Finding the spine on a rod blank is really easy. Nearly all rod blanks are made by wrapping graphite and/or fiberglass around a mandrel. The wrapping process causes a slight deviation in the material which results in softer and stiffer sides to the rod blank—commonly referred to as the spine.
The spine basically determines which way the blank prefers to bend or flex. The best way to understand the spine is to think about your own backbone spine. Your body really only wants to bend in one direction—which is very similar to a rod blank. Every rod blank has a spine. Some are more defined and than others. Using a Spine Finder will help pinpoint the exact location on your rod blank.
—Wrap masking tape around the middle of the rod blank. This is where you will mark which side of the rod blank the spine is on.
—Support the rod near the top with an open palm so that the section of the rod blank is resting at roughly a 30-degree angle.
—Apply downward pressure to the rod blank as you roll the blank along a smooth surface.
—You will notice that the blank tends to pop or spring into a pronounced curve, and is difficult to role the blank out of that curve. You just found the spine of your rod blank.
—Now, mark the outside of that curve on the tape with a pen. We will refer to that side as the spine (or spine side).
—If you are building a multi-piece rod, repeat this process for all rod sections. Keep in mind the rod always wants to bend in that direction and has the most lifting power.
—If you are building a spinning rod, your rod guides will be located on the inside of the curve 180-degrees to the spine.
—If you are building a casting rod, your guides will be located on the outside of the curve on the spine.