bit of a puzzle, thoughts please

[stuckinthemud]

I took the drawings of a fragment of an 11th century bow (probably elm) and used it to attempt a replica. It has a small recurve and a side-nock.  The recurve worked best being bent in with dry heat and the angle I got using dry heat exactly matched the angle of recurve in the drawing. As I was using heat on the recurves, I also heat-treated the rest of the bow, figuring they used a bed of embers or similar to heat the bow .  I built a few to check my findings.  The drawing showed the bend for the recurve was 10mm thick and regardless of wood type, and taper, the bow came in VERY underpowered - the average for the three bows comes in at 50lb, and performance is dreadful. 

I think my bow design is the issue here.

The recurves are definitely done deliberately and not as an attempt to save a weak bow.  They are very short at 2" each, and only at an angle of 25 degrees.  How do you tiller a recurved bow when the recurve only begins to work in the last couple of inches of travel?  I used a constant taper from centre to tip, and had a lovely profile right up to close to the end of the draw, then the recurve started working, the tips tighten up and a massive hinge suddenly appears in the inner third.? I built the bows straight, should I have reflexed them?  How would you have designed the bow - bare in mind the centre is only 30mm wide (1.25")and the end of the working section is 1" wide.

Now I know there is no direct evidence for sinewing bows in Europe in the 11th century, but I am struggling to think of a good reason for going to a lot of trouble to build a recurved bow that is limited in power by the extremely thin end of the working sections.

Current theory is the bow is a training bow for a youngster, and it very well could be, but apart from they look good, and increase bow performance a tiny bit, why go to a lot of trouble when you could just use a nice piece of yew instead - yew bows were found on the site . I am really struggling to understand why they liked this design, but there are several bow fragments to this pattern at the site.

Anyway, have at it, I really need your thoughts on this puzzle.

[superdav95]

It does appear that the tiny recurves are working a little at the end of draw.  Can’t be sure how much but they do look like they are.  The drawing of the crossbow shows a representation and obviously not to scale.  Based on this pic the temptation may be to add the recurves early on in the build thinking they will hold their shape as you tiller.  If it were me I might just tiller the bow near completion to get that inner and mid bend the way you want then add your tiny recurves in.  It would change things too drastically I would think.  Any change would be minor and could be addressed with a few scrapes or sanding perhaps.  To me those tiny recurves need to be non working being so small.  I think they are there mainly for the string nock angle and keeping the string on the bow.   I’ve never made a crossbow so take this advise for what it’s worth.  Good luck and forge ahead.   

[stuckinthemud]

Just re-read my post, I made a misleading description.  When I said the recurves only began working at the end of the draw, what I should have said was they only began to have any affect on the tiller in the last inch or so of the draw, they are static recurves.  There seems to be a sharp change in the behaviour of the bow in the last inch of draw.  I figured that was caused by the string lifting ? The force draw curve, for anyone interested is very linear at 8lb per inch

[Jim Davis]

" I am really struggling to understand why they liked this design."

With only a drawing of part of one example, there is no evidence that they DID like the design. Maybe the part that was found was the remains of a discarded effort.

For elm, it seems it would need to be wider to get more draw weight.

[stuckinthemud]

Actually Jim, I agree with you, except the site has the remains of two bows of this design, but, hey, I’ve built three and there’s a pretty good chance they’ll all go in the firewood pile.  I did go wider with one of the elm bows but draw weight only went up a few pounds.  There is a theory that archaeologists only find the stuff that didn’t work and got thrown out in disgust 

Also, one of the current replicas by Andreas Bichler got round all the problems by putting in a 110lb yew prod, which makes the question of “why bother?”  even more puzzling, I mean like really, why would you build a bad recurve bow when you have yew- there were yew bow remains on the site.

I am undecided whether to have another go, or see what happens if I sinew one.  Maybe sinew will transform the speed of cast, and it would turn into a useful wild-fowl/squirrel bow.

[DPGladius]

stuckinthemud, I like the shape of your bow. It re-curves are not as aggressive as the drawing, but it is hard to tell from the drawing.

During my crossbow builds, I had many failures trying to get a short bow to draw far enough to get good power. My final results relied on laminated oak and hickory 1/16 inch thick. A total of 6 I think. These were glued in a jig adding de-flex and reflex at the tips. The crossbow string only has slight tension at brace height, This allows it to remain strung all the time. This also allows to wood to start working when drawn which is what I needed to get an 11 inch draw length.

I added a picture of two failed attempts using a solid wood bow.

David

[organic_archer]

I saw Clay Hayes recently dropped an all-wood crossbow build using yew on the Tube. I haven’t watched it because I don’t personally care about crossbows, but you might find some inspiration in it. He knows his stuff.