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Rear Weighting Stiffens Spine??
In a few threads lately some people have suggested adding weight to the rear of the arrow to stiffen the spine. I can’t grasp the physics of this. Adding a wrap will marginally stiffen about four inches by adding wall thickness and same for longer or more vanes. But that effect would be negligible. Some have suggested weighting the nocks. The tiny bit of velocity lost by the incremental weight change would be unmeasurable either.
I don’t get how adding weight to the back of an arrow can change the spine. To me, only changing what the shaft is pushing affects the dynamic spine.
What am I missing?
I believe the notion is that the extra weight slows down acceleration which reduces the dynamic flexing of the shaft at release. I tend to agree the impact is likely nominal when adding things wraps, heavier vanes, or lighted nocks.
I tend to agree.....maybe this has a significance if you are a Olympic or Pro shooter, but for most it`s just over tinkering. Getting a arrow to come out of a bow to kill an animal proficiently isn`t that difficult.
Basically adding weight to power stroke without affecting spine. It slows your arrow speed so it may seem like it stiffens spine.
The same effect would be if you added a heavier knock
Sorry.....I was slow The guys before me got it covered
It does have an impact but much less so than changing weight at the tip. It changes the dynamics of the arrow by increasing/decreasing inertia at the tips of the arrow.
It will most definately change the deflection of the arrow on impact, the opposite effect of adding weight upfront during the powerstroke of the bow system.
HDE. Not sure what you mean. Are you saying it does change dynamic spine?
JTV, that is the exact reason I went with the Velocity XT shafts myself. Lighted nocks in back, 125 grain heads up front. 420 grain arrow shooting just under 300fps. Works like a champ.
The thing is this… An arrow gets flexed twice on every flight: once on launch, and a second time on impact.
The heavier tail end on impact has the same affect as a heavier front end on launch. And if your arrow is not flying straight on impact, then a heavier tail section can try to force the nose off of its original line of penetration, which increases resistance. That’s why high FOC can be helpful, and why tuning well such that your arrows are flying perfectly point on is a big deal.
Also why mechanicals do not penetrate nearly as well as a tapered two-blade. Just a lot more resistance upfront.
I understand that GF, but I'm talking about adding weight to the rear of an arrow to supposedly stiffen the dynamic spine for flight characteristics. Terminal performance is not part of this question.
But mechanicals kill way better, so that's more important to me :)
Matt covered that fer ye: more weight = slower acceleration = fewer Gs flexing the shaft at launch.
But you did ask about what HDE posted...
I think it’s important to realize that you are never changing the spine of an arrow. That remains constant. Spine is the actual deflection in a shaft. You do however, need a stiffer spine to stabilize if you are either adding more weight or increasing arrow length. In the adding weight category this is THE most pronounced at the tip. This makes it incredibly whippy as you are only pushing from the back end of the arrow with lots of weight at the opposite end. Adding weight to the back of the arrow, has almost no effect (moderately speaking) as that is directly where the torque is being applied. So,. You likely don’t need to go to a stiffer spine if you are adding weight at the back, but you would to add weight to the front... OR you could potentially stabilize a shaft by adding more weight to the front if it was too stiff to begin with. Of course then there is the whole arrow being slower thing because you added weight to the back, in turn lowering your FPS, but for real world application, I can’t inagine that would change the spine required, unless you’re throwing in a 100gr insert back there or something
Ambush, I realize you know most of this, I am just also explaining for potential newer bowhunters that are following along
You are never changing *static* spine of an arrow, but that isn't the relevant thing here. Dynamic spine is, and that IS changed by adding weight at either end of the shaft.
Other than adding mass to slow the acceleration of the string (it would take a lot to be appreciable) how can it affect the dynamic spine?
I think that is it. Ergo, not appreciable.
So let's buy a speed bow then slow the arrow via adding weight to tune the spine. This is over thinking things imo unless a guy is into competition.
All good info. Bottom line avoid adding weight to the rear of the arrow at all costs 99.9% of the time.
Ambush, you are correct in that a shaft has a static spine that can be measured. The ASTM measurement for static spine is 28". The old AMO method is 26". And the static spine doesn't change. But the dynamic spine does change based on a number of variables. By adding 15 grains to the nock end of an arrow I can increase the dynamic spine by a few pounds. Stu Miller created a calculator that is based on recurve bows but still shows the effect changing one or more variables has on the dynamic spine of the shaft. YOu can find it here just for reference - https://www.3riversarchery.com/dynamic-spine-arrow-calculator-from-3rivers-archery.html
Here's a blurb from Easton about dynamic spine.
