Forward of Centre - how to calculate? plus n00b fletching Q.

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buzz · #1 Post by **buzz** » Sat Apr 01, 2006 8:22 pm

With a set of Keith's fine Ash shafts in the shed I thought I'd quickly check a few things before I started work on them.

I went through two books I had just casually lying around (Chuck Adams Bowhunters Digest & Byron Fergusons "Become the Arrow") when I realised that they measure FoC differently.

Chuck took the overall length, point tip to back of nock, halved it, then took the distance from balance point to centre and divided it by overall length. He was looking for 7 to 10%.

Byron measures length from nock slot to end of (alloy) shaft, not tip. And if it is a tapered shaft or uses additional weight then the maths gets funky. He measures from the centre of the shaft to the balance point then divides it by half the shaft length. Aiming for a 10 to 15%.

Now, in practice both come out in the ballpark of each other. However people here quote FoC numbers - so what numbers do folk recommend and how do you measure it?

Next, I just started doing my own fletching - replacing the 5" gateway parabolics with 4".... not hard to do as they were falling off with monotonous regularity. Using a JoJan with a right wing helical clamp all was going well until I noticed that I seem to be having colouring from the fletches marking where they brushing past my side of the bow, just near the shelf (Internature Viper longbow) - I don't think I noticed this happening previously. What I did realise was that if I were to move the feather up the clamp towards the tip it will wrap around the arrow in a spot a little rotated around from where it would have been otherwise....

Cutting to the chase.... where should I be placing my fletch?

MaylandL · #2 Post by **MaylandL** » Sat Apr 01, 2006 10:24 pm

buzz wrote:...so what numbers do folk recommend and how do you measure it?

...not hard to do as they were falling off with monotonous regularity. ...

Cutting to the chase.... where should I be placing my fletch?

G'day Buzz

I use about 10% as a general rule of thumb and use Chuck Adam's method.

I now bind the tip and ends of my fletches. In terms of positioning for the arrows I use, when I nock the arrow the tip of the fletch does not touch the bow. To date I haven't lost any fletches.

I am also looking forward to getting some of Keith's excellent shafts too. Expecting them to get here next week.

Happy shooting

#3 Post by **Stickbow Hunter** » Sat Apr 01, 2006 10:50 pm

G'day Buzz,

IMO the method you say Chuck Adams used is the correct one. The AMO and Byron Ferguson use the other method as does Ed Ashby in his recent article on the FOC of an arrow. I donÃ¢â‚¬â„¢t believe the latter method to be correct because doing the calculations using only the arrow shaft length DOES NOT give you the FOC of the arrow.

Using the AMO method will give you a FOC reading approx 3 to 5 per cent higher than if you use the full arrow length method.

The AMO method does not fully take into consideration the difference that the arrow head length makes. The longer the arrow head is, the longer the distance that the mass of the head is distributed over.

As an example I will give you the FOC of one of my arrows I use out of my self bow. The arrow weighed 600 grains including a 160 grain head by the way.

Using the full arrow length (Chuck) method the arrow with a 160 grain field point had a FOC of 11.3%. This same arrow but with a 160 grain Ribtek broadhead had a FOC of 9.5%. The reason for this was that the broadhead made the arrow 29mm longer.

Using the AMO method this arrow with the field point had a FOC of 14.1% and with the broadhead 14.6%.

As you can see there is a considerable difference in the FOC figures when using the two different methods and the FOC percentages certainly do not come out in the ball park of each other.

For my hunting arrows I like to use a FOC of around 10%.

In regards to feather placement, just check that your jig is placing the feathers in the correct position. I remember using a Jo Jan jig many many years ago now and I did not glue the nocks on the arrows until after I had fletched them because the jig placed the feathers in the wrong position. The cock feather would have been in the 11 oÃ¢â‚¬â„¢clock position instead of the 9 oÃ¢â‚¬â„¢clock position.

Jeff

buzz · #4 Post by **buzz** » Sun Apr 02, 2006 10:33 pm

Thanks folks. If trying to calculate overall length of a big long pointy stick to work out what weight is needed to make it fly right then you would think that balance point versus centre could only be determined with all weight and length in place. And if the sum is wrong, then you swap tips and measure again, and again until "right".

I plan to only be using field points, and locally I have only been able to source 125grain tips. So it will be fun, obviously a shorter tip can be a tad lighter than a longer broadhead to get the same balance, so I think I might be up for a magical mystery tour of fiddling and tinkering.

Jeff ~ you were saying the cock feather should be at 9 o'clock ~ with a helical fletch it twists around the shaft, so in your opinion and experience.... should the front, back or middle of the feather be at the 9 o'clock position?

