Cable Backed Bow

[GrahameA]

Evening All

For some time I have been thinking about Cable-Backed Bows. So it eventually got to me. I did have a Rock Maple Flatbow that was a bit disappointing in pondage and performance and thus it was selected for the transformation. The standoffs were from some pine crate pieces that I had in the just-in-case timber pile. They must be at least 50 years old. A quick trip to the Trition and I had some appropriately sized strip which were then cut to the correct size and dished so the cable would lay in the centre. The Nock for the binding were fashioned with a small saw cut and then a bit whittling with a Stanley knife. I guesstimated how many standoffs I would need and and made a complete set. These were then attached with a bit lashing to the back of the bow.

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I made the cable from artificial sinew. I was thinking about using B50 but opted not to as I was, and stil am, concerned about crushing the timber. To anchor the cable I made some extra nocks and whipped them on.

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So having all that done I fabricated a new handle piece to go over the cable so I was not grabbing it when using the bow. and then made a small tool (tommy bar) to wind up the cable.

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Having all the preliminary stuff done I put together the cable like an endless bowstring except the join is tied with a couple of blood knots - just like joining fishing line.

Getting the cable was the easy bit as I wound it up it kept on coming off the nocks. That issue was solved by tying the bundle of cable strands together at the nocks.

So now I could wind it up and did so until all of the current set in the bow was removed. And then I tried to string it. Not as easy as it use to be but with a bit of perseverance and the unwinding of the cable - counted how many turns - I managed to get the bowstring on. So wind it up again plus another twelve and take it outside to shoot. Its' original arrows spined at 25lb now were hitting the riser when they were shot. I tried some others and eventually settled on some that were spined at 40lb.

Voila a shootable Cable Backed Bow.

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So how does it shoot? A darn sight faster than it use to and there is little shock.

The little bit of work has transformed something that was a wall hanger into a useful bow and I am happy with it. Will have to take it along to the club just to make everyone do a double take. There is something nice about having a bow that no one else artound the place has.

[Stickbow Hunter]

Very interesting Grahame. It looks good. What poundage was the bow before and after fitting the cable?

Jeff

[Moss]

I Like That Bow

Do you know the speed and final poundage?

[perry]

Good stuff Grahame . Cable backing can be used on bows to remove excess string follow as well . I never wound any of my cable backed bows up more than 5 to 10 pound of draw weight . I am a little surprised you used artificial sinew though - it stretches more than B50 .Next effort you should try some Linen or TSi or similar Grahame

Regards Perry

[greybeard]

Grahame,

The experiment with an old bow appears to have been successful.

Are the cable supports equal in height or do they reduce towards the cable nocks?

Did you have to play around with the positioning of the cable supports to keep the bow in tiller?

I suppose only time will tell if the belly of the bow can handle the extra stress applied by the cable backing.

Cheers,

Daryl.

[GrahameA]

Good Morning All

Short answers as I am about to head for the range in about 30 minutes.

Jeff

It started at around 30lb. And at the moment I have it at 40lb. A few more twists and you would gain a few more pounds.

Moss

Don't know the speed. Chrono' is next on my list of things to buy. However it appears to shoot faster than the equivalent selfbow and that makes sense due to the lower llimb mass.

Perry

Strange you should mention the artificial sinew. Note that the handle piece is only lashed in place and I can easily remove it. I was initially going to use B50 but I was very uncertain about the whole thing. I remember a certain chap telling me how he destroyed a bow by winding it up a tad to much. Thus I opted for the artificial sinew thinking, "Well if I get a bit cariied away it will stretch". To replace it with other materials is not a big job. TBB suggests Linen and I do have some however that may be a bit of an overkill so it may be some B50. Hilary Greenland I seem to remember also suggests such materials. Still very much in the experimental stage.

Darryl

All the supports are close to the same height but they do get a little lower towards the tip. They have been placed at an equal distance apart and the bow is a bit out tiller - but it was out of tiller to start with and it really is no worse. And you can always move your hand. As I have said to Perry this is very much experimental.

A general comment. This method gives you an increase in bow poundage with little gain in bow (limb) mass. All things being equal that should give you a faster bow as there is less mass to be acclerated in the limbs and less hysterisis. So far for me that does seem to be the case.

