Speculation please

[bradsmith2010]

Sleek that was so confusing,,

[SLIMBOB]

Let me translate...........................no idea.

[sleek]

The string tension is higher at brace because you have a very low string angle. As the string angle increases, leverage increases against the tip. As leverage increases, the required force to do work decreases, which is why the string tension drops. Its literally stacking in reverse.

Basically, think of it like this. How hard do you have to push straight down on a bows tips to get it to bend an inch? If the bow is perfectly straight, you can't push down hard enough, its impossible.  Put a little bend in it, ( changing the angle of force on the tips aka string angle ) and it becomes possible to bend it that one inch. The more bend you put in the bow, the easier it becomes to bend it an inch more. This is because your angle of applied force gives you leverage. The angle of applied force is why a braced bow has more string tension than a drawn bow.

So, why doesn't a bow become easier to bend as you draw it then? That would be because of the bend in the string at the arrow nock point. As the string comes back, that angle becomes less, and when combined with your tip string angle it decreases your overall leverage. The combined angle of your string at the tips and the arrow nock, added to the load of the bow, gives you your final draw weight.

[Badger]

That's a pretty good explanation sleek

[DC]

I've been wondering about how to roughly calculate the strings mechanical advantage. I looked at bunch of websites and got a lot of formulae that I didn't understand. I wanted something like figuring the MA of a lever or gear, something I can get my head around. Something in Sleeks post triggered a thought. If you use the limb length as one arm of the lever and the distance from the centre of the handle to the string as the other arm it seems to work. As the string gets closer to the bow the MA starts to get close to infinite and vice versa. Thanks Sleek, that gets me closer to understanding how these things work.

[avcase]

I don’t believe higher string tension is a useful indicator of performance. 

At very low brace with a longer string, the bow limb has a lot of leverage on the string which will drive a very high string tension. But ignore you look at it from the string’s perspective, it has very poor leverage on the limb. This becomes important when shooting the bow. If the string doesn’t have sufficient leverage on the bow limbs, then the string will just stretch at the end of the shot, and it will not be able to effectively transfer and slow the forward momentum of the limbs to the arrow. Efficiency will be reduced, and the limbs will have a lot of post shot vibration.

Raising the brace height improves the leverage the string has over the limb, but reduces the power stroke, or amount of energy that can be stored in the draw.  There is a happy medium where the bow has the best combination of efficiency and stored energy, and that depends on the properties of the string and design of the bow.

I was chronograph testing a new set of bow limbs over the weekend and had some interesting results that are related to this discussion. I had two strings. One was longer and the brace height was just under 6” and had very high tension at brace. The second string was shorter for a brace height of just over 7” and had lower tension at brace. When I shot a very heavy arrow, the lower braced configuration was faster. But with a lighter 3 grains per pound arrow (this is for flight shooting), the higher brace height string (with lower tension at brace) produced faster arrow speeds. And much lower post-shot vibration too.

Alan

[avcase]

I've been wondering about how to roughly calculate the strings mechanical advantage. I looked at bunch of websites and got a lot of formulae that I didn't understand. I wanted something like figuring the MA of a lever or gear, something I can get my head around. Something in Sleeks post triggered a thought. If you use the limb length as one arm of the lever and the distance from the centre of the handle to the string as the other arm it seems to work. As the string gets closer to the bow the MA starts to get close to infinite and vice versa. Thanks Sleek, that gets me closer to understanding how these things work.

The string mechanical advantage is the ratio of the horizontal brace height to the neutral plane at the bow center, to vertical lever arm from the center of the bow to the string contact point. I hope that makes sense.

Alan

[DC]

Perfect sense, it is a simple machine after all

[bradsmith2010]

ok I can understand that, trial and error to find best brace,,