Primitive Archer
Main Discussion Area => Bows => Topic started by: DQ on September 04, 2011, 11:56:09 am
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I'm working on this osage selfbow for the fall hunt. It's 64" ntn. Right now it's 62#@28"with about 1/8" positive tiller on the upper limb. I've shot it a few hundred times and nothing has changed. It shoots okay...but not great performance. Should I flip the tips on this bow? What would be the advantages? What would be the risks? (Upper limb is on the right in the photos) Thanks for any response. Darryl
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An Osage bow at 62# should really spit an arra so there is more to your problem. How wide are the limbs? How wide are the tips? Too much mass will slow down an Osage bow. Lets see some unstrung pics and a front profile pic. As far as flipping the tips goes it should not be a problem, you may have to re-tiller some but maybe not. Flipping usually gives some benefit.
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Absolutely... Your string angles are near 90* and probably stacking some. You will likely get more of the energy you put into drawing the bow back by reducing the string angle (flipping the tips). If you aren't happy with the performance, you might also consider heat treating the limbs.
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Jon bet me to it. We need some unbraced side and back profile pictures.......Art
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I personally would not. My question; is there hand shock? If there is, then there's performance to be gained by working the limb tips. That design should shoot very well without flipping the tips. It seems to me that something else is wrong or your performance comparison is flawed? Flipping the tips will for sure make the draw more pleasant but I think the performance gain will be small. You could pike an inch or 2 off the bow, narrow the tips and flip them and you might see some improvement.
If I were going for the peak performance out of an osage selfbow, I'd make it a lot shorter than that one (60" max) and have it bend through the handle. I'd work hard at making the tips as small/narrow as possible and I'd flip them to help with string angle.
You've built a lovely bow and if it draws smoothly, has little hand shock, and delivers an arrow on target quietly it would exceed all my requirements as-is.
George
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Thanks for your input everyone. I value your responses.
The bow is 1 5/8" at the fades, 1 9/16" for about half the length of the limbs, tapering to 5/8" at the string nocks. The bow started out with reflex. It now stands quite straight although the upper limb has taken just a touch of string follow after shooting.
Another question: I tillered this bow last week during sort of high humidity. I worked in the basement with a dehumidifyier going. My cheap-mart humidity gauge said it was around 50% relative humidity. Is this a problem?
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Do you have it pulled back further than 28" in the photo? I ask because you should not be hitting a string angle like that with a 64" bow at 28" draw.
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Gordon, I just went and measured. It is drawn 28 inches from the back of the bow. Thanks.
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You really need to see it at full draw being pulled by a person.
IMO.
Your tiller rig, having two supports doesn't allow it to pivot like it does in ones hand. I favour a single curved support on the tiller rig.
If you support a bow where it is gripped and pull the string back where the fingers will be the bow often looks to start off canted over to one side*, this dissapperas as the bow is drawn.
At the start of the draw there is V little force and the bow is being supportedjust below the centre and pulled from just above the centre lines.
The shorter the bow, the more critical it is to check a realsitic draw, rather than relying just on the tiller.
Del
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Narrow it down at the string knocks if it is 5/8" wide. That will help you a bunch. Dean
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Gordon, I just went and measured. It is drawn 28 inches from the back of the bow. Thanks.
If, by the back of the bow, you mean the part furthest from the string, you are correct. If you mean the part of the handle closest to the string (where the bow is touching the wood on your tillering setup), then you're drawing the bow more like 30" on the tree. That would explain the string angle and mean that it is in the neighborhood of 56-58lbs at 28" which would poorly compare with another 62lb bow.
George
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The problem lies more in your tiller and width in the outer limbs as far as performance goes. There is too much bending inner limb for as wide as your limbs are at mid limb which should be doing more work. Then your tips are far too wide for optimal performance. I don't think flipping the tips alone would help very much. It may help to add more stored energy by decreasing your string angle but the inner limbs will be stressed more at the same time which could lead to increased set. Heat treating to increase stiffness and then adjust width and tiller would do more good than flipping the tips IMHO.
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I would narrow the tips a bit more. It is scary, I know but the osage can take it. I would narrow from your current mid-limb to a 7/16 -1/2-inch tip. Or, you could leave the bow just as is and send it to me. I think your tiller looks great.
On string angle, this rule-of-thumb for the minimum NTN length of the bow, works for me:
Bow length: 28"
Double it: x 2
Equals: 56"
Stiff Handle: + 7"
Equals: 63" NTN
Any shorter than the equation above will near the 90 degree angle (plus be difficult to succeed without sinew). If I have a bendy handle bow, I remove the "Stiff Handle" part of the equation.
With your bow at 64" you had a bit of room. It doesn't appear quite at 90 to me, but it is close.
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It is drawn 28 inches from the back of the bow
Ok, but it still doesn't look right ???
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Wow. Thanks everyone. I appreciate your insight. This forum is a great place to learn. (and I have a lot to learn.) You've given me plenty of food for thought.
gstoneberg - Handshock is an elusive critter for me to tame. Yes, I'd like to reduce handshock on this bow, but it really isn't bad for an osage selfbow. Maybe narrowing the tips will help. "comparison is flawed?" - Probably is. With crops in the fields I don't have a place to shoot it for max. distance, which is the only comparison I have. This bow just "seems" a little slow.
Del - Your post has me re-thinking my grip. I do have a curved support like you described that I use on my tiller rig. This bow has equal length limbs. When supported dead center, it pulls to full draw smoothly with no rocking. I'm gripping the bow near center and shooting the arrow 1 7/8" above center. I'm fairly new at this, so I welcome suggestions.
Dean, ryoon and Matt W. - Okay, okay, I'll narrow the limb tips. I didn't want to whip-tip the bow so I stayed at 5/8". Thanks for the formula, Matt.
String angle? Hmmm. This is a whole new mystery for me. Never gave it a thought. What are the "rules" for string angle?
Like I said, plenty of food for thought. Now...back to the shop.
Darryl
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No rules, just physics. ;D It is about leverage. The angle where your sting meets the bow is almost 90 degrees. Pull the bow to about 20 inches and you will see that the angle is a lot smaller, maybe 60 degrees. The closer a bow gets to 90* the harder it is to pull. How hard it is to pull is a symptom of "stacking". A lot of energy that you are putting into the bow is being wasted and will not be transferred to the arrow. If you get beyond 90* you risk the string popping off, and get really low energy return. Recurving, or in this case flipping the tips reduces the string angle and makes leverage work for you making it easier to draw and putting more of the energy you used to draw the bow back into the arrow on release. On longer bows, the string angle doesn't get near to 90 so they draw easier. On short bows, you have to be careful of this.
I thought the same thing Gordon. The string angles seem really high for that draw length and that length of bow.
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according to your original measurements your bow should be tillered eliptical . it is tillered arc of circle now and that is why your having issues with hand shock and performance. hope you can work it out though it is a beautiful bow
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Justin - Thank you for a great explanation of the string angle thing. Makes perfect sense.
ken75 - I now agree with you. I was trying to use more "working limb". Oops. :-[
Live and learn! I've learned a few things today. Now I'm thinking of flipping the tips a little to releave the string angle and narrowing the tips a bit to lower mass weight. It's not a bad bow right now, but I think it can be improved. I'll let you know how it turns out.
Thanks, everyone.
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Make sure you flip the tips before you narrow them. That way you can shave off one side only to keep the tips lined up with the handle.
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String Angle being the angle measurement in degrees between the bow limb and the string at full draw. When your string angle at full draw exceeds 90 degrees there is the danger of the string slipping out of the string grooves and off the end of the bow. The results would be catastrophic.