FOC question

[Deerstalker]

This may be a dumb question but it seems that all of my arrows come up with a FOC of 15%+ and I am curious if others get similar results. I measure from the nock to the insert for 'A' input and subtract the length from the balance point to the insert for the 'B' input to this formula. Thanks for your input & I believe that my arrows are about 450/460 grains total.

http://archerycalculator.com/arrow-fron ... alculator/

[PublicLandHunter1]

I got an FOC of 14.86% with a 125grain tip and 16.21% with a 150 grain tip with an arrow that weighs 387 grains (without the tip). I measured the same as you did; don't remember the website the FOC calculator was on though.

[ThackMan]

https://www.goldtip.com/Resources/Calculators.aspx

[nchunterkw]

An easy formula for FOC is:

(Distance from balance point to center point)/(Total Length) X 100 = %FOC

Total length is usually debated as to whether or not to include the tip, but just be consistent in your method for measuring overall length so you can compare different arrows.

I do not inclyde the tip in my overall length measurement.

EXAMPLE

Total Length = 20"
Center Point = 10" (from nock)
Balance Point = 14" (from nock)
Distance from Balance Point to Center Point = 4"

FOC = (4)/(20) X 100 = 20%

Hope this helps

Use something rather thin to balance the arrow on for most accuracy. Something like an old saw blade or short flathead screwdriver.

[janesy]

Why wouldn't you include the tip? Wouldn't a longer or shorter tip increase or decrease the % of you FOC.
I could see to balance your arrows when building them, but that all changes when a tip is added, but we don't shoot without a tip anyway

Just curious, I've never thought about it before

[nchunterkw]

Jansey,
To be sure, I include a FP or BH when making the FOC measurement, but I do not include its length in the overall arrow length calculation is all. It just makes comparing different arrows more consistent to me. A 1/2' long 100gr FP and a 1 1/4" long 100gr BH give the same FOC using my method. Where as if yu include the tip it will be different.

Example:
Tip not included
Nock to end of insert = 20"
Nock to Center = 10"
Nock to Balance = 14"
FOC = (4/20) X 100 = 20%

Tip1 = 1/2" FP
Nock to end of tip = 20.5"
Nock to Center = 10.25"
Nock to Balance = 14"
FOC = (3.75/20.5) X 100 = 18.3%

Tip2 = 1 1/4" BH
Nock to end of tip = 21.25"
Nock to Center = 10.625"
Nock to Balance = 14"
FOC = (3.375/21.25) X 100 = 15.88%

Since FOC also plays a role in penetration (see Ed Ashby) by determining where the shaft flexes at impact, I try to be consistent in my measurement methods so I know where that point is on the shaft on different arrow setups.

Hope that helps
Keith

[janesy]

Jansey,
To be sure, I include a FP or BH when making the FOC measurement, but I do not include its length in the overall arrow length calculation is all. It just makes comparing different arrows more consistent to me. A 1/2' long 100gr FP and a 1 1/4" long 100gr BH give the same FOC using my method. Where as if yu include the tip it will be different.

Example:
Tip not included
Nock to end of insert = 20"
Nock to Center = 10"
Nock to Balance = 14"
FOC = (4/20) X 100 = 20%

Tip1 = 1/2" FP
Nock to end of tip = 20.5"
Nock to Center = 10.25"
Nock to Balance = 14"
FOC = (3.75/20.5) X 100 = 18.3%

Tip2 = 1 1/4" BH
Nock to end of tip = 21.25"
Nock to Center = 10.625"
Nock to Balance = 14"
FOC = (3.375/21.25) X 100 = 15.88%

Since FOC also plays a role in penetration (see Ed Ashby) by determining where the shaft flexes at impact, I try to be consistent in my measurement methods so I know where that point is on the shaft on different arrow setups.

Hope that helps
Keith

I don't understand. If the length of the arrow changes when you have a tip installed. That would effect the balance point, no? So if it effects the balance point then it obviously changes the FOC. Why wouldn't you want to know what the FOC was with the individual arrow as it is set up to fly through the air?

I think I'm missing something LOL

[nchunterkw]

In my experience, an arrow balances at the same place (maybe 1/32" difference) with a 1/2" long 100gr FP or a 1 1/4" long 100gr BH installed. FOC is how far forward of the center of pressure the balance point is. For our purposes, the center of pressure is also the center of the shaft. I want to know far forward of the center of pressure the balance point is.

Measuring the overall length either way is fine. Just be consistent if you want to compare different arrow setups.

And FWIW I do measure FOC on arrows setup exactly as I will hunt with them. I just do not include the BH length as part of the calculation for FOC.

[janesy]

Interesting , I assumed that the balance point would have changed far more than that.

Thanks Keith.

[nchunterkw]

Consider that most of the BH weight is in the ferrule. It gets much thinner the further away from the insert that you are.

Good Luck!!

[Ekkie]

... FOC is how far forward of the center of pressure the balance point is. For our purposes, the center of pressure is also the center of the shaft. I want to know far forward of the center of pressure the balance point is.

Hmm. I'm not sure I quite believe that. The vanes/fletching should move the center of pressure towards the rear of the arrow... so it should be further back than the center of the shaft. At least to my way of thinking!

~E

[nchunterkw]

... FOC is how far forward of the center of pressure the balance point is. For our purposes, the center of pressure is also the center of the shaft. I want to know far forward of the center of pressure the balance point is.

Hmm. I'm not sure I quite believe that. The vanes/fletching should move the center of pressure towards the rear of the arrow... so it should be further back than the center of the shaft. At least to my way of thinking!

~E

E
You are right but how would you measure that? I think that is why the AMO settled on the center of the shaft for the standard. Center of pressure is where the forces on the arrow equal out, so it is somewhere between the center of the shaft and where the fletching is located, and will depend on dynamic spine and fletching size and how the fletches are mounted, etc. So for practical purposes we use the center of the shaft.

[longbow joe]

I stopped worrying about all the science and statistics. On a carbon arrow (bolt) a 110 grain brass insert and 100 or 125 point will fly like a dart . Or a excalibur bolt with a alluminum insert with at least a 150 grain point will have the same similar foc. Just my worthless experience flinging many bolts .

[Ekkie]

E
You are right but how would you measure that? I think that is why the AMO settled on the center of the shaft for the standard. Center of pressure is where the forces on the arrow equal out, so it is somewhere between the center of the shaft and where the fletching is located, and will depend on dynamic spine and fletching size and how the fletches are mounted, etc. So for practical purposes we use the center of the shaft.

I've never worried about measuring it. I have tried different vanes through the years and just settled on the ones that gave me the best overall performance (adhesion, minimal weight, rail clearance, arrow steerage, etc).

That being said though, if you do want to measure it, you'd get yourself a big fan and an arrow all built up. Figure out a way to allow the arrows to rotate about a specific point of the shaft. For example you could take a tiny bit and drill a little hole through the shaft, and insert a needle through it as a pivot point. Put the pivot point about where you think the COP is, place the arrow horizontally in front of the fan, turn it on, and see which way the arrow rotates (to point into the fan or away from it). If your pivot point is aft of the COP, the arrow head will rotate away from the fan. If your pivot point is fore of the COP, the arrow head will rotate to point into the fan. There will be no rotation at all if you're exactly on the COP.

It seems like a lot of hassle for not much gain. I'd rather just shoot some arrows to make sure they fly like I want.

~E