Deadeye101,

What I was attempting to point out in my last post is that I believe your 260 ft/sec speed for your Razortec is quite reasonable. I’ll have to disagree with Buck Magnet a bit. Here’s why:

Given IBO conditions (350gr;70lb,30”) the Razortec is rated at 310 ft/sec. That translates to a kinetic energy of 74.7 ft lbs. A very reasonable estimation of efficiency during the launch for this arrow mass is 78%. That’s about 100gr of virtual mass and again reasonable. This translates to 95.8 ft lbs of stored energy for the initial IBO condition.

Now the question is, how much stored energy will a Razortec at 64 lbs and 25” draw contain? A good way to estimate it is to scale down the force draw curve proportionally. The Razortec has a 7 ¼” brace height so its power stroke for a 30" draw is 21”. The maximum energy a 21” wide by 70lb peak force draw curve can contain is 122.5 ft lbs. but given the initial conditions of 95.8 ft. lbs. this means that this shape draw curve contains 95.8/122.5 or 78.2% of maximum. So now if you take 78.2% of the max for a 16” wide by 64lb draw curve it turns out to be 66.7 ft. lbs of stored energy in your bow for your conditions.

Finally, how fast will 66.7 ft. lbs. of stored energy launch an arrow. Well if the arrow weighs 350 grains and the bow is equal in efficiency to our initial conditions (i.e. 78%) it will launch the arrow at 258.7 ft/sec. That’s’ pretty close to what you said. Now if we put a 320 grain arrow on this bow and use the same virtual mass it calculates to 77% efficient and a speed of 266.3 ft/sec.