Heat bending deflex or letting it take set??

[Woody roberts]

I have an Ash bow that looks very much like that. The stave gull winged naturally. I put 2-1/2” of reflex in the tips. About half of which pulled out. The limbs took a lot of set and now it looks like the OP pic.

Why it took so much set ( I think ). This was a dead tree so I thought it might be workable sooner. Dead is not dry. Not sure how long from cutting to bow but no more than a couple weeks. Didn’t heat treat the belly.

This bow pulls 55 lb at 27” and shoots the same arrow 10 fps slower than my 40 lb bow with straight ends.
You could say I’m not a fan of this design

[bassman]

Here is a natural bending Hickory stave still green that sort of takes on a gull wing design. I followed the grain width wise , and I will shape it better on a form. Throw it in a hot box, heat treat the belly, and tiller it as a Gull wing design. May fire harden it to try, and get better results.

[bassman]

I can not get the pic to come up.

[scp]

Don't tourture the belly cells any more than necessary.  If you let it take set, it's that much closer to chrysaling.

The real question is whether a heat induced permanent bend is same as a set caused by over-pulling. Usually heat treating wood makes it weaker in some way. However heat treating the belly of a stave appears to make it stronger in compressive strength. Still it might be making the stave weaker as a whole by reducing its tensile strength. That's probably why kiln dried wood is said to be weaker than air dried and well seasoned wood, mainly because kiln drying reduces the tensile strength without increasing the compressive strength much.

It appears that causing set by over-stressing a stave makes it worse; while heat induced set makes it better as a bow material by changing its side profile to be better or by changing the ratio of its relative strengths in tension and compression. The question is, as suggested, whether causing set by over-stressing is much worse than causing it by applying heat. A set is a set whether it was induced by over-stressing or intentional heat treatment, boiling, steaming or otherwise. It basically make the stave weaker where it was affected.

But heat treating of only the belly is an exception. It increase the compressive strength of the belly, at least relative to the tensile strength of the back, thereby making the stave a better bow material. In other words, over-stressing makes the belly weaker while heat treating the belly makes it relatively stronger.

As for the change of side profile, over-stressing always makes it worse, while heat bending might improve it depending on the design. This is the way my "primitive" mind works without the benefit of math and physics.

[Digital Caveman]

I think you're agreeing with me.  I think that cells crumble with set, but they get 'melted' into a new form in heat tempering.

[bradsmith2010]

I get a feeling,, I dont know, and would have to make some bows to tell,,but its a great question,, I just wonder if it would translate into a loss of performance either way,,,but very interesting,,

[scp]

I think you're agreeing with me.  I think that cells crumble with set, but they get 'melted' into a new form in heat tempering.

Yes, proper heat treating appears to be hardening the belly wood into something with more compressive strength. But it is also possible that heat treating also weakens the tensile strength of the back somewhat and makes the belly comparatively stronger, thereby evening out the relative share of work for back and belly, making the bow safer to bend. Just a speculation at this point. We need a wood scientist here. I don't mind being found out to be silly so long we learn something. Thanks in advance.

[Digital Caveman]

I think Marc St. Louis's tests are some pretty strong evidence that heat tempering improves the overall performance of the bow.  I think heat tempering clearly alters the belly wood a lot more than the back wood, so I think the belly is made stronger more than the back is made weaker.

[scp]

Actually for wood like Hickory that is relatively too strong in tensile strength, it might not be a bad thing for the back to be weakened in tensile strength a little by heat treatment. If the belly's compressive strength becomes stronger as well in the process, Hickory might become a better bow material like Osage Orange through heat treating. I was mainly looking at the hardening of the belly wood, just weeks ago. But it might be better to look at the change of relative ratio of back's tensile strength and belly's compressive strength. While kiln drying might just weaken the tensile strength of the whole board or stave, heat treatment of the belly works both ways to make it the better bow-wood.

[Digital Caveman]

The relative strength of the back and belly is important, but remember that this is in context of the limb mass.  It's important to maintain as much strength as possible per mass unit.

[txdm]

Speaking about a conventional R/D design (not the inverted like in the original picture)

In TBB1, Tim Baker says:

The cause of set, deflex, or string follow is irrelevant to a bow's performance

I'd like to see if there's any difference in the F-d curves between two all-things equal bows, where one has been allowed set and the other was given deflex with heat bending during tillering.

My theory is there in fact would be a slight advantage to intentional heat deflex vs set. And it would be more pronounced in a case where the stave was cut and spliced to have deflex. But in either case heat treating the belly would diminish the difference.

[Digital Caveman]

I understand the wood gets closer to its failure point as it takes set. 

[Don W]

I believe the best way to make that design for the best performance would be clamp it to a form green, let it dry to the shape, then heat treat the belly.

If I heat treat a hickory or HHB bow that is fully tillered or close to 50#@28", it will gain so much weight I can no longer string it. I've heard guys say they get a 5# gain, I don't know if it's different wood type, but I always get a lot more gain.

I'm fairly new to bow building, but not building with wood, and coaxing wood is usually not bad, forcing it always is!

[Selfbowman]

Speaking about a conventional R/D design (not the inverted like in the original picture)

In TBB1, Tim Baker says:

The cause of set, deflex, or string follow is irrelevant to a bow's performance

I'd like to see if there's any difference in the F-d curves between two all-things equal bows, where one has been allowed set and the other was given deflex with heat bending during tillering.

My theory is there in fact would be a slight advantage to intentional heat deflex vs set. And it would be more pronounced in a case where the stave was cut and spliced to have deflex. But in either case heat treating the belly would diminish the difference.

I don’t totally agree with Tim. Set is not good in any wood bow but if it happens in the fades to mid limb it’s not as bad for performance. Put the set in the outer limbs it’s a dog. Arvin

[bownarra]

Set in the inner limbs is worse as it moves the tips proportionately further. Easy to prove.
Set and heat treated deflex are very different animals.