Bow Performance 55 @ 28 vs 55 @ 30

[Ragnarok Forge]

I am wondering what the difference in speed is for a two identical design bows with one bow that is 55 lbs @ 28 inches and another that shoots 55 lbs @  30 inch draw.  

Does anyone know if there is a general number for increase in arrow speed per extra inch of draw length?  I did a search and could not find anything covering this one.  I know I have read in posts somewhere that the longer draw length makes the 55 lb @ 30 inch bow the equivalent of a higher poundage bow drawn to 28 inches.

[Christov84]

For speed tests they use a 30 inch draw usually. I always thought it was to make the bow 'look' faster.

In addition does anyone know if limb length changes the speed of a bow?

[ishoot4thrills]

I know with wheelie bows it runs about 10 fps per inch of draw length.

With trad bows I think it runs about half that or about 5 fps per inch of draw.

[hybridbow hunter]

For the same grain per pound arrow, speed will increase about 3 fps for each inch of draw over 28". Thus from 28" to 30" gain is about 6 FPS.
While Dynamic efficiency remains almost identical, the store energy per per pound of draw force will increase about 3 to 4% for each inch over 28" , that's how longer draw length will increase arrow speed.

[atatarpm]

Longer limbs shorter risers equil faster bows.

[Friend]

Stu's calc doesn't reflect effects of power stroke length on velocity while all other factors remain constant. He is aware of it and would be a good source to address your question.

If you do follow-up with Stu, then please post you feedback.

[John Havard]

With precisely the same grains per pound of arrow weight the difference in two inches of draw is very close to 10 fps for almost every bow I have ever tested (5 fps per inch of draw).

[guk]

ok with that being said how much draw weight is need to add the same 10fps

[swampthing]

Take a look at the difference between 26-27" to 28 "

[John Havard]

guk, it depends on the bow and how efficiently it stores energy and how efficiently it delivers that stored energy.  

swampthing,  for a while I tessted all bows at 26", 28", and 30" AMO (all with 9 grain per pound arrows at their respective draw lengths and corresponding draw weights).  The difference from 26" to 27" and from 27" to 28" is the same - right at 5 fps per inch of draw.

[Ground Hunter]

Draw weight is but one measure of a bow. As John says, bow design matters more.  A 45 lb self bow is not a 45 lb ACS.  Over generalizations are misleading and unproductive.  There may be some patterns, but specifics will vary greatly by design, materials, etc. etc...

[ishoot4thrills]

Originally posted by Ground Hunter:
Draw weight is but one measure of a bow. As John says, bow design matters more.  A 45 lb self bow is not a 45 lb ACS.  Over generalizations are misleading and unproductive.  There may be some patterns, but specifics will vary greatly by design, materials, etc. etc...

Correct.

But this discussion is more about draw length and it's relation to arrow speed gained or lost as draw length is increased or decreased, not as much about draw weight comparing from bow to bow.

[Ground Hunter]

[Ground Hunter]

[Ground Hunter]

 

Originally posted by guk:
[qb] ok with that being said how much draw weight is need to add the same 10fps [/b]

Seems to me to be about draw weight - and to that I added my comment.  I hope that's ok with you?

[pdk25]

OK, I am a little confused why we are referring to gpp when the bows are both supposed to be 55#.  Is this what you are saying, John , that say a 55# acs @ 28" will be 10fps slower than an acs that is 55# @ 30"?

And what, roughly, would the increase in speed be for an acs that is 55#@28" drawn at 28" versus 30", assuming the same total arrow weight, but adjusted for.proper tuning.

[lpcjon2]

So how would the 2 inches of arrow(for the 30" draw)compare for weight and drag to the 28" arrow draw.

[ishoot4thrills]

Originally posted by Ground Hunter:
     

Originally posted by guk:
[qb] ok with that being said how much draw weight is need to add the same 10fps [/b]

Seems to me to be about draw weight - and to that I added my comment.  I hope that's ok with you? [/b]

Sorry dude, I didn't notice the comment about the draw weight. Nothing personal intended. My bad.    

I was referring to the original post, which is what this topic was originally intended to be about and I didn't think anybody was overgeneralizing that.

[ChuckC]

John,  I think there is an apples / oranges thing going on here.  

By using the same bow, you are also increasing horsepower with each inch of draw.  

I believe the question compares different bows, one 50 @ 28  one 50 @ 29  one 50 @ 30, all else being equal.  

There is all sorts of talk of power stroke length, but I am questioning all that myself.  I think it all depends on a bows design and quickness to recover.  Heck,  build a 95 foot tall bow and using that logic with a 90 foot power stroke that arrow might hit light speed !

Anyone that is actually smart (knows about this stuff, instead of my feeble guesses) got any answers based on testing and not theory ?
ChuckC

[Christov84]

Originally posted by John Havard:
With precisely the same grains per pound of arrow weight the difference in two inches of draw is very close to 10 fps for almost every bow I have ever tested (5 fps per inch of draw).

In reference to Clay's original question, is this roughly applicable to 2 bows, one 55@28 and the other 55@30, all else being equal?

I think someone mentioned power stroke earlier on and this term sounds like it fits the question at hand.

Thanks for the input guys!