I used polyurethane wood glue to glue the bits together. This glue creeps pretty deeply into crevices in the wood. Clamped it really tightly. Realizing the old back wouldn’t be able to work in tension anymore I backed it. First, the broken area I covered with a good layer of raw flax, so as to move the neutral plane right up to the original back of the bow. On one side I also laid a layer of flax to avoid lateral movement of the tip.
Wrapped it really tightly, then unwrapped it and let it dry for a couple of days.
Next, I backed the entire bow with sisal fibers I retrieved from hay bale cordage. Sisal has a bit lower stiffness than bamboo (but more than most woods), but at least twice the stretching capacity. Applied it like you’d back with sinew, although it’s stiffer so the glue needs to be tackier too. Tension tests sisal indicate it can easily stretch 2.5% before breaking, and my own DIY-tests confirm this. On the bow I’m repairing it would never need to stretch more than 1.2%, so that back shouldn’t be at risk of breaking, unless a fatal belly hinge develops and the belly collapses at that point.
All sisal was applied in a single session, some 25 g in total plus 5 g of dry hide glue. Wrapped very tightly with bike inner tubing, heated with a heat gun to reliquefy the glue and ooze out the excess while letting the tubing pressing all fibers against each other and against the back in a homogenous glue mix. Removed the tubing half a day later and let the bow dry.
Monitored weight losses, which told me it was at equilibrium a week later.
Back to the tiller tree then! This was pretty exciting. Half expected it to blow to pieces, but it stayed together. All required tillering was essentially to reduce weight of the bow and get the fades bending some more. Just the old bow with the backing on drew 50# at 15”. My target was 45-50# at 24”, about the original draw weight before breaking (48#).
During tillering, I heat treated the belly three times over coals (put the embers from the fire stove in an old pan) to make the belly stronger for the much stronger than original bare back. This was rather superficial toasting (literally), so after scraping and scraping the belly to reduce draw weight I had to repeat it.
Followed Dances With Squirrels’ advice to make the bottom limb a bit stronger (now it makes sense to me why this should be done, thanks DWS). Tiller may not be perfect, but it shoots nicely (shot some 50 arrows this evening), and I don’t want to tamper with it too much right now. It now draws 48# at 24” at exactly the same physical weight as before breaking the unbacked bow (351 g), but after sanding and smoothing I guess it will drop a few more pounds, which his fine for me.
The sisal back is a bit rough. You can’t finish it as smoothly as flax or sinew, so I still need to think of a way to finish that in a more attractive way.
I'll try to to some flight shooting this weekend and do some chronographing too. Wanna see if it still shoots 220 m
Joachim
