Bought some douglas fir shafts in 55 60 and had 165 grain broadheads and they don't shoot the same as my 55 60 cedars with 135 grains up front. I cant imagine that 30 grains would make them shoot that different, they always seem to shoot off to one side or other. My bow is in between 50 and 55 at 28 and I draw a full 28.
30 grs equals over 5 lbs of spine.
5 pounds heavier?
With wood on a bow 55#@28 I would be shooting 65# spine arrows or even 70# and I have 160 gr Hill heads that when I shoot them on the same shafts as my Zwickeys(135) they do not fly the same either.
So if i were to shoot a 65 pd at 28, what spine would i need for a 160 grain head and what for a 135 grain head?
Taking that your bow shoots 55@28 65-70# would handle 165gr heads fine, with 125-135 grn heads you could shoot 55-60# that is dependent on what you cut them to or keep them full length. My 70@30 hill shoots 85-90#spine shafts with 160 grn heads and 80-85# with 125-135 gr heads my arrows are all full length. But if you take a 31" shaft @65# and cut it down to 29 then you have increased the spine. Some guys have calculated what every inch on or off the shaft increases the spine by, I dont.
If your draw is 28 then what are your shafts cut to?
30 inches, I always leave them long to be able to fix broken tips.
If you cut one to 29 and add the 165 head and see how it flys. head weight can weaken the spine and when cut shorter it can stiffen the spine. you may be good if you cut them to 29.