Does a higher brace height make for higher draw weight at a given draw length?

jonUK76

Member
Just wondering, at the risk of sounding like a noob... All things being equal, at a given draw length, does increasing the brace height (i.e. shortening the string) result in a higher draw weight? I've heard people say it doesn't make a difference to draw weight - just the power stroke (higher brace height = shorter power stroke). I understand that the power stroke is shorter. But at the same time, the limbs are bent back further at the braced position, and so I think I'm right in saying that the limbs also will be bent further back at full draw? This should actually increase the draw weight, should it not?

Any thoughts?
 


geoffretired

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I think that you are correct. However, it isn't quite as simple as it first seems. As the string is drawn back, the angle the string makes with the limb itself, changes . The angles are bigger for the shorter string from start to finish of the draw. The string is pulling on the limb and bends it, but bending a leaf spring is easier when the bending force is applied at right angles to the spring. As we draw a bow the initial angles are small so a relatively large force has to be used, compared to a force applied at right angles. The shorter string gives slightly larger angles(closer to the right angle) all through the draw which compensates a little for the extra bend on the limb.
This explains why it is so difficult to string a longbow by trying to slide the loop up to the nock in the top limb. The string angle with the limb is very small, so we are doing more of a compressing of the bow rather than applying a force at right angles.
 


Timid Toad

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What you are inerested in is how far they are bent back at full draw, which will be the same.
So with a high bh you have some energy in the bow (at brace) and you put some in yourself (through the draw).
With a low bh there is less energy already in the bow (at brace) but you put more in yourself (through the draw, to anchor, which is the same in both examples)
*however*
When you loose the energy is rapidly imparted to the arrow. This increases until the string is straight and the arrow is flicked off. So a low bh will impart more energy to the arrow. In addition the residual energy has to go somewhere, so with a high bh your bow has more to absorb, which is mostly taken by the limbs. So higher bh can damage limbs, and it is extremely important to follow manufacturer's recommendations. The safest bh for limbs is no tension in the string at all, but that comes with it's own set of problems!
 


jonnyboyws6

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Some real world figures I've just measured agree with JonUK and contradict Timid Toad:

58" bow nominally rated at 55# with a 7 5/8 BH pulls 59# at my 30"ish according to my scale. Swapping the string for one from a 60" bow gives 6 5/8" BH and measures 56# at full draw

Shortening the string to increase brace height also increases the amount of bend on the limb at full draw, and therefore increases the draw weight
 


Timid Toad

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Shortening the string does *not* increase the bend significantly, and 55 to 56lbs, is well within the error of most bowscales, even electronic ones.
Cut a draw stick to 30". Pop it on the bow, hang on the wall and draw round. Change strings, repeat. It's an interesting exercise.
 


jonnyboyws6

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Sorry think you have misread the figures in my previous post. The bow is rated 55# @ 28" and one inch extra brace height takes the actual weight from 56# to 59# at my draw length.

As the rough rule of thumb states 3# per inch of extra draw length, it suggests to me the extra inch of brace height roughly equates to an inch of extra draw length.

So if we take the "interesting exercise" a step further, I reckon if you lengthen the draw stick by an inch, the bend will be exactly the same.

Anybody else care to measure and report your findings?
 


Del the Cat

Well-known member
It is self evident that if you shorten the string and then draw to exactly the same draw length the bend on the limbs MUST be different, unless the string is stretching or you have changed something else!
E.G. if you have a square with sides 3' long and you shorten two of the sides which meet each other it will no longer be a square!
Del
 


jonUK76

Member
Thank you for the info. Geoff, I hadn't considered string angles at all, and that is surely a factor, although I guess in the real world the "mechanical advantage" from a bigger string angle would be quite slight with "normal" brace height ranges.

Jonnyboy - thanks very much for the hard info. 3 lbs is certainly a meaningful difference. I can try this out myself, as I have a bow scale, and I have a few different strings (and bows for that matter), although I haven't rigged up any sort of draw length measurement aid yet..

