Glass Bow Taking Set Mystery

[Steve Graf]

I like to make Hill Style Bows. I put about 1 1/2" of backset in the limbs. The mystery started when I noticed that the current bow I am shooting has lost about 10 fps in arrow speed (But no loss in draw weight of 55lbs). While investigating I found that it had lost about 1/2 inch of backset.

I have made 4 bows on this form. They all have .050 glass front and back. They all have many 1000's of shots through them. 3 of them have red cedar cores, 1 has a bamboo core. They are identical in all other respects. One of the cedar cored bows and the bamboo core bow both show a loss of backset (same loss of 1/2 inch), while the other two bows don't.

The only thing I know I did differently during construction is that for the 2 bows that developed backset, I left their lams in the oven at 100 deg overnight to dry out. I am haphazard about drying the lams. Sometimes I do, sometimes I don't. And I didn't record this step in my notes, so I am not 100% confident about it. I am 100% confident that I dried the bamboo lams, just not 100% confident about which cedar bow I dried the lams for.

The loss of backset seems to be evenly distributed though the outer 2/3's of the limbs. Although this may be an optical delusion.

Any Idea's? Here's what I can deduce from the facts:
-I know there must be an answer as 2 of the bows did not take set.
-It can't be a material issue as both a cedar and a bamboo took set, Yet 2 cedar bows did not.

I thought drying lams would improve performance. But I am wondering if that is what led to the backset issue. I have a theory about it, but it's weak so I won't bother sharing it.

I didn't think Glass Bows could take set. Silly me I guess. All suggestions are appreciated!

[tjdeerslayer37]

Could be a couple things I think. Did you use the same number of lams in each bow? Are there less/thicker lams in the ones taking set? If you have a thicker chunk of wood in there I think its more likely to take set than a bunch of thinner ones.

Was all of the cedar from the same board? Even if it was, every piece of wood has inconsistencies and the potential to bend differently.

Did you let the bows cool the same length of time? Did you leave them in the form to cool? Sometimes if you pull it from the form while warm still it can relax a bit.

Were the bows that are setting left in different environments after or during building? Extra heat to one could be a culprit.

Also, I've never laid any of my bows back onto my form and had them fit perfect. They always develop at least a little bit of set.

[Crooked Stic]

All good points Tj brings up. The same number of lams in each bow will give a constant springback .

[Steve Graf]

Thanks or the replies!

Lams - Yes I used the same number of lams - 4 and they were all about the same thickness

All wood from same board - No. But the actionboo bow took a set too. So I don't think it's a wood problem

Cool Time - I let them cool in the form overnight usually before taking them out. Always room temp before removing from form.

Laying Bows in form to see set - Yep, they all take a bit of set. But what I did was take these 4 bows and lay them next to each other so I could compare bows. 2 of the bows took 1/2 inch more set than the other two...

I was talking with another bowyer friend - he's the real thing, I'm just a hack. He suggested that maybe it was creep in the glue. This makes some sense to me as I have been wondering how long it takes the form to really heat up. I start my 6 hour clock when the thermometer in my hot box gets to 150 deg. But if it's like a baked chicken or turkey, it takes way longer for the internal temp of the form to get up to that temp. So maybe the glue isn't getting a full cure and is creeping a bit. I'm going with that theory if I don't hear anything better.

My next bow will sit in the hot box for 12 hours once it reaches temp. We'll see how that does...

Thanks again for your thoughts. Keep em coming if you don't buy this last idea.

[Crooked Stic]

Never heard of glue creep. Been building bows for about 12 years now. The fact is you don't even have to cook the bows for them to cure in 24 hours at a temp of 70 degrees. All of them will have some spring back right of the form. If this is happening after awhile you got something else going on.

[stagmitis]

Steve,

Hill bows are unique animals compared to recurves,hybrid or flatbow type designs. Depending on the core material used, glass thickness, degree of trapping and other design features, you can expect to lose up to 1/2" straight out of the form. Another slight loss may occur after the bow is shot for a short while. This secondary loss will recover a few moments after the bow is unstrung. Pics of the bow profile would be helpfull as well as measurements: Length,width at fades, taper rates etc..

[Steve Graf]

The mystery is that 4 bows out of the same form have taken different set, even though they were built basically the same. I usually wait about 2 weeks before taking any speed readings and backset measurements. That way they should settle down and be stable.

Overall length : 66 inches nock to nock
Width at Nock : 5/8 inch
Width at Fade : 1 1/8 inch
Core thicknesses : about 0.094 inch on 4 lams
Taper : 0.004 inch/inch
Glass Thickness : 0.050 Front and Back
Trapping : Not much. A bit more rounding on back than belly as bow is brought to draw weight and tiller.

[stagmitis]

I think glue creep is very real and one of the reasons we use smooth on. It has a good balance on the modulous of elasticity scale. Not too stiff and not too weak. Mike (Crooked stic) on the R/D../ recurves you build it is less notable because your glass to core ratios are much higher than a deep core Hill bow.

Steve, the only thing I can think of is that your rate of taper is too slow for a 66" bow and that along with the 5/8 nock (wider side profile) creates excessive internal pressures on the glue lines. That in conjunction with possible variations in the spine of the core material. (I never beleived this until I started to spine my lams).

I would humbly suggest the following changes on your build.....Increase your taper rate to .006, decrease your width to 1.090, drop your tips between .475-.490, keep .50 glass on belly and .43 on the back. I think besides doing away with backset issues youll have a smokin bow!

Hope this helped.

[Crooked Stic]

Ok I follow what you are saying just never heard that term before.

[Steve Graf]

II would humbly suggest the following changes on your build.....Increase your taper rate to .006, decrease your width to 1.090, drop your tips between .475-.490, keep .50 glass on belly and .43 on the back. I think besides doing away with backset issues youll have a smokin bow!...

Thanks for the insights. I've thought about going to a .006 taper rate, but it seemed a step too far. I started with parallel lams, went to .002, tried .003 and .004. Each change in taper brought a sweeter shooting, but slower bow. I made maybe 4 or 5 bows at each taper rate before deciding to move on. I have not tried .006 cause I figured the tips would just get too whippy. I've seen some really whippy looking bows and I don't like it. I do like to shoot heavy arrows (12 to 13 gpp) so those whippy tips don't help.

I'm going to mull it over. If I go with .006, I might drop a lam and go with 3 lams at .002 each. Less work is attractive... I've got some 0.014 carbon coming sometime this week and thought I'd try to get that in a bow and see what happens. I used to put in in my R/D bows and it seemed to help some. I thought I was done with carbon, but I guess I just got the bug again.