10 replies to this topic

[anewdef]

how many grains does it take to equal 1fps lost or gained on a typical crossbow? 5? 3?

[Highlander]

From a Dealer site:

"Arrow Weight vs. Speed: The rule is—a six grain change in arrow weight will equal about one foot
per second of arrow speed. This means that if your chosen shaft weighs nine grains per inch
trimming an inch off will make about a 11¼2 feet per second difference in arrow speed. In fact
switching from a 100 grain point to a 70 grain point will increase speed about 5 feet per second.
What this means is once you select a particular arrow shaft you will have to make some pretty
drastic changes to that arrow to affect the speed, with all other factors being equal. The original
shaft choice has the greatest bearing on an arrow's potential speed with a given bow."

[anewdef]

cool, thanks! may I ask, what site this info is from? thanks

[Highlander]

We are not allowed to post links to seller's websites in the Forum pages. However we are allowed to send you info by PM.

PM sent

[SPECIALIZED]

HighlandHunter, on 09 February 2012 - 07:56 PM, said:

We are not allowed to post links to seller's websites in the Forum pages. However we are allowed to send you info by PM.

PM sent

Thank you very much.

[Highlander]

You're welcome.

FWIW, this is standard practice for many forums as well and I understand why and agree. I do try my best to respect all rules.  

Especially when you have retired law enforcement enforcing said rules. lol

Edited by HighlandHunter, 09 February 2012 - 08:03 PM.

[Jack Pine]

anewdef, on 09 February 2012 - 02:45 PM, said:

how many grains does it take to equal 1fps lost or gained on a typical crossbow? 5? 3?

Anewdef,

Variations of the same formula used to calculate KE will give you the answer.

Mass [grains] X Velocity² [fps] ¸ 450240 = Energy [ft lb]


First weigh your arrow; let’s assume 450 grains. Then shoot it through a Chrony to find the speed; assume for this example 300 fps.
Then calculate the KE by arrow weight X velocity² , then divide by 450240 to get your KE in ft lb. 450 X 300 X 300 ¸ 450240 = 89.952025 ft lb KE.
Next reduce fps by 1 and recalculate. 89.952025 [KE] X 450240 [constant] = 40499999 ¸ 299 [speed reduced by 1 fps] = 135451.5 ¸ 299 [fps] = 453.01505 [grains] minus 450 grains = 3.01505 grains added to reduce fps by 1.

To verify this, use the KE formula. 453.01505 grains X velocity squared, divided by 450240 = the KE.
453.01505 X 299 X299 ¸ 450240 = 89.952014, or virtually the same KE of 89.952025 to start.
450240 is a constant and the KE was a known value, so the only changes were the arrow weight [450 to 453.01505] which affected the speed by reducing it 1 fps [300 to 299].  So... to answer your question, about 3 grains will affect speed by 1 fps, all things being equal.  
BUT if you change flight characteristics in any way by changing heads, arrow diameters, fletchings, WFOC, or other things, this will skew your results and you will then have to recalculate the whole process all over again with real values to find real answers.

The other day I saw an ad for X brand crossbow, stating that their speed was 350 fps and their KE was 95.  No mention of arrow weight so you could tell if the speed was with a real-world hunting weight arrow.  How do you find out what arrow weight they used to arrive at these figures?

95 ft lb X 450240 = 42772800 ¸ 350 fps = 12208 ¸ 350 fps = 349.16571 grain arrow.
[to verify]: 349.17 grains X 350 fps X 350 fps = 42773325 ¸ 450240 = 95.001166 ft lb KE, same as the ad.  Don't be fooled by claims of blazing speed and bone-smashing KE, check out the arrow weight to see if it is valid.

What is the real speed of that X brand crossbow when using a real hunting arrow of 450 grains?
95[KE] X 450240 = 42772800 ¸ 450 [grains] = 95050.666. Ö 95050.666 = 308.30288 fps. Still respectable, but not the 350 fps they want you to focus on.
[to verify]: 450 grains X 308.30288 X 308.30288 ¸ 450240 = 94.999993 ft lb KE as advertised.

In the first example, we found 3 grains meant 1 fps difference in speed. In this example, 101 grains difference yielded a reduction in speed of 42 fps. 101 ¸ 42 = 2.4047619 or 2.4 grains per fps reduction instead of 3.  
Why the difference? Because the formulae to calculate KE, speed, and weight cannot produce precise values, they have a slight variance.

Jack ><>

Edited by Jack Pine, 12 February 2012 - 08:07 AM.

[Aimo]

Very nice and important info..thank you very much

[anewdef]

man! ive never thought of figuring it iit that way! niiiice thanks for the info guys. see, the paperwork that came with my Horton SuperMag said "470g @ 230fps." idk, I have a hard time believing that! I read on here once that a printed add said 410g @ 230fps.

who knows. ill have to see if they give a KE reading... im trying to figure out if a lighter.arrow will benefit me enough to justify a switch - but I don't want to loose too much "punch"

thanks all!

[Jack Pine]

anewdef, on 10 February 2012 - 03:53 PM, said:

man! ive never thought of figuring it iit that way! niiiice thanks for the info guys. see, the paperwork that came with my Horton SuperMag said "470g @ 230fps." idk, I have a hard time believing that! I read on here once that a printed add said 410g @ 230fps.

who knows. ill have to see if they give a KE reading... im trying to figure out if a lighter.arrow will benefit me enough to justify a switch - but I don't want to loose too much "punch"

thanks all!

Yah, if you can find the advertised KE, it's a simple matter to see if 470 or 410 is the correct weight since you know the speed already.

Jack ><>

[XBOWED]

"Wow" Very impressive.........