Thanks Del, in my own mind I knew that but nice to have it confirmed.
 


geoffretired

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The mechanical advantage does increase as we get closer to full draw. The angle is closer to 90 deg wanting to bend the limb rather than compress it as it does to some extent at brace height. And it's helping when we need it most.
One aspect that bothers me a little is an idea that I have read about or thought about( not sure which now)
If you load a single leaf spring with a weight, the leaf bends a certain amount. Put on another weight, the same, and it bends the same amount again.( Hooke's law I think)
If we had one weight on the leaf already, and pulled down on the end of the leaf, until it reached the position it had with two weights on; would we need to pull down with a force equal to one weight.( the first weight is still working to bend the leaf ,yes?)
Could this lead us into thinking that shortening a string does some work on the bow limbs( like the first weight) so when we pull we don't have to use as much force as we expect? I can't quite see whether I am correct or that there's a flaw in there that is hiding from me.
Imagine the bow at brace height. Draw 1" and there's an increase in draw weight, let's say 2lb to hold the 1" draw. Draw a second inch and the holding weight is now 4lb. repeat in 1" increments and get 2lb increments in weight. I think DFC show an almost linear increase.
Use the same bow with a shorter string. Draw 1" and the weight goes up by 2lb? or 4lb because the limb is more or less where it would have been on the longer string drawn 2".( 1" already for increased brace height and 1" from drawing the string.) The limbs are the same so 1" gives 2lb each time.
In other words if every 1" is worth 2lb then that applies to every draw within the bow's working limits.
If we say the draw weight has gone up by 2lb because of the shorter string then at full draw, the holding weight is 2lb more than with the longer string let's say 40lb instead of 38lb. Going down from 40lb in 1" increments; will be - 2lb increments. and reaching 0lb would happen after 20" of letting down.
Letting down from 38lb at 2lb per 1" would take a 19" let down leaving the brace height 1" higher than with the short string.( The string at full draw each time was at the same distance from the bow grip.)
 


Whitehart

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Then of course increasing draw weight in this way is a bodge as more draw weight does not always equal better arrow speed a shorter string (higher BH) and a longer draw length but still within guidelines will increase holding weight but can flatten out the bend of the limb and make it less efficient and stable so more draw weight less limb efficiency. As the limb flattens out the string tension will also be less so again less efficient (transfer of energy to the arrow) a lot of this also depends the design and parameters of the limbs.
 


geoffretired

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Hi Andrew, a lot going on when the limbs are drawn, yes?
I think the OP was wanting an answer to a specific question; does draw weight increase if you shorten the string but pull to the same draw length.
I feel that it isn't quite resolved yet.I know I am still not clear on what is happening.
My ramblings about 2lb per inch of draw are clearly flawed as they don't give the right results at the end. Where is the flaw, I wonder?
There is more to this ,I feel.
 


jonnyboyws6

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Geoff, I agree with your Linear scale theory, but have to add that the first inch is always much higher as the bow is already under tension when braced. I'm getting about 8# and 10# for the first 1 1/2" with the different brace heights, as accurately as I can measure such short lengths and low poundages with my cheapo scale anyway. So again it seems the extra inch of brace height is much the same as pulling the bow an inch. If we could make a string which is the exact length of the un-braced bow, we could possibly see a perfectly linear increase of 2# per inch, or possibly string angles would come into play and completely skew the results! This is the bit I'm struggling to get my head round...

Example: using my same 58" 55# bow, what if I use the string from my 68" American longbow? I get to about 15" of draw length at 0# just taking up the slack, but then things get surprisingly heavy. I can see at full draw the limbs are hardly bending, but the scale reads over 50# ?!?! I'll admit I was a bit shocked it was so high. On the face of it I have just blown my longer string = less draw weight statement out of the water. But I can also see the unusual angle of the string is giving me no leverage, so the bow is effectively stacking purely due to string angle even though the limbs have hardly moved.

Whitehart I agree. There is no more energy output to be had, we are not increasing anything, just moving it from one place to another. Even before we consider the positives and negatives of the limb working efficiently, and string angle, we have just traded more draw weight for less power stroke.

