miniature crossbow, trouble with such a small stave

[pal]

Hello all,

I've been fiddling with a mini crossbow since I got quarantined inside. I'm still green at bowmaking, but it didn't stop me from blowing through 4 tiny staves. I'm hoping by posting my struggles y'all could help me to figure out a better way, and perhaps some of you will be entertained or get learned off it.

First off, here's a video of my rapidly failing most recent attempt to give you an idea of the whole project.
https://www.youtube.com/embed/qAqqurQkm5A

Now here's the first rounded-belly bow of osage that whose back broke under tension. I confidently reflexed it and microwaved it for 40 seconds three times because I was excited to try the crossbow out. It's a little over ten inches long.



The second was of black locust with an almost equilateral triangle profile down the length. It also ultimately broke its back, though before that I could see a number of areas of compression on the belly. It's about 12 inches long.



The third one was more of a flatbow, but when it broke I was so frustrated I snapped it a few more times and now I can't find it.

The fourth is the one in the video and still in the bow. For this one I glued together with titebond II 8-10 pieces of (maple?) veneer I had lying around and backed it with a piece of silk. Early in tillering I noticed a belly area with some compression where the glue had failed between two pieces (where the most obvious bend is now). I attempted to hit it with glue and slapped a strip of bamboo on the belly in hopes of fixing that spot, and that worked for the day before it finally gave in, the first layer of veneer on the back breaking under the silk. It is 12 inches long.





So I hope you found that all interesting. Are any of you seeing my patterns of failure? Something I could do better? I seem to break them all close to the same spot, and I'm unsure of what might be my best strategy for making such a small bow - would you think there are any problems with such a small size? I end up using just a small portion of the stave, barely two growth rings, if that. That why my last try was composite, to try and get more "growth rings".

[DC]

Are you scraping the bow down to one ring on the back(the side away from you)?

[pal]

As best I can, yes

[stuckinthemud]

I love messing with mini crossbows!
 Design your bow to match your draw length and build a mini tillering stick and carefully tiller it the same as a full size bow. The double draw length plus 10 percent  is a minimum rule for crossbows, so a 10 inch lath should draw no more than 4.5 inchs as a max, but I wouldnt take it past 4.  Your fades are much too steep, try to get a bendy handle shape with only the 1 inch at the centre not bending.  Your not going to get away with those reflex hooks, they are simultaneously reducing the working section and increasing the stress on rhe much reduced working section.  If you lose 2 inches to the hooks and 2 inches to the fades your working section goes from 10 inches to 6, so max draw length becomes  2.5 inches....A layer of sinew adds a whole new level to the fun factor.  Keep us posted, I love these

[stuckinthemud]

The double draw length plus 10 percent works for bows that are drawn and then released immediately , crossbows are not fired for a few seconds after drawing and so are much more stressed than archery bows.

[Del the cat]

Make the whole prod bend, it should have a smooth continuous shape with no cut out or thick section in the middle. Deffinitely no reflex. A bit of deflex is probably helpful.
Bamboo would probably be a good option.
https://bowyersdiary.blogspot.com/2011/10/crossbow-prod.html
Del

[pal]

Thanks stickinthemud, that analysis makes sense. The draw length on the stock is about 5", so I figure I'll bump the bow length up to about 14 or 15", and as Del suggests, aim to have it bend along the whole length.

And funny thing Del, I just slapped up some lengths of bamboo from a mat to make some sort of rough bundle bow. Take a look:

[stuckinthemud]

the centre inch where the stock hits the lath needs to be stiff to prevent the edges of the stock digging into the lath as it is bent, but apart from that, you need as much of the bow working as possible

[Del the cat]

the centre inch where the stock hits the lath needs to be stiff to prevent the edges of the stock digging into the lath as it is bent, but apart from that, you need as much of the bow working as possible

I disagree, if the edges of the stock are slightly rounded and have a sheet of leather over the face it's fine.  IMO
Del