Primitive Archer
Main Discussion Area => Bows => Topic started by: Badger on June 21, 2014, 11:01:04 am
-
Braced bow measured 46#@26"
string loose to 6" 45#@26"
String loose to 8" 44#@26"
string loose to 11" 41#@26"
String loose to 16" 38#@26"
Moral of the story, if the string is just a tad longer than the bow you can get fairly accurate weight readings at actual inches. You have no need to account for tip movement or anything else just see what weight it reads.
-
So if the string is hanging loose to about 6" or brace height it will read pretty accurate, past this point you loose roughly .8 of a pound per inch of hangdown. If you are just going for a clse figure to know when to brace the bow just keep your long string at less than 8" hangdown and you should be good to go.
-
I would think string angle could influence that reading. I am prepared to be wrong here, but my guess would be that the readings would differ between straight staves and recurves??
-
Slimbob, I never use a long string for straightbows besides this test. Recurves is all I have been using it for. As long as the string hangs down somewhere around brace height it works the same for recurves.
-
Don't most bows, especially straight profiles, add weight pretty linearly? If that's the case, what you're measuring is just starting at a different point along the draw. Think of it as measuring from a negative brace height and follow the line as-is. I can see how this wouldn't apply the same with a severely curved profile because the F/D curve would look different depending on where in the draw you measure, but even most recurves have a fairly linear F/D based on what I've seen in TBB. There may be some semi-standard deviation you could account for based on the angle of the tips relative to the handle unbraced or something.
As a purely conceptual point, could it look something like this?
(http://i5.photobucket.com/albums/y161/Ishkerbibble02/Drawonvariousbracejpg.jpg)
-
BL, It won't look like that. very easy 5 min test you can conduct yourself. The graph will look like the one I posted.
-
I would think on a straight stave you would lose the mechanical advantage early on in the draw which would cause a higher reading with a long string. Again, I am just thinking out loud and am prepared to be corrected here. It still may prove your point that it's not enough to be overly concerned with.
As a note, I never weigh one on the long string so this is academic for me. I get them low braced and then weigh them.
-
Thanks, Badger. Guess that confirms what I found. Good peer review. :) Jawge
-
It confirms yours at the length of long string you use and it confirms mine at the length of the long string I use. I was wrong aboout the length of the string not mattering. Up to about 10" hangdown my method will wrok pretty good.
-
So there is hope for my hackberry yet? I am gonna take it real easy and slow from here out. This is really cool info. Thanks Badger.
So then if I had hang down of 6" and drew 40# at around 10" I should have a little room left but not much?
-
Dubois, you missed it completely, if your string is hanging down 8" loose you read your weight at like 24". Read it just like you would a braced bow.
-
I guess we could take a bow with a straight profile that we have already made, already knowing the draw weight and put a long string on it and see what we come up with...This is interesting info and would be good to know...Would keep me from turning blue in the face from stringing to early. I learned early on to string as soon as possible, but with some figures I'd know approximately where I'm at on the long string...
Don
-
Not surprised I missed it ???
Let me give it another try.
Total inches from back of bow when drawn is 24" either braced or with string hanging slack 8" before drawing and weight is same either way?
-
WOW!
Here's a very interesting and sort of contradictory result here :o.
I'm testing a bow I'm currenty working on (it's about 60# at 20" at the moment)
With a 3" brace it drew to 15" at 50# (that's 15" measured as conventional draw from back of bow)
I loosened the string so it would just slip onto the bow easily. The string hung to the 8" mark
I pulled it back to 50# and the string pulled back to the 16.5" mark.
So that's just 8" of draw on the long string vs 15" (or 12 if you subtract the brace height) on a long string :o.
Why the discrepancy between my result and Badger's?
My guess, two three possible reasons:-
1. The bow isn't finished and isn't being pulled to full draw length (so angles are different)
2. I'm pulling to a fixed weigh, not a fixed length.
3. Maybe there is some difference in measuring draw vs sting movement ? I think we need to spell out v carefully exactly what we measure.
It would be interesting if Badger would repeat my test, see if he sees the same thing..
This is getting V intersting!
Del
-
Del, try it with the string hanging down around 6", just meant to be close not exact, you don't seem that far off to me. I guess the bottom line is the shorter the string the closer it will be. Yours read 60#@20, pull it to 20 with your long string and see what it reads.
-
I'm a tad confused here.
When you say pull it to 20" you mean the 20" mark? Or 20" string movement.
I think I see where my confusion is coming from. If you are pulling to the 20" mark then the readings happen to be close.
Bur to my mind that's mere coincidence! If the long string hung down 19" then pulling to 20" wouldn't be much at all.
I think there's a danger of people believing long string measurement, which to me are totally deceptive.
From my previous result the string movement braced is almost twice what it is on long string! Yet when I do your test i get the same result as you!...
