Force draw on bow?

[Badger]

      Efficiency will usually be in the 65% to 75% range for wood bows. The stored energy on that bow is unreasonably high. Geometry is the best predictor of FDC. Very unusual for a bow to exceed 100% SEPDF ( Stored energy per draw force) 95% would be considered good and usually produce a bow that shoots in the 170's. A couple of years ago I built a very radical bow big 12" hooks that were set back 12" behind the back. This bow gave me about 115% SEPDF which was very high but the design could not be considered to be any kind of a standard or practical application.

[gfugal]

just measuring "one the way up" vs "on the way down" makes a difference.

Yeah, this is the difference between work-in and potential energy i was mentioning earlier. Measuring weight on the way up is work-in. Measuring on the way down is the closest approaximation for determining potential energy. To me i just elimanate the extra step, unless someone can offer an explanation that bows will have drastic variabilites in in their efficiency differences from work-in and potential energy. I'm figuring the decrease in effecieny for work-in calculations will be similar for all bows.

Efficiency will usually be in the 65% to 75% range for wood bows. The stored energy on that bow is unreasonably high. Geometry is the best predictor of FDC. Very unusual for a bow to exceed 100% SEPDF ( Stored energy per draw force) 95% would be considered good and usually produce a bow that shoots in the 170's. A couple of years ago I built a very radical bow big 12" hooks that were set back 12" behind the back. This bow gave me about 115% SEPDF which was very high but the design could not be considered to be any kind of a standard or practical application.

Is that 65% to 75% efficiency obtained from potential-energy/kinetic-energy or work-in/work-out. If it was from potential it would be a higher efficiency than what i've been getting. I would love to see some of your force draw curves and analyze them. I'm trying to figure out what to expect from bows. Maybe you could direct me to some links.

Yeah i'm getting about the same for SEPDF. I think i figured a straight line to be around 90%, so anything ocer 90% is a great, and if you can get above 100% its excelent.

[Badger]

   Efficiency is a fraction of the work in which would be work out. Same as stored energy.

[gfugal]

Yes. What i'm saying is that there are two different force curves. One if you measured the poundage on the way up, and another if you measure the poundage on the way down. The force curve measured on the way down will show less stored energy due to energy lost in the friction in the limbs during draw, heat lost in the muscles, and etc. I define the force curve measured on the way up "work-in" and the one measured on the way down "potential-energy" or "stored-energy". So the efficiency for the one on the way down will be higher and the efficiency for the one on they up less. What i'm asking is which curve you measured your efficiency from. The one on the way up or on the way down?

[Badger]

      The force draw curve should be measured only on the way up for a reading of SEPDF, the losses you mentioned will be reflected in the efficiency of the bow. Honestly, If I get a bow that reads more than about 1# difference it is going in the burn pile anyway.

[Selfbowman]

Steve can I have all the burn pile bows. I might learn more from those than I already know.  Arvin

[gfugal]

Wow! Those are high efficiencies then! I haven't seen a force draw curve with that arrow speed yet. If thats the case how do you get it so high. Could you or somebody else provide a link of one i could look at, maybe i'm calculating things differently.

[willie]

those up vs down differences in spring scale readings are from friction in the sprng scale " bearings" I think.

there are very real differences between measured force draw and what gets applied to the arrow, as you point out, but any difference attributed to hysteresis, with a slow "way down" type measurement would be meaningless.

the energy delivered is limited by limb return speed, and lighter weight arrows can increase limb speeds, however, it can be shown that reducing arrow weight past what the limb is capable of, does not add any speed.

perhaps badger's comment about the burn pile bow, was a bow that cannot be measured at the same weight, at the same spot, if it has been held there too long and begins to weaken?

[Selfbowman]

those up vs down differences in spring scale readings are from friction in the sprng scale " bearings" I think.

there are very real differences between measured force draw and what gets applied to the arrow, as you point out, but any difference attributed to hysteresis, with a slow "way down" type measurement would be meaningless.

the energy delivered is limited by limb return speed, and lighter weight arrows can increase limb speeds, however, it can be shown that reducing arrow weight past what the limb is capable of, does not add any speed

perhaps badger's comment about the burn pile bow, was a bow that cannot be measured at the same weight, at the same spot, if it has been held there too long and begins to weaken?

