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Are alloy arrows a viable first choice? Part 1
By Konrad Lau
Konrad Lau writes why he prefers aluminium arrow shafts to the more popular carbon shafts used by most bowhunters today – a viewpoint that is sure to raise a few eyebrows.
This article covers my reasoning for choosing one shaft material over the other and some little shared information regarding both aluminium and carbon composite arrow shaft construction. Both materials have application benefits and problematic technical issues that must be dealt with in order to use either product safely in archery pursuits. For convenience, I have broken the subject into "topics" addressing misleading commentary I have run into over the years during my investigations. At the end I explain the conclusions I have reached in my testing.
I returned to using aluminium arrow shaft material after having a fling (no pun intended) with the "state of the art" carbon composite shafts a few years ago. I too had heard all of the gurus' claims of higher speed, flatter shooting, better penetration, durability and retention of straightness. Mind you, I had been using a set of heavy XX 75 alloy shafts for over a decade. My primary target comprised tightly bound hay bales backed with a hard polyurethane sheet. I had experienced no issues other than a mysterious problem with the occasional arrow wanting to disappear into the brush, never to be seen again (according to popular myth, I should be able to blame those old, sub-par, "bent" arrows, shouldn't I?).
Topic number one
I purchased a dozen of the latest and greatest carbon composite shafts along with my newest modern single cam compound bow. Let me qualify the following by saying that I spent almost twenty years in the elastomer moulding industry. My specialty was engineering polyurethane products for the mining, petroleum and sporting goods industries.
< Nock-end marks from other arrows. Super UNI Bushing still OK (one is more close up).
Polyurethane chemistry is the DNA blood brother to epoxy chemistry and as such all the problems associated with polyurethanes show up in epoxies – particularly their habit of disintegrating when exposed to temperatures over 150°C. Knowing this fact, one of the advertising claims that drives me absolutely "three weasels in a tree crazy" is: "We only use pure carbon in our arrows. Beware of manufacturers using carbon composite."base visible)
In fact, arrows constructed using carbon fibre technology are always carbon composites. The carbon fibres are bonded to one another (or a substrate) with an epoxy matrix or "binder" (we have all heard of epoxy glues). Therein lies the root of the temperature problem referred to earlier. If you are using flame or high heat to remove, install or adjust inserts that have been bonded with an epoxy adhesive and you get that adhesive hot enough to release, you have also more than likely got the epoxy-binding matrix used in the construction of the shaft hot enough to break down.
As epoxies are not thermoplastics (they do not become liquid with heat and then re-solidify to their original properties) the basic structure of the epoxy is partially or even completely destroyed. Regardless, this heat damage seriously weakens the shaft even though there may be no visual evidence. Hydrogen bonds on a molecular level are what have been broken and those bonds are what hold the whole package together.
Topic number two
I have also read "expert" commentary stating that "a carbon shaft is either straight or broken, but never bent." My own personal experience shows this to be patently false. Within a few short weeks of starting to use my shiny new carbon shafts (I was so proud of my finally joining the 21st century!), I began to notice that the towel I was using to wipe the arrows clean after each shot began dragging around the area of the insert (old wet hay can be nasty and who wants that debris all over their new bow?).
> Carbon nock-end damage (stripped Blazer vane)
At first I thought nothing about the small dragging sensation I was feeling, but then one day threads were pulled from the hand towel in a large snag. It was then that I noticed what appeared to be a splinter running lengthwise from just behind the insert for about half an inch. Thank goodness I wear synthetic leather gloves when shooting! Under magnification I discovered that the shaft had multiple fractures extending rearward around the shaft circumference. I retired that arrow – chalking the failure up to a manufacturing glitch and resumed shooting still wondering what had been happening.
About two quivers later, I pulled an arrow only to find the insert had been driven into the shaft and there were splinters poking out in all directions! (See photographic insets.) An inspection of the remaining arrows in my quiver under magnification revealed 35 per cent of all my new shafts were damaged to some extent or another. Only the one that had experienced complete structural failure was bent beyond factory specifications; however, the others – while still being technically "straight" – were eminently not safe for shooting any longer. While a composite shaft will try to return to its original shape after glancing blows or hard impact(s), fractures in the binding agent and/or the fibres themselves can produce conditions potentially dangerous for the shooter and those nearby.
< Carbon point-end collapse after impact with plastic pallet
Another, to my mind much more preferable, way that arrows incur damage is from nock-end impacts. My carbon shafts had only the plastic nock for protection and I eventually lost a few from this issue (see photographic inset). Aluminium alloy shafts are either supplied with a ground swaged taper (very effective at knocking aside all but the most direct of hits) or tapered bushings of the UNI or Super UNI-bushing design.
