WHY are short bows slower?

[RyanY]

If shooting 10gpp at the same draw weight and draw length, best designed short bows won't shoot as fast as well designed longbows simply because they can't store as much energy. I think what happens is that, if you were able to get a short bow to store as much energy as its longer counterpart, the attributes which give that short bow more stored energy would also hinder its speed, most likely due to excess limb mass. I had a discussion on PP a while ago about bow mechanics and it seems that bow length is proportional to a certain arrow mass where if a longbow shoots a 10gpp arrow with a certain efficiency a shorter bow will shoot the 10gpp arrow with greater efficiency but less speed. Shooting the shorter bow with an arrow which brings down efficiency to the same as the longbow with 10gpp would raise speed to the same as the longbow with its arrow. Sorry if this explanation is confusing.

Bucksbouy, what you're saying about wood and speed is just wrong. This may be true if each wood is compared in one exact design but if each bow is designed to match the wood, all woods shoot equally well. Design is everything.

[Kegan]

If shooting 10gpp at the same draw weight and draw length, best designed short bows won't shoot as fast as well designed longbows simply because they can't store as much energy. I think what happens is that, if you were able to get a short bow to store as much energy as its longer counterpart, the attributes which give that short bow more stored energy would also hinder its speed, most likely due to excess limb mass. I had a discussion on PP a while ago about bow mechanics and it seems that bow length is proportional to a certain arrow mass where if a longbow shoots a 10gpp arrow with a certain efficiency a shorter bow will shoot the 10gpp arrow with greater efficiency but less speed. Shooting the shorter bow with an arrow which brings down efficiency to the same as the longbow with 10gpp would raise speed to the same as the longbow with its arrow. Sorry if this explanation is confusing.

All very true but not what I'm asking- or even arguing. I'm working under the assumption all that is understood and accepted. The shorter bow has a higher efficiency due to less mass. If you compensated for the lower stored energy by reflexing it why doesn't the short bow NOT outshoot the longer coutnerpart? Does the extra mass due to reflexing just cancel it out like it does in a longer bow or does the shorter bow lack leverage or is it something else entirely?

Is it all just about mass? If the short bow is reflexed enough to increase the stored energy does that extra mass now lower the efficiency down to longer bow levels? This wasn't something I accounted for, and I'm out of town so I don't have my TBB vol. 4 to review the Mass principal chart I'm asking an uneducated question... man I hate that.

[RyanY]

I believe that is the case. I just think shorter bows can't be designed to store as much energy and handle the same draw weight and draw length as longer bows with out being over encumbered by the extra mass needed to do so.

[bow-toxo]

No one has mentioned the arrows. They have to be shorter for a short bow, and of course thinner for the sams draw weight as a longbow.

I assume then,that you are not familiar with the Archery,of the West coast Native people and how they applied it to their shooting.

 No, I'm not. Enlighten me.

[bow-toxo]

I believe that is the case. I just think shorter bows can't be designed to store as much energy and handle the same draw weight and draw length as longer bows with out being over encumbered by the extra mass needed to do so.

   I don’t understand . The  energy stored by any bow would be the energy needed to pull it to full draw. How would that be different in two bows of equal draw weight but different lengths ? Also, would a lighter arrow be faster than a heavy one, at least on a bow of less than warbow draw weight ?

[RyanY]

bow-toxo, two bows of the same draw weight and draw length can store different amounts of energy depending on their design. Draw weight doesn't equal energy storage. This is why recurves can store much more energy than straight bows and have the potential to shoot faster. It has to do with how the string pulls on the limb. I'm sure we can all agree that short bows stack earlier in the draw than longbows and this is because a longer bow allows the string more mechanical advantage at higher draw lengths. FD curves show the difference in energy storage where a longbow may be a straight line to its desired draw weight the short bow would be a curve below that line. It still ends up at the same draw weight but the space between the lines is the difference in stored energy. This is hard to explain because I don't fully understand exactly how the mechanics of string angle affect stored energy and that all this would be easier explained with graphs. 

[Kegan]

I believe that is the case. I just think shorter bows can't be designed to store as much energy and handle the same draw weight and draw length as longer bows with out being over encumbered by the extra mass needed to do so.

