FOC

Barry C

New member
I have just refletched with some ff187. I measured the FOC. My arrow is 29 inch long. The distance from nock grove to balance point is 20 Inches. According to the Easton program that gives a foc of 18.9%</p>

Before with spin vanes the balance point was 20.5 inch from the nock grove to balance point,?which indicated an FOC of 20.6%! I previously made it about 18... I must have been incorrect the first time. I have triple checked everything this time.</p>

Have I made a mistake?</p>
 


Barry C

New member
This is a really strange set of arrows. The guy in the shop commented how light they feel, but whats more I can tune them bang on with 50 pounds on my fingers... The bareshaft right into the group at 18m.?I havn't put them into the crono, but they seem much faster than a guy's whose flay at 220fps and similar to my brothers compound.
 


jerryRTD

Well-known member
220 fps is not that fast for a compound. I estimate that my arrows take about a second an a quarter to cover 100 yds.I must se about getting the bow on a crono.
 


Barry C

New member
I know, i meant mine are much faster than my friends at 220 he shoots recurve, mine are?about as fast as a compound my brother shoots, also with very underspined arrows. he shoots 560 acc with 82gr pound, 29 inch arrows at 55 pounds.
 


Marcus26

New member
220 is terribly slow for a compound of those specs. I get 260fps with my Bowtech at 55# and a 360 grain X10 with full weight in the points. I only have a 28" draw so with a 29" draw I would get 270fps.

I don't see the need to fuss over FOC, none of the top recurve or compound shooters I know bother with it at all. Full weight points and that's it. More weight = less drift.
 


joetapley

New member
I don't see the need to fuss over FOC, none of the top recurve or compound shooters I know bother with it at all
Australia is about the only place in the world where the bow was not invented - and as you can see they have not yet caught up! :)
 


Marcus26

New member
Yep we just win with em while the english try to overthink things.
Sorry Joe but you have never proved IMHO that FOC matters. In my own testing I can vary the weights of my points by over 100 grains and the things still go where I aim them. The biggest impact is the point weight in relation to wind drift, but without wind my CXLs with 60 grain points shoot 10s at 70 just like my 110 grain point X10s do at the same arrow length, yet their FOC are much different.
Certainly adjustment of 2% makes no difference. You'll lose more points from increased drift.

Sorry guys, there is no magic formula, tune your bow poundage to the spine of the arrow, execute the shots correctly, aim at the 10 and you will hit the 10. Changing the FOC setting by 2% will not change your groupings if the above is held true.
 


joetapley

New member
Yep we just win with em while the english try to overthink things.
I wish that were true but it's the Koreans, East Europeans, US and yes Australia etc. who win the (recurve) medals because they long ago moved (not entirely but to a significant extent) from top of the head opinions to trying to put things on a firm basis - would you scrap the AIS?

As for FOC the effects are masked by by the 'forgiving' aspect of the compound and the high arrow speed so you can get away with a lot. With lower arrow speeds from a recurve the main arrow flight performance properties are length, diameter, mass and FOC. You need a good combination of all four.
 


morphymick

Supporter
Supporter
American Shoot
Furface - 19/9/2005 7:31 PM

OK, on the basis that there is no such thing as a stupid question
What is "FOC"?
Forward of Centre.

It is the %age of the length of the arrow, "Forward of Centre" at which the arrow balances. See 2nd post on this thread for Determination of FOC.doc, includes a useful Excel titbit.;)

Mick
 


Marcus26

New member
Sorry Joe I know all too well the AIS stance on FOC and their processes as I coach one of their shooters and visit there often. They do not measure it at the AIS, they don't care, full length points.
If they turned to my archers and told them to not use full weight points in their arrows then I would support their closure quite happily.

If you read my post carefully you will see my requirements for a well tuned arrow, these are recurve requirements and there isn't more than that. This has been backed up by Australian representative recurve archers in field testing.
Recurve Arrow flight is governed by matching bow poundage to the spine of the arrow. Compound arrow flight is governed by contact to the rest. No voodoo to it and no secret combination that will suddenly produce a 'forgiving' setup.
 


joetapley

New member
Furface

OK, on the basis that there is no such thing as a stupid question
agreed - there are only stupid answers

If you download the (free) Easton Shaft Selector program (a useful thing to have anyway) it includes an FOC section including an arrow FOC calculator.

Unfortunately understanding the importance of FOC requires some basic understanding of mechanics. Just to present the highlights FOC affects arrow flight in two ways, it controls how drag forces act on the arrow and it effects the fishtailing/porpoising behaviour of the arrow. The basic rule is that the higher the the FOC the smaller the arrow groups become (this like all one line statements is a bit of a simplification). That's the reason elite archers will use specially made heavy tungsten piles, you get a few percent more FOC.

