler less cumbersome. They did use a little steel connector but very simple and lighter

Ivo wrote:

About the rubber: After testing different kind they find out that to
realise a maximum of force in a minimum of time it wash better to use
many smalls rubber rings then a massive one (don't know if they used the
same material then yours:silicone or else?)There a test result paper of
different type of rubber (OSS crossbow)I don't know how to interpret
but maybe someone know here.

Interesting table. Some of the items are easy to understand abbreviations, while a few others are quite the puzzles.

Scale:

Good

Fair(OK)

N.G.(Not Good)

Are probably the abbreviations we see through out the table:

G=Good

F=Fair

NG=Not Good

Hyst. Loss

Mod.

Perm Set.

Tear Resist.

Abras. Resist.

30 to 50 Cycles #70 at 360%

Type Hitch

Thickness Size

Letter Date

Life

"Hyst. Loss" probably means "Hysteresis Loss"

-- SNIP --

Mod. - Modulus (a measure of the resilience or stiffness of a substance)

Perm Set - The amount of "permanent" stretch (set) the rubber takes during use.

Abras. Resist. - Abrasion Resistance

Tear Resist. - Tear Resistance

30 to 50 Cycles #70
at 360% - I would bet that this is the description of testing conditions -30 to 50 Cycles...load 70 pounds...stretching to 360%. Just my guess.

Type Hitch - I'll get back to it *

Thickness Size - Dimensions of the rubber being tested.

Letter date - Probably the date when report was received.

Life - Number of stretch/relax cycles that the sample can withstand until it breaks (or until the test is terminated)

Here's an enhanced version of the table image. Sorry about the residual distortion, but that's due to the camera angle and lay of the pages when the picture was taken; trying to straighten it out would cause further degradation of legibility.

I am very fortunate to have known a man (Gil Frey) who actually owned one of the few examples of this weapon known to exist. He lived in Gaithersburg, MD and worked for the N.I.H. or some other federal agency. He was a crossbow guru, right up there with Pop Bailey and George Stevens. He was instrumental in the design of the Jennings Devastator crossbow. Before he died, he donated his Big Joe to the C.I.A.'s Covert Ops Museum. I don't know if that museum is even open to the public. Thanks for bringing up some fond memories of a truly unique gentleman.

jeep wrote:This is the main problem whit rubber: when you stretch it quickly it produce some heat and lost lot of power and if you wait before realising the power drop even more!!

I researched very much in this regard, both by studying scientific publications and by doing a great deal of experimentation.

I can now say that - as long as you don't exceed a stretch of factor 5 (eg. 10 inches relaxed band length, 50 inches stretched length) - the hysteresis effect SOLELY comes from the temperature effects.

If you stretch rubber, it warms up. If you relax it, it very quickly cools down again. The energy is stored thermally, unlike any other known propellant media for crossbows.

So if you keep a rubber crossbow cocked, the initial draw force is very high. However, the temperature drops, and the draw force drops as well. So your 550 lb will only be, like, 300 lb after a few minutes.

It is not so bad, because this means that once the temp of the environment has been reached, the draw force won't go down any further.

I have created a crossbow with integrated heating system, it works fine.

So the main disadvantage of rubber is that it will only shoot accurately if you either always wait a few minutes after cocking, or by keeping the breaks between cocking and shooting about similar.

Rubber based weapons really only work well between 5 and 38 degrees Celsius. Dr. Brunner has included documents into his book that support my observation in this regard.

You are absolutely right Jorg,It is probably wy the OSS crossbow have an incredible draw pull: BJ5=550# for a killing distance 20 to30 yd or even lot less at night time (it is an elimination weapon) at this distance the trajectory is flat and what ever is the hysteresis your a dead duck.....But it was certainly a problem to use it in European winter or in the moisture of the New Guinea jungle. I watch your work with a lot of interest about heating rubbers , I may try to use thermal film in a polar fur bag to wrap the in swinger rubbers units.