I recently switched string material on a couple of my bows, and was getting a somewhat erratic flight, so I decided to check my arrow tuning.

The bows are the 61# Leon Stewart and 63# ADM in my signature. Out of both, I've been shooting 14 strands of B55, shooting a 29" 400 spine carbon with 225 grains up front.

The new strings are 8 strands of Ultra Cam (big difference), and after some testing they shoot a 28" 350 spine carbon with 260 grains up front. No big problem for me, I'd prefer a heavier arrow, just need to buy more more 350 arrows.

Is such a change of spine normal when changing strings, or is there another factor i'm missing?

Going from dacron to a skinny fastflight type string will change tune for sure, how much not sure, others will certainly chime in.

What he said^^^^^^^

Bisch

Youe bow will tell you what it likes. Some times theres no big change, sometimes there is. Ive had a big change just by swaping string silencers on one bow, but on the next no change.

Ran the numbers through Stu's calculator and it appears it is normal.

Just for the hell of it I made one up for my 55# Bamabow and checked, it shoots the  400 spine/225 combo perfectly with the 8 strand FF string.