Assigning a Grade of Flex

The Standard Flex Grades

This table is constructed from words in the Golfsmith and GolfWorks catalogs. But they should really refer to swing speed and strength, not gender, age, or skill.

The reason I put "standard" in quotes is that, while the names are industry de-facto standards, there is no standard measurement of the stiffness represented by each of the grades. Yes, that means that one company's "R" flex may be stiffer than another's "S" flex. In fact, there are cases that approach this between two shaft models from the same company; measurements show the True Temper Dynamic Lite "R" shafts for irons to be stiffer than the True Temper Gold Plus "S" shafts.

Nevertheless, until the industry agrees on flex standards or we all buy Dynacraft's DSFI book (which publishes detailed measurements for most of the shafts available), the standard flex grades are all we have. So let's pretend they are truly standard and continue.

The reference catalogs (Golfsmith and GolfWorks) have a number of tables that suggest measurement to determine flex for a given golfer. Here are some sample tables:

We should recognize this as the tables we used to estimate clubhead speed "translated" into recommended flex. I think this is easier to see graphically, so I've reproduced the tables as graphs.

                  Shaft Flex vs. Clubhead Speed

GolfWorks  LLLaaaaaaaaaaaaaaaRRRRRRRRRRRRRRRsssssssssssssssXXXXXXXXXXXXX

Clubhead   5....:....6....:....7....:....8....:....9....:....1....:....1
Speed MPH  0         0         0         0         0         0         1
                                                             0         0
Golfsmith  LLLLLLLLLLLLLLLLLLLLLLLLLaaaaaaaaaaRRRRRRRRRRssssssssssXXXXXX

                  Shaft Flex vs. 5-Iron Carry Distance

GolfWorks  LLLLLaaaaaaaaaaaaaRRRRRRRRRRRRRRRsssssssssssssssXXXXXXX

5-Iron     8....:....1....:....1....:....1....:....1....:....1....
Carry Yds  0         0         2         4         6         8
                     0         0         0         0         0
Golfsmith* LLLLLLLLLLLLLLLLLLLaaaaaaaRRRRRRRRRRRssssssssssXXXXXXXX

Note that GolfWorks tends to recommend a somewhat stiffer shaft than does Golfsmith, for the same golfer characteristics. I can think of a couple of possible reasons:

• Ralph Maltby of GolfWorks and Carl Paul of Golfsmith are experienced and opinionated on all facets of club design and construction. That doesn't mean they always reach the same conclusion. In this case they don't.
• Following in this vein, perhaps the experts disagree on whether to shade their ratings in favor of distance or accuracy. We know, within a range of reasonably-fit shafts, a stiffer shaft will give more accuracy and less distance than a flexible one.
• Of course, it may be that they just have different favorite shaft models, and that those models have substantially different characteristics for the same nominal grades.

"Studies into golf club design and playability show . . . that under almost all circumstances, golfers will perform better with lighter swingweights and stiffer shafts.... "Contrary to popular opinion, studies show that rather than 'jumping' into the ball, the softer more flexible shafts tend to lag, leaving the clubface out of alignment with the hands and therefore promoting misdirected hits. Stiffer shafts, on the other hand, tend to better maintain the relationship between clubface and hands through impact, and therefore provide straighter shots."

The latter point of view has become much more mainstream lately. In fact, a number of the most popular OEM clubs are going to softer flexes without changing the nominal designation (e.g.- "R" or "S"). The idea is to improve the players' performance without trashing the players' ego.

Let me cite my own experience. On the GolfWorks chart, I'm a definite "S". On the Golfsmith chart, I'm between an "R" and an "S". From a fair amount of experimentation, I consistently hit better with an "R" than an "S". Moreover, the best results for my own swing come when I custom tip-trim a shaft to somewhere between an "R" and an "S". So I know which chart I believe.

You pays your money you and takes your choice.

Other Flex Grades: Frequency

This is a good idea, but requires overcoming a few problems. The most significant is that frequency depends not only on stiffness but also on length (longer shafts vibrate more slowly) and weight (more weight for the same stiffness causes slower vibration). When Dynacraft and Apollo jointly did the study that resulted in "The Modern Guide to Shaft Fitting", they standardized on length and weight. They cut the shafts to a standard length and swingweighted the clubs consistently to make the measurement.

