Questions for the math geniuses here

DaGriz · Post by **DaGriz** » Sun Mar 30, 2008 7:54 pm

I have a CB that shoots a given weight arrow from 180lb draw weight with a 13&3/8" power stroke at 300fps

If I increase the draw length/power stroke by 1 inch how much faster will it shoot the same arrow and what would the draw weight increase be??

I have searched for the formula to calculate this but can't find them any place and I am extremely out of practice on physics.

DanO · Post by **DanO** » Sun Mar 30, 2008 8:16 pm

I sent this to a physics teacher friend to see if he knows how to calculate the answer. I'm a biology guy so I have no clue on this one.

Dan O.

xbowking · Post by **xbowking** » Sun Mar 30, 2008 9:14 pm

Hi , i figure your draw weight will increase 20-25 lbs & your speed will go up about 20 fps.

ComfyBear · Post by **ComfyBear** » Sun Mar 30, 2008 9:16 pm

Griz, from the specs. you're describing I'm thinking that your CB is a compound, but I don't want to assume.
Whether it's a compound or a recurve more information is needed in order to determine and arrive at the answers you're seeking.

For instance, type/ shape of cams, string material used, and so on. If it's a recurve, then width, length and thickness and well as material used for the limbs would have to known.

In other words, there's no simple answer.

xbowking · Post by **xbowking** » Sun Mar 30, 2008 9:21 pm

Is this xbow an Excalibur ?

Stash · Post by **Stash** » Sun Mar 30, 2008 9:24 pm

No way to tell without tons more information.

If it's a compound you have to figure out the type of cam (for example, no letoff, 50% letoff?) and the draw-force curve. Maybe you CAN'T get another 1" of draw out of it.

If it's a recurve, the limbs may "stack" past the 13 3/8", and you might get extra stored energy (draw weight), but less efficiency. The limbs may end up bending to parallel.

If you need to know exactly, you have to rig up some sort of draw board with an accurate scale, and take the bow off the crossbow so you can draw it the extra inch to weigh it.

Probably no way to test the speed without making a whole new stock/rail.

Assuming it is an Excalibur, though, and based on an Exocet200 being 1" shorter than an Exomax, you can figure that the extra 1" of power stroke would give you approximately 25# and about 25 fps more.

xbowking · Post by **xbowking** » Sun Mar 30, 2008 9:27 pm

Hey Griz your post has a bug in it , LOL

groundpounder · Post by **groundpounder** » Sun Mar 30, 2008 10:02 pm

With relating it to Excaliburs Exocet versus Exomax the only difference being between the two xbows is one inch of power stroke. The Exocet has a pull of 200# and is shooting 330ft/sec with a 15.5"powerstroke. While the Exomax is a 225# xbow shooting 355ft/sec and a 16.5" powerstroke. That will give you a ball park guess but if you were to graph out the increases your graph would not be linear. So only a ballpark guess will be all you can get with the info given.

DaGriz · Post by **DaGriz** » Sun Mar 30, 2008 10:06 pm

My bow is't a compound It a 10 Point Curve. The draw weight and power stroke are standard. I am thinking of extending the length from the front-end to the stcok by 1 inch which wil give me an inch longer power stroke. I'm trying to calculate what the gain in FPS and draw weight wil be.

groundpounder · Post by **groundpounder** » Sun Mar 30, 2008 10:12 pm

How's their warranty when it comes to splitting limbs? Don't know that I would try it. It was engeneered and produced the way it is for a reason. I like tweaking with some things but some areas I know not to mess with.

Stash · Post by **Stash** » Sun Mar 30, 2008 10:42 pm

Right.

You might find if you push the bow forward, the limbs have to come back an extra inch and they might be overstressed, or even bend past the point where the string might slip off.

I don't want to put words in his mouth and I may be wrong, but I seem to recall Bill T mentioning that he wasn't interested in making a bow stronger than 225# by lengthening the rail any more, for that very reason. He would have to make the limbs wider.

saxman · Post by **saxman** » Mon Mar 31, 2008 12:23 am

I'm fair at math.
The add on's and take away's aint to bad but those gazzinta's really mess me up.You know-4 Gazzinta 8 and 5 Gazzinta 15.I'm not good at those

ehntr · Post by **ehntr** » Mon Mar 31, 2008 1:17 am

Your getting approx 13.5 lbs for every inch of draw length (180/13.25). Providing that the bow has a consistent power curve...i.e., it doesn't start stacking up as draw length is increased.... you should get about 193.5 lbs. There is no formula that will give you a precise answer as the dynamic of the limbs are not constant across its draw length. Don't know how much more speed but I wouldn't be suprised if it was in the vicinity of 15+ fps..................Thats how I see it...

Stash · Post by **Stash** » Mon Mar 31, 2008 4:02 pm

ehntr wrote:Your getting approx 13.5 lbs for every inch of draw length (180/13.25). Providing that the bow has a consistent power curve...i.e., it doesn't start stacking up as draw length is increased

Unfortunately, a recurve crossbow doesn't have a consistent power curve. The recurve on a crossbow is very slight compared to that on a regular, well-designed recurve bow, and the shortness of the crossbow limbs is such that your draw-force curve is nowhere near a straight line - it starts to curve up very quickly.

So while you might get and AVERAGE of 13.5#/in over the course of the draw on your bow, the first few inches would be lower, and the last few inches would be much higher than 13.5#/in. Probably on the order of 20-25#/in, as it is on the Excalibuir recurves.

I have a draw board and scale here at work, and if I get a chance I'll do some measurements. Unfortunately, the scale only reads to 100#, but I'll see what I can measure and report back.

DaGriz · Post by **DaGriz** » Mon Mar 31, 2008 4:17 pm

I appreciate the info from everyone. I've been doing some research on the limbs and also a bit of experimenting. I can draw the string back over an inch farther than is needed to lock in the trigger mechanism so I don't forsee a problem by adding 1" to the draw length/power stroke.
I believe the linmbs will handle the extra inch with no problem. I would like to have an idea how much more draw weight that would generate. I can find the FPS by shooting through my chronograph.

I'm hoping the 1" longer power stroke will allow me to shoot heavier arrows in 305 - 315fps range
I've modified some XX75 2219 arrows by adding a 110gr brass insert. They should hit quite hard and provide excellent penetration on large critters like moose and elk, Which is what I am striving towards.

The other questions that can only be answered by shooting is how much will virbation and noise increase.

I could easier go to a 200lb Excal CB but it's more fun "tinkering" and trying to "improve" things.

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