[Len]

Very clever bow Grahame, well done. Could you use something like hemp/sisal rope/cord for the cable for more of an 'old' look ?

[perry]

Many primitive peoples around the world developed cable backed bows to a fine art , the Penobscot in my veiw the best example of this .

It is not 100 % necessary to raise the cable above the back of the bow but it has the advantage of the cable doing more work and less stress on the bow if it is elevated at key points along the bow . You can also cut the nocks to secure the cable further down the limb or even at the tips though this is a pain for stringing the bow

One people in Asia used cable backings on reversed split pieces of bamboo using the nodes as bridges to support the cable . All of a sudden a single piece of tension strong , weak compression bamboo made a servicable bow from everyday materials quickly .

Clever buggers these so called primitives . Who says the greatest advancements in archery have come in modern times - we just put a modern slant on things .

regards Perry

[Stickbow Hunter]

I suppose only time will tell if the belly of the bow can handle the extra stress applied by the cable backing.

I think Daryl has raised a good point. Rock Maple doesn't seem to be good in compression from the self bows I have seen. Anyway an interesting project so keep us up to date with how it all goes.

Jeff

[Glenn]

Good poost Grahame, I have couple of rock maple selfbows myself that will benifit from a cable backing like you have done...Glenn...

[perry]

Due to the cable backing shifting the neutral plane towards the back the bow is less strained as the cable also does tension work leaving a greater thickness of wood to handle compression

regards Perry

[yeoman]

Ah, but Perry, when you move the neutral plane closer to the back, the outermost belly fibres are of course further from the neutral plane, and stress in the outer fibres increases as the square (or cube, can't remember off the top of my head) of the distance from the neutral plane.

However, I personally don't think that the increase of stress on the belly is all that much with a cable backing, and is a perfectly safe venture to use within reason. Well done Grahame! Good stuff.

If you serve the ends of the cable like an endless string, you shouldn't have a problem with them slipping off the nocks.

Dave

[GrahameA]

Hi Dave

Consider it to be a "Post-Stressed Timber Beam".

Regarding serving the end - but then you have to get onto the bow. Yep, I did try that, you fix one problem and create another. I will take a photo so you can see how I have bound, as distinct to served, the cable together.

[Buford]

Hi all,
I saw this bow first hand today and it is indeed an interesting bit of gear..... cannot say how it shoots as i was having anough trouble getting arrows to hit a target from a bow that I'm supposed to already be familiar with!

[perry]

Doubters - why would primitive folks create a tool that did not work or was not reliable and use it to help them survive for thousands of years . Have a look at the eskimo's , no choice of bow wood , cold climate ruling out using animal glues as it would gel almost instantly . A cable backing doing the tension work opened up other possibilities for them so they used segmented pieces of antler cut thin enough to bend slipced together and bound on with sinew cordage . Wind up the tension of the cable and the antler belly stored more energy .

This example interests me Grahame as sinew twisted into a cable would stretch considerably but the recovery from that stretch would be snappy as compared to modern synthetics . The cable was used with their composite bow to do the job a sinew backing did in a dry warm climate .

Penobscot Indians advanced the cable backing and raised bridges idea to a short bow and cables on the back of a man length selfbow as they lived in a cold wet enviroment for a lot of the year and needed a more powerfull bow to hunt the Moose that lived in the swamps they where surounded by and the available arrow woods where heavy . A sinew backing in this enviroment would be useless .

Wood does have its limits with commpression but more wood area available for compression coupled with a cable backing doing the tension work will help make a tension weak timber into a fine bow .

One thing I learned with cable backed selfbows is that a more whip ended tiller was called for as the cable loads up the mid limb and handle otherwise basically creating a bow with too stiff limb tips A whip ended bow distributes more stress throughout the bow more evenly and the cable forces the bow into a circular tiller loading up the mid limbs more .

regards Perry

[GrahameA]

Hi Perry

The artificial Sinew seems to work. I really have no feel for the material as such but it does stretch a fair a bit and returns to its original length - ie elastic. For the moment I will probably just sit and ponder the whole thing. I do not want to break this bow.