Mind it does tie in with my assertion that a "long string" should still be as short as possible to just slip on.
I think I need to lie down.
Del
-
Del, forget about string movement and forget about tip movement just look at your tiller tree marks like you have a braced bow. Read it just like it is braced.
-
Del, forget about string movement and forget about tip movement just look at your tiller tree marks like you have a braced bow. Read it just like it is braced.
Yeah, that's ok, but only for 'short' long strings :laugh:
My 78" long string I use on warbows :o gives bad results on a 66" bow ;)
Del
-
I agree, I admitted that, the closer the string the more accurate it is. But it does show an easy way to know present draw weight before braceing a bow, thats my only point. I don't like to brace them too strong and it takes some of the guesswork out.
-
Yeah, must admit, I prob' do brace 'em a tad early, dunno if that causes set, but at least I'm never under weight ::)
Good to discus this stuff 'cos we all get a bit set in our ways (you only have to look at that shirt I always wear ;) )
Del
-
There is some technical support for the results Steve is seeing. While this probably won't be as true for recurved tips as for longbows, the basic geometry of the limb-string relationship would indicate that the long string (up to a point) would generate a force-draw curve that would be slightly shallower than the braced bow.
To explain: Longbows typically use a string about 2-3" shorter than the NTN dimension, and when drawn, an idealized limb bends in a circular arc. Now, only the force that is perpendicular to the limb tip bends the bow limb. That force directly related to the tension in the string and the angle of the string to the limb. Then, the draw force is directly related to the angle of the string to the dirction of the draw.
A long string that is only a few inches longer than the braced string, doesn't change the geometry all that much, so the force-draw curve is close to the same. As the long string gets longer, this becomes less true.
Also, what is different between the long and full brace string, is the compression that is put down the limb from the braced string. This would be the parrallel force that is the result of the tension in the string at brace and the angle the string makes with the limb. This initial compression diminishes as the bow is drawn and can get down to zero if the string goes to a 90 deg angle to the tip at full draw.
Ken
-
Del, my method works for 26" with a string about 1.5 x the bow's length. The weight reading with the long string at 10" will equal the weight of the bow at 26".
That is why I give it 5# over target weight with the long string at 10".
I got confused reading your post but saw 20" probably because I am tired.
For other than a 26" draw one can set up a proportion.
Jawge
-
Jawge, if your bow is 66" long do you use a 99" long string?
-
As long as the long string is not hanging down more than 8" you can check your present draw weight anytime you like simply by pulling it down and reading it like a braced bow. If you like to brace your bow about 10# heavy just get your target reading about 4" before your full draw. I don't see how it could get simpler as long as the long string is not too long, anywhere from 4 to 8 inches slack will work fine. Read it the same way braced or long string.
-
My long string is 75" and I use it on bows from 62-66" ntn. Don't make them longer than that usually.
Sorry about the confusion (mine).
Jawge
-
Goerge, I am going to give you the old student to teacher challenge here. I have no doubt you have a solid system. You have succesfully located a sweet spot where you can get a fairly accurate reading of draw weight. and the amount inches of draw needed to get it. I feel the reason your system is flawed and requres some tuning is beacuse it is based on a false premise.
The false premise is that the inches of string movement cannot be accurately gaged unless they are gaged relative to the geometry of the bow. By simply using a string about 1" longer than the bow you eliminate that entire step, you can read your weight the entire trip just by reading it like a braced bow. I was wrong about the length of the string not mattering much, by doing this test I was able to refine my own system. I always use a short long string anyway so wasn't really familiar enough with a long long string.
-
Steve, I put the string on the nocks and the bow on the rope and pulley system. I stretch the string and mark its starting point. I then measure 10" down and mark that spot and slowly inch it down to the the ten inch mark all the while removing wood and checking the bend.
When I get around 5# over target weight at 10 inches, I get the back glassy smooth and string it up. This gets me at around 10# over target weight.
Reflexed staves and longer draws change the figures but this is just a ball park method to make sure the stave is not too heavy for stringing.
Next time I make a bow I'll check all figures.
My description of it has been faulty. My apologies.
Jawge
-
In thinking about it, I am wondering if measuring the finished bow with the long string is really an accurate gauge of the potential target weight of the stave at 10" with the long string.
There is really no way to measure the potential of the bow at that early stage of being a stave.
The goal should be to get the stave at around 10# over potential target weight before stringing.
The proof is in the outcome.
Time for bed, my friend.
Jawge
-
One more thing, Steve. I did try your method years ago and it worked well. It will work better with a very short string for the long tiller. I never liked using the short string because I feel it puts too much stress too early on the stave.
The plan all along was to go just a little past brace height to make sure stringing with a bow stringer would be easy combined with making sure the stave was not to strong for stringing.
I didn't like going much beyond 10" because strung bows behave differently from bows on the long string.
Perhaps you should try my method someday? You may like it.
Jawge