Shaft flex: useful starting point, terrible gospel

Flex letters are marketing categories, not measurements. The thing that can actually be measured is frequency, in CPM, and it tells the story. Inside one shaft line, about 10 to 12 CPM separates each flex, so a driver regular sits near 230 to 245 and a stiff near 250 to 265. The problem is that the ranges overlap across brands by roughly a full flex, which means one company's stiff can measure the same as another's regular. That is why we show overlapping speed bands instead of hard cutoffs.

Speed sets the starting band. Roughly, under about 74 mph is ladies or soft, 70 to 85 is senior or A, 80 to 96 is regular, 92 to 106 is stiff, and 103 and up is extra stiff at the driver. The bands overlap on purpose, because the charts themselves disagree by 5 to 10 mph at the edges. Weight shifts the same letter, too: a light 90-gram regular can play like a senior, while a heavy 120-gram stiff plays closer to X.

Here is the part the charts hide. On a robot, flex barely moves ball flight at all. In motion-capture testing of the same golfer hitting regular against stiff, the softer shaft bent about 2° more forward on paper, yet the loft actually delivered to the ball differed by only 0.4°, which was not statistically significant, because the golfer's hands quietly compensated. Flex is felt and timed by a person, not stamped onto the ball, which is exactly why speed charts break on real golfers: in test panels the slowest swinger's best flex has come back X, and a 100 mph player has fit best into regular.

None of that makes flex optional. Being one band off is a starting-point argument; being two bands off tends to cost timing and dispersion, which is why our tool flags a shaft sitting well outside your speed range. The letter is simply a coarse tool, and the torque number stamped beside it is coarser. Torque is mostly a feel variable: the common advice that faster swingers or hookers need lower torque did not survive human testing, where players chose by feel rather than by their miss pattern. Fit the flex to your speed and tempo, and do not chase the torque label.

The averages are small but the personal fit is not. Across a six-player panel, one flex step was worth only about 2 to 5 yards on average, while individuals gained up to 16 yards on their own best flex. For irons the founder's bench rule is to start at stiff rather than extra-stiff below about 90 mph of 7-iron speed, since low-torque tour shafts already play firmer than the letter suggests, and iron flex letters vary by brand even more than driver shafts do. Treat any of this as the rack to start from. Tempo, transition, and release decide the rest, and that is the part of shaft fitting a launch monitor and a fitter settle faster than a chart can.