FPS vs bow weight?

[StxHunter]

I know this can be a vague question, with lots of intricacies, but roughly how many fps per 5lbs of bow weight would you typically gain? Let’s say the same 10-12gpp arrow between 450-550gr shot out of 45lb, 50lb and 55lb bows. How many fps would you gain with every 5lb increase in bow weight?

[KentuckyWolf]

Depends on the style of bow and construction.

I can’t get to it right now but there was a really good section on this in one of the “bowyer bible” books

[Orion]

If you use the same gpp, arrow speed shouldn't change much at all as bow weight goes up, all other things being equal.  You will be shooting progressively heavier arrows at the same speed. I've read, but haven't tested, that a# of bow weight increase yields a 1-1 1/2 fps gain in speed. I don't know if those who offered that figure accounted for a change in arrow weight.   

[Kirkll]

All bows are not created equally, and you will have a lot of different answers to this question that will mostly be speculation. Draw weight and energy storage, vs how much energy actually gets transferred to the arrow totally depends on the bows limb design.

Even using the same identical bow design in different draw weights can make a big difference even if the GPP (grains per pound) are closely matched. I can give you a good example...

Take an ASL design bow with very low preload in the limbs. These bows draw very smooth in the first few inches of the draw and slowly gain pounds per inch throughout the draw cycle. These bows typically have very poor performance in lighter draw weights compared to higher preload limb designs like some recurves and hybrid long bows do....  But... once you start increasing the draw weight on these ASL long bow designs, the performance increases a lot due mostly to higher preload, or more tension on the string at brace.

One of the key components of a bows performance is not just how much energy is stored in the limbs, but how much of that energy is transferred to the arrow. Some bow designs are much higher in performance than others, but even high performance designs have different performance levels depending on draw weight. for example... a 30-40# high performance recurve or hybrid bow shooting 300 to 400 grain arrows. (10 GPP) drawing 28" will have significantly lower performance than that same exact limb design at 50-60# draw weight shooting 500-600 Grain arrows. The heavier limbs with heavier arrows may be 10-15 FPS faster in some cases.

Why?   I could write a full chapter in a book explaining the details, but the bottom line is the ratio of limb mass to preload is much different, which results in less energy being transferred to the arrow. or simply put... it's the lower draw weight limbs not having enough string tension at brace to stop the limbs clean and transfer the same amount of energy.

You can get into some other of the same factors with longer draw lengths vs shorter draw lengths and different limb designs too....   

So to answer your question, there is no rough formula with so many different factors involved.

Kirk

[LookMomNoSights]

I would get this one over to the Bowyers Bench forum......   great people over there talking about all that technical stuff and plenty of them REALLY know what they are talking about......... they can probably answer your question better than you expected.
 A certain poundage pulled may be the exact same from 1 bow to the other,  but geometry is just 1 of the sciences involved in a bow build and not all say 50lb draw bows are created equal,  bank on that!
 

[LookMomNoSights]

Kirk, we posted at the same time 
So case in point OP ...... listen to Kirk and the other great bowyers we have hanging around here ....... lucky to have them 

[Kirkll]

With this primarily being a hunting forum, i would say that feet per second or FPS is a trajectory factor that isn't that important to many hunters that keep their yardage inside of 15-20 yards.

The shot placement and penetration is vastly more important than how fast that arrow is traveling. This can be accomplished with lighter draw weights and heavier arrows with good sharp broadheads when that arrow hits the sweet spot...

With that being said... Shooting the heaviest bow you can comfortably draw will give you higher kinetic energy, which comes in handy when you hit ribs on larger animals. Having a higher performance bow has advantages that give you more FPS & more kinetic energy drawing less draw weight.  The difference between a 45 & 55 pound bow is significant on your average bow.

Where arrow speed and having a flatter trajectory has a significant advantage is shooting longer distances. A slight miss calculation on distance can make the difference of 2' low and a total miss, vs hitting 2" low.... A faster flatter shooting bow also has the advantage of being able to shoot though smaller openings accurately.

I personally like a higher performance , flat shooting bow. But keeping the arrow weight up to at least 9-10 gpp is important. A 10-12 gpp is even better.     Kirk