Chronograph Readings

[Big Boy Blue]

I was shooting my new G3 limbs over the chronograph today and noted that the arrow speed being measured was quite a bit above the estimated arrow speed given by Archers Advantage.

I was expecting around the 155-159 fps but the chronograph measured 185-188. This represents a fair amount of error. Does anybody else see this sort of difference or is my chronograph out of calibration ? I was told in the shop when I bought them that using G3 limbs, was like using a limb that was effectively 3-4lbs above what is printed on the label.

That raises another question which is how do you check the calibration of a chronograph.

Phil

[Rik]

Originally Posted by Phil Hodgins

I was shooting my new G3 limbs over the chronograph today and noted that the arrow speed being measured was quite a bit above the estimated arrow speed given by Archers Advantage.

I was expecting around the 155-159 fps but the chronograph measured 185-188. This represents a fair amount of error. Does anybody else see this sort of difference or is my chronograph out of calibration ? I was told in the shop when I bought them that using G3 limbs, was like using a limb that was effectively 3-4lbs above what is printed on the label.

That raises another question which is how do you check the calibration of a chronograph.

Phil

What sort of chrono? The kind which work by light (rather than the radar variety) can be fooled by the lighting...

Out of interest I ran some figures through the Easton software again.
Assuming a 68inch bow and 38lb on your fingers, it was estimating arrow speeds for the aluminium shafts it suggested at between 171 and 194 fps. So it looks like AA might be out of whack on this one. You might like to check the data AA was basing it's prediction on - it might have a daft point weight or something.

[joetapley]

Don't actually know the arrow/setup details but a rough estimate comes out about 180fps,between the two values you quote

Would guess the chrono reading high but nearer the mark.

[Big Boy Blue]

Guys thanks for the replies.

I am using 28" 1913 shafts which have a total weight of 335 grains. The arrows bareshaft nicely and when I tried a 1916 shaft the bareshaft was off to the right showing it was to stiff. So far so good

I have measured the poundage with a calibrated bowscale and it was 36lb on my fingers which is about right as me draw length is 27".

How are you coming up with the higher speeds ? The only software I have that calculates the arrow speed is AA. I have hunted around the Easton site and I cannot find anything that provides this data.

You just never stop learning and I had no idea that the sport could get so technical, which I have to say I do enjoy be that way inclined.

Phil

[joetapley]

Speed estimate was based on a 28" 2013 XX75 so wrong arrow picked. However still would put the AA value as too low - Still an estimate is just that , a guess.

I'd only worry about speed if there were sight mark issues.

[Rik]

Originally Posted by Phil Hodgins

Guys thanks for the replies.

I am using 28" 1913 shafts which have a total weight of 335 grains. The arrows bareshaft nicely and when I tried a 1916 shaft the bareshaft was off to the right showing it was to stiff. So far so good

I have measured the poundage with a calibrated bowscale and it was 36lb on my fingers which is about right as me draw length is 27".

How are you coming up with the higher speeds ? The only software I have that calculates the arrow speed is AA. I have hunted around the Easton site and I cannot find anything that provides this data.

I'm using the old "Easton Arrow Flight Simulator 2.0". I ran it through again with the revised figures you supplied and it came up with an estimated speed of 183 fps. That isn't a bad guesstimate, given that the software was never designed to cope with the newer limbs. And so many other factors can push the speed up and down, that you'd have to give it an error bar of around 10fps...
Incidentally, did you mean 27" draw? Or 27" to the button (1" of shaft past the button)? 2.75" would be quite a bit of shaft overhang...

[Chase Archer]

Phil,

I shoot G3's with 39lb on my fingers, 28 1/4'' A/C/C 680s and last week I had between 178 and 190 fps on the chrono. It's the type sold by Bowsports for around £60

Dunno if that helps or hinders, is good or bad, but that's what I found.

[Big Boy Blue]

Rik

It is 27" to the centre of the button with 1" beyond it, so the arrow tip is just behind the front edge of the riser window.

Phil

[Big Boy Blue]

Joe

I guess the other thing it could be is the equations used to calculate arrow speed in the program are wrong and giving incorrect values. I will send and e-mail to Perry at AA and ask the question.

Phil

[joetapley]

Quick estimate of arrow speed is given by

v = Sqrt(DWGE/M)

Where
v =speed; D = draw length; g= gravity acceleration; E = bow efficiency and
M = arrow mass. Just need to be consistent on units.

So eg. in your case took D as 28-1-9 = 18" (overhang 1" & bh 9") and convert to 0.457 metres.
E taken an 0.75, M taken as 0.0197Kg (338 grains); g = 9.81 and w =38/2.205Kg