Penobscot (exploring the puzzle) SUCCESS

[half eye]

OK Fellas,
    I'm at a "....Houston we have a problem" moment. Please bare with the explanation cause I could use some input here.

    When I drew the shorter bow with the twisted backbow cables it felt really "spongy" and the cables developed a "droop". After getting over the initial "what the hell" moment I started to cogitate on the original model bow, and then the flat cable bow then this one.....over and over. So I went back to all my different drawings and pictures. So here is the initial breakdown....

1)  The model bow had the artificial sinew strings and shot like a rocket at 50#.....but....I needed to retie the backbow strings several times to keep them "taught". (Chalked that up to stretch). I then changed over to flat rawhide backbow strings and the weight went up  but blew a string near full draw (chalked that up to a crease defect in the strap). So that brought me to the twisted rawhide backbow string which was spongy and way stretchy.....so by this time I got me a bad case of of "what the......".
2) When I looked at the drawings I happened to notice that the profiles were all with the bows braced. (The one exception shows the back highly reflexed and no strings at all.....it occured to me that there really are no pictures of a healthy bows that is un-braced. So that leaves with a couple of alternatives....

     I can put the backbow strings on while the bow is braced, so that at brace the strings are just starting to carry load....which means they will have some slack when unbraced, unless the type is where the backbow is highly reflexed, either overall or the tips and the backbow takes up the slack when the bow is unstrung.  This "take up the slack" answer is fine for the type but is not an option on the type where the backbow is wrapped nearly tip to tip.
    So back to the original problem/ question....should the backbow strings be somewhat slack at rest and just start to tension at brace, or   should they be tensioned tight at reat....personally I think they should be tensioned when braced and if they are a little slack when unbraced then so be it.

Does anyone know for sure? Especially, has anybody seen an example that is a unstrung working bow? Going to be cogitating for a spell so I'm sorry your gonna have to wait a bit for the draw shots and performance indicators.

sorry fellas for the delay
rich
 

[Stoker]

Cool prodject.. Very interesting indeed.. The only ones I've seen are FG repoes..
Thanks Leroy

[burchett.donald]

  Rich,
          I agree with you about tensioning after brace...It is probably something that had to be constantly monitored being it was a natural product "rawhide"...Seems it would continue to stretch, weaken and finally break if you keep it tight unbraced, has to be tremendous tension...I also think in time it would cause set or weaken the smaller back bow by keeping tight unbraced...I think there has to be a middle margin there somewhere...You are definitely exploring the puzzle, great info from you 
                                                                                                                                        Don

[willie]

I can put the backbow strings on while the bow is braced, so that at brace the strings are just starting to carry load

I would guess that this would be the most workable arrangement, but I have no actual experience with how the originals or any replicas work

Rich-

do you think that the design calls for the back string to stretch and do work that contributes to the energy stored under the f/d curve?
 its also possible that it was only expected to act as a "link" or "connecting rod" between the two limbs

willie

[H Rhodes]

This is a great thread Rich.  I think you are going to be thoroughly educated with the properties of rawhide bowstrings when you flesh this problem out.    To me it would make sense for that backstring to remain under little strain until well into the draw and then add it's oomph during the last part of the draw.  The few rawhide bowstrings that I have made lacked the elasticity of the modern stuff - it was a strong string that either held or it broke, but it didn't stretch much after break in.  Mine were fox squirrel and I have no idea how it compares with elk hide.  Maybe being slack when unbraced is a good thing.  I am following along to see how this turns out.   

[Oglala Bowyer]

I would think braced would be the best go at this. You should be expecting a delivery from me so you may have mixed results

[half eye]

Did some brainstorming yesterday with a couple of friends here on PA ( thanks Jon and Don). And came up with roughly the same idea ....that the backbow string should be nearly zero or very little tension at brace.

