The bamboo rod needs the wider casting arc.

The Second Essential says the size of the casting arc must vary with the length of line past the rod tip, and the mechanics chapter states the same relationship from the other side: match the arc to the rod bend. A bamboo rod is the more flexible of the two, so for similar loops it bends more under the same line mass 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 to keep the tip path straight.

Seeing it on the lawn

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