As a result, the annual Dynacraft DSFI "Shaft Fitting Addendum" is the most objective comparative measurement of shaft flex available.

Later in this section, we'll see that some manufacturers (specifically Royal Precision, which was originally Brunswick) grade their shafts by frequency. Now some small, high-quality graphite shaft makers (e.g.- Apache and Composites Dynamics) are also beginning to specify target frequency for the club.

Other Flex Grades: DSFI and RSSR

DSFI is the result of a huge study by Dynacraft and Apollo, related in the book by Summitt and Wishon mentioned earlier. They took 400 driver shafts and a similar number of 5-iron shafts, and:

• Measured all their objective characteristics.
• Got experimental, subjective reactions to them from real golfers.
• From high-speed pictures of golfers' swings, found the speed at which the shaft was JUST straight; that is, the maximum-accuracy speed for the shaft. (This was done for only a few of the shafts.)

Then they came up with a mathematical formula for predicting, from the measurements, the observed rank ordering. They applied to that formula a few correction factors, and it also gave a good approximation to the clubhead speed for a straight shaft. They called the formula the Dynacraft Shaft Fitting Index, because it promised to give a single number that would fit a shaft to any golfer's clubhead speed.

The most important elements of the DSFI are frequency and torque. In fact, DSFI is directly proportional to frequency divided by the fifth root of the torque. (That is, the longitudinal stiffness of the shaft is much more important than the torsional stiffness, but the torsional stiffness does contribute something to the feel of the shaft.)

Dynacraft annually publishes a book of shaft measurements for most commercially available shafts. The 1995 version rates over 1200 shafts. In addition to the DSFI, the "Annual Shaft Fitting Addendum" gives data such as frequency, actual cut weight, and measured torque (manufacturers' specs tend to list a stiffer torque than is factual). I find it an important tool in my design work.

When Wishon left Dynacraft to become Golfsmith's chief technical officer, Golfsmith immediately began publishing a spec they call RSSR in their catalog. This is a range of clubhead speeds for which the shaft is presumably appropriate, such as "70 mph - 80 mph".

My own experience with RSSR has been so bad that I'll continue to buy the Dynacraft DSFI book rather than put any credence whatsoever in Golfsmith's free data. Early this year, I made up a trial 5-iron with a Brunswick Rifle shaft. Knowing that my favorite True Temper shaft is a Dynamic Lite, trimmed to be somewhat stiffer than "R", I wanted to find out which Rifle was the most similar to it. The DSFI rating for the Rifle was not yet available, so I looked at the RSSR. Here are the ratings (both RSSR and, now that I know it, the probable DSFI and frequency):

From the RSSR, I decided that the Rifle "6.5" would be the best match my favorite Dynamic Lite. The DSFI and frequency numbers, however, strongly indicate the Rifle "5.5". Sure enough, the 6.5 club turned out to be the stiffest, harshest iron that I or any of my friends had ever hit. And the price of that one wrong shaft would pay for a year and a half of the DSFI Annual Addendum.

Other Flex Grades: True Temper Letter-Number

True Temper charges about 50% more for this tighter spec, which is the only difference between the Dynamic Gold and the Dynamic, or between the Dynalite Gold and the Dynalite.

But wait! True Temper doesn't actually flex-match the subgrades. What they do is manufacture a shaft to a design of, say, an "S". Then (without any flex testing at all; they assume the shaft has indeed come out an "S"), they weigh the shafts, and sort them by weight into slots. The heaviest become "S500", the lightest "S100", etc. This tends to make them closer by flex than if they had just been randomly selected from the "S" output. However, measurements show that this does not assure a monotonic flex progression nor a real flex match, just a good weight match. But several catalogs still refer to them, erroneously, as "frequency matched."

Other Flex Grades: Brunswick Frequency Coefficient Matching (FCM)

Because the flex grades are derived by frequency matching, Brunswick claims that a set of their shafts is matched better than their competitors' within a matched set of shafts. They do charge for the matching. The shafts go for about $12 apiece, comparable to the most expensive steel shafts available. In other high-priced steel shafts, you're paying for light weight; in the Brunswick Precision, you're paying for flex matching.