Learning the hard way that crowned backs are risky

[joachimM]

I’ve recently written about putative benefits of trapped backs, or crowned backs.

Since then, I’ve made six crowned-back bows from branches up to 3 cm (1.25”) wide, and toasted bellies. All between 50” and 60” (3x Plum, 2x Hawthorn, 1x Rose), I tried to test them extensively, monitor all kinds of data, shoot them a lot and chrono them. Since then, I’ve broken three of them.

Incidentally, these three were the most reflexed bows with the lowest set (retaining up to 1.5” net reflex after shooting, losing less than 0.5” relative to resting profile), and with calculated back tensions that were very close to the expected limit (0.9 to 1.1% back elongation). These were branch bows with some pins, and two of them clearly broke due to tension failures that involved these pins. A third one was "just" overstrained in the back. These broken bows shot very well according to my standards (>170 fps@ 10 gpp, 30-35# bows). One of them shot 178 fps at 10 gpp, twice in a row, when I pulled it one inch further (27 instead of 26”) than intended for the bow. The third time, it broke . Pushed the limits too far.

The still intact branch bows are decent but not exceptional shooters, shooting around 160-165 fps at 10 gpp (or 150-155 fps at 13 gpp). Incidentally, most of them took some set in the round branch handle, as the belly there is narrow (round), yielding a weak belly spot resulting in net handle deflex. Set in the limbs themselves is rather limited. This results in a lower strain design for a given draw length. (the last bow, made from a wild Rose branch took 1.5" of set in the limbs).

So what have I learned? Probably what you all already knew, but what I still needed to experience first-hand. Crowned designs spare the belly (low to absent set), but heavily stress the back. Crowned backs strained close to their limit will eventually fail, especially when they are littered with imperfections such as pins. The difficulty in dealing with crowned backs lies in defining acceptable back tension limits and associated degrees of back crown. Trapped backs, with at least a nice flat plateau, seem safer.

What’s next? I’ll continue making some branch bows, but will decrown them a bit and add a more tension-safe plant fiber backing to replace the wood that I removed. Hopefully, this will allow me to keep these bows shooting 170 fps instead of breaking them within the first 100 shots

Joachim

[Del the cat]

Great post, thanks for sharing.
It's all about compromise and getting to know the various woods... even then no two bits are the same.
One thing I keep having to re0learn is "listen to the voice in your head" especially when it tells you "that wood just doesn't look right"
Del

[SLIMBOB]

Just a thought to add here. For me, high crowned backs won't get any heat tempering. My thoughts are (right or wrong), the belly won't need it and the back won't take it. A back with problematic knots won't get a tempering either. The belly giving way somewhat might just keep the knots from splitting. These are very general rules which I can break if I am so inclined. It's all very edisonian for me and each build is viewed individually, but generally speaking. Keep the results coming. Enjoy following what your doing.

[joachimM]

That's a very valid point, Slimbob.
Since these bows are so narrow and I want to get some draw weight (at least 30#, but preferrably something over 35#) I give them a slight  belly toasting just a couple of minutes till I start getting the toasted smell. This allows me to add some reflex. But off course, you risk overpowering the back this way, and this clearly happened here a few times.
Del: yes, that voice in my head keeps telling me not to do things. I tend to ignore it  , but will start to listen

[half eye]

I usually no longer engage these rethorical discussions because I'm neither a scientist nor a maximum performance freak. At the risk of the coming ridicule please explain why you did an experiment where-in you exagerated a weakness and then are suprised at the failure outcome?   

You take a weak back situation then make the belly even stronger and wonder why the back failed. Making the back stronger to catch up to the overpowering belly is an excercise in futility because when your all done all ya got is a cobbled up stick doing what it has no right to do. If ya want a 200mph AA fuel dragster, there is no sense in starting with a converted VW.

Anyway I'll butt out now so fire when ready Gridley.
rich

[Springbuck]

3 cm is darned small.  In the previous discussion, I was imagining bows made from 4" logs.