That makes sense. My original example completely overlooks all the mass needed to get the 56" bow to hold severl inches of reflex. In which case the short bow would be function more like a longer bow: higher stored energy but with lower efficiency due to mass.

Thanks for the help

[Badger]

      Flight bows are very short bows, they tend to store good energy for their length but really capitalize on being more efficient. Generaly speaking what we make are hunting bows that shoot hunting weight arrows, 10 grains of arrow weight per pound of draw weight and a 28" draw are used as a standard for testing. Recurves around 60" long tend to be the fastest here give or take a bit and longbows at around 66" to 68" tend to be the faster ones. Designs are still actively being improved upon for the flight bows as no limit is put on how light an arrow can be, the length of an arrow they use is 23" so usually about 150 grains is about the lightest we can get them and still maintain enough stiffness to shoot straight. The lighter the arrow is the more the bow tends to rely on efficiency based on "virtual bow mass". Likewise if a bow were required to shoot extremely heavy arrows designs would get longer and bows would rely more on pure energy storage. Mark St Louis has become very good at redcucing outer limb mass in his recurves as is rewarded with the best of both worlds getting high energy storage as well as high efficiency. Longbows and R/D bows have gone more elyptical in the tiller effectively shortening the working limb and have gained a lot of performance here in recent years.

[Kegan]

Thanks Steve! How big a difference is there in speed between flight bows and "hunting" bows at 10 gpp?

[Eric Garza]

...FD curves show the difference in energy storage where a longbow may be a straight line to its desired draw weight the short bow would be a curve below that line. It still ends up at the same draw weight but the space between the lines is the difference in stored energy...

Not always.  Here is an FD curve i made of a short, 47 inch hickory bow that i finished last year and traded.  As you can see the FD line is a curve, but it curves above the line connecting brace height (0 pounds draw) to the final draw weight of about 59 pounds at 23 inches.  If you compared this line to the straight line connecting brace height to full draw it would show that you have stored more energy.  The bow was 3 cm wide at the handle and tapered to 1 cm nocks, if my memory serves me.  No Eiffel Tower shape to the limb tips.

http://www.uvm.edu/~egarza/47_Inch_Hickory_FD.jpg

[RyanY]

Eric Garza, I agree that my said example is not always the case but say your bow was drawn to a full 28". If a straight bow it would most likely take a large spike in the last few inches of draw where a longer bow would not, yielding more stored energy. Also, although it has a good FD curve, the curve shows nothing in the way of speed.

[Jesse]

I have also noticed that a short working limb on a longer bow seems to be a great combination of speed and shootability. Here is a picture of a fdc of Kens bow. Ken lives real close to me and has been helping me out with my bow building. As far as I know he made the fastest bow officially tested at WTT.  @ 199.7 fps 10 gpp @ 28" draw . There where a lot of bows real close in speed and some recurves were even right up there. I guess it comes down to personal preference. I seem to like the feel and shoot the best with a 64" to 68" longbow.

[Eric Garza]

Eric Garza, I agree that my said example is not always the case but say your bow was drawn to a full 28". If a straight bow it would most likely take a large spike in the last few inches of draw where a longer bow would not, yielding more stored energy. Also, although it has a good FD curve, the curve shows nothing in the way of speed.

The bow was originally straight, but took perhaps 3/4 an inch of set by the time it was shot in and I measured its FD curve. 

Do you mean to say that the short bow's FD curve would spike towards 28 inches and yield a sudden rise in stored energy?  I can't easily interpret what you're trying to say. 

And of course you're right, the FD curve says little about arrow speed.  It tells us how much energy the limbs store, but not necessarily how much they can deliver into the arrow upon release.  Release style and quality have a lot to do with that, as well as string weight and other things.

[RyanY]

Well say you built the bow to draw to 28". The FD curve would be shallow to a certain point and raise suddenly in the last few inches of draw to reach the full draw weight (this is what we know as stacking). This curve would be below a straight line curve produced from a longer bow thus storing less energy.

[crooketarrow]

   If mass is equal to the design stored engery in the limb the longer the limb the more stored engery. The more efectively the engerys put into the arrow =ing more speed. Not sure if that makes sence. Way to many vairables and WAY OT TECTIAL FOR ME I build selfbows.