Mark

#5 Post by **Stickbow Hunter** » Mon Apr 03, 2006 12:08 pm

Buzz,

I don't know your bow poundage or draw length but if you are using Keith's shafts that are spined over 60# I would think that a 160 grain head would help balance your arra better.

The 160 grain heads are hard to get in Australia but Townsville Indoor Archery may have some and I'm pretty sure John McDonald in Sydney (02 9875 3032) would be willing to get you some.

I use a lot of helical in my fletches and I place the back of the fletch so the right side of the fletch lines up with the index on my nock - in other words the thickness of the feather left of 90 degrees. I didn't have a good photo showing this but the one below may show you enough.

Jeff

buzz · #6 Post by **buzz** » Mon Apr 03, 2006 5:59 pm

Thanks Jeff.
The bow is nominally 45lb, and we've measured it to be 52~53 lb at my 30" draw. I am trying 60/65 spined 5/16th shafts.

I am a southpaw so it might take a little fine tuning but I will start with the back of the cock fletch lined up with the nock index and go from there.

Mark

#7 Post by **Stickbow Hunter** » Mon Apr 03, 2006 7:53 pm

I am a southpaw so it might take a little fine tuning

Mark,

So am I and it shouldn't cause any arra flight problems.

In fact they say us lefties should shoot right wing feathers because of the rotation but I don't think really matters.

Jeff

dan76 · #8 Post by **dan76** » Tue Apr 04, 2006 8:37 pm

I just tried working out my FOC on 2 arrows.

Both are 32 inches long.
1 has a 125grn ribby and the other has a 145 grn ribby.
I get 2 and a quarter inches forward of centre on the 145 for about 7% and 1 and 3 quarters forward of centre on the 125 for about 5.5%.

Does this sound about right. Just for interest i shoot them out of both my long bow and recurve with good results. Recurve 55 pounds, longbow 65 pounds, 29 inch draw 60 pound spined 5/16 keith forrester shafts. but was curious to see how much i could go up in weight in the tips for my next longbow specific arrows in 23/64 65 to 70 pound spine.

Dan

#9 Post by **Stickbow Hunter** » Tue Apr 04, 2006 8:53 pm

Dan,

Your sums would seem to be correct. I like to have more FOC than that especially so with the 125 grain head ones.

With the new shafts I would think that the 190 grain Ribbies would do just fine. They should increase your FOC slightly. Although they are heavier they are longer so I don't think your FOC would increase a lot.

Jeff

dan76 · #10 Post by **dan76** » Tue Apr 04, 2006 9:03 pm

Thanks jeff, i had thought of giving the 190's a go because i heard you mention them on here once before. Can i ask why everyone wants around the 10% mark, i understand about penetration on game and such, want as much weight as is practical, but are there any other advantages.

I use the 125's only for rabbits cause if they hit rocks and such i wont be missing a heavier arrow if a dog or pig walks by.

Do you get your ribbies straight from the maker.??

Dan

#11 Post by **Stickbow Hunter** » Tue Apr 04, 2006 9:13 pm

Can i ask why everyone wants around the 10% mark, i understand about penetration on game and such, want as much weight as is practical, but are there any other advantages.

I believe the arras fly better and aren't affected by wind as much. Ed Ashby is finding that an arra with a lot of FOC penetrates better.

Do you get your ribbies straight from the maker.??

Yes mate I do.

If you want to try a 190 on ya arra just give a holler as I have plenty here.

Jeff

dan76 · #12 Post by **dan76** » Tue Apr 04, 2006 9:18 pm

Thanks jeff will do as i have some leather coming too, just feel a bit funny about intruding on peoples home life, i even felt funny about going around to keiths place, just shy i guess

You wouldn't think it to look at me though.

Dan

#13 Post by **Stickbow Hunter** » Tue Apr 04, 2006 9:50 pm

Dan,

No worries at all just drop around when it suits as I am home most of the time. Come around the back of the house as we often don't here people at the front door.

Jeff

dan76 · #14 Post by **dan76** » Wed Apr 05, 2006 8:55 pm

Definately jeff, shoot me a pm with your address and i will come around for sure.
Thanks
Dan

#15 Post by **Stickbow Hunter** » Wed Apr 05, 2006 9:26 pm

Dan,

PM sent.

Jeff

dan76 · #16 Post by **dan76** » Wed Apr 05, 2006 9:33 pm

Thanks again jeff, see you some time over the next couple of days.

Dan

Ozbow

Forward of Centre - how to calculate? plus n00b fletching Q.

Forward of Centre - how to calculate? plus n00b fletching Q.

Re: Forward of Centre - how to calculate? plus n00b fletchin