Brace Height and Arrow Speed

[lonbow]

When I was in Dresden, I saw some ottoman bows that were braced with their original strings. The brace heights were huge compared to modern standards. There must be a reason for it.


So I thought about the pros:

- You can use a greater variety of different arrows (spines), without having tuning problems.
- Ottoman bows have a lot of reflex, which means they can be unstable and twist, if not treated in the right way. A big brace height adds some stability.
- Bows with big brace heights are more accurate.
- The bowstrings of the Dresden bows are really short, which makes them lighter. ->More arrow speed?
- Hand shock is the result of foreward movement of the bow limbs being stoped by the string. With more brace height, the bow limbs don't move so much in the same direction of the arrow. Instead they move more outwards. There should be less handshock then. Has anyone tested this?

The only cons are a higher strain of the material and smaller amount of stored energy. The last point seems to be neglegtable according to badger.

What do you think?

lonbow

[Del the cat]

When I was in Dresden, I saw some ottoman bows that were braced with their original strings. The brace heights were huge compared to modern standards. There must be a reason for it.


So I thought about the pros:

- You can use a greater variety of different arrows (spines), without having tuning problems.
- Ottoman bows have a lot of reflex, which means they can be unstable and twist, if not treated in the right way. A big brace height adds some stability.
- Bows with big brace heights are more accurate.
- The bowstrings of the Dresden bows are really short, which makes them lighter. ->More arrow speed?
- Hand shock is the result of foreward movement of the bow limbs being stoped by the string. With more brace height, the bow limbs don't move so much in the same direction of the arrow. Instead they move more outwards. There should be less handshock then. Has anyone tested this?

The only cons are a higher strain of the material and smaller amount of stored energy. The last point seems to be neglegtable according to badger.

What do you think?

lonbow

The reason is the material they are made of and the design.
Horn and sinew are much more elastic than wood. They need a huge deflection to get the best out of them, hence the vast reflex and high brace.
Del

[jeffp51]

That armament museum in Dresden is awesome. I have some pictures of that bow somewhere that I need to post.

[DC]

DC, have you measured string tension on your bows? 
I never did but have heard from glass builders that string tension is close to double draw weight in some designs.  I thought your bows would have higher string tension.

Well surprise surprise! I have two spring scales. One 50 # and one 77# It pinned them both. I was using my fastest bow https://www.primitivearcher.com/smf/index.php/topic,64730.0.html
42#@28" and on the 77# scale it went past the 77 and right out to the end of travel of the scale so it's more than double the draw weight. I could only get it to a low brace so that would make the reading higher and the string was still laying on the belly quite a bit. Not sure what that would do. I have my digital scale that goes to 300 but I'm not sure if I could push the button and get it braced before it timed out. I'll probably give it a try. The spring scales have about 2" of spring travel so you have to pull it way past brace height to get it braced. The digital has a load cell(think that's what it's called) in it so it stays a constant length. That will help.
Anyway I am very surprised. Can this be tied directly to early draw weight? Is this high string tension the reason this bow is fast? Or am I doing something wrong to get such high numbers?

[Badger]

  The biggest problem I have with skinny strings on flight bows is that they break at brace especially if I have a low brace. Never too much unless it starts twisting your bow in half like it does on bows with long straight outer limbs with about 10" of near string contact with the limb.

[DC]

I managed with the digital. It was a bear to get it braced for some reason. Anyway I got two readings at 4" BH the string tension was 100# and at 8" it was 90#. I guees what you heard was right Ben. Still surprised me but the last time I did this was around when I started six years ago. That time I got 45# or so on a 40# bow. Guess I'm makin' them better

[Bayou Ben]

Nice Don.  I would say you are much better at making bows than you were when you started.  Not many people in the world can make wooden bows hit the numbers you are getting. 

I've seen them setup with a turnbuckle connected to the scale so that it's easy to adjust for different brace heights.
 
The guys that are recording string tension report that with only a few exceptions high string tension bows are good performers.  There's obviously more than just string tension, but if I ever get more into chasing speed, I surely will be recording it. 

[Bayou Ben]

 Can this be tied directly to early draw weight?  Yep, I would say so

 Is this high string tension the reason this bow is fast? One of the reasons 

 Or am I doing something wrong to get such high numbers? I doubt it, that's consistent with the numbers that they report. 

 

[bradsmith2010]

wow

[DC]

I had a turnbuckle in there but it was a cheapy and with that much strain on it I couldn't move it. If I decide to make this a regular thing I would get a better one. I doubt that it would be a regular thing though because the chrono is right there, it's easier to use and I think it would tell me the same thing. The string tension would tell me that this bow may be fast but the chrono would verify it. I can't see a way of using the string tension as a tool.

"The guys that are recording string tension report that with only a few exceptions high string tension bows are good performers."
It would be interesting to know what the exceptions were

[DC]

wow

Yeah, those were my sentiments exactly

[Bayou Ben]

"The guys that are recording string tension report that with only a few exceptions high string tension bows are good performers."
It would be interesting to know what the exceptions were

Too much mass is the 1st thing that comes to mind. 

The times when string tension and speed line up, yes, I agree it wouldn't be that helpful.  But it would be nice to understand those exceptions like we mentioned; you got good speed but string tension wasn't all that high, and vice versa. 
The string tension tells you if you are storing good energy, and the chrono tells you if you are transmitting it to the arrow efficiency.  Not necessarily the same.

[DC]

Good one. I'll think about that.

[avcase]

The effect of brace height on bow performance is pretty interesting. It is true that a bow will give the most energy storage for a lower brace height. How much does depend on the bow design, and it can be pretty significant. But the reason many bows quiet down at a higher brace height and do not lose much, if any speed, is because the efficiency of the higher braced bow also improves. If you could track how much limb mass is moving for each inch of draw, you would find that there is more limb mass moving farther for each inch of draw for the first several inches of draw for a low brace height, compared to a bow at higher brace. You can think of it like two cars in a race. One has a more powerful engine, but can’t shift past 3rd gear, the other car has a less powerful engine, but can keep shifting to a higher gear.

There comes a point where higher and higher brace height does no good. For natural material bows, the higher braced bow is being held under higher bending stresses with no relief.

Alan

[avcase]

You can quickly test how string changes as the bow is drawn by plucking the bow string while drawing it and hearing how the pitch goes down. Lower pitch equals lower tension. This leads to some others interesting phenomena.

How about this, not only should the string push arrow all the way from full draw to the brace height of the bow, the string actually pushes on the arrow a little beyond brace height when shot!  How much depends on how light the arrow is, the brace height of the bow, and the elasticity of the string.

Alan