> Carbon point end collapse close up
If you look at the pictures provided, you will see numerous marks on the bevelled face of the Super UNI-bushing installed on this one subject shaft. Virtually all of the arrows in my current quiver display similar impact evidence. One can only imagine how many times the nock end of my arrows would have been obliterated had there only been a square face of carbon composite for the incoming point to glance from! The happy news is that the nock can easily be removed and replaced and if the hit was more perfect than usual, the entire bushing/nock assembly can be removed and replaced resorting that arrow to "as delivered" condition in 99 per cent of instances.
Of course, there are carbon composite shaft manufacturers who provide "outserts" or "collars" for nock-end protection, but we are forced into analysing the cost/benefit ratio of initial purchase price vs practicality using these designs.
Back to the safety aspects of the discussion...
Every arrow manufacturer now suggests careful inspection of each and every arrow after each and every shot regardless of its construction material. They suggest the additional inspection procedures for composite shafts prior to every shot by twisting and flexing the shaft and listening for cracking sounds or feeling a relaxing of the shaft. As stated earlier, I have also found that cleaning each shaft with a couple of strokes from a gloved hand or towel will quickly reveal most shaft damage related to impacts.
Danger: Do not use suspect shafts!
Resist the temptation to use shafts of unknown integrity. It may just save you, one of your family members or friends a trip to hospital. Scavenge the inserts, bushings and nocks for later use and break the damaged shaft so there can be no confusion or temptation later on. I know, breaking expensive shafts makes me cry too. If you are still not convinced, search the internet for "exploding carbon arrows". A word of warning: Have all of the faint-hearted, squeamish types (like me) and children leave the room before doing your search. The pictures are not pretty. Of course, shooting underspined shafts in high draw weight, hard cam bows can result in the same thing but in this instance we are discussing attempting to shoot damaged arrows. Be safe!
Topic number three
I have also heard "experts" say, "Carbon arrows are more accurate than aluminium."
I suggest we review some factors that contribute to intrinsically accurate arrow shafts aside from using a shaft that is correctly spined for your bow:
Consistent weight from arrow to arrow and lot to lot.
Shaft wall concentricity tolerances.
Most archers are familiar with the need for controlling completed arrow weight and straightness, but an overlooked specification ignored in advertising literature and pro shops around the globe is wall concentricity. If one can imagine peering down a shaft with no nock, insert or point; measuring the wall thickness around the shaft's circumference (ID to OD) and finding that wall thickness not the same all the way around you will have something like two circles, one within the other, but not centred. The thicker side of the wall will not only be heavier, but stiffer as well.
Years ago it was discovered in the firearms industry that minor imbalances in bullets (voids or out of round projectiles) dramatically affected bullet flight and group sizes. Rifling was incorporated in firearms design to deal with those inherent bullet imperfections as rotating the imbalance around the axis of its flight path averages those weight imperfections around that axis and much improves accuracy. The advent of copper jacketed bullets reintroduced this concern about concentricity and great strides have been made in the industry to remove the problem from the equation. In fact, many projectile manufacturers advertise the concentricity of their bullet jackets. Those striving for the utmost in accuracy (match shooters) do pay attention.
The construction process used in carbon fibre lay up generally produces an overlap along one side of the shaft. If one measures the outer diameter of modern carbon shafts, one will find that dimension is quite well controlled (usually by a grinding/polishing/coating process); however, it produces a stiff side in the shaft. That stiff side is referred to as the "spline". Traditional archers using wooden arrows have long oriented the grain of the shafts to achieve similar flight from all the arrows in their quivers. Usually, arrow construction oriented the spline or stiff side in the 12 o'clock position (when viewed from the archer's perspective). In many internet forums the term spline is often confused with "spine".
An arrow's static spine is an arrow shaft's resistance to flexing, measured in thousandths of an inch of deflection, when a weight (typically 1,94 pounds) is hung from the centre of the shaft with the shaft supported on either end at a specific distance (typically 28 inches). If one were to take the time and measure the spine of a carbon shaft at different points around its circumference one will find a spot where the deflection is less than in other spots. This is the place where the composite is thickest and stiffest along the shaft wall – its spline.
