I must be blind or stupid....

OwnerITO · Post by **OwnerITO** » Thu Mar 08, 2007 10:13 pm

Before I post a topic for discussion I look to see if I can find the answer in this or other forums. So far, no luck at all. Here is the question and I definitely need an explanation on it. What the heck is F.O.C??? Everyone seems to know what it is and so everytime I see it used it is always like this (F.O.C.) and never in its full form. Sorry for being the only one who has no idea what this means and what its bearing is with arrows; even though it seems to be important. TY all in advance, Owner.

XbowMisiu · Post by **XbowMisiu** » Thu Mar 08, 2007 10:27 pm

F.O.C is Front Of Center... Take a look at this site it should explain some things about it.

http://www.huntersfriend.com/2007-Carbo ... guide4.htm

awshucks · Post by **awshucks** » Thu Mar 08, 2007 11:12 pm

It has to do w/ arrow balance. There is a search function here somewhere, lol, should get several days reading on that topic if you can find it. Here's a very loose way to figure it. Balance an arrow w/ tip on it, mark same. Measure arrow total length divided bye 2. Multiply the difference by 5% per inch, ya got your foc. Example: 20" Gt L II w/ 4" vanes and brass insert w/ 100 gr tip. 1/2 of 20"=10". Balance point will be darn near dead on at 14". 14-10=4x5%=20% foc. [FOC = Front of center balance point] There are tables available which I'm sure you will get references to which will show you this same configuration at about 18% or 19%.

Matthias72 · Post by **Matthias72** » Fri Mar 09, 2007 3:34 am

Interesting review on FOC importance:

"The FOC value for an arrow indicates how far forward of the centre of the shaft the centre of gravity (COG) is located, expressed as a percentage.

If 'L' is the length of the shaft and 'D' is the distance from the centre of the shaft to the COG then the FOC = 100 x D/L.

e.g if the arrow is 80 cm long and the FOC = 12% then the COG is 12*80/100 = 9.6 cm in front of the shaft centre.

The FOC relates to two different aspects of shooting arrows, how the arrow behaves on the bow when being shot and how the shot arrow flies through the air.

Manufacturers publish recommended values for the FOC e.g an FOC of 7-9% for aluminium shafts, 11-16% for ACE carbon shafts. These values are largely based on the standard pile weights available for the shaft. In practice recurve archers often use higher FOCs by e.g. using specially made heavier tungsten points.

The reason the recommended FOC values are higher for ACE then for aluminium shafts is because the carbon shaft is much lighter then the aluminium for the same shaft stiffness. As the shaft is lighter the COG is further forward & the FOC is larger.

While the FOC value is limited by how the arrow behaves on the bow it also affects how the arrow flies through the air. This is related to the arrow total drag and the fletching action. As covered in the section on drag, the drag force on an arrow is split between two separate forces; one which acts through the arrow centre of gravity which acts to move the arrow and one which acts somewhere else, roughly around where the fletchings are which acts to rotate the arrow.

The principle drag effect on the arrow which makes you 'miss' with a bad shot or a gust of wind is the drag on the shaft. The drag area of the shaft with respect to moving the arrow about depends on the arrow FOC. If 'L' is the length of the arrow shaft and 'A' its diameter then the shaft area Fa which relates to drag movement of the arrow is approximately given by:-

Fa = LA(1-FOC/50)

This is only an approximation because any rotation (fishtailing) of the arrow will affect the value of the shaft drag area.

e.g if the arrow is 80 cm long and has a 0.5 cm diameter then:-

with an FOC of 8% the shaft drag area is around 80 x 0.5(1-8/50) = 33.6 square cm
with an FOC of 16% the shaft drag area is around 80 x 0.5(1-16/50) = 27.2 square cm

or to put it another way each 1% increase in FOC reduces the shaft drag area by about 2%.

The overall fletching area with respect to how the arrow flies comprises three elements:

- the effective area of the fletchings
- the shaft fletching area
- vortex shedding torque (expressed as an area)

The shaft fletching area is determined by the position of the COG i.e. the value of the FOC for the arrow. The shaft fletching area = 2 x D x A = 2 x FOC x L x A / 100. ( A, D and L as defined above). In other words the higher the FOC value the higher the shaft fletching area.

For example suppose you have a 80 cm long arrow with 0.5 cm diameter.

with a 7% FOC the shaft fletching area = 5.6 square cms
with a 11% FOC the shaft fletching area = 8.8 square cms

In practice the higher the arrow FOC the smaller the diameter is likely to be and also the size of the fletchings will probably be smaller (compare the typical fletching size/diameter of aluminium arrows to carbon arrows).

The FOC value also effects where the axis of rotation of the arrow is located as it fishtails etc. about. The arrow rotation point is always in front of the COG and as the COG moves forward increasing the FOC the axis of rotation moves forward. The overall speed of response of the arrow to fletching torque (its angular acceleration), i.e. how fast it straightens up, depends not only on the area of the fletchings but on the fletching torque and the 'rotatability' of the arrow, its moment of inertia. As the FOC increases the effective fletching area increases and the 'lever arm' increases. At the same time the 'rotatibility' of the shaft decreases (higher moment of inertia). Overall the arrow fletching response increases with FOC.

