Deflex? Maybe one for Sleek??

[stuckinthemud]

Yes it’s a crossbow, but it is NOT a horn bow, it is a yew/sinew bow.  It was well used over a prolonged period but eventually fell off something damaging one end and knocking it a little out of its bridle.

Please don’t leave this thread because you don’t know about crossbows, it’s just a yew/sinew short bow.

My main question is this, do you think it was likely to have been deliberately deflexed when new? One crossbow builder I know and respect strongly advocated deflex in heavy bows like this but I am far from convinced.  I would love the thoughts of y’all.

The bow is 40mm by 40mm thick/wide at centre, half that at the nocks, the bow is somewhere around 85cm from tip to tip (1.75” wide /thick, 32” long) draw length is 25cm (9.5”) giving a length to draw length ratio of 3.5:1

[sleek]

Any bow has a maximum bend radius it can obtain before takinflg damage. Most folks refer to it as draw length, but thats not 100% whats happening, as it ignores and deflex or reflex in the bow that changes how much the bow is actually bending. So,  a reflexed bow pulled to 26 has a tighter bend radius than a deflexed bow pulled the same.

In the case of this sinew bow, deflexing it to allow it to reach its desired bend radius without taking set is logical. Include that it has no horn, and it becomes a probable reason. The fella who made that was pretty smart id say.

[Hamish]

I got no idea. Are there any pics of this bow, or a similar bow unstrung?
That is one thick bow. Sleek's logic sounds good to me, but I have no idea how much deflex would need to be put into it, before sinewing. Sinew then draws some deflex out. Then tillering, and usage puts some back in.
Its too massively thick for me to accurately speculate, as I have no experience with such a bow. Unless a contemporary bowyer has made replicas, I imagine it would take a couple of attempts to work out the best way to do it.

[Selfbowman]

Hamish  this is a shot in the dark. But if you put the deflex in the center of the bow. The amount of reflex drawn into a straight limb bow should be about the same. Adjust the deflex in the handle center according to the reflex In your design . Not a perfect fit maybe because of the unknown amount of drawn in reflex caused by the sinew.  maybe I am missing the whole topic.

[Tuomo]

So,  a reflexed bow pulled to 26 has a tighter bend radius than a deflexed bow pulled the same.

No. It is not possible to say from fully drawn bow that it is straight, deflex or reflex in rest. The bend radius is same. But the stress (and strain) is different - reflexed bow is stressed more than deflexed if the draw weight is same. Deflex design with sinew backing in this kind of bow is very reasonable – high draw weight with minimum stress.

[stuckinthemud]

I am having trouble with resizing the images.  If an admin wants to step in and quarter them I’ll be very happy. 

The advantages of deflexing the bow are said to be the ability to remain strung for months and to remain drawn for prolonged periods without taking set.  Remaining strung and drawn for a long time are not normally possible with wooden crossbows but I’ve copied three historic bows and they all had straight limbs but then they are all light or medium weight. A heavy bow might be very different.

This bow is almost unique, sadly there is no similar artefact and no-one has built one.  Digging around in my records I found these two images: