Excellent and was hoping I wouldn;t have to chime in as I can not put it as well.
Mark A Patton [EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]> -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of Dave Tutelman Sent: Tuesday, April 19, 2005 6:09 PM To: [email protected] Subject: Re: ShopTalk: MOI Matching Guys, I hate to be a wet blanket, but I'm going to be anyway. Too many of these posts are taking an approximate rule of thumb and trying to use it to six decimal places. When I was a young engineer I once did that, and my mentor compared it to "polishing a turd". Now THAT'S an image that will stick with you over the years! Anyway, let's try to set some of these rules of thumb in order: (1) When you're MOI matching, the BEST rule of thumb you can use is... Match swingweight to a slope of one swingweight point per inch. This works very well if you're using the same model shaft across the set. If you switch shafts at some point (e.g.- steel to graphite), you have to add or subract up to one swingweight point at the discontinuity. (I don't remember which way it should go. But you can figure it out yourself if you have a good spreadsheet.) Note that this does not give an exact MOI match. But it is about as close as you will get from direct measurement of pendulum period, club length, balance point, and total weight. IOW, it's about as good as you're going to get in the shop, and certainly as good as you'll ever need (from a point of view of feel or performance). (2) My approximation of 0.4" per club across a set of irons is an approximation to the sloped-swingweight approximation. So it's an approximation of an approximation. (3) The rules of thumb for swingweight (6 points per inch, 1 point per 2 grams of head weight) are also approximations. So using these to match MOI is also an approximation of an approximation. If you're trying to MOI-match, the most accurate way (short of devoting bench space and capital to a pendulum setup and computer in your work area) is to use your swingweight scale and build to a swingweight slope. Don't depend on #2 or #3 above. They are more for acquisition of parts and rough trim. The final trim should be with an actual swingweight scale. Now, let's look at what happens if you try to get too precise with an approximation to an approximation.... At 12:39 PM 4/19/2005, [EMAIL PROTECTED] wrote: >After all the figuring and re-figuring trying to work within those >parameters, I get back to my basic swingweight chart, and it tells me that >an 1/2" decrease in length results in a 3 point swingweight loss, but the >7 gram increase throughout the set in headweight results in a 3.5 point >gain, therefore I have a 1/2 swingweight point gain thru any set using the >same shafts and grips. But I cannot find a swingweight calculator that >will bