Amentum

[PMBardunias]

Think you know how an amentum works? I did until recently. I have read many times that the amentum aids in throwing because the thong lengthens the "lever arm" of the thrower in a manner akin to an atlatl or a sling or catapult. Sounds right, so I really didn't give it much thought. Then I read a paper on the atlatl (Howard 1973) that shows an atlatl and a sling have nothing in common in terms of the physics of their behavior.

A sling adds velocity, and thus energy, through a simple mechanism. If the hand holding the sling moves in an arc with a radius of X, then a stone in a 3' sling held in that hand has to move through an arc of X + 3' in the same time the hand takes to move through the arc shorter radius. Since velocity is distance/time and the time can't be changed and the distance the stone travels in that time is greater, the stone has to move faster.

But this is not what happens with an amentum. If it did it would be a cestrophedone or dart-sling. Instead the amentum acts to increase the time that the javelin is in contact with the hand accelerating it. Longer acceleration leads to a faster missile. The diagram below hopefully makes this clear.

The problem is that you have to release the javelin at a specific point in the arc of your arms motion in order to make it take on the appropriate trajectory. Once you reach that release point, you can no longer accelerate the javelin. With the amentum, your hand can continue on its arc of motion, but stay in contact with the javelin beyond the release point. I have tried this, and you can feel the secondary "boost" as the amentum first becomes taut. It is very difficult to keep from imparting the "sling-like" rotation which leads to your javelin nose diving or tail spinning when you first try, but this is a matter of finding the exact spot at which to tie the amentum. Alternately I suppose you could push the amentum towards the rear if the shaft was extremely flexible like an arrow or atlatl dart, but mine were not.

There are some consequences of this (embarrasingly obvious once you are told!). The amentum length is governed by the arc of the arm throwing the javelin, and you gain nothing from any additional length in the manner that a longer sling makes for faster missiles. As you can see below, you have to let go of the amentum before you cease adding forward acceleration and start simply pulling it downwards. Second, I don't know if winding the amentum imparts sufficient spin to stabilize the javelin, but wrapping it around the shaft allows you to hold the shaft near its balance point, which greatly aids throwing.

[Giannis K. Hoplite]

Very interesting Paul! Thanks for the info,and the diagram is very helpful. What i'm not sure is if one is actually capable to control when the loop will leave his hand. What he can constrol is the movement of the arm. The loop is not held with the fingers,it is just pushed with the inner or side part of the two fingers,and this means that you can't actually hold it. But if your arm follows the correct arch then the loop should leave your fingers at the correct time. Or so me thinks,before trying to throw knowing the mechanics you just explained.
By the way,jusvelins were not meant to be flexible at all,since Xenophon suggests dogwood for them. It' sone of strongest and least flexible timbers.
Khaire
Giannis

[Paullus Scipio]

Whist fairly correct, your diagram is not quite right....the javelin is launched at an upward angle ( optimum 30-35 degrees for max range, depending on conditions) and released before the hand commences it's downward curve ( see my diagram p.50 "Warfare in the Classical World" 1980 of a peltast throw).

A leather thong wound around a wooden shaft has enough grip to apply lateral spin, which not only improves accuracy but can increase distance (the conservation of vertical angular momentum means the javelin precesses a little to the left for a right-handed thrower just as a gyroscope precesses. This precession induces a lateral lift and drag but resists the downward pitching motion.....but we are getting a little too technical !! hock: )

As a boy I spent many a time wiling away an afternoon throwing 'gypsy arrows' with the aid of an amentum. As an archaic sport, it is still practised in Yorkshire with specially large arrows/small javelins....

[PMBardunias]

By the way,jusvelins were not meant to be flexible at all,since Xenophon suggests dogwood for them. It' sone of strongest and least flexible timbers.

That answers the question then. Mine were stiff and the amentum needed to be near the center of balance, which was greatly aided by wrapping the thong around the shaft so I did not have to grip it far forward.

As to "letting go", it sort of just falls off your fingers if you don't hold it wrong, you're right. You should try it.

Whist fairly correct, your diagram is not quite right....the javelin is launched at an upward angle ( optimum 30-35 degrees for max range, depending on conditions) and released before the hand commences it's downward curve ( see my diagram p.50 "Warfare in the Classical World" 1980 of a peltast throw).

You are correct, I show a flat trajectory (slavishly copied from the atlatl paper). As you can see though, whatever angle used the amentum only becomes more useful as the release become earlier. An interesting fact is that the 30-37 degree release is not the optimal angle for the trajectory of a missile, it derives from the angle at which our musculature can impart the most force. Its a trade-off. If you were shooting a crossbow, you would use a less acute angle.

Something that might be apparent, is that when a spear is used in an overhand strike, we actually make better use of our musculature because the "release", which never occurs, is at an even more acute angle.