[Badger]

  Very good observations. I possibly look at back tension wrong but I see it as if the back is twice as strong as the belly in tension all that really means is that when the belly bends so far it will either collapse or start stretching the back. In other words the tension is only twice as strong while the stave is at rest, once we start to compress the belly it starts catching up to the back and if it passes the back the bow breaks in tension.

[joachimM]

I usually no longer engage these rethorical discussions because I'm neither a scientist nor a maximum performance freak. At the risk of the coming ridicule please explain why you did an experiment where-in you exagerated a weakness and then are suprised at the failure outcome?   

Hi Half eye.
Well, first of all I had no stats whatsoever on hawthorn as a bow wood. Also for plum, few stats, let alone rose branches. Since 99.9% of what you find of hawthorn bow wood are branches, you go for them. Then, you find that crowned-back hawthorn branch bows shoot rather well, and you try to tweak them. So you explore the leeway, by toasting them a bit.
I've found out that I can make rather good bows out of it, and I've also found out about some of the limits of that particular wood. But not enough yet.

Second, the best way to learn is from mistakes. Not somebody else's, but your own.
Third, since I'm not a hunter (no bowhunting allowed here), neither a target shooter, I go for speed or distance. Frankly, I don't care about having 200 medium speed bows, so I try to go for faster ones each time, and break a lot of bows. Doing so, you bump into limits, and you may also venture into less known grounds and discover or rediscover things.

[PEARL DRUMS]

Your third reply is what I have to constantly remind myself when I scream, "Why would you do that!" at my computer screen. Then I remember, there are many factions in the wooden bow world. We all build for different reasons, but happen to use the same ingredients.

[George Tsoukalas]

I know, joachim. We have to learn first hand from our mistakes.
I've made quite  a few during my bowyer times.
The only way I've been able to get staves with heavily crowned backs to become bows is to leave them a few inches longer.
Jawge

[jesswprater]

Hey Joachim! Where do you live that doesn't allow Bow Hunting? I'm getting an education here. I can't imagine. I'm getting into bowhunting because the rifle deer season here is too damn short and you have to wear all that neon orange clothing! I am starting to feel like a peacock when I hunt.

[duke3192]

check profile and post yours.

[steve b.]

I would argue that there is not putative argument about crowned or trapped backs.  I've been studying this for a long time and, despite everyone having their opinion, the jury is out on what benefits each type of back actually has.  Truth is, everyone wants to know.
I would argue too, that jumping to the conclusion that a crowned back is, such and such, and trapping is such and such, based on broken bows that break at pins, is a non sequitur.  Weakened backs because of pins on branch bows may or may not have anything to do how a bow behaves made from trunk wood.  It doesn't mean that backs are stressed because they break at pins and therefore high crowns must mean, such and such.  Just because a branch bow breaks, because it has a high crown, with pins, means nothing in the overall scheme.
Pins are one thing, but crowned backs are a very subjective parameter based on, type of wood, relative width to belly and length of bow, etc. etc.  There pins are actually throwing the wrench in. 

I personally look for the balance between back width to belly width, starting always with the belly width in my bow design and matching everything else up to it, depending on wood type, weight desired, draw length, etc.  I'd rather have a decrowned back with rawhide then a crowned back with pins or defects.   It has worked for me.  I put no stock in the bow that bends in half, with not set, with the high crown.  I want to see that bow 1000 arrows later, then I'll decide.
But its a known fact that defects on the back cause splinters and breaks.  That has little to do with the crown.  A clean back that is crowned can be the ideal design for a particular bow.  Its no different from taking a rectangular profile that shoots wonderfully and cutting a 1/10mm slice across the back and then coming to some conclusion about bow design when that bow breaks there.

Having said all that, I think this type of testing is important, when done right, and might someday answer the question that we all have as to the degree of crown necessary to make the bow ideal.