The measured version of a bend profile: a shaft's local stiffness (its "EI" — modulus of elasticity times moment of inertia) plotted every few inches from tip to butt. Where kick point gives you one word and bend profile gives you a rough shape, the EI curve is the actual data a real fitting lab pulls the shaft through a deflection machine to get. Read left to right, it's what lets a fitter say things like "stiff tip, soft butt" with precision instead of a hunch, or spot that two shafts sold as the same flex have genuinely different curves. You'll almost never see the raw curve on a retail spec sheet — it's proprietary to the maker and the testing lab — but it's the ground truth every marketing term (kick point, bend profile, tip stiffness) is trying to summarize in fewer words.

The measured version of a bend profile: a shaft's local stiffness (its "EI" — modulus of elasticity times moment of inertia) plotted every few inches from tip to butt. Where kick point gives you one word and bend profile gives you a rough shape, the EI curve is the actual data a real fitting lab pulls the shaft through a deflection machine to get. Read left to right, it's what lets a fitter say things like "stiff tip, soft butt" with precision instead of a hunch, or spot that two shafts sold as the same flex have genuinely different curves. You'll almost never see the raw curve on a retail spec sheet — it's proprietary to the maker and the testing lab — but it's the ground truth every marketing term (kick point, bend profile, tip stiffness) is trying to summarize in fewer words.