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Discussion Starter #1
This past weekend I was tuning a Mathews Z7. The guy who brought it to me said he could not get rid of a left tear. I watched him shoot it through paper, he had good form and a good grip, but a left tear through paper. I took the bow and shot it through paper as well, with the same left tear. So, I took the bow and added a couple of twists to the left yoke, and took a couple of twists out of the right yoke. The tear got a little better, but still left. I moved the rest to the right a little at a time, with very minimal change to the left tear. This was after I ran his numbers through OT2 and it said his arrow was spined properly. But, I decided to try different spined arrows. I shot a 400 shaft, his GT 75/95 shaft, and a 300 shaft. The 75/95 is a 340 spine. All left tears!

So, I added another twist to the left yoke, and still a left tear. Here is where it gets interesting. I then grabbed a Harvest Time HT-1 350 spined shaft, and shot it through the paper at 6 feet. Perfect bullet hole! I moved to 12 feet, perfect bullet hole, then to 15 feet, perfect bullet hole! With this setup it shot perfect bullet holes at every distance. I then took his GT and shot it through the paper, and guess what, left tear. I spent some time with his bow, the starting cam position is perfect, the bow is in perfect spec, etc.

The reason I didn't try the HT-1 350 right off the bat is because, it is just about the same spine as his GT. But I decided to try it anyways after having a hard time tuning his bow. Boy was I surprised when it got perfect holes. The only thing that I can think of is this. The 350 has a slightly weaker static spine, it was a little longer, and it has the 46 grain insert/outsert. These things make the arrow a little weaker than his GT.

Would this bow be that sensitive to spine?

What do you guys think?

Here is his setup:
Mathews Z7
29" measured dl
70.2 lbs
402 grain arrow 296 fps

Oh, and I also ensured that there was no fletching contact.
 

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This past weekend I was tuning a Mathews Z7. The guy who brought it to me said he could not get rid of a left tear. I watched him shoot it through paper, he had good form and a good grip, but a left tear through paper. I took the bow and shot it through paper as well, with the same left tear. So, I took the bow and added a couple of twists to the left yoke, and took a couple of twists out of the right yoke. The tear got a little better, but still left. I moved the rest to the right a little at a time, with very minimal change to the left tear. This was after I ran his numbers through OT2 and it said his arrow was spined properly. But, I decided to try different spined arrows. I shot a 400 shaft, his GT 75/95 shaft, and a 300 shaft. The 75/95 is a 340 spine. All left tears!

So, I added another twist to the left yoke, and still a left tear. Here is where it gets interesting. I then grabbed a Harvest Time HT-1 350 spined shaft, and shot it through the paper at 6 feet. Perfect bullet hole! I moved to 12 feet, perfect bullet hole, then to 15 feet, perfect bullet hole! With this setup it shot perfect bullet holes at every distance. I then took his GT and shot it through the paper, and guess what, left tear. I spent some time with his bow, the starting cam position is perfect, the bow is in perfect spec, etc.

The reason I didn't try the HT-1 350 right off the bat is because, it is just about the same spine as his GT. But I decided to try it anyways after having a hard time tuning his bow. Boy was I surprised when it got perfect holes. The only thing that I can think of is this. The 350 has a slightly weaker static spine, it was a little longer, and it has the 46 grain insert/outsert. These things make the arrow a little weaker than his GT.

Would this bow be that sensitive to spine?

What do you guys think?

Here is his setup:
Mathews Z7
29" measured dl
70.2 lbs
402 grain arrow 296 fps

Oh, and I also ensured that there was no fletching contact.
yep..and i've found that just 25grs of tip weight or a 1/2" dif in shaft length or so little as 2lbs draw weight can make the dif between tune-able or..not...and it all plays a role in getting it right.
 

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Discussion Starter #3
Oh, and I will add this as well. When I was scratching my head trying to figure out what was going on, I thought about nock fit. His shafts fit too tight on the factory serving. So, I redid the center serving on the bow to get proper nock fit. Still a left tear. The only way I could get this bow to shoot bullet holes at all distances was with the HT-1 350 shaft.
 

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Discussion Starter #4
yep..and i've found that just 25grs of tip weight or a 1/2" dif in shaft length or so little as 2lbs draw weight can make the dif between tune-able or..not...and it all plays a role in getting it right.
I can see that with this bow, but usually when I get one tuned up, I can get good flight out of a wider range of spines.
 
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