Couple of questions for the experienced archers. I have a z7extreme 70# 29″. What spine should I be shooting and what are the average speeds of this bow? In my opinion it is slower than a friends exact same bow and year. Shot thru chrono at 273 with 400 spine arrows 100g points.
Spine is how stiff and arrow is, measure by how much the shaft bends/deflects under a given weight. Spine is affected by the combination of outside diameter, wall thickness, and what the shaft is made of or material properties.
Speed is affected by the total weight of the arrow, regardless of spine.
For example, a Gold Tip 3555 (7.4 gpi, .500″ spine) may be too weak spined for a bow, a GT 5575 may be properly spined (.400″)but weigh more than the GT 3555 and be slower, but a GT Velocity 400 (7.4 gpi, .400 spine) would be as stiff as the GT 5575 and weigh the same and be as fast as the GT 3555.
All .400 spine arrows don’t weigh the same. You’d have to know the total weight of your friends arrows and the DL of his bow to make any comparison with your setup. To really make a fair comparison, you both should shoot through the same chronograph.
You should be shooting a .340 spine or maybe a bit stiffer arrow for best results. However, 273 fps doesn’t sound right, too slow for a Z7X, and would equate to an arrow total weight of about 476 grains…to get that kind of weight out of an .400 spine arrow you’d have to be shooting an Easton XX75 2117 aluminum shaft.
To find out approximately what speed your bow should produce, use one of the online calculators:
Thanks for the reply. My friend shoots the same arrow. GT 5575. 8.2 gpi 400 spine. Same length too. Just wondering if my bow isn’t tuned right or what the deal is. How did you figure out what spine is ideal for a specific bow??
Going at least one box to the right of a DW of 69-70 lbs and an arrow length of 28″, GT recommends a #4 region shaft (.340 spine), like the 7595 shaft, and maybe into the #5 shaft region if the arrow is longer than 28″.
Update: took it to the shop today. Was drawing 66 maxed out. Timing was off a little too. One twist in bus string jumped it up to 70# draw weight and corrected the timing. Shot thru the chrono 3 times and averaged 305. Changed arrow as well. Went to maxima blue streak 350.