Curious if anyone knows what the velocity increase is per lb of bow weight with the new high performance cams and also the velocity gain per inch of draw weight with the modern high tech cams and the bow being in the 70 lb range?
Take care
Dave

The general number is 2 feet/second for each additional pound of draw weight.

Sorry Dr. C but your info is incorrect. Modern bows are shooting at velocities of plus minus 320fps at their optimum of 70pounds to find the increase in velocity in relation to pounds you divide fps by velocity. 320/70 = 4.57fps/lb

Sorry James but I believe you have also got it wrong. As you increase the poundage of your bow, lets say a pound at a time, the increase in velociy is greater for each pound than the previous pound gained. What you said means that the velocity increase is directly proportional to the increase in draw weight. But because the draw weight is getting more and more the velocity is increasing more than the previous. Picture it as a exponential graph.

Just to follow up I bought a PSE X Force bow and when I went from 65 to 66lbs the velocity went from 298 to 303 so your info is pretty close. The arrow weight is 402 grains. The other thing I'm still having a problem with is realizing how quiet this bow is. My old bow was an 80 lb High Country Supreme and the PSE bow is a wisper compared to it. It is also so smooth upon release with no recoil due to the opposing limbs. I'm having such a good time shooting this bow that I might have to put a night light on it. It is definately a full cam compared to my old bow.
Thanks
Dave

Velocity loss per inch of draw probably varies from bow to bow/cam to cam. That said, I took a split limb single cam 70lb bow from 30" to 27" for a friend and it dropped 40-45fps, so I would say you could figure a 10-15 fps loss per inch with the same weight arrow. IBOs are given based on a 30" DL and 70lb w/ a 350 grn arrow. I happen to shoot a 30" DL and have my bows maxed out(70-73lbs depending on the bow) and while shooting 385-395 grn arrows I get from 15-20 fps of the IBO speed.

Good Luck

Reloader

Take the poundage you want to shoot, square that number. Divide it by the poundage you currently shoot. Divide it by the poundage you currently shoot again. Take the square root of that number and multiply it by your current arrow speed.

The answer should be pretty close to what it will actually end up being unless you go outside the spine tolerance for the arrow.

HI Big Guy
I think it's easier to set up my chronograph and bow scale.
Take care
Dave

quote:

Originally posted by 470 Mbogo:
HI Big Guy
I think it's easier to set up my chronograph and bow scale.
Take care
Dave

Or you could just take your current arrow speed divided by your current draw weight times your desired draw weight.

Exact same answer.

This is still just an approximation but more accurate than some arbitrary scalar per pound adder.

Walker, You are assuming his set up is shooting 320 fps. Not likely! Very few people shoot an arrow as light as is used to come up with IBO ratings, and his draw length is not likely what is used to calculate IBO speeds.
If you want to be specific then shoot through a chrono and do the calculations, but certainly don't use Walker's numbers.

Bottom line,

To approximate the increase in velocity achievable by turning up the poundage. You must know exactly what your poundage and velocity is NOW before you make the change.

I agree with Dr. C. IBO ratings are for 350gr arrows, 30inch draw at 70lbs only.

My draw length is 30 inches the bow poundage at the moment is 66lbs, The arrow weight is 402 grains and the velocity is 303 fps. I will crank it up to 70 and shoot it through the chronograph again soon.
Take care,
Dave

Well I couldn't stand it any longer. I cranked the bow up to 70 lbs. and the velocity went to 309 fps. Not worth the extra grunt at all. So it's back a 66 lbs. and is shooting at the same 303 fps. I'm using my 35P chronograph which has 4 ft. screen spacing. I guess this doesn't really follow any of the rules . The bow bottomed out at 70 lbs also.
Take care
Dave

I suspect your arrows may be underspined at that poundage increase. They are flexing too much and robbing you of the speed. 4 pounds should have got you a good bit more than just 6fps.