Why do we use "FOC" instead of balance point relative to length?

Dartwick · April 02, 2009, 11:55:00 AM

Im just curious if theres a good reason.

It would seem easier more obvious to to just measure the length then measure the balance point fom the back(or front) and just express it as a decimal or percentage.

Don Stokes · April 02, 2009, 01:53:00 PM

Dartwick, it's traditional!

WildmanSC · April 02, 2009, 01:57:00 PM

Bartwick,

The balance point is measured relative to the center of the arrow and that tells you how much FOC the arrow balances. It's impossible to determine FOC without first determining the balance point and then measuring how much forward of the center of the arrow the balance point is.

Bill

PS And it is traditional!

Dartwick · April 02, 2009, 02:14:00 PM

Right I understand how FOC is calculated. It just seems like FOC is a complicated way of presenting a simple concept.

O.L. Adcock · April 02, 2009, 03:18:00 PM

Dartwick, I hear you and have NO clue why they refference it to the middle. In aviation it's usually refferenced as CG and given as a % of cord or total length. A 30" arrow that balances 12" (40%) behind the point is a 10% FOC. Both indicate the same place. I think the length of the point should be counted as well. All one needs to know however is the further forward the better.

...O.L.

WildmanSC · April 02, 2009, 03:31:00 PM

QuoteOriginally posted by O.L. Adcock:
Dartwick, I hear you and have NO clue why they refference it to the middle. In aviation it's usually refferenced as CG and given as a % of cord or total length. A 30" arrow that balances 12" (40%) behind the point is a 10% FOC. Both indicate the same place. I think the length of the point should be counted as well. All one needs to know however is the further forward the better. ...O.L.

O.L.,

If I'm including the weight of the point in the arrows weight, I would include the length of the point in the computation. Otherwise, you would be doing the computation and giving more weight to the shaft with inserts and feathers than it actually has. The only way to accurately calculate the FOC is to include the weight of the point and therefore, the length, too, IMHO.

Bill

O.L. Adcock · April 02, 2009, 03:48:00 PM

I agree Bill but look at Easton's FOC calculator, they include the point weight but only use the shaft length. In reality it's only a percent or two off either way usually but if I have a real short 150gr field tip and a long narrow 150gr BH, the BH will balance significanly further forward but not enough to see any tuning issues. So maybe that's why they don't count it? Doc Ashby doesn't include the point in the length either. I don't really care one way or the other, it's only trying to discuss it for everyone to be on the same page. I've since started using just the shaft length to stay on the same page as the Doc and Easton...O.L.

wtpops · April 02, 2009, 03:51:00 PM

QuoteOriginally posted by WildmanSC:

QuoteOriginally posted by O.L. Adcock:
Dartwick, I hear you and have NO clue why they refference it to the middle. In aviation it's usually refferenced as CG and given as a % of cord or total length. A 30" arrow that balances 12" (40%) behind the point is a 10% FOC. Both indicate the same place. I think the length of the point should be counted as well. All one needs to know however is the further forward the better. ...O.L.
O.L.,

If I'm including the weight of the point in the arrows weight, I would include the length of the point in the computation. Otherwise, you would be doing the computation and giving more weight to the shaft with inserts and feathers than it actually has. The only way to accurately calculate the FOC is to include the weight of the point and therefore, the length, too, IMHO.

Bill [/b]

I agree with you and i beleave that is what OL is saying but the standerd calculation dose not inclued the point lenght, if im not mistaken.

Dartwick · April 02, 2009, 03:53:00 PM

Which length should be used is a separate issue - but a more important one than mine

Which length is most useful partly depends on if you are trying to make calculations about parallax(in which case both have value) or calculations about penetration and aerodynamics(in which case total length makes the most sense.)

Daddy Bear · April 02, 2009, 10:04:00 PM

There are several formulas for %FOC which will give you different answers. Everyone I know speaks of %FOC in the same terms as Dr. Ashby. He uses the Standard AMO %FOC formula. You can use whichever formula you desire, but when speaking in terms of extreme %FOC, we should all be on the same page and use the same math.

AMO Standard % FOC as used by Ashby:

(Length of balance point with Broadhead measured from throat of nock) divided by (length of shaft from throat of nock to BOP) equals the decimal%. (Decimal%) subtract (.50) equals the decimal fraction. (Decimal fraction) multiplied by (100) equals %FOC.

Best,
DB