There is really two ways to look at these bows, and while they both come into play we got to decide where Dons' "happy medium" is.
1.) the back bow can be utilized to assist the recovery rate (speed) of the main bow....such that at full draw the main bow limbs have strain AND compression at the same time because the backbow string is pulling one way and the mainbow string is pulling just the opposite.....so at the release the main bow Accelerates forward because of it's stored limb energy ....but that force is assisted further by the backbow string tension.
2. There needs to be movement in the back bow limbs to compensate for some of the stretch distance. In the case of a very heavy backbow (old-style or heavybow new style) the layout of cables and mainstring are going to be of length that does not exceed the elasticity of the backbow string material.

Maybe a simpler way to put it would be this way....the newer style (I think it's the improved version) is using a backbow that is weaker than the main bow and it is acting like a take-up spring for any slack in the backbow strings. This is accomplished by having greater re-curve in it's tips so that ther have more travel untill contact with the main bow limbs.  The "old style" where the entire backbow is tied down you have a different dynamic....sort of a "in your face strength" thing. In this case I think the weight of the bow is increased and expected to provide all the power.....and the backbow strings are serving as in a cable bow to simply keep the now weaker area (mid limbs and out) from breaking, taking set etc.

I rebuilt the shorter bow, took it apart and made the backbow weaker (still strong though) and thinner,( also gave it a bit more reflex) and then re-attached the two bows with a 7"rawhide grip wrap. I took pictures (without backbow strings to see what kind of movement we were up against. The ruler in the pics is blurry so I redid all the measurements....the unbraced bow is 13-1/4 from back bow knock to main bow knock....that distance goes to 13-3/4 at brace......and 15-1/2 at full draw.

On the bow that the strap broke on...the strap was very tight in the unbraced position...so that may be the defective part ( the bowyer....me). You cany reaslly stretch a piece of rawhide a couple inches , let it dry in place and then expect it to stretch another 3 to 3-1/4 inches more.....especially when the little bugger started out just over a foot long....

I really appreciate Don and Jon calling, I respect your opinions and thoughts fellas and helping the old guy straighten out the issues....thanks

Thank you Oglala, none of this would be happening without your considerable generosity, sir

Howard, ya know I got the utmost respect for abilities, sir. I whole heartedly agree about the strings. I'm gonna build this one with all rawhide just to do it.....but in the future I'm gonna goe to linnen or the art sinew so I dont worry about if "THIS SHOT" is the last one for any particular string. Me and the fellas kicked that around yesterday as well, while a rawhide and sinew string is very capable they do require a lot of constant attention.....I think most moder people are not that dedicated to clean, tweek, repair,regrease and oils a bow every day....so ya, in the future it's going to be "old-looking strings 

Willie, I dont really know enough at this point to say about that. I'm still trying to get my head around this design.   For just 2 sticks, and a couple strings them fellas got this old guy scratchin his head....then again I'm too stupid to quit

Enjoy the pics, will keep you up on the latest.....good or bad.
rich

[Springbuck]

"Maybe a simpler way to put it would be this way....the newer style (I think it's the improved version) is using a backbow that is weaker than the main bow and it is acting like a take-up spring for any slack in the backbow strings. This is accomplished by having greater re-curve in it's tips so that ther have more travel untill contact with the main bow limbs.  The "old style" where the entire backbow is tied down you have a different dynamic....sort of a "in your face strength" thing. In this case I think the weight of the bow is increased and expected to provide all the power.....and the backbow strings are serving as in a cable bow to simply keep the now weaker area (mid limbs and out) from breaking, taking set etc."

I thought his very thing yesterday after reading your long post from 12/30, and had an idea, but didn't have time to type it out.  When you mentioned more reflex in the back bow it hit me that maybe instead of, or in addition to, adjusting the length of the backbow, and the tension and angle of the cables, perhaps adjusting the length of the handle/tie-down wrappings counts just as much.  Longer and shorter handle wrappings would affect the amount of reflex or recurved tips in the back bow, the amount of back limb working (and thus the back string tensions, angles of attack, springiness and movement of the  "tensioning spring" as you put it, the amount of stretch the back cables are forced to take, etc...

So, what I mean is, you make a long, or reflexed back bow, get it all put together, and then adjust the reflex or length and strength of the working back bow limb, and back string angles somewhat by tying more or less of the back bow down.

