String tension

[Crooked Stic]

String tension is higher at a lower brace height. Why is that?
And if you if you had a bow strung low string just tight would the tension be higher also.
In a discussion on another forum-----

[mmattockx]

String tension is higher at a lower brace height. Why is that?
And if you if you had a bow strung low string just tight would the tension be higher also.
In a discussion on another forum-----

As you lower the brace height the string angle to the limb gets smaller and smaller, which means more of the string tension is pulling straight down the limb and less is acting to bend the limb. The leverage the string has for bending the limb is just getting smaller and smaller. If you take this to its logical conclusion, with a brace height of 0" all the string force will go straight down the limb and none will bend the limb.

I'm not sure I understand the second part, are you talking about having a very small brace height, like 1/2"? If that is correct then, yes, the string tension will just continue to go up as you lower brace height. At some point the leverage will get so bad that the string won't be strong enough to hold brace and will fail due to the tension load on it.


Mark

[Crooked Stic]

Thanks

[Kirkll]

I cant explain the physics of this, or relate it to string angle at all, but every limb design you build has a different shape and a different amount of pre load on the limbs at brace height. The limbs also have a different shape at brace height. Meaning some have more bend to the working portion on the limb at brace than others.

As you lengthen the string the string, and the limbs start straightening out, the tension increases until that working portion is straight. At this point, it starts going the other direction. Once you reach that point a limb pad angle adjustment can get that slight bend to it again and retain your pre load without loosing performance.

There is a sweet spot at a point right before that limb straightens out completely you want to find. The funny thing is.....A completely straight limb will give you a higher string tension, but....It  will not stop the forward momentum of the limb mass as well a slight bend in that limb at brace.  It's called "limb bulge".

In high speed video you can see the limb bulge issues with a straighter limb at brace even with higher string tension, and you start seeing a drop in arrow speed too because more energy is staying in the limb and not being transferred to the arrow.  You will also feel a buzz in the grip when this occurs, and a slight increase in  noise too.

Bottom line is highest string tension you can get lengthening a string, doesn't necessarily equate to higher performance.... Its a balancing act...    Kirk

[Crooked Stic]

I have another guy on FB told me I was very wrong by saying string tension increases with lower brace hieght. I asked if he had ever checked it. That's when the bush beating started.

[Kirkll]

I have another guy on FB told me I was very wrong by saying string tension increases with lower brace hieght. I asked if he had ever checked it. That's when the bush beating started.

There a lot of guys out there blowing smoke about things they know nothing about, only what they've heard.

I have to admit though... It does kind of boggle the common sense looking at it.

More bend = higher limb tension... Right?

More bend = higher brace height = higher tension on the string.... Right? ......  WRONG!!!

Yes you have more limb tension on the limb, but you actually have less string tension as those limbs bend further.

But once you have used an inline scale and actually measured the difference at different brace heights and different amounts of bend in the limbs by changing the limb pad angles and string lengths, you'll see how much difference it makes.

You want to go deeper with these tests?... Try building two sets of limbs exactly the same draw weight for the same riser. One has medium length limbs, and the other has long limbs. Everything else is identical except longer wedges and 2 more inches in total length....  Now take these two bows and do a DFC chart on them, measure the pre load with a scale,  and test them at different draw lengths on a shooting machine.

Well i'm not going into test results, but the differences in these two exact draw weight limbs , coming out of the same form, and tested on the same riser will surprise you.....   Nuff said.