"Then there is dynamic spine, which describes the way an arrow reacts from the stored energy of a bow as it is shot. Too many factors determine the way an arrow is going to react when shot out of the bow, and because of the nearly unlimited variables in determining dynamic spine, Easton hunting arrows are measured using static spine. You can manipulate the dynamic spine of an arrow and make it act stiffer when shot from a compound bow by decreasing peak bow weight, point weight or the point/insert combination, using heavier bow string material or adding more strands to the string, heavier vanes, heavier serving material and/or nocking point and shortening the length of the arrow."
Ambush - I think your question to me has already been answered in other posts after mine and your question.
“So let's buy a speed bow then slow the arrow via adding weight to tune the spine. This is over thinking things imo unless a guy is into competition.”
Or unless a guy wants to actually TUNE his bow instead of saying “Aw, screw it! I’ll just shoot a mechanical instead.”
There’s a lot more to this whole deal than FPS. And penetration is high on the list.
I shoot a .400 spine with 190 grains of point weight. I draw 28"s and shoot a 27.25" arrow. I am shooting 58#s this year. They spine well but I add a 7"wrap and a 25 grain lighted nock it does stiffen them a tad. (Dynamic) spine. I am shooting a new Traverse. A 420 grain arrow at about 270. It blew through a big does shoulder the other day and passed through her daughter from last year like butter. Speed and some weight are nice. Shawn
Ok Shawn, then answer me this. How do you like the Traverse? It’s a bow I have my eye on, but I like products to have time to develop a history before I buy.
I think Elkman summed up my belief.
And other than slowing down the string (power stroke) you cannot stiffen the spine by adding weight to the rear of the shaft. If you are under spined and you can’t shorten the shaft or reduce point weight, then new shafts should the solution.
You're still confusing static spine with dynamic spine. They aren't the same. You cannot change the static spine of a shaft. You absolutely can change the dynamic spine.
“If you are under spined and you can’t shorten the shaft or reduce point weight, then new shafts should the solution.”
In the Stickbow world, you can always build out the sideplate a touch; a bow cut farther from center needs less spine, so basically you can tune the bow to the arrows you’ve got. But Tune being equal, a stiffer shaft is better suited to hunting applications. And the lighter the rear of the shaft, the less it will force the front end off line upon impact.
Phil, I am well aware of the difference and am not confusing the two at all. Are you saying that the simple act of adding weight to the rear of an arrow will give the same affect (or nearly so) of subtracting the same weight from the point? And I don't really consider miniscule, theoretical changes valid in a real world application.
I should also preface by saying that I'm referring to modern compounds, using a release and usually shot off a drop away rest.
No matter what you shoot adding weight to the rear stiffens dynamic spine. Really don't notice a difference til ya add 50 grains or more IMO. The Traverse is for me a great bow. There is absolutely no vibration or handshock at release and it is very quiet. I shoot it well at least for me. 30 yd group here. Shawn
Ambush, "Are you saying that the simple act of adding weight to the rear of an arrow will give the same affect (or nearly so) of subtracting the same weight from the point? And I don't really consider miniscule, theoretical changes valid in a real world application. "
Yes I am saying that. Again, from Easton -
"You can manipulate the dynamic spine of an arrow and make it act stiffer when shot from a compound bow by decreasing peak bow weight, point weight or the point/insert combination, using heavier bow string material or adding more strands to the string, heavier vanes, heavier serving material and/or nocking point and shortening the length of the arrow."
GoldTip makes a weight system for the nock end - https://www.3riversarchery.com/gold-tip-weight-system-nock-adapter.html
You can add weight in 20 grain increments.
Phil, everything mentioned in the Easton link as per rear weighting of the shaft can be accomplished by putting the same weight in crimp on metal nock collars on the string. Simply put, slowing the string by inhibiting the power stroke. Counter productive, IMO.
Not if it gets you tuned correctly.
An arrow that comes off the bow flying off-line will scrub off a chunk of that speed you’re after very quickly, and then you can end up with compromised accuracy/penetration and no significant gain in terms of trajectory.
And as far as “practical implications” go....
JMO, if this stuff matters at longbow speeds, then you speed-freaks should be paying mighty close attention...
That's true. There are a number of ways to achieve the same goal.
I have a set of GoldTip nock weights that I tried one time because I had a some arrows I wanted to shoot. Never kept them on and ended up changing arrows. It's far easier to go up one spine and play with the length and/or weighting the front end.
Or helical fletching ...;)
Don’t confuse people, Junior!
Big, helical fletchings can HIDE a multitude of sins; tuning RESOLVES them.
I like how big helicals look, but they’re noisy and create a lot of drag. With most of my bows, I’m tuned to where I need no fletching at all out to 20-some yards, but I’d like to get dialed out to 30...