Question about cast.....

[slayer1]

I currently have a 56 lb Dwyer longbow. I just ordered another 65lb Dwyer made the same way just at a higher poundage. I want to set the bows up to cast the same. Where do I start. I shoot a 505 grain carbon out of the 56lb bow(9 grains per lb). If I do the same with the 65 lb(shoot an arrow at 9 grains per lb) bow will the cast be the same?

[vermonster13]

It should be similar so long as you draw it to the same anchor. Though wood bows can vary because no two pieces of wood are ever the same.

[Orion]

Think you'll need to go heavier than 9 grains per pound with the heavier bow, though don't know how much.  We talk about the draw weight/arrow weight curve/graph line as if it's perfectly linear, but it isn't.  You'll have to experiment.  Good luck.

[slayer1]

Thanks for the replys. There should be a mathmatical equation that could help determine a starting point. We know the distance, speed, draw weight of the bow and the weight of the arrow. Seems to me that if you could determine the affect of gravity at a given distance, say 20 yards, you could get close to the arrow weight needed for the higher poundage bow. Any physics guys around? Come on I know there are some smart guys around here.

[Dr. Ed Ashby]

Slayer1, the efect of gravity (the downward acceleration, or rate at which an object falls) is a constant, regardless of object 'weight'. It is known as the 'gravitational constant', and is a rate of acceleration of 32.174 feet per second per second.

Regardless of arrow mass weight, arrows will fall (drop) at the same rate, hitting the ground at the same TIME after launch. What may differ is the distance traveled in the TIME PERIOD required for the arrow to fall to the ground.

If of equal external dimensions and with equally good flight, at the same launch velocity a heavier arrow will retain more of its absorbed energy downrange. In other words it slows down slower. AT EQUAL LAUNCH VELOCITY the heavier arrow will be going faster than the slower one as you get farther downrange, and will travel a greater distance before falling to the ground.

The easiest way for you to get equal cast between the two bows will be by trial and error. Start at the same grains of arrow mass per pound of draw force. If bow efficiency is equal between the two bows you will have a starting point that is close, but it's likely you will need to adjust the heavier bow's arrow mass upwards (more grains of arrow mass per pound of draw force) to achieve essentually equal trajectory across hunting ranges.

High efficiency bows generally show more gain at higher arrow mass than do bows with slow limbs and a lower efficiency. That's one reason I do much of my testing with older-style straight end longbows. There is little difference in trajectory across a wide range of arrow mass weights. That's a great asset when testing a wide verity of differing arrow setups. Makes it easier to hit where I need to.

Hope that helps some.

Ed

[DarkeGreen]

If bow #1 shot its arrows at 180 fps it would take about .33 of a second to cover 20 yards. If bow #2 shot 190 fps (10 fps gain which you likely will not get) it would take .32 seconds to cover the same distance. At 10 yards the difference is .003 seconds. In theory if you used the same arrow and ignored a bunch of other stuff you could find the difference by taking .01 second or .003 second from the number Dr. Ashby provided and find the answer. Human error should provide a much greater difference in point of impact than the 3 or 4 fps you might gain from the heavier bow.

[Bard1]

wow, I feel smarter after reading this thread.  

[slayer1]

Thanks again for the input. I will let you guys know how it turns out. I forgot to mention the reason I was doing this in the first place. I really am not concerned with increased arrow speed from the higher poundage bow. What I am really after is the KE gained (at the same cast of the lighter poundage bow) for the higher poundage bow.

[DarkeGreen]

I found some information that shows modern longbow of high end design could gain as much as 13.5 fps when moving from 56 lbs to 65 lbs. Using an arrow that is heavy enough to offset that difference in speed would give you the type of gain you are looking for. The question is whether or not it is enough. Your bow materials will likely give you a gain a little smaller than the example above. You could go to a force curve chart to see how much heavier an arrow you need to select once you now the true speed increase you gain with your new bow.

[slayer1]

Darkegreen should I shoot the 505 grain arrow out of the 65lb bow to check the fps first, then do the calculation? to see how much weight should be added? Just so I make sure I understand what you are saying: If my fps is 172 with the 56, then my 65 should be 185.2 using the same arrow? Is that correct? The bows will be identical except for the poundage. I know they will still have differences(because they are made from different woods), I guess I will have to wait and see.

[slayer1]

[DarkeGreen]

That's the way I would do it but I'm a rookie.