For starters you might check out Ashby’s latest update to the 2008 studies, just posted to the library a few days ago (at the bottom of the contents list). While not specifically addressing the concept, it contains some nice summaries of how tip weight/FoC reduces spine flexing in flight and on impact thus increasing penetration. The Doc seems to be taking another break but I’d hope before long he’ll be back to address this good question and likely point you elsewhere in his studies. dave

Clay Hayes wrote: can someone point me to a good source to bone up on my limited knowledge of this subject? I’m mainly looking for a good description of the relationship between tip weight and how it affects arrow behavior (flex, spine, whatever).

As I recall, a change of 35-50 grains of tip weight or 1″ of shaft length will change dynamic spine by 5#.

Clay, below is my post from the “A Starting Point for EFOC” thread. It might have some of the answers you are looking for.

RE: A Starting Point for EFOC Arrows?
Posted Friday, February 5, 2010 at 12:55 PM

The best starting point is with some carbon shafts. It’s much easier to build/tune an EFOC arrow on carbon shafts than on any other shaft material.

You don’t mention anything about the degree of center shot on your bow, but I’m guessing it doesn’t have much of an arrow shelf cut into the bow. I’d suggest that you start with a light weight carbon shaft, such as a GT Hunter of Ultralite in 55-75 spine. Next get some 100 grain brass inserts and some 125 grain steel broadhead adaptors and some 190 grain glue-on field points. Mount that combination onto ONE full length BARE shaft and see how it shoots. If it shoots ‘weak spine’, begin shortening the shaft (from the shaft’s nock end) in 5mm increments and shooting a number of shots after each cut is made. See if you can get this combination to ‘tune’. i.e.; get the point of impact of your ‘group center’ to be neither right nor left of your aiming point. (Remember, you’re shooting a number of shots after each shortening of the shaft, so make you adjustments based on the center of the group you’ve shot.)

If you get to your minimum arrow length and the bare shaft tuning is still showing weak spine, remember that you still have things you can do to stiffen the shaft’s dynamic spine. First, unless you have no room to do so, you can thicken the arrow plate. This lets a weaker spine tune. Second, you can add an Internal Footing. This shortens the ‘working section’ of the arrow and stiffens the dynamic spine.

If that doesn’t work, then you need to try using a lighter tip weight. Try changing the steel BH adaptor to a lighter one and/or going to a lower weight for the field point.

If, on the other hand, the full length shaft tunes ‘strong spine’ your options are fewer. One is to increase tip weight by using a weight back of the insert. Some inserts accept screw-in, add-on weights. You can also add a short section of 6mm diameter steel or brass rod back of the insert … or any of many other weights you might have available, such as a 6mm diameter rifle bullet or the shank portion of a 6mm diameter bolt – but be sure to keep track of the exact weight of the add-on piece(s).

Another option, when the spine is too strong, would be to cut your arrow shelf deeper into the bow; but not many will want to take that option.

As a last resort you may have to try another shaft, in a different spine range. Also remember that you can change to a shaft IN THE SAME SPINE RANGE that has a larger or smaller outside diameter. This effectively changes the degree of center shot of your bow. Using a smaller diameter shaft (of the same dynamic spine) is like cutting your arrow shelf deeper, it lets a stiffer dynamic spine tune to the bow. Using a larger shaft diameter (of the same dynamic spine) is like building the arrow plate out; it lets a weaker dynamic spine tune properly.

Hope that wasn’t too confusing, and helps a bit.

If it’s a ‘magic formula’ that says increasing point weight ‘X amount’ results in ‘y amount’ of change in dynamic spine your pretty much out of luck. Besides the bow design factors affecting the change you’ll get, there are the pure mechanics involved. This involves the the changing center of mass and the center of pressure as FOC is altered. These alter the flexional characteristics of the arrow.

As arrow FOC goes up the balance point of the arrow moves forward. Think of your arrow shaft as a see-saw having one side of the balance beam longer than the other. Because of the leverage involved, every time the ‘short side’ gets shorter and the ‘long side’ gets longer it will require a greater amount of weight added on the ‘short side’ to move the balance point a set distance closer to the front end of the arrow (the short section).

Conversely, every time the ‘short side’ gets shorter and the ‘long side’ gets longer it requires less weight added to the ‘long side’ to move the balance back towards the arrow’s rear. That’s why, as you get closer and closer (and into) Ultra-EFOC it becomes more and more important to reduce weight at the shaft’s rear; removing just a little weight from the shaft’s rear gives a change in FOC that’s equivalent to a much large increas in weight at the arrow’s tip – and the closer to the arrow’s rear you can remove that weight the more effect it has on the FOC. Ergo, one of the big (FOC) benefits of using the smallest fletching possible when you’re dealing with arrows that already have very, very high FOC.

In effect, adding 50 grains of point weight to an arrow having normal FOC arrow has a far greater effect on the balance point than does adding 50 grains of point weight to an arrow that already has EFOC or Ultra-EFOC, and removing 10 grains at the rear of an Ultra-EFOC arrow can yield an increase in FOC equaling that obtained by adding an additional 50 grains to the tip weight.

To reiterate, any change in arrow FOC means the arrow’s center of mass has changed and the center of pressure will be altered. This alter the flexional characteristics of the arrow, affecting the dynamic spine when shot from a specific bow.

Was that the info you were looking for? If not, let’s try again.

Ed

Dr. Ed Ashby wrote:

Conversely, every time the ‘short side’ gets shorter and the ‘long side’ gets longer it requires less weight added to the ‘long side’ to move the balance back towards the arrow’s rear. That’s why, as you get closer and closer (and into) Ultra-EFOC it becomes more and more important to reduce weight at the shaft’s rear; removing just a little weight from the shaft’s rear gives a change in FOC that’s equivalent to a much large increas in weight at the arrow’s tip – and the closer to the arrow’s rear you can remove that weight the more effect it has on the FOC. Ergo, one of the big (FOC) benefits of using the smallest fletching possible when you’re dealing with arrows that already have very, very high FOC

Dr. Ed

Has someone tried having the knock stay on the bow string when the arrow shoots? That would loose 10 grams on the very back end every time.

Tom

Tom,

Speaking of nocks, I do cut 1/8″ or so off the male portion of the nock. It reduces the rear by 2 grains. As Dr. Ed said, every little grain counts.

Proper weight management = most efficient FOC

Exactly what the doc, said is what I have been experiencing with my trying to get to 35-40% UEFOC, I added another 245 grains to my all ready front end loaded arrow and it only moved vary little, my shaft is to heavy.

It is a combination of a super light back end and a super heavy front end, to get to the UEFOC.

I am thinking of going to ether a 400-500 ultra light carbon arrow, if I go with the 500 it is 6.4 grains an inch, that’s only a 179.2 for the shaft weight 😯 I think I will need to do an internal footing to stiffen the spine, to be able to handle the heavy front end.

The 400 is 7.4 and that’s 207.2 for my 28 inch draw, you might say that’s only 28 grains, more that the 500, yes but it’s 14grains off of the back end of the arrow and that will move my UEFOC up.

With the 500 it is lighter and has a softer spine, yet I can stiffen it up using internal footing as I have done with my current arrow.

It’s all fun, I will get to 35+ UEFOC and I will post the results.

Kingwouldbe wrote:
It’s all fun, I will get to 35+ UEFOC and I will post the results.

Can’t wait to see what you come up with, Kwb.

I’m waiting too, David. Not being able to do much these past 19 months has me very frustrated. So much I want to be experimenting with and yet am unable to do so.:x

Ed