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Discussion Starter #1
So...what's the theory here.

For every inch of draw length I get how many fps? (Assuming draw specific cams)

For every lb of draw weight I get how many fps?

For every 5grains of arrow weight I get how many fps?

I'm curious. Thank you.
 

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So...what's the theory here.

For every inch of draw length I get how many fps? (Assuming draw specific cams)
Approximately 10fps per inch of draw length.

For every lb of draw weight I get how many fps?
Approximately 2fps per 1lbs of draw weight.

For every 5grains of arrow weight I get how many fps?
Approximately 1fps per 3 grains of arrow weight.

I'm curious. Thank you.
In my experience those approximates are generally pretty accurate.
 

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Discussion Starter #3
So if someone claims 15fps for an inch and 3fps for an lb of draw weight...it'd be safe to assume they're trying to blow smoke up my rear?
 

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So if someone claims 15fps for an inch and 3fps for an lb of draw weight...it'd be safe to assume they're trying to blow smoke up my rear?
Not necessarily.

The estimates are general rules of thumb, and not always exact. Take the Hoyt spiral cams for instance, since they are DL specific, and since they do not use modules for the DL they do not loose the estimated 10 fps, as a rule of thumb.
 

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Discussion Starter #5
So what do these draw specific cams lose as a rule of thumb? Saying they only lose 10fps...would be the same as saying they'll gain only 10fps which an increase in draw length correct?
 

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So what do these draw specific cams lose as a rule of thumb? Saying they only lose 10fps...would be the same as saying they'll gain only 10fps which an increase in draw length correct?
On my spiralled bows I'd say they lose 6-7fps per inch. It's why so many people post speeds that are adjusted to well above IBO.
 

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Ditto very close to the same specs that the other posters have written on this thread about Spiral cams...I've found about 8 f.p.s., within certain draw specs...I've also found that the larger "Speed Bow" cams that take a draw length module , but use the same size cam base seem to have a draw length range that suits the larger cam base, but falls off sharply as draw length decreases...To the tune of 13-14 f.p.s....This data is from actually testing bows, all on the same Chronograph, same shooter, and adjusting arrows for same grains per pound....No eccentric design stays linear for performance when draw length changes, even round wheels...Thus different base sizes for different draw length ranges just makes sense, to me anyway...Take care........Harperman
 

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FEDIE 316: I'm definitely not an expert. I probably misunderstood but If I go by the guidelines in red above I'm shooting 500 fps! My draw length is 27", my draw weight is 55#, and my arrows are 360 gr.. Where did I go wrong? Please help me understand this because I've often wondered what fps I'm shooting but don't have cash for a chronograph. Any advice would be greatly appreciated, thanks!!
 

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......That is fun to play with, and see how it matches up with real world , but most Hoyt's will be off that chart....I punched in the specs for my Son's CRX 32, and it is off by a large margin...At the charted specs I entered,, it should be 270 f.p.s., but we are getting over 280 f.p.s., and havent tweeked the bow yet....it is pretty close on my new PSE Dominator Pro....But still a few f.p.s. slow....Thanks for the link, that is pretty cool to play with.......Take care!..........Harperman
 

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Your Bear, just deduct those numbers, not add them. :lol:

Start with the IBO numbers. Subtract 30fps for DL, subtract 25-30 for arrow weight over IBO (275 for 55#), and subtract 1fps for every 3gr of weight added to the string. Then subtract 10-29 from that since you're running such a large cam and have it turned down shorter (DL specific cams would save a lot here).

So if the IBO rating is 330fps, you'd be at 255fps if you're lucky.
 

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Hunter177, You lose 30 fps with your draw weight not being at 70 lbs. Then -30 fps for the 27" draw length. You then, lose 7 fps for the d-loop and peep. And, about 3 fps for the arrow not being 350 grains. Given all this, you are getting 229 fps and 42 ft. lbs of kinetic energy. I find that those archery calculators are pretty accurate. I used one to estimate my speed, and got 265 fps. Then, I went to the local pro-shop and shot through a chronograph and it said 262 fps. Keep in mind, they are only there to estimate your speed and kinetic energy based on the numbers you enter. The only way to get accurate numbers, is to shoot through a chronograph.
 

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Thanks for the help. I figured I did something wrong because I knew 500 fps was wrong. Thanks again!!
 

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So...what's the theory here.

For every inch of draw length I get how many fps? (Assuming draw specific cams)

For every lb of draw weight I get how many fps?

For every 5grains of arrow weight I get how many fps?

I'm curious. Thank you.
Send me an email to [email protected] and I will send you a copy of Excell spreadsheet for calculating bow speed based on IBO rated speed of your bow.
 

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For arrow weight you add 1 fps for every 3 grains over IBO wt and deduct 1 fps for every 3 grains under. Example: you shoot 280 fps with a 450 grain arrow. Arrow is 100 grains over IBO so 3 into 100 = 33. 280 + 33 = 313 fps. Works out pretty well.
 

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Whitetail101, a member here, posted a thread about estimating bow speed from IBO. He had it down to a cam specific science. Search his profile and find his Started threads... Its there...
 

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I did some playing around this weekend and extended my DL by 1/2 inch using a new module.

I gained...... Wait on it...... Here it comes..... NOTHING. The bow is much more comfortable but I did not gain any speed. I may bump it up another 1/2 inch this weekend just to see what happens.
 
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