  I know you touched on this idea before and how it adds to the whole system, but I'm coming at it from the adjustability aspect here.

[Springbuck]

It IS a puzzle, huh?

The more I look at it, and following some of the basics of bow design we all know, what I WANT to happen is for this system to both improve the F/D curve and the efficiency.

 In my mind, I WANT the back bow to increase EARLY draw weight, rather than later, for a fat F/D curve, AND protect the relatively short and slim main bow from set or breakdown, AND somehow help compensate for string angle leverage loss in the later draw (like all short-to-medium bows face), all while making the whole system less massive, less vibration prone, faster dry fire speed, or whatever else

And once again, it seems like getting one or two of those is pretty easy, but getting three or all four is not.

[half eye]

Exactly Springbuck.....a puzzle for sure. Welcome to the head scratchin party....I am with you on the length of grip wrap also....you retain more reflex (backbow) the shorter the wrap. So all the cable/strap issues, amount of reflex, tension, and where and when all that happens is what I am muddling through right now.

Like I said....a whole bunch of cogitation for 2 sticks and some string
rich

[riverrat]

i still think they were made from small diam. saplings. the reason for the back bow was to give a relatively weak bow a little extra strength. if you take the time to study pictures of them its easy to see that the main bow is tillered from the back to bend towards the belly. the belly haveing a natural crown to it. why? the back can take more tension this way its flat. o.k. but now the belly is weak in compression because its round. yep thats why that back bow is on there. to give it a little extra strength/protection i.e. to "help" it. i do not believe it was some kind of compound anything. just to give it more strength. kind of like eskimo bows, they used junk that washed up on shore for bow material. so they put a cable on the back to "help" the bow out. otherwise it wouldnt work good enough on its own. not saying you cant put any bow on the back of any other bow and make it stronger, yea thats pretty much the concept. i think the size of the back bow was in exact accordance with how much the main bow bent through the center. hear me out, if you split a sapling and if you tiller it by tillering the back,  basicaly "decrowning" the back/split. i.e. keeping the grain straight. without the grain fading like on a bow where the belly is tillered, so it dont lift the grain when its bent, well you end up with a bow that bends too much in the center. o.k. you take other split out piece make it long enough and thick enough to compensate for this. wala! you got a bow properly tillered that will have more power than a sapling that was done the traditional way of tillering the belly and leaving the back crowned. just my thoughts....Tony

[half eye]

Hey fellas,
      Put the loose fit rawhide straps on last night....got up this morning to find them tight as all get out.   So I remade all the strings from artificial sinew and stretched them on tight and then drew the bow several times to full draw. Even weighed it at 50# @ 25". Then let it down and unbraced it. The artificial sinew stretches but is not elastic so I could measure the necessary length for future rawhide. The backbow string looked just like I suspected it would....it was totally slack at rest, lightly stressed at brace and had the backbow limbs down flat to the main bow and so was the string.
     I'm convinced that I need to learn how to prepare the rawhide strings so they wind up the length I need without screwing up the shrinkage thing. Right now I'm working on some twisted rawhide string that is twisted tightly, stretched and weighted down until dry......then we'll see about fiting it to the bow with the right length.

Here are some pics from today. I plan to shoot some tomorrow, and take some pictures at regular intervals of draw.....try to document the way the limbs work during the draw sequence.
rich

[ajooter]

This is great rich! I want to work on a rawhide string myself...but for one bow not two attached together!  I'm sure you will figure this out sooner then later.  Keep the posts coming.

[willie]

Rich-

     I'm convinced that I need to learn how to prepare the rawhide strings

Rawhide that I have worked with seems susceptible to dampness and stretching, and I can imagine that keeping those back strings in tune would be very difficult without some way to easily make adjustments. Could it be possible that the originals were tanned? perhaps made from sinew? or used in combination with some other primitive string or bast?

willie

[burchett.donald]

   Rich,
             Looking great    Can't wait till tomorrow for the draw sequences and your report...By the way, what's that white stuff streaking all around in the pics
                                                                                                                                          Don