Having a high FOC for an arrow provides two principal benefits - better arrow groups and reduced wind sensitivity. When you aim at the gold but the arrow ends up in the black something must have changed the direction of the arrow. An arrow mechanically has to leave a bow going in the direction it was pointed and with its axis very closely aligned with the direction it's going. The arrow changes direction after it leaves the bow and the cause is arrow rotational energy (cartwheeling). The arrow flies in a curved path until this kinetic energy is removed by fletching drag (the stabilisation distance). Having a higher FOC results in faster energy dissipation (more fletching action) and because the drag area moving the arrow is smaller the amount the arrow direction is changed is reduced. The result is more forgiving arrow to bad tuning or a poor shot leading to reduced group sizes. In a wind the smaller drag area that moves the arrow results in reduced wind drift.

The downside to a higher FOC is because the offset angle between the arrow axis and the direction it's going will in general be smaller, the drag on the pile will increase; lift from shaft drag will be reduced and probably the arrow will be heavier and hence going at a lower speed. All these factors result in 'loss of sight mark'.

A recent example of how FOC affects flight comes from throwing the javelin. Javelins don't have any fletchings and because of the tapered end don't have any vortex shedding torque. Javelin rotation relies solely on shaft drag. The problem was that there was insufficient 'fletching' and javelins were often landing flat and skidding. Also because of the low FOC a lot of drag lift was being generated. Competitors were throwing javelins over 100m which was too for far safety at most stadiums. A couple of years ago the regulations were changed increasing the required FOC value. Now javelins rotate and stick in the ground nicely. The increased rotation rate has reduced the vertical drag component and the distances being thrown have been reduced to within safety acceptable distances."

groundpounder · Post by **groundpounder** » Fri Mar 09, 2007 3:40 am

Way too much information

I'll just buy my bolts from Greywolf and be happy

wabi · Post by **wabi** » Fri Mar 09, 2007 6:26 am

groundpounder wrote:Way too much information I'll just buy my bolts from Greywolf and be happy

I've got to agree that FOC gets a little too much emphasis is many cases. As long as I'm getting good arrow flight I don't worry about it. If arrow flight is bad it is one of many things that should be checked. Not saying it doesn't matter, but it isn't the only thing that affects arrow flight.

BOB VANDRISH · Post by **BOB VANDRISH** » Fri Mar 09, 2007 7:01 am

Put this in your browser-
Javascript FOC calculator.
The chart that comes up will allow you to determine the Front Of Center of any arrow or bolt.
You need to be aware that a broadhead needs a higher percentage FOC than a target point to fly the same as a target point.

sumner4991 · Post by **sumner4991** » Fri Mar 09, 2007 7:20 am

groundpounder. . . doesn't that go against your moto?

BOB VANDRISH · Post by **BOB VANDRISH** » Fri Mar 09, 2007 7:47 am

Try "Javascript Archery Ballistics Calculator V 02.08",if you want to learn more about arrow/bolt flight.
You can plug in all the numbers from your equipment and this calculator will automatically plot arrow drop,energy at various distances.
Just note the differences in flight when changing from feathers to plastic vanes as an example,without changing any other variable.
Quite interesting!

pokynojoe · Post by **pokynojoe** » Fri Mar 09, 2007 7:52 am

Mathias

You're scaring me! I think I actually grasp most of what you are saying. I only wish that you had been my Physics professor about 35 years ago when I was at university. I should think I would have gotten a better grade!

Ciao
Joe

BOB VANDRISH · Post by **BOB VANDRISH** » Fri Mar 09, 2007 7:56 am

Add "arrow front of center calculator",and now you can let the calculator predict the FOC for any combination that you are using.

BOB VANDRISH · Post by **BOB VANDRISH** » Fri Mar 09, 2007 8:05 am

Sorry guys!
That last link,as I have it posted,does not get you to the calculator that I wanted to send.
I will try to figure out what went wrong,and send the correct one later.
It allows you to plug in all the parts of a completed bolt,and then it will generate an FOC based on what is entered.

BOB VANDRISH · Post by **BOB VANDRISH** » Fri Mar 09, 2007 8:16 am

OK,here is the right way to get to the chart I wanted to send.
Go to : www.bowjackson.com.
The top item is " Jackson's Archery and Hunting "page.
Open it up and then click on the Arrow FOC and Weight,and then click on FOC calculator by components
Now you can do the calculations.

groundpounder · Post by **groundpounder** » Fri Mar 09, 2007 4:39 pm

sumner4991
PostPosted: Fri Mar 09, 2007 8:20 am Post subject:
groundpounder. . . doesn't that go against your moto? Laughing

I guess in a way it does. There are some things in life worth stressing over but I feel with this one I'll let someone else figure it out and I'll reap the benefits. I feel my brain can only hold so much and to add this new info in I'll have to flush something else out.

rutman · Post by **rutman** » Sun Mar 11, 2007 9:33 am

Been huntin with a crossbow for over 20 years and just learned in the past year what FOC meant. Didn't stop me from killin a S load of deer!. I still haven't done anything with the new found knowledge. Why try to improve on perfection? BILL ALREADY DID THE WORK FER YA!