I showed the bow to a few people yesterday and comments ranged from "Wow an Engineered Bow" to "It's ugly" to "Beautiful smooth draw". It has achieved several things for me:
- I have built one - and that has a lot of self satisfaction. And just think of of the looks you get with it.
- I have learnt a lot more - actually it is amazing the things you learn when you try to do something different. Even making the little standoffs was a great learning experience.
- It drives home the message about low limb mass
- It once again makes me marvel at the sheer genius of people from so many years ago.

When I made the Meare Heathe I went through 2 bows before I got one that worked and this may be a similiar story. I have this partially complete bow over in the corner and everytime I look at it the mind starts to churn - and this time around I already know a lot of what not to do.

Rock Maple was probably the best choice but the bow was there.

The Cable is a real pondering point. If you use a material with low stretch when you bend the bow you may find that the forces rise rapidly and overload the timber. But if you elect to use less strands the cable will break. At the moment I am thinking that your advice regarding crushing the bow and me being a little conservative may have actually paid dividends.

[greybeard]

Hi Grahame and Perry,

I may have been on the wrong tram when I was thinking about the compression forces on the belly of the bow.

My initial thoughts were along the lines of what can happen when you back a selfbow with a material such as bamboo.

Am I correct in my thinking that with the cable backed bow, because the cable is independent to the bow the limbs can maintain their stretch/compression ratio?

Daryl.

[GrahameA]

Hi Daryl

No. Your original thoughts are correct.

What you have with a cable backed bow is exactly the same as a bow backed with any other material.

However, the result of of having the cable running along a set of standoffs is that the neutral axis may - and hopefully will - run along a line just above the the surface of the timber with the result that all of the timber is in compression.

Another way to think of this is imagine a bow with the neutral axis running down the centre of the limb (beam). When the bow is loaded that above the neutral axis will be in tension and that below the neutral axis will be compression.

If we spit the limb and replace the timber in the top section with a cable on some standoffs taking all of the tensile loads you will have a cable backed bow.

So all things being equal and with a bit of careful thought you should be able to use all of the timber for compression - this is called "Suck and See Engineering", there is a bit of educated estimation/guessing going here. If I was to get all carried away and actually do the calcs it would be real Engineering.

If the cable does not stretch sufficiently you can easily end up in a situation whereby the neutral axis migrates further away from the timber rapidly increasing the compressive force which will lead to compressive failure of the timber beam. I spent a few hours thinking about what happens before I built it. Perry had given me some clues on what to look out for - he previously having established how to break a bow.

Does all that make sense? All of this is the same principle as pre or post stressed concrete beams. You use the concrete (or in our case wood) to take the compressive loads and the cable to take the tensile loads.

However - you get a substantial performance gain as the limb mass has only increased a little whereas the bow force (bow weight) has increased significantly.

[GrahameA]

Morning All.

The 10 second update - just as a result of impending questions (getting ahead of the pack).

The bow stil exists. It still shoots much the same as it did when I made it - however, I have not shot it for a couple of years.

It has not been crushed on the belly - which after some know legible comments I expected. I am putting this down to the stretch factor of the artificial sinew being a limiting factor in the movement of the neutral axis. If I had used a material with les stretch the neutral axis would have moved away from the wood, the outer section would have had it's compression limit reached and a cascading failure would have occurred across the limb. In simple terms I fluked a good choice. I would only build another if the interest was stirred - there are too many other interesting bows to build.

[greybeard]

Hi Grahame,

You could have a play with it down at the range on Wednesday.

Daryl.

[GrahameA]

Afternoon All.

An update. Today I took a couple of my bows along to the club and shot a few ends of indoor with them. One of them was the cable backed bow. So a bit of an update on the bow.

Since the last time the bow has aged nicely and has retained all of the performance it had when it was first made with no string follow. My comment would be, "I need to build another, but bigger". Need to think about the timber carefully. I want something that is good (strong) in compression and light.

So here is shot of Scott shooting it this morning.

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[bigbob]

Very interesting process there Grahame. enjoyed following your progress and successful out come.

[yeoman]

I've been thinking about cable backed bows a little lately.

On whip tillering the bow:

The reason the bow should be whip tillered is because as you reach the tips, the cable becomes proportionately larger as a percentage of total limb cross section. This makes for more tension strength at the tips, which are thus stiffer. A normally tillered bow will appear stiff tipped, so to compensate must be made weaker.