Because the relative density of that material overlap is different than the other parts of the wall section (usually heavier), there is also an inherent weight imbalance in the shaft and/or lack of wall concentricity. The stiffer the spline, the more weight is off centre. In most respects a bent arrow effectively has the same weight distribution problems as when a portion or portions of the arrow's mass are off centre. This condition produces difficulties in stabilising the shaft rotationally as well as problems in duplicating arrow flight from arrow to arrow. Even when a shaft's wall thickness is tightly controlled, if the material consistency is not tightly controlled (more binder and less carbon fibre or vice versa) around the circumference of the shaft, weight distribution and dynamic characteristics are also negatively affected.
I am sure you all have also heard of the process euphemistically referred to as "arrow tuning" whereby one shoots arrows from shooting machines like the exceptional Spot Hogg Hooter Shooter. Through a trial and error process one rotates carbon shafts' nocks so all the arrows in a batch leave the bow and hit the target in the same way.
The common practice of "tuning" carbon arrows is essentially a work-around solution attempting to compensate for manufacturing imperfections within the carbon shaft wall. Traditional archers shooting wooden arrows can only drool at the prospect of turning nocks.
Topic number four
The gurus will also tell you that "lighter, faster arrows shoot flatter so you don't have to estimate range as accurately".
Yes, by and large, carbon composite shafts can be constructed with similar spines as alloys and achieve lighter shaft weights. But when looking at objective data found in the Easton Archery 2013 Bowhunting catalogue and elsewhere we find many of the "carbon" offerings similar in gpi weights (grains per inch of shaft length, there are 7 000 grains per one English pound) to their alloy counterparts. For example, the 2413 XX-78 shaft weighs 10,4 gpi. Whereas the Carbon Injexion weighs 8,9 gpi and the widely popular Axis N-Fused Realtree weighs 9,8 gpi. When viewed as a percentage, the lightest of our examples provides only a 14,43 per cent reduction in weight:
29 inches X 10,4 gpi = 301,6 grains total weight of the 2413
258,1 grains weight for the Injexion X 100 divided by 301,6 = 85,57
100 – 85,57 = 14,43 per cent reduction from the 2413 shaft
Using the same calculations the 9,8 gpi Axis is only a 5,77 per cent reduction in weight. The questions then follow: Does a reduction of six per cent in shaft weight translate into a six per cent increase in velocity or a six per cent reduction in trajectory and how does that reduction in arrow mass affect kinetic energy and momentum or penetration potential?
For the sake of our discussion we will base velocities on an IBO bow speed of 318 fps (feet per second), a 70-pound draw weight, a 29-inch draw length, 200 grains for point, fletching, insert and nock with no weight on the string.
My calculator shows arrow velocities of:
XX-78 2413: 500 grains = 258 fps
Axis: 484 grains = 263 fps
Injexion: 458 = 272 fps
The speed gained by the theoretical Axis arrow translates into a 1,93 per cent increase, but using the Injexion shaft boosts speed by 5,4 per cent. So the answer is "yes, kinda". A radical reduction in weight does produce an appreciable increase in arrow speed; however, does that apparent increase in speed actually produce a flatter shooting arrow?
Once again, whipping out the calculator we find when using the previous data and "zeroing" the bow at 20 yards and then shooting at 40 yards with the 20-yard pin (I know it can get a little confusing, but I don't hear any weasels... yet):
XX-78: 500-grain arrow, 258 fps = 22,1 inches of drop from 20 to 40 yards
Axis: 284-grain arrow, 263 fps = 21,3 inches of drop from 20 to 40 yards
Injexion: 258-grain arrow, 272 fps = 19,8 inches of drop from 20 to 40 yards
Using the same equations as before, the Carbon Injexion shows a 10,4 per cent reduction in drop from 20 to 40 yards. On paper this sounds like quite a bit, but reviewing the differences in inches we find a savings of only 2,3 inches. The "standard" accuracy minimum that I have adopted for bowhunting is Chuck Adams' recommendation equating to one inch of group diameter per ten yards of distance (ie one-inch group at ten yards, two inches at 20, three inches at 30 and a four-inch group at 40). If that specification is good enough for him, it should be good enough for me too! However, I must admit that some days I think it would be easier just to go back to the 375.
While a reduction of 2,3 inches of drop may sound like a considerable amount, it has been my experience that at forty yards many if not most archers have difficulty in maintaining the prerequisite four-inch group size. Since the vast majority of game animals are killed within a 20-yard radius, I think the 2,3 in reduction in trajectory becomes a moot point.
I prefer a heavier weight arrow for all applications. You will note that in the beginning of the carbon shaft craze, all of the hoopla was about light and fast. Now the trend is toward heavier and heavier carbon shafts. Why do all the top archers use heavy for diameter shafts then? I'll tell you why: better wind bucking, more retained energy down range, greater durability and the supposed flattening of trajectory is statistically minimal.