I believe when experimenting like this, it pays to look at the extremes to be able to see the full picture clearly. Within the recommended/sensible/usable range of brace heights the effects may be smaller and harder to see, but they are still there. The original question was whether or not changing the length of the string changes the draw weight. Whether or not, or how, this effects arrow flight is a different question!
 


geoffretired

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Thanks for those figures, jonnyboy.
I think the bit about no extra energy output to be had, might not be exactly correct. I think we can get a string length that gets more energy from the system because the power stroke changes in length with brace height. At some point the length of the power stroke when put with draw weights being generated, give a maximum. Again, an extreme case is helpful; very large BH and very short power stroke to the same draw length give very poor energy output.
I like your use of an over long string. It does highlight how important string to limb angle is in the weights being felt on the fingers.
Trying to work out how much the angle changes the holding weight, is not within my abilities to calculate. I think the higher BH means the angles are bigger at every equivalent draw length, than on a lower BH. I guess that the advantage of the bigger angles doesn't fully compensate for the extra bend put into the limbs. Another guess is that the advantage of bigger angles, isn't linear. I would think the advantage increases faster as 90 deg is reached.Or put another way, the extra weight you spotted on the first inch or two of draw isn't maintained all the way to full draw.
On a similar line of thinking; putting more pre bend in the limbs using limb bolts, increases draw weight but in a different way from a shorter string due to the angles being different.
 


jonnyboyws6

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Perhaps we're saying the same thing but confusing it with different words! I'm not convinced the total energy output can be changed, but the system can certainly be "tuned" for more efficiency, in other words to impart more of the energy from the bow into the arrow, instead of wasting it making more noise and vibration.
 


geoffretired

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Perhaps we are taking about similar things but not exactly the same,heehee.
Let me try to clarify what I am hoping to say.
I am using force times distance as a way of comparing two string lengths on the same bow.
With the higher BH the distance over which the force is applied is reduced and the average force from the string is slightly higher. Go to an extreme BH and the power stroke is very short. Because the string to limb angles are close to 90 deg I don't think the force from the string on release is sufficient to make up for the short distance travelled.
I am also considering that some archers adjust their BH to get best sight marks/ faster arrow speed.
 


jonUK76

Member
I believe when experimenting like this, it pays to look at the extremes to be able to see the full picture clearly. Within the recommended/sensible/usable range of brace heights the effects may be smaller and harder to see, but they are still there. The original question was whether or not changing the length of the string changes the draw weight. Whether or not, or how, this effects arrow flight is a different question!
Yes indeed. As I said, I once heard the assertion that changing brace height does not affect draw weight. It would seem that is false. The question of how it affects the amount of energy transferred into the arrow, and how it affects arrow flight is different (but is an interesting discussion none the less :) ).

I think the actual idea of increasing brace height beyond normal range as a bodge method of increasing power is not a good one, is probably not effective (for reasons yourself, Geoff and Whiteheart mention) and has potential to damage equipment and/or the archer (as Timid Toad mentioned, the energy in that pre-load has got to go somewhere, and the limbs will take the brunt of it..).
 


geoffretired

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I'm not sure where the idea of increasing power by increasing power came from. I can see that it would be easy to assume that a shorter string will bend the limbs more etc etc and it would follow that the draw weight would go up and make the bow feel like a heavier one and perform like one. It seems this thread has shown that thinking to be incorrect.
When the shot reaches the stage where the arrow leaves the string, is it true to say that the energy that is still at work within the bow will be directly related to the speed of the string/arrow/limb tips at that point? In other words, the faster the arrow is travelling at separation, the more energy there is within the bow at that point.
So, lighter arrows would go faster, meaning the limbs and string do the same.
So, longer draw lengths on the same bow, and same arrows, will give faster arrow launches/faster limbs at launch.
Is the possible damage to the bow the result of how much stored energy there is within the highly braced limbs; or is it about how fast the limbs were going when they were brought to a sudden stop?
Will a high speed launch of a weightless arrow( dry fire) do more damage even at normal brace height?
What about a dry fire with the extra long string that jonnyboy used? Or an extra high brace height where the power stroke is really short.
 