[Steven M. Peffley]

So, how *do* you calculate the length of the amentum? And does this apply to the type that stays in the hand ( on the finger ) as opposed to the type attached to the javelin?
I question the idea that javelins are not flexible. It is a matter of relative flexibility - they are not as flexible as an atlatl dart, but i have thrown javelins with Ash wood shafts up to about an inch ( 2.54 cm ) in maximum diameter and observed an oscillation in flight; these flew straight but still ocillated vertically for the first third or so of their flight before the vibration dampened out. These are tapered and balanced javelins, with balance points varying from .333333333 to .466666667 of the total length measured from the point of the javelin back along the shaft.

[Paullus Scipio]

They would be Olympic type javelins "old" and "new" style ?.......except 'ash' ????

[Steven M. Peffley]

No Paullus, they are javelins that i have put together for reenactment, various forged iron/steel points mounted on Ash wood shafts.

[Paullus Scipio]

I see....I had imagined 'tapered' to mean like a modern javelin - from front point to balance point, where they are around an inch or so diameter, and then a long taper back to a pointed tail. So just how are yours 'tapered'?

[Peter Raftos]

To be a total pedant the Greek word for the Latin amentum is ankyle (??????). A nice Illustration appears here:
[url:6f1g849u]http://upload.wikimedia.org/wikipedia/commons/e/ef/Javelin_thrower_Louvre_G384.jpg[/url]
From memory some material also appears on the Slinging.org Forum:
[url:6f1g849u]http://slinging.org/forum/YaBB.pl?num=1258917669/0[/url]
[url:6f1g849u]http://slinging.org/[/url]

[Paullus Scipio]

The javelin thrower practises with a 'headless' ( as was customary) and rather long javelin - then as now, an Olympic 'sports' javelin did not resemble the real thing much.......

[Giannis K. Hoplite]

However,the sports javelins were tappered to be heavier near the point. They seem to have been pointy in both sides. Pretyy similar to modern javelins! Their size s no much longer than what military and hunting javelins appear to be. Those are not tappered but they have a spear point which of course draws the weight to the front. So,i don't see so much difference,especially in the tecnique they would use.
I have an excellent dogwood shaft in apropriate measures for a javelin (around my eye level with the spear point and less than 2cm thick) and i'm duying to try it,but i need a better spear point. I have a manning imperial small head,which is cast and too thin. I have tried it in the past and it bents too easily when it stacks to the ground. And i'm in exam period...

[Paullus Scipio]

However,the sports javelins were tappered to be heavier near the point. They seem to have been pointy in both sides. Pretyy similar to modern javelins! Their size s no much longer than what military and hunting javelins appear to be. Those are not tappered but they have a spear point which of course draws the weight to the front. So,i don't see so much difference,especially in the tecnique they would use.
I have an excellent dogwood shaft in apropriate measures for a javelin (around my eye level with the spear point and less than 2cm thick) and i'm duying to try it,but i need a better spear point. I have a manning imperial small head,which is cast and too thin. I have tried it in the past and it bents too easily when it stacks to the ground. And i'm in exam period...

the flight characteristics of a 'double'tapered ( at head and tail) without a 'warhead/spearhead' are very different to a parallel shafted weapon with head....and technique and angle of launch will vary also. Few true javelins exceed 50 inches (1.27 m), and your 'eye level' weapon with a shaft slightly less than 2 cm thick is really more in the 'longche' class.....

[PMBardunias]

So, how *do* you calculate the length of the amentum? And does this apply to the type that stays in the hand ( on the finger ) as opposed to the type attached to the javelin?

Well, the way I did it was mostly through experimentation rather than calculation and it will be different for each arm because of the difference in the arc the hand travels. As you can see from the image above, the limiting factor is the point at which the hand is moving more perpendicular to the vector of the shaft than along it. Figure the first 45 degrees of the arc after release.

I question the idea that javelins are not flexible. It is a matter of relative flexibility - they are not as flexible as an atlatl dart, but i have thrown javelins with Ash wood shafts up to about an inch ( 2.54 cm ) in maximum diameter and observed an oscillation in flight; these flew straight but still ocillated vertically for the first third or so of their flight before the vibration dampened out. These are tapered and balanced javelins, with balance points varying from .333333333 to .466666667 of the total length measured from the point of the javelin back along the shaft.

In terms of flexibility I was pointing out that unless the shaft is flexible enough to go through something akin to "archer's paradox", it probably cannot be pushed from the rear because it will inevitably tail-spin. Getting to neither flip up fromthe back, or nose-dive takes some practice with amentum placement.

[PMBardunias]

Few true javelins exceed 50 inches (1.27 m), and your 'eye level' weapon with a shaft slightly less than 2 cm thick is really more in the 'longche' class.....

What do you think about amentum on longche? This seems to be the case with the spears on the Chigi vase.

[Paullus Scipio]

Few true javelins exceed 50 inches (1.27 m), and your 'eye level' weapon with a shaft slightly less than 2 cm thick is really more in the 'longche' class.....

What do you think about amentum on longche? This seems to be the case with the spears on the Chigi vase.

No essential reason why not....though above a certain size, a heavier weapon puts a severe strain on the two fingers in the amentum.