Jerk strain

[stuckinthemud]

Sorry about the title but that's what it's called,  the strain imposed on a bow, the nocks and the string when the string passes its brace height after the bow has been fired. I saw a video last night where the host suggested a rule of thumb whereby this strain is 5 times draw weight, so a 90lb bow places a 450lb stress on its string.  Now, this was on a fg crossbow 36 inch ntn.  I'm guessing a similar strain is put on every kind of bow, my question is, how do different materials and types of string affect this.  Continuous loop linen with no twist should stop quicker than a nylon string with Flemish twist ? Does this stress the string more or less?

[Del the cat]

There is AFIK a huge difference between the stain on the string and on the bow.
You get a huge strain on a string at brace and at the end of the power stroke, but it's virtually all in compression on the bow, there is little bending force, so it doesn't hurt the bow, unless the nocks are poor and it splits one out!
There is some weird geometry going on that is beyond my pay grade, all about angles etc.
Del

[mmattockx]

I saw a video last night where the host suggested a rule of thumb whereby this strain is 5 times draw weight, so a 90lb bow places a 450lb stress on its string. 

The impact load on the string is very dependent on the bow design, arrow weight, string stiffness (and weight) and limb dynamics. I am not sure any rule of thumb is going to be much of a useful thing. In general, the stiffer the string, the higher the load. You then have to determine how the different strings rate in terms of their stiffness.


Mark

[bownarra]

Put a dynometer on your string

[Dances with squirrels]

"I saw a video last night where the host suggested a rule of thumb whereby this strain is 5 times draw weight, so a 90lb bow places a 450lb stress on its string."

That sounds like a lot at first glance, but the strain on each limb tip at resting/static brace is twice what it is at full draw, so when it slams home at the shot, I don't know how much it is exactly, but I wouldn't bet against the statement above.

[George Tsoukalas]

Pretty established that a string should be 5 times bow weight. Rediscovering what is well known can be a bit tedious and keep one from making shavings which is how you learn.Jawge