Dynamic Spine Test
Equipment
A few years ago on a discussion about dynamic vs static spine, one poster demonstrated a method where he put his shafts in a clamp to find the stiff side. The discussion ensued that too many forces were at work to skew the results. Then on the current Four Fletch thread someone mentioned that they always use a spine checker to put the stiff side of the static spine in the same position. Static spine and dynamic spine may be different on the same shaft and it is dynamic spine at work when we launch an arrow.
It just happens that I'm doing up a dozen new Easton Axis shafts. I quit static spine checking the Axis shafts after doing several dozen for myself and friends. Any perceivable difference was so minute as to be negligible for consideration.
When prepping shafts, I cut one inch off the nock end and precision grind it. I cut to length, foot and precision grind the insert end. For this experiment I put a ball bearing in each end and put the shaft in a pipe clamp as parallel as possible. I applied pressure both slowly and quickly (same result) and moved the shaft end around on the clamp to eliminate that variable. Once I determined the consistent stiff side, I marked that, then released and reapplied pressure while turning the shaft in quarters. A couple were consistent, a couple pretty consistent and the rest somewhat unpredictable. The inconsistent ones would be the ones with a very nearly perfect spine tolerance.
I also tried a couple of ICS Hunter shafts and two cheap carbons. A definite stiff side showed consistently on them. Which I somewhat expected as I used to static check them and a stiff side was usually very apparent. I had built a very accurate static spine checker years ago, but have since repurposed those dials and parts, so I couldn't check the dynamic spine results against static spine.
I believe the results are legitimate and maybe useful for any one shooting shafts with less consistency. Easy test to duplicate.
Shaft in clamp
Shaft in clamp
Can’t add pics to previous post.
Both ends have a ball bearing to eliminate radial deflection force.
Both ends have a ball bearing to eliminate radial deflection force.
Did you spot drill the clamp faces to help locate the ball bearings? I would have arrow shafts and ball bearings flying everywhere.
That is pretty smart thinkin' Ambush. I would assume that the stiff side would be the concave side of the shaft ?
Midwest, no dimples. Anything that would prevent the ball from rolling with as little force as possible may alter the results.
buckeye, that is a good question about which side is the stiff side. From my steel fabbing days, the weak side of a column would collapse first but I'm not absolutely sure that applies here. But at least you could put them all the same.
Ambush, pretty ingenious! I do have a question on dynamic spine. My impression is that static spine is the deflection in a shaft when a given weight is placed on a shaft that is supported at two points at a given distance apart whereas dynamic spine is how a shaft deflects when accelerated from a bow. Two different models of arrows may have the same static spine but a different dynamic spine. A heavy arrow may deflect less when shot than a lighter arrow with the same static spine because it is being accelerated more slowly all others things being equal. The weight distribution on the arrow may also impact the dynamic spine. IMO your device is an ingenious way to measure the weak side of the shaft or to determine if there is one. If the deflection was measured at a standard shaft length and with a standard load it could also be another method of measuring static spine. I think that the shaft has to be accelerated to measure dynamic spine. I really appreciate your efforts and the results of your test as I too have stopped trying to measure the stiff side of a shaft and just buy higher quality shafts. Thank you.
Yes, the weak side collapses in and the stiff side extends out. If this were a complete round. The inside diameter would be the limber side and the outside diameter would be the stiff side(at rest). I say at rest, because while in an arc the two sides have "equal rigidity". The weak side compresses and the stiff side stretches(as far as the molecules are concerned) to keep all things balanced. If you could measure the arc with accuracy, you would also see variances along the length of the shaft where there are stiff spots longitudinally.
This was a unique test method but, not without flaws. Those clamp faces are not perfectly parallel. It would be more accurate with properly sized dimples for the bearing balls. And they would need to be lubricated as best as possible. The best method for replicating static spine and to some extent the affect of dynamic spine is with an actual spine tester, using the center of the arrow with a dedicated weight and a dial indicator. You can measure the deflection as you rotate the arrow. Or, use roller bearings at all three contact points so the stiff side self-centers.
Ambush,
Nice engineering talent you have. You're definitely providing "food for thought".
My approach to arrow building is via applying the KIS method that begins with a high quality, small diameter arrow, and build each one as identical as possible.
My objective to to build arrows that are accurate/repeatable as possible and with the least amount of potential winddrift. For me, this is most critical while still hunting.
With shooting small diameter arrows, penetration is improved and winddrift is reduced. However, the margin of error regarding spine to force applied is reduced (I believe), I feel it's imperative to orientate the stiff side of the arrow the same on all the arrows. (should reduce the step of nock rotation during tuning)
Just my philosophy.
You could just shoot each bareshaft and index it until it hits with fletched shafts. Put a folded piece of trimmer wire in the nock that matches vane total weight. No substitute for actually shooting each shaft.
Xman, setting the ball in a dimple will create frictional resistance which lube will only lesson, but not eliminate. On a flat surface the ball can roll with no frictional resistance. The ball will find the centre of the shaft and will transfer that exactly to the contact end of the ball. I could go through the trouble of machining two pointed inserts, but I don’t believe there would be an appreciable difference in outcome.
As others have said, buying the highest quality shaft you can afford is the best solution for ninety nine percent of us.
RT, ideally we would shoot all our shafts from a Hooter Shooter and index them before fletching.
You have to tune the shafts to your hand and torque, not a machine.
I'm not sure if Randy Ulmer spine tests as you are attempting, but one thing he does do is adjusts his D loop with a tied serving nock set inside the bottom of the D loop to create downward pressure on the front of the arrow pushing down on the rest. He said that helps "force" the arrows to all begin to flex the same direction upon release, which helps the arrows fly the same.
I may not be explaining it very well but he wrote about it in one of his Dead On columns.
smarba, you are exactly right. Although I think I used to tie under the nock for something to do with applied torque.
I really don't shoot good enough to be overly concerned, beyond buying quality shafts. I just had the opportunity and time to fiddle with something that was in the back of my head.
Ambush, "the ball will find the centre of the shaft" but does the clamp face find the center of the ball. In my mind, if the clamp faces are not exactly perpendicular to the axis of the shaft then they may contact the ball bearing somewhere other that dead center. Which would impart "point contact" forces not perfectly in line from one ball to one another or directly through the centerline of the shaft. Food for thought.
rooster, all that has been taken into account and mitigated for, within reason.
And Rod’s pipe clamp can make the basis for a heck of a decent bowpress for low $ if he tires of arrow testing.
Neat test....and valid per my engineering mind with the care he took to square the shafts and inserts. Bet my old Goldtips would really show a lot of stiff-side and variations of actual spine. Much poorer than the Injexions I shoot today.