The one slightly dubious information you come across regarding FOC is to do with distance. In the Easton Tuning and Maintenance Guide for example Don Rabska says that FOC becomes important at longer distances (70/90m). This is not strictly correct. FOC mainly 'does its thing' in the first 20 metres of flight. However the further distance you shoot the bigger the error becomes in where the arrow hits from what happened in the first 20 metres. FOC is still marginally relevant shooting 18 metres.
 


Mick

New member
Ironman
"FOC is still marginally relevant shooting 18 metres."
Joe so it's not a complete waste of time to add loads of weight to my X7 2114 points for the indoor season? (shooting 29 1/2", 47lb on my fingers)
 


Marcus26

New member
Unfortunately understanding the importance of FOC requires some basic understanding of mechanics. Just to present the highlights FOC affects arrow flight in two ways, it controls how drag forces act on the arrow and it effects the fishtailing/porpoising behaviour of the arrow. The basic rule is that the higher the the FOC the smaller the arrow groups become (this like all one line statements is a bit of a simplification). That's the reason elite archers will use specially made heavy tungsten piles, you get a few percent more FOC.
Having talked to a number of engineers about FOC and they disagree with you.
The one slightly dubious information you come across regarding FOC is to do with distance. In the Easton Tuning and Maintenance Guide for example Don Rabska says that FOC becomes important at longer distances (70/90m). This is not strictly correct. FOC mainly 'does its thing' in the first 20 metres of flight. However the further distance you shoot the bigger the error becomes in where the arrow hits from what happened in the first 20 metres. FOC is still marginally relevant shooting 18 metres.
Again, our top recurve archers have proven that the FOC is not important, the arrow mass is. What the heavier weight does at long range is increase your downrange velocity and prevent the arrow from being hit by drag closer to the target. A light pointed arrow starts off fast but gets moved off course easier. This all has NOTHING to do with FOC and all to do with mass. Result may be the same, but it isn't going to suddenly make you group better. You might reduce the effects of wind drift, but chances are the reason you missed the 10 last end would be the same next end.

Once again, grouping is determined by draw weight in relation to spine. You get this right and the arrow will straighten up very very fast and totally negate the reasonings you have for FOC. Too many archers fail to tune their arrows correctly and then try to hide the effect by messing with plunger tension and searching for the magic FOC percentage value.
My wife has been ranked #1 in Australia recurve and can change between spin wings and flex fletch without effect on her groups at 70m. This is with recurve. There is a few extra FOC % points right there.

I'm not an enginner, but I do train some top compound and recurve archers and talk alot with many others. Not a single one of them cares about FOC. None of them even bother checking what their FOC values are. Yet they are world class archers shooting over 1340 FITAs. (compound and recurve)
Now I may well be wrong in all this, but at the end of the day, the actual physical testing I have seen does not support you.
 


joetapley

New member
Mick

so it's not a complete waste of time to add loads of weight to my X7 2114 points for the indoor season?
Well :) you will get a marginal improvement assuming you can get a good tune with the heavy points (That's why people do it). Result in most cases would be better by using cheaper carbon arrow e.g. ACC/Triple. The idea that very wide arrows are better for recurves (line cutting) is rarely true. The amount the better arrow will hit nearer the X for a given shot is greater than the difference in the diameter with the worse performing fat aluminium arrow.
 


joetapley

New member
RE: Indoor Setup

Mick
Good advice from Vittorio F. on Sagi

This subject comes out every year, and many people continue to say that they switch to aluminum for indoor for budget reasons.
Well, In my club,for the same reason, nobody (recurve) changes anything at all. Affording a second set of arrows, a change of the arrow rest, a second plunger and string (if you want to shoot outdoor as well) is really too expensive not only in terms of money, but time, too. And, aluminum arrows are much more fragile than ACE/Navigator/ACC and bend quite easily, so need a continuous maintenance.
 


TJ Mason

Soaring
Supporter
Fonz Awardee
American Shoot
RE: Indoor Setup

The other side of this, though, is that the horrible hard materials used for indoor bosses can abrade carbon, leading to the arrows becoming steadily less precise. Not that you'd notice with my shooting...
 


Marcus26

New member
Exactly correct TJ.
An archer I know tested his X10's before and after using them into stramit for 3 months. He lost 5 grains of weight per arrow. I did the same test with alloys and obviously lost nothing.
 


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