question about arrow speed math,,?

[BowEd]

Wonder what the increase you would get pulling it to 28"?I've read 5# per inch but think it's a little more than that.I should tape an arrow off to find out I guess.

[bradsmith2010]

Beadman ,, the bow I used in the test was made by a friend ,, and it does shoot really nice for the draw weight,,I checked it a couple of times ( two days in a row)to makes sure I was getting a good reading,, then I got interested in being able to calculate speed of different arrows, to compare with different bows,,, anyway,, thanks for all the informative input,, that particular bow seems to be light in mass for the draw weight and is tillered very well,,, I will have to get a scale to weigh it ,,,, as far as pulling it to 28,, increased set might offset increased performance ,,,, I think it is at the "sweet spot" now,,
there is a chart I think in Volume 4,, where tim increased the draw until the set offsets the performance,, very interesting chart and informative,, I have made some sinew bows that shot hard say at 25 inches,, and tried to increase the draw,, the set made them shoot about the same fps,,even though the power stroke was longer,, I went past the sweet spot on some,,

[DC]

The way I figure it, it's the draw weight divided by the power stroke length. Most bows are more or less linear and ignoring the set that Brad mentioned. If you have a 26" draw and brace height is 6" then the power stroke is 20". A 40# bow will be 2# per inch, an 80# bow will be 4# per inch. This is just ball park but they can't all be 5#, the arithmetic(I don't do math ) doesn't work out.

[bradsmith2010]

your right DC,, sometimes they start out at 2# per inch, and as you get closer to full draw it may go up per inch,, or "Stack",,
also don't forget, on the 80# bow, the draw weight starts out higher,, so it can go up 3# per inch of draw and the math will work,,

[Badger]

The way I figure it, it's the draw weight divided by the power stroke length. Most bows are more or less linear and ignoring the set that Brad mentioned. If you have a 26" draw and brace height is 6" then the power stroke is 20". A 40# bow will be 2# per inch, an 80# bow will be 4# per inch. This is just ball park but they can't all be 5#, the arithmetic(I don't do math ) doesn't work out.

  As a ballpark that will get you pretty close but it will tend to run higher than actual stored energy. Really hard to ballpark a force draw becuase that is one of those things where precision is a little more important if you are trying to figure efficiency and all the other figures we play with some time. Even careful measurements don't usually work out all that accurate without some kind of machine that will stop and weigh at precise increments. Using the linear method you will get about 105% of peek draw weight to stored energy. In reality most self bows are closer to about 90% of peek draw weight at 28" draw. 72" ELBs with an arc of the circle tiller will come out pretty close to linear.

[DC]

Mostly I was just trying to point out that there is not one weight per inch for all bows. I've seen people mention numbers that seem to indicate that there is a "one number fits all". Recurves with fat FD's and stacking bows with thin FD's will change everything but not that much. IMHO. Which isn't worth that much

[Badger]

Mostly I was just trying to point out that there is not one weight per inch for all bows. I've seen people mention numbers that seem to indicate that there is a "one number fits all". Recurves with fat FD's and stacking bows with thin FD's will change everything but not that much. IMHO. Which isn't worth that much

   You are correct, all bows are very different, even bows with similar profiles.

[BowEd]

Yea don't remember what I was thinking there.To say 5# per inch is way wrong.A 47# bow would'nt gain that at all.From 26" to 28" depending on length and profile of bow it should'nt gain much more that 4 to 5 pounds.
Brad....I believe your right in your thinking about that sweet spot/set etc.,but that's why tillering means so much for extra draw length and good profile.At a 7" brace[outside of handle] a 21" power stroke with same profile set back bow will always shoot faster than a 19" power stroke.

[bradsmith2010]

unless you have to increase mass to hold that profile at the longer draw,, right?

[Badger]

 Brad, I have increased my mass quite a bit over the years to hold profiles better. Most of my 50# self bows weight about 20 oz now, if they have a couple inches of reflex smetimes 21 oz. Years ago I made them between 16 and 18 oz.

[bradsmith2010]

thanks Steve, thats very informative,