So there lies the reason for quick release on selfbows? Just asking. Arvin

[gfugal]

those up vs down differences in spring scale readings are from friction in the sprng scale " bearings" I think.

there are very real differences between measured force draw and what gets applied to the arrow, as you point out, but any difference attributed to hysteresis, with a slow "way down" type measurement would be meaningless.

the energy delivered is limited by limb return speed, and lighter weight arrows can increase limb speeds, however, it can be shown that reducing arrow weight past what the limb is capable of, does not add any speed.

perhaps badger's comment about the burn pile bow, was a bow that cannot be measured at the same weight, at the same spot, if it has been held there too long and begins to weaken?

Changes in the springs may be a factor, but there are real differences. With any energy transfer your going ro loose some energy to heat. The stored potential energy is always going to be less than the work-in you put to get it there. But as Badger mentioned, that difference should be prertty small. I didn't realize how small. It makes sense not to ever use the way down method cause some difference could be the scale not the bow, plus you can't know how much more will be lost do to slower limb return with a light arrow.

As for arrow weight, lighter arrows definately go much faster than hevier ones, but that doesn't mean they are more efficient. Quite a bit of the energy is lost in the limbs and only so much is transfered to the arrow. Lighter arrows in fact decrease this transfer, and hevier arrows increase it. If you think about it, if you have too light of an arrow you will be essentially dry firering the bow and get more recoil. Dry fireing is an example of no energy transfer. A hevier arrow may go slower but the bow is dampered by it and you have much less recoil. The bow limbs may go slower than before, but that doesn't mean they imparted less energy, if anything they gave more because if an arrow is too light it starts moving faster than the limbs can recover. Here's an article that talkes about it. http://archeryreport.com/2011/02/bow-efficiency-care/
They are using modern compound bows, so expect some differences. For example their efficiencies are much higher than what we are going to get, but thats due to their freak bows and the fact that they have better scales so they can use the potential energy instead of work-in. However,  the principles should b the same.

[gfugal]

Again i wanted to point out that efficency does not equal arrow speed. If we want faster arrows then lighter is better (up to a certain point like you said) but for more efficiency hevier is better.

The use of having a more efficient but slower arrow is up for debate though. You could argue that it would be better for hunting cause it has more energy behind it, but like i said its still debated.

[Springbuck]

    A couple of years ago I built a very radical bow big 12" hooks that were set back 12" behind the back. This bow gave me about 115% SEPDF which was very high but the design could not be considered to be any kind of a standard or practical application.

Sorry to hijack ...but, Badger...

 I hate to trouble you, but do you have a picture or link or name of a thread for that bow?  I'd be very interested in seeing what you did, because my fastest bows to date have consistently been deflex recurves with string bridges, and what most consider too much curve for a wooden bow.

 Also, I recently broke a 60" R/D BITH bow before I got too far into it, but it had a big rounded deflexed middle, maybe 40% of the bow's length. Unstrung from handle to deepest deflex was 4" and the tips came back to rest almost 2" behind the handle.  Strung, it looked kinda like the Karpowicz "low stack" design, but much bigger middle. The string sat really parallel to the outer limbs and almost touched it for 25% each end.  That thing had INSANE tension on the string when braced.

So, I want to learn more about this.

[willie]

plus you can't know how much more will be lost do to slower limb return with a light arrow.

hmm...
but if you shoot increasingly lighter arrows, you can see with a chrono, when speed gains top out?

[gfugal]

plus you can't know how much more will be lost do to slower limb return with a light arrow.

hmm...
but if you shoot increasingly lighter arrows, you can see with a chrono, when speed gains top out?

No doubt that's right. Sorry what i was refering to was when your measuring a force curve on the way down, your going down so slow between meaurements that your going to get no information on the recovery speed or whether or not that energy will be available due to too slow recovery. This is because the rocevery speed can really only be identified in the way you described.

[Selfbowman]

Springbuck no worry about hijack here. I am guilty way to often myself. Arvin