I collect old lefty Hogan irons. Today's irons look huge compared to the 1970 Hogan One Plus and 1972 Apex I own.
Technology - Golf Balls versus The Clubs Themselves - Page 2
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On the other hand, I had one friend who was using a old callaway big Bertha (2000) version and switched to new 460cc to gain like 50 yards.
I think it's a combination of both. And I think USGA regulations play a significant role here as well.
I think the limits placed on balls are actually a little stronger than those placed on clubs, so in recent decades there is going to be more improvement from clubs. The club face can't really get any more efficient, but there are still things they can do with newer materials that make them lighter and allow higher swing speeds.
With balls, the initial velocity test prevents some improvements, but they can still do things that cause different trajectories and spin to get more distance for many of us. And lower compression balls are increasing distance for those of us with swing speeds below where they actually test at. For the bigger hitters though, the overall distance standard is a harder limit to get around, preventing improvements no matter what the technology involved. There has been an overall distance standard since 1984, and an initial velocity limit since 1942. They didn't put a .83 COR limit on the club face until 2002 (the current test for "characteristic time" during which the ball remains in contact with the face accomplishes much the same thing).
So I think for the top players, more of the improvement has been in the clubs. For more average joes swinging 80 mph, it may be as much in the balls.
A new golfer and I were on a par 3 tee box one time (mostly messing around) and talking about his game, or lack thereof.
He had a very old set of blade irons that his uncle had given him. I asked him if I could hit one of them, fully intending to show him that his irons were far inferior to mine. I hit one shot and it sailed right over the top of the flag stick and landed on the back of the green. I had figured there wasn't a chance in the world I could hit that little thing worth a crap.
An anomaly I'm sure but instead of making fun of his irons I simply handed it back to him without saying a word.