The solution: Make a really awesome cable from reverse twisted fibers like a flemish twist string that drops threads towards the tips. To tighten the cable, you'd have to take it off and twist it up like a bowstring, and not use a little lever in the middle...or else you'd untwist one half of it.

On historic use:

I think it was the Bhutanese and/or the Nepalese that used cable backed bamboo slat bows. I intend on making one of these myself sometime soon.

On compression: When you back a bow, if the modulus of elasticity of the backing is greater than the belly, the neutral plane moves towards the back. This places more tension on the belly. Backing a pine bow with bamboo will amply demonstrate this and result in massive set and chrysaling. The reverse is true too. If the backing has a modulus of elasticity lower than the belly, the neutral plane moves towards the belly. If you back an ironbark bow with thick leather, the leather will stretch and eventually want to tear.

On stresses:

In a cable backed bow, the cable often has a lower MoE than the belly (being nylon, rawhide etc). To compensate and make best use of this stretchy stuff, it should be elevated off the bow's 'real' back to bring the neutral plane closer to the middle of the bow as, ironically, the back will be under more stress otherwise (this may contradict my earlier assertions but I'm confident I'm at least sort of correct in this instance). The stiffer (that is, the more stretch resistant) the cable backing, the closer to the surface of the bow's real back it should be. This is analogous with having two wood backings of different tension strengths: higher tension strength wood backings require thinner backings.

I would like to see an experiment between two bows of the same timber. One a plain selfbow with no backing, the other made exactly the same with the same draw weight and blah blah, except for the presence of a cable backing. I can't speak as to the overall mass, but I would hazard a guess that the bow with the cable actually has more hysteresis. Due to the fibers of the cable being able to rub against each other as opposed to wood fibers which are electro-statically bonded together. I could be wrong though. A chronograph would help some.

Grahame, very glad to see it's still shooting so well. You must make another one soon. A pyramid designed bow will probably be the quickest and most expedient to make. When I make a bamboo slat one soon I'll share what happens with that.

I hope no one minds my meandering and wordy perorations. Not too much anyway.

D

[GrahameA]

Hi Dave.

The solution: Make a really awesome cable from reverse twisted fibers like a flemish twist string that drops threads towards the tips. To tighten the cable, you'd have to take it off and twist it up like a bowstring, and not use a little lever in the middle...or else you'd untwist one half of it.

No.

Make a endless loop string. Attach it to your bow.insert your toggle in the middle ot the string or somewher close. Wind it up. Both ends are twisted in the same direction and thus tightend at the same time.

[yeoman]

But then you have a cable that is the same diameter overall throughout the length. By making a reverse twist cable, it can taper in diameter.

[GrahameA]

Hi Dave.

But then you have a cable that is the same diameter overall throughout the length. By making a reverse twist cable, it can taper in diameter.

However, you do have a cable that you easily adjust and you do not have to dismantle the bow to change. At the moment I would not build a tapered cable - I would just vary the height of the stand-offs along the length of the bow.

[greybeard]

as opposed to wood fibers which are electro-statically bonded together.

Dave,

This is a new one on me.

I worked in the timber/wood pulping and fine printing paper industry for thirty years and have never heard of this before. Can you quote a reference source?

Daryl.

[yeoman]

Well, er, no. Electrostatic bonding is what I assumed the force of attraction to be between the fibers.

If not through that means, what force holds the fibers together?

Whether it is or is not electrostatic bonding, the fibers of wood are more tightly bound than the fibers in a twisted cord.

[greybeard]

Dave,

These are just two of several substances that help bond the cellulous fibre in wood.

http://en.wikipedia.org/wiki/Lignin

https://porttownsendwiki.org/Mill_Tour% ... ip_Pulping

http://en.wikipedia.org/wiki/Resin

Tar and silica are other substances found in wood and the content will vary with species.

Daryl.

[greybeard]

To the best of my knowledge cable backing was used to enhance the capabilities of a bow made from wood and other materials that had marginal bow making properties.

The cable backed Inuit bows were made in all shapes and sizes.

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If you have quality bow wood why would spend the time and effort to cable back it.

Daryl.