The rod that actually bends more in the cast needs the wider arc, other relevant factors being comparable. A softer bamboo rod often bends more than a fast-action graphite one under the same line and force, so the usual answer is the bamboo — but the material name alone does not settle it.

The Second Essential says the casting arc must vary with the degree to which the rod bends, not with line length alone; the mechanics chapter puts the same relationship the other way: match the arc to the rod bend. A bamboo rod is often the more flexible of the two, so for similar loops it bends more under the same line and force. More bend needs more arc to keep the rod tip path straight. A fast-action graphite rod resists bending — the bend sits closer to the tip — so less butt rotation is needed for a straight tip path. Line weight, line speed and carry can all change which rod bends more in a given cast.

Seeing it on the lawn

• Rig both rods with the same line and false cast about 40 ft.
• A straight fly leg on the bamboo usually comes from a wider arc; on the graphite, from a narrower one.
• Force the same arc on both: if the bamboo bends more, its arc is now too wide and it throws a wide loop; the graphite’s arc is too narrow and it tails.

The guide does not compare the two materials; this follows from matching arc to rod bend.

Source

From the CI Study Guide (2025) (PDF), the Second Essential (the casting arc varies with the length of line past the rod tip) and the mechanics chapter’s arc-matches-bend rule; terms from the FFI Fly Casting Definitions.