Will riser length affect draw weight in Hill-style longbow?

[kiltedcelt]

So, I'm finally getting around to shaping my riser so I can do a glue up. I bought some lams from John G. at Old Master Crafters in Waukegan, Illinois a few weeks back. I told him I wanted to build a Hill-sytle of longbow about 70" total length and about 55lb or so. I wanted Osage in the bow and an osage riser. To balance things out and keep it from getting too sluggish from having all osage, John suggested some hickory. What we came up with was:

Back and belly .038 brown glass
4 lams of .085 osage
4 lams of .105 hickory

For the riser he suggested 18 or 19 inches length. I didn't think much about that at the time, since this is my first glass bow that I'm trying to build without doing anything stupid crazy. (Ask me about "stupid crazy" glass builds later)

Anyway, there's nice chart on Dick Wightman's page somewhere that had the lengths of numerous actual glass Hill bows, and most had risers of 15 or 16 inches length. From what I've read various places, and on that Hill-build-along on Dick's page, a longer riser will shorter the working limb length and maybe make a faster bow. What I want to know is will it affect the draw weight - ie. make it go below 55# (projected), if I go with 16 inches instead of 19 inches?

Also, the layup was going to be glass, all the osage and riser, then the hickory all going on the belly, coming up the fades. Now that I think about it though, it might look nicer to have glass/hickory/osage-riser/hickory/glass. The order of the lams shouldn't make any difference either should it? They're all .001 taper with no parallel. I want to get this riser laid out today so I can hit my little worksop early tomorrow to get the riser shaped and get gluing.

[jsweka]

In general, yes, a longer riser will make for a greater draw weight - all other things being equal.

I use 16" risers in my bows, and 16" is already on the long side for a true "Hill Style" bow.

I assume when you say "4 lams" you actually mean two lams - a pair of 36" lams butted together to make one 72" lam.  Your total stack thickness would be 0.456 with 0.038 glass.  With a 16" riser, I'd estimate you'd end up with about a 40 lb bow if your limb width was 1 1/8" at the fadeouts tapering to 1/2" at the sting grooves.  Now moving up to a 19" riser would probably increase the draw weight about 7-8 lbs, so you'd be in the  47 - 48 lb range.

How wide were the limbs to be?  Wider limbs in increase the draw weight some.

[jsweka]

Oh - I just re-read your original post and noticed you intend to build a 70" bow.  I'd subtract another 5 lbs from my estimated draw weights (I was thinking 68" which is what I typically build.)

[kiltedcelt]

Yeah, we were thinking 70" NTN (72" TL), and the lams are 1 1/2" wide. And yeah, I realized as soon as you mentioned it that those were pairs of lams to make a single full length piece. No wonder I was ending up with a ridiculous stack thickness when I was adding all that up!  :D

[kiltedcelt]

So, what would I need to do to get a 50-55# bow if I cut the riser down to 16"? Presumably I'll need to go shorter, like maybe 68"? Of course now that I think about it, we could've been laying it out for a 68" bow with a TL of 70". I guess whatever will get me in that weight range with a 16" riser. The plan was also to do a straight taper from the fades all the way to the tips.

[jsweka]

The lams are 1 1/2", but you'll be closer to 1 1/4" thickness by the time you clean the edges up and shape the limbs.

You could cut the length down from 68" and assuming you gain about 5 lbs for every 2" you cut off, decreasing the length to 62" might get you there.  However, 62" on a bow of this design wouldn't draw very nice and will stack at a 28" draw length.

My recommendation would be to add another lam.  In my bows with a 1 1/8" width, a total stack thickness of 0.520 gets me in the mid 50 lb range.  But I also use 0.050 glass and reducing glass thickness has a greater effect on draw weight than reducing wood thickness for a given reduction.  So, I would say add another lamination of 0.080.

Did John G. give you other dimensions for your bow other than just the riser length?  Maybe he does have a good recipe that will get you the weight you want if you follow all his dimensions.

Lots of things affect your final draw weight - bow length, limb width,  how the width tapers, total thickness taper, riser length, and even how much you round the corners of the limbs once the width profile is cut.  You could always just go ahead and build it and whatever draw weight you end up with you end up with.  Take good notes on all the dimensions, and then in your next bow if everything stays the same, you only need to adjust the stack thickness to hit your desired draw weight.

[kiltedcelt]

Yeah, I'm gonna have to call John G. tomorrow and read those numbers off to him and go over exactly how I was supposed to be laying out this bow to get the draw weight we were talking about.

[canopyboy]

Yes, riser length is definitely a player, but it's not huge. The longer the riser, the more you have to bend a shorter section of limb and the higher the draw weight.

Like John said though, 15-16" is on the long side for a Hill bow. I prefer to build mine with 13". I think even with a long riser, you're going to be way under your desired weight. Remember that a Hill style glass bow is only about 1.125" at the fades. You can pick up weight by keeping the width wider, but then it really isn't a Hill style.

What's your draw length where you want 55#? I'm assuming over 28" if you're looking at 70" bow.

My calculations show that you're going to want about 0.610 total stack for a 70" bow with a 16" riser to achieve 55@28. This assumes 1-1/8" width at the fades. Will very a little with backset/vs straight/vs string follow as well things like trapping, etc. If you use thicker glass (0.050 like John mentioned), you can ease up on the stack a touch.

If you decide to add an extra lam to make up the stack, I would recommend 0.002 taper for a total taper of 0.006. Hill bows shoot smoother with more taper, especially as the poundage and length goes up. Reducing weight at the tips on bows of this length pays dividends.

I'm also showing your current stack coming in around 39@28 with a 16" riser. Going to a 19" riser only adds a pound or two.

[kiltedcelt]

I really NEED to get this stuff sorted out with riser length and stack because I have a four day weekend coming up and I wanted to get this bow at the very least glued up and ground out to shape, even if I still have to spread out the shaping of the riser and such over the next few evenings or something. I REALLY want this bow to be close to the sort of profile of a typical Hill-style bow. I want the shorter riser - 16" max, but really something more like 14 would be better I think, and limb width on the finished bow of about 1 1/4".

Okay, and to further complicate matters, I just measured the glass again. I marked out 1 foot increments and took a measurement with my digital caliper at each measurement. One piece had all five measurements at .039 while the other had three measurements at .040 and one at .041 and one at .038

I took multiple measurements at each mark to make sure I was getting the proper thickness. So, it looks like I've got two pieces of glass with different thicknesses. Is there any issues with using two different thicknesses of glass on a bow? Should the thicker go on back or belly, or does it matter? I guess at this point, unless I can get a hold of John G. and have him send me another lam, I'm gonna have to buy a single set of tapers or maybe a parallel from somewhere close by and hope it can be shipped to arrive before that this weekend. Any thoughts guys?

Oh yeah, draw length is a nominal 29", but I think I usually settle for drawing around 28"

[kiltedcelt]

Well - everything has come together. Some talking to jsweka has gotten me on the right track. I'll be building up my bow with the materials I have. More details to follow this weekend.

[BenM]

John Girardi passed away about a week or so ago.

[kiltedcelt]

Yeah - unfortunately I only found that out talking to John Sweka this evening. Kinda put a damper on the evening.

[BenM]

What kind of form are you using for your Hill bow?

[kiltedcelt]

BenM - since I'm going for a straight limbed Hill-style bow I'm just laying everything out flat on my straight form, riser facing up.