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.

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.