Flat belly turned concave by sinew backing?

[Frodolf]

So, I'm working on a recurved pyramid sort of bow. Yesterday I backed it with sinew. I reverse-braced the bow to 7", applied the sinew, and then wrapped and heated the backing to get excess glue out of it (á la Pat M's method).

Now that the sinew has been drying for a day, I discover that the previously perfectly flat belly has become concave. The opposite of crowned, that is. I've never had this happen to me before, but then I've never used deer sinew before, only cow sinew. Deer sinew seems to expand much more when soaked and should therefore, logically, result in more reflex as the sinew pulls the bow more than cow sinew does. But even if that is the case, I can't quite wrap my head around what's going on. Any ideas? Similar experiences? Is it fatal?

BTW, the back of the bow pre-backing was crowned. The stave came from a 6" trunk. The wood is rowan, well seasoned.

[wizardgoat]

it sounds like the sinew has pulled reflex, and as it shrunk it also pulled the sides up. I've heard this can happen if the limbs are very wide, or I've also heard it can happen if too much sinew is applied at once, instead of a layer or 2 every week or so. Just a couple ideas, not sure if they are true

[Ryan C]

I'm with wizard I bet your limbs are too wide and thin.

[PatM]

You guys are misunderstanding his description.  The belly is concave because the back is temporarily wider due to moisture and an increased poisson effect.

[Dakota Kid]

If that's the case, it should go away as the MC stabilizes?

[PatM]

 Yes, when the sinew matrix and the now temporarily damp  wood shrink again their width decreasing will flatten the belly again.
 Very common if your limbs are wide and flat and of a softer wood. Happens with elm all the time.
 You aren't likely to notice if you use an even slightly crowned belly.

[Dakota Kid]

I never backed a flat bow with sinew, just crowned osage. Good info to know. I would have been concerned if I ran into the same issue.

[burchett.donald]

Frodolf,
             I had the same thing happen to a couple of 2" wide Hickory bows after applying green/freshly peeled snake skins with hide glue...After drying, the belly flattened back out...Before it dried I held a straight edge on the belly's cross section and it was concaved 1/16" at center limb, middle third only...Had to be excessive moisture in my case...They are flat now, took a couple weeks in my airconditioned home...
                                                                                                                     
                                                                                                                                           Don

[Frodolf]

If I'm not mistaken, the opposite effect – when the sinew pulls the sides up, what wizard and Ryan are talking about – is mentioned in TBB? Don't they write about how it can even crack the belly lengthwise? At least I've heard about it before. That's why this thing was so surprising! The depth of the concave surface is about 2-3 mm. That's about 1/10"

What you're writing, Pat M, makes total sense! Thing is, I didn't have acetone at home so I had to prep the back with soap and hot water. I've done it before, and it works great, but I always let the bow dry out longer than I did this time before putting the backing on. So the bow was a tad wet even before backing it. And for some reason, I also sized the bow twice, also not really letting it dry out for long. So a lot of extra water there...

And it's great to hear that it will (most likely at least) go away by itself. Thanks for you comments, Donald! Good to know that it can happen with other backings as well. Rowan is one of the most beautiful kinds of wood around where I live, yet I've never had as much bad luck with any wood as with rowan. I REALLY wanted this one to work out. And now it sounds like it will, even though it gave me a good scare!

[Badger]

  I think moisture is the culprit more so than the sinew at this early stage.

[LittleBen]

Yeah I think you just have some moisture in there. It will go away.

Just remember, when the belly becomes flat again, it's probably not all the way dry yet, it just means the belly and back have the same moisture. My guess is you're gonna eventually have a crowned belly when that sinew pulls the sides of the limb up a bit. I've had this as well as the longitudinal cracking on the belly.

I really like to give sinew time to cure.

[Eric Krewson]

I made a reflex deflex osage bow and sinewed the back. I saw a side crack in the handle when I started the bow but though it wouldn't be a problem. It has been my finding that staves that have side cracks in them are better cut up into firewood as all the side cracked wood I have used has failed.

Anyway, lots of work to make and sinew the bow and this happened; When the sinew dried it "delaminated" the bow from one end to the other.

[DC]

Is there maybe a limb width/thickness limit for sinew?

[burchett.donald]

So the belly would be convex...Concave is inward
                                                                                                                                          Don

[PatM]

 Concave, it is bowed inward.
 There is no limit for limb width or sinew thickness really. Bows have been made with every combination from wide limbed paddle bows to narrow and thick plains bows with varied sinew amounts as well.