[Ed Ashby]

in reply to: Dynamic spine #58108

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

[Ed Ashby]

in reply to: ABS Ashby single bevel BH #58065

Dave’s right; today we have several excellent single-bevel BH’s from which to choose, the new Grizzly 200 gr. among them, and I hope we have even more in the future.

Ed

[Ed Ashby]

in reply to: Steel Force Broadheads #58062

Can you post a picture? If these are the same as the ones I’ve seen they don’t have a very desirable ferrule fade in. It’s bulbous and abrupt. I’ve tested several of the double-bevel Steel Force, but have not yet tested their single bevel version.

Ed

[Ed Ashby]

in reply to: Experimenting with EFOC is Expensive! #55455

Richie, you’ll virtually always find a difference in the dynamic spine when going from a lower GPI shaft to a higher GPI shaft (even when they are of the same brand). Here’s why. The inside diameters (ID) are generally the same (so the same diameter inserts will fit). The shaft’s extra GPI are generally gained by use of either a larger outside diameter (OD) or use of a somewhat different material and/or construction technique.

To change the GPI manufacturers have add weight somehow. That changing the material or construction technique would influence the dynamic spine is obvious, but the other type changes are more complex.

A larger shaft OD means it effectively reduces the degree of center shot of your bow. When “all else is equal” that means you’ll need a weaker dynamic spine (or more tip weight) to tune properly. Offsetting that is the fact that the higher GPI shaft has greater shaft mass towards the arrow’s rear. This means more ‘push’ from the rear due to the shaft’s mass, which results in greater shaft paradox, and slower recovery from paradox. The greater paradox resulting from more weight at the shaft’s rear then requires a stiffer dynamic spine or reduced tip weight to tune at the same shaft length. When the ‘new shaft’ is both larger in OD and has a greater GPI it becomes a balancing act to counteract the opposing forces affecting the dynamic spine caused by the difference in the net effect between these shaft dimensions.

The explanations can become even more complex. Sometimes there is a tiny difference in shaft ID too. The ID between the lower and higher GPI shafts will still be close enough that the same inserts can be used, but they won’t measure exactly the same ID. Use of a Varner caliper will tell you what’s been changed.

Static spine is merely a rough guideline. Every specific shaft is going to tune slightly different. It’s precisely like handloading a rifle cartridge; changing any parameter; even just the primer or brand of case; changes everything about how the load performs.

Hope that explains your question,

Ed

[Ed Ashby]

in reply to: Experimenting with EFOC is Expensive! #54863

Sapcut wrote: Well I now have the GT Ultralight 300 at 31 5/8″ from groove to end of insert, (then another 1/4″ to back of broadhead). 825 grains and 32.5% Ultra-EFOC shooting from a 71 lb @ 31 inch Widow recurve. Very simple loaded arrow. It just gets better and better.

CORRECTION:
829 grains and 32.3%

Richie

Stick a good BH on the end and I’d shoot buffalo all day with that setup, Richie. Now go and shoot something big with it so we can see what it does! 😀

Ed

[Ed Ashby]

in reply to: Experimenting with EFOC is Expensive! #53204

M, you need to see what the bare shafts do, in comparison to your fletched shafts. With that much fletching area ANY EFOC arrow is going to be flying straign before it hits the target, even if the dynamic spine is WAY, WAY OFF.

Ed

[Ed Ashby]

in reply to: A Starting Point for EFOC Arrows? #53093

Bert’s ‘spot on’. This is where it starts. If we want to get an assortment of information on what EFOC/Ultra-EFOC arrow setup(s) are working from verious bows it has to be through a lot of folks cooperating and sharing their results with each other. That includes the results they get with the fletching setup too.

Even when we have the information, you’ll still need to remember that it will still just be a starting place. Many small factors; such as thickness of your arrow plate, minor differences in the degree of ‘center cut’ between 2 ‘like bows’ or quality of your release; can alter your individual ‘best arrow setup’. The information from others will be just like the ‘load data’ information in a reloading manual; “here’s what might work, but it might not be the ‘best load’ for your individual weapon. Start here and do you own load development”.

The nice thing is, once you have your individual ‘pet load’ it won’t change, so long as you bow setup stays the same! You only have to develop your ‘pet load’ once. But there’s nothing to keep you from looking for other ‘pet loads’ if you like. It’s always fun.)

Ed

[Ed Ashby]

in reply to: A Starting Point for EFOC Arrows? #52626

Sounds like you’re all set to do some bare shaft work, chad!

Ed

[Ed Ashby]

in reply to: tungsten-footed woodies; considerations #49348

Dave, I didn’t address it because I haven’t tried it! I’m trying to learn what happens from your experiments! Wood doesn’t behave like the synthetics, because of it’s cellular structure. The rod should definitely strengthen the section of shaft it’s in, and that could be a big plus. The most common fracture on wood shafts (on hard, real tissue impacts) is just where the taper starts fading into the head’s ferrule. Will the rod create a new weak spot just back of where it ends? Don’t know, but the wood will have a better chance at NOT showing this weakness (where an Internal Footing ABRUPTLY ends) than the synthetics do.

Ed

[Ed Ashby]

in reply to: Excel Spreadsheet Momentum and KE Calculator #48859

Str8arrow wrote: Ed, why would someone want to know their KE?

1) It can tell us what KE is NOT useful for.

2) It’s what’s used to measure bow efficiency.

3) If you chronograph at the bow and again downrange it can tell us a lot about the flight efficiency of an arrow setup.

Those are the main reasons arrow KE can be a useful thing to know.

Ed

[Ed Ashby]

in reply to: tungsten-footed woodies; considerations #48240

Dave, there’s no such thing as a failed experiment. ➡ From each we learn something; each expands our knowledge base. 💡 I’m following your weighted wood shaft experiments with great interest! 😮 You’ve already shown that you can get into the EFOC range :shock:; perhaps with a bit too much overall arrow weight for your needs, but it’s useful information for folks shooting higher draw weight bows! 😀

Ed

[Ed Ashby]

in reply to: Experimenting with EFOC is Expensive! #48237

A significantly lighter shaft will give you a big boost in FOC. As the FOC gets progressively higher the more gain you can make (in FOC) by reducing the fletching size – and the longer rear leaver (the fletching’s ‘stearing arm’) your arrows will have at the higher FOC will let those smaller feathers exert substantually more pressure than they would at lower FOC.

Ed

[Ed Ashby]

in reply to: Excel Spreadsheet Momentum and KE Calculator #48233

Let us know if it works there too.

Ed

[Ed Ashby]

in reply to: Excel Spreadsheet Momentum and KE Calculator #47674

I don’t know the formula formats for that program. It should be very similar, but the ‘formula format’ (the “=Sum” part of the equation) might be different.

Ed

[Ed Ashby]

in reply to: A Starting Point for EFOC Arrows? #46919

Steve, I’ll second that WOW! That is truly outstanding. Wish there were more archery shops delivering such service!

Ed

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