Driver Length and Weight (4)

Acknowledgements

Notes:

I mentioned at the beginning of this article that the optimization of clubhead weight is an obvious question for designers, and the first one I tackled when I started analyzing golf with physics. Here is more confirmation of the question's importance. My own personal copy of Cochran & Stobbs was a gift from the Callaway Golf R&D library, during a meeting at Callaway headquarters which included Alastair Cochran himself. The book had obviously been used as a reference by Callaway's engineering staff when it was in their library, because there were some turned-down pages and a few notes in margins. The most-annotated page in the book was the one with this study, and several of the unit conversions I added to the table above were already penciled in the book.
Actually, it is surprising that the carry distances are as close as they are. A few reasons we might expect bigger differences:

Cochran & Stobbs used a very simple straight-line approximation to go from ball speed to carry distance. They didn't even do anything to optimize the loft.
Golf balls have come a long way (pun definitely intended) between 1968 and 2012. We would expect a modern golf ball to travel a few yards farther than the one C&S used in their study.

We compute MOI from a simple approximate formula: MOI = L² (H + S/3) where L is overall club length, H is head weight, and S is shaft weight.
Here is a typical example of Baymiller's position on swinging a long driver, from a post to ShopTalk on Jan 17, 2005.
"If you want more distance on a driver, you can get it by building the club longer and lighter. BUT, you have to change your swing timing to hit it consistently on the fairway. Any player with a late release will not get the club head square unless he adjusts his release point a bit earlier. You can still swing hard and fast, but you have to get the wrists releasing sooner and play the ball more forward."
Bernie has posted this opinion in similar words many times over the past decade.
Here are the ways in which the graph is not exactly what Baymiller is saying:

He has claimed 25-30 yards increase going from 45" to 48", while lightening the driver to keep the heft manageable. (The graph assumes a 27-yard increase.) I have gone a step beyond that, keeping the heft perfectly constant (using MOI as the measure of heft).
Another way I have gone beyond Baymiller's claim is to assign a straight line between the points he talks about. The the points at 173g and 200g would be his claim (assuming he kept the heft constant, which he did not).
Finally, I am using his 25-30 yard claim on a base distance of 232 yards. Actually, that is probably more reasonable than his claim, because his base yardage is a lot lower. (He pointed out that, at 75 years of age, the longer driver does not get him to 260 yards.) That means his added yardage is a larger percentage of the base, making it even less likely. For instance: 25 yards on top of 232 yards is an increase of 10.8%; 25 yards on top of 150 yards is an increase of more than 16%, a tougher goal.

Here is Baymiller's account, from a private email of November 28, 2012.
I made a lot of long drivers for senior women from 1994 for the next few years. One day a 70 year-old senior woman about 5'7" called me and said she was "tired of having the girls drive by her" and "could I build her a driver to get more distance." At her course range, I saw she was almost completely an "arm swinger" with very little wrist cock and release. But, her swing was almost "automatic," every drive was about the same trajectory and distance with a nice little draw. She was using a 45" Cobra driver, I think, and hitting the ball about 150 yards. She tried my 46" demo and gained little, my 47" demo worked a bit better, but she still wasn't happy, so I pulled out my 48", 295 gram driver with an D9 swingweight and handed it to her. Frankly, I didn't think she could get the face closed with that much swingweight and the first swing she hit a big slice. Her second swing was just like those with her own driver and the ball reacted the same way...nice trajectory with a bit of a draw. She hit again and again with the long driver and the results were all the same...only the distance was now 185 yards, a 35 yard gain with 3 additional inches to the club length. "I want one like this," she told me. I warned her about the E0 wearing her out before she could get around the 18 holes, but later found out she had no problem with the swingweight. That's how I learned "arm swingers" could probably adapt their swing to almost any swingweight in a few swings at the range. It took me 2 hours on the range to figure it out when I first went from 46" to 48" club length. [She] went to her winter Florida home course a couple of weeks later and won their biggest tourney playing that driver.
Kenney's results are not universally accepted. Paul Glazier has published a critique of the work, which has been responded to by Kenney. I am relegating this discussion to a footnote. I believe -- though I could certainly be proven wrong by future events -- that, while Glazier's comments are well-placed and valid, they do not alter the general shape of the conclusions. Additional work will provide additional details, but I doubt if the main results will be greatly altered. Of course, this may be bias on my part; Kenney's results are in line with my own mathematical model and experience.
The Tour players' insistence on shorter-than-average drivers requires a few more words:

Yes, there are occasional tour players who need the extra distance enough to use a longer driver. Danny Seng's story of Euan Walters is a good example. And you occasionally see a maximum-length driver on the women's tour. But those are the exceptions, not at all typical. If the players with the longer drivers were not special cases, you would imagine that all the pros would be going that way.
According to the research paper by Kenney, et al., the extra distance comes with no penalty for a highly skilled player. If that were so, you would imagine that all the pros would be going that way. But they are not. Perhaps test results on a driving range do not challenge consistency the way a competitive round does. But all that is speculation on my part. If the research results prove to carry onto the course, then we might see a mass switch at some future date. It might happen, but I'm not holding my breath.

That would be 50" by the LDA rules. Long drive competitions use a different way to measure length than does the USGA, illustrated in the diagram. A maximum-length golf driver would be measured by the USGA at 48". The same driver measured by the LDA rule would be almost 50" -- probably about 49.5". When you see mention of 50" drivers at LDA events, they are close to USGA-legal; that is, using the USGA measurement protocol, they would come in at about 48.5". So a maximum LDA driver is about a half inch longer than a maximum USGA/R&A driver. That half inch extra is good for maybe 2 yards of distance, which matters to an LDA competitor but almost not at all to a golfer.
Some of you sharp-eyed readers may have noticed that there are eight red data points and only six blue ones. (Actually, nine red data points; the rightmost point represents two nearly identical hits.) The plan was nine hits with each driver, switching between clubs every three hits. Charlie, being who he is, decided after the second round of three hits with each club that I needed more head weight on my short driver. He confiscated it while I was hitting the long driver, and added about six grams of lead tape on the sole. Consequences:

Obviously it was useless for continued comparison with the long driver; it was now a different club.
I did hit three more shots with it in the last round; they are just not on this scatter plot nor table.
I looked at the scatter of those three data points. They showed a drop in clubhead speed of just under 2mph. The computer model predicts a drop of about 1mph, not too far off, given the statistical variation in a small sample. I consider this further confirmation that the computer model corresponds pretty well to reality, or at least the reality of my swing.
Contrary to expectations, it did not improve the quality of impact; in fact, the smash factor was lower than the first two rounds.

In 2017, Padraig Dooley posted in his blog a test of two drivers. His goal was to compare a 2004 driver (Titleist 905) to a 2016 driver (Titleist 917) to see how much technology had accomplished in a dozen years. The drivers were almost identical except for the vintage ot the clubhead. Almost, but not quite, as we shall see. Dooley's conclusion was that very little had changed. The newer driver got a few yards more, but even that was suspect. As Dooley pointed out:
The 917 did have a little more club head speed which could be attributed to the 1 inch longer shaft. But the ball speed from the 917 should be more than 1.1 mph faster due to the 1.9 mph club head difference and the 1 inch longer shaft might be leading to a bigger discrepancy in strike and as a result a lower than expected ball speed.
Dooley is right; the ball speed should be almost 3mph faster if the quality of strike was the same. With only 1 extra inch, he did not actually lose ball speed -- as I did with 3 extra inches. But he gained far too little, given the extra clubhead speed.

Last updated Apr 5, 2017

Driver Head Weight and Club Length

Conclusion

Acknowledgements

Notes: