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Tenpoint FPS and arrow weight question


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#1 sargent

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Posted 08 June 2013 - 06:35 AM

Hello,

 

(Note I have this question in the tenpoint forum area thought I might get a second opinion)

 

I have a 2010 GT Flex set up in the long (180lb) position. The chronograph at local Sport Shop has been broken for some time now and in conversation with one of the workers there it sounded like it was not much of a priority to fix it.

 

I’d like to look into a HHA Optimizer for my CB and need to know the FPS in order to set it up correctly.

I would also like to know the weight (grains) of the arrows I’m using.

The arrows are Easton Aluminum XX75 with a 125 gr field tip and 125 gr Grim Reaper mechanical broadhead.

Questions

:

What is the weight in grains for this arrow setup?

 

What would be the FPS of an arrow shot from this CB?

 

I looked in my owners man and the grains and FPS were listed for a 100 and 75 gr tip not 125gr.

 

Anybody in the Oceana Co. area have a chronograph I could use?

 

Thank you for your time

sargent

 



#2 TNjohnboy

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Posted 27 June 2013 - 07:37 PM

I haven't ever used the HHA Optimizer, but I have been looking at them online.  I don't think you need all that info to set them up for your equipment.

From what I read - The unit has a 'calibration tape' on the dial when you receive it.  You set the dial to the start point, shoot at 20 yds and adjust your optics to get the proper POI.  Then you shoot at 60 yds and adjust only the dial on the Optimizer to get the proper POI.  Next you read the number on the dial (calibration tape) and select the corresponding tape and replace the calibration tape.  It's now supposed to be 'right on' from 20 - 80 yds.



#3 bobbyd1947

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Posted 27 June 2013 - 08:21 PM

I think UL does a video on this subject. Check under videos on top of page



#4 sargent

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Posted 29 June 2013 - 09:34 AM

After messaging tenpoint crossbows and others with the GT Flex, I've determined that my CB with the 460gr. arrow shoots in the 260-275 range and so I should be able to use that range of tapes that come with the HHA.

 

bobbyd1947, you answered my question in the tenpoint discussion area but I was wondering if there was a math formula for determining FPS ?

 

Thanks for the replies!

 

sargent



#5 Jack Pine

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Posted 29 June 2013 - 10:51 AM

  ....I was wondering if there was a math formula for determining FPS ?

sargent

 

Howdy Sarge,

Yes there is, it will usually get you to within + or - 10 fps of a true Chrono reading, but you have to know some specifics, and you have to understand that there will be variances due to fletchings, friction, limb stress, etc.  The formula can't compute PRECISE values but it will be close enough to be workable and reliable in predicting what different arrow weights will do to your speed. 

 

For example, my Middleton 375SS has its speed of 375 fps rated using a 350 grain arrow.  Using the formula to compute [E] Kinetic Energy, I multiply [M] 350 grains times [V] 375 fps times 375 fps [or, V squared] to find my [kinetic] E of 109.31 in ftlb. 

Now, what if I make up a heavier arrow and want to know about what speed to expect?

 

Using the KE of 109.31, multiply by 450240 [a constant], and then divide by the new, heavier weight [the new one weighs 462 grains, over 100 grains heavier] of the arrow. 109.31 times 450240 divided by 462 [the new mass weight] is equal to the velocity squared, or 106527.56. The square root of that is 326.38, so the expected result would be around 326.38 fps.

How close is it? 2 shots through the crony produced 324+ and 325fps shown in the pic.

 

If I wanted to increase the speed, I know that I would have to reduce the arrow weight, so I make a shorter, lighter arrow that ends up weighing 388 grains.  How fast can I expect it to be?

 

109.31 X 450240 / 388 = 126844.67. The square root of that is 356.15 and is verivied by a Chrono reading of 350 +.

 

Jack ><>

Attached Images

  • 462 grain.JPG
  • 388 grain.JPG


#6 sargent

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Posted 30 June 2013 - 09:52 AM

Hey JP,

 

Math was not one of my strong points.  Even with my calc. I can't come up with your numbers.  I was however looking at the Tenpoint 2010 Complete product guide and in the 2010 Crossbow Specs and Reference Guide section it shows the GT Flex in the 180lb (long) position 270 fps using a 420 gr arrow and 68 foot pounds.  So looking at these specs my bow should be even slower with a 460 grain arrow?

 

How far are you from Oceana county?

 

thanks for your time

sargent


Edited by sargent, 30 June 2013 - 09:55 AM.


#7 Jack Pine

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Posted 01 July 2013 - 09:02 AM

Sarge,

Assuming the numbers 10pt gave you to be correct:

420 X 270 X 270 divided by 450240 is indeed 68 fllb of KE. So….

 

68 X 450240 divide by 460 [grains of the arrow you mentioned] = 66557.21... The square root of that is your new speed of… 257 fps.

 

The good news is: 460 X 257 X 257 / 450240 = 67.48 ft lb. In other words, although you slowed some, you retain almost all your KE at 67.48 which is twice the required energy to take out 2 ribs on each side of a deer and pass completely through.

 

Don’t fall into the speed trap, I’ve killed deer at speeds under 200 fps. The 400 fps muscle bows shoot flatter, but the difference in the time of impact between 257 and 400 fps is less than most people would suspect.

 

I’m quite a ways north of Oceana county, in Kalkaska County, about 25 miles SE of Traverse City.

 

Jack ><>



#8 sargent

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Posted 01 July 2013 - 09:42 PM

Jp,

 

thanks for clearing that up.  One person @ tenpoint gave me the FPS for my configuration @ 250 fps and the other said about 270. 

 

Does anyone know of a crossbow friendly sportshop in the Muskegon (MI)  area with a chronograph?  The one in Hesperia is broken and I checked at the bow shop in Ludington and they said Muskegon. Any leads would be appreciated.

 

thanks all

sargent



#9 Jack Pine

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Posted 02 July 2013 - 08:23 AM

Jp,

 

thanks for clearing that up.  One person @ tenpoint gave me the FPS for my configuration @ 250 fps and the other said about 270. 

 

Does anyone know of a crossbow friendly sportshop in the Muskegon (MI)  area with a chronograph?  The one in Hesperia is broken and I checked at the bow shop in Ludington and they said Muskegon. Any leads would be appreciated.

 

thanks all

sargent

Sarge, the 250 fps guy is closest, but in order to know, you're going to have to have real data: the true grain weight of your arrow and the true speed of your arrow, and as you know, you need a chrony for that.  You might visit a gun or reloading shop, or best, buy your own, I think I paid about $60 -$70 for mine at Wal-Mart online.  They have them on sale there quite often, you may have to wait awhile to get that price though, and then they ship it free to your local store where you can pick it up.  Well worth the investment IMO.

Jack ><>



#10 sargent

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Posted 03 July 2013 - 11:32 AM

Hey jp,

 

good advice.  you know I just may do that (get my own chrono) didn't know they were that cheap.  Is that where you got yours (the one in your photo) walmart?

 

Thanks for your time.

 

sargent



#11 Jack Pine

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Posted 03 July 2013 - 04:24 PM

Hey jp,

 

good advice.  you know I just may do that (get my own chrono) didn't know they were that cheap.  Is that where you got yours (the one in your photo) walmart?

 

Thanks for your time.

 

sargent

Yah, it's from Wal-Mart and is one of the least expensive because it doesn't store your data; turn it off and then on again for every shot.  Works fine for what I use it for.  I see it listed for $89.65 there now.

Jack ><>


Edited by Jack Pine, 03 July 2013 - 04:28 PM.