Del the Cat

Well-known member
One has to realize that even the most learned mathematical papers on the subject (Kooi, Hickman etc) have had to simplify often to the point of considering a bow to be two rigid limbs hinged at the centre connected with a big spring!
The problem is much more complex than appears at first glance and most of the simplifications and assumptions made in this thread haven't really been much help. (except mine of course ;) )
A better way to increase draw weight while maintaining the power stroke, draw length and string length is to crank up the limb bolts. I daresay that modern materials will withstand it (it would be poor design if they couldn't take the full range of available adjustment at a standard draw length)
Anyhow, at the end of the day, it is the performance that is the arbiter. My experience with wooden bows is that trying to chase increased draw weight or speed by increasing brace height is counter productive.
However:-
Adjusting brace height upwards gives less angle of arrow against grip and causes less flexing, but a lower brace gives a longer power stroke an more chance of a whack on the wrist!
This is what tuning is about and is particularly pertinent with wooden flight bows where there is some serious arrow flexing.
Del
 


geoffretired

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Del, I think you are right about there being more complex than we imagine.
From my point of view( not necessarily that of the OP, but I feel it is similar) my interest is not about gaining power or arrow speed with a higher BH; but knowing what happens and why, in simple terms. I mean simple enough not to need formulae to prove them. The formulae may show the proof but there must be simpler versions of that truth.
Shortening BH increases holding weight at the same draw length? True or false? It seems we are not all agreed.
Sometimes, I feel statements are made that are a bit misleading. NOt incorrect; just not quite what was wanted/needed.
I can imagine someone adding a couple of twists into their string to do some testing and noticing the bow felt just the same. The decrease in string length was so small that other measurements seem the same. However, more extreme changes might demonstrate a clearer truth.
Winding down the limb bolts to increase draw weight, seems just as logical to me as shortening the string ; both are adding more bend to the limbs before the draw is made. The fine details show they are not exactly the same thing. The differences in the changes of each may not be that obvious. The archer concerned may never notice. BUT, I would want to know, just for the record.
My earlier post about 1" increments and draw forces, when worked backwards from full draw, gave the wrong BH for the two strings. Clearly there is a flaw in what I thought was logical.
I find it very difficult to leave things alone when they have reached an interesting stage like that.
 


jonnyboyws6

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Geoff, I think I get what you mean in your similarly worded similar conclusion! Ok, I'm assuming a linear increase of 2# per inch. With a short (but sensible) brace height let's assume the first inch pulls 10# and by the end of the draw we've pulled through a 20" power stroke and ended up at 50#. Then if we increase brace height by an inch, we start at 12# and end at 52# but the power stroke is only 19" *this doesn't quite add up, even on paper, but it's close enough for this example, I think* So, we haven't just lost 2# at the beginning and added it on again at the end, we've traded 10# at the beginning for 52# at the end, therefore the energy stored in the bow at full draw is 42# more, and on release all we're losing off the power stroke is that final inch were there is only 10# acting on the arrow anyway. I can't explain mathematically, but my gut feeling says it can't quite work as simple as that...

Consider if we plot a graph of brace height v energy of the arrow. I imagine at excessively low brace height the graph starts off low, because we aren't bending the limbs enough to store much energy. Then it rises. Then we get to excessively high brace heights and it falls again because we haven't got enough power stroke to impart the bows potential energy into the arrow. I don't think the graph goes up to a sharp point, like a triangle. I think it's a curve which fattens out in the sensible brace height range. Yes I suppose there has to be a peak, I'm just not sure it's a distinct one. We need a volunteer with a chrono, a hooter shooter, and a lot of free time, to test this out for us!

My interest in this topic (I rarely post on forums) comes from having long arms and short bows. I like short brace heights because, with a longer string, I believe I'm being kinder to my bows by not bending the limbs so much, and my back-of-a-fag-packet calculations say I'm not really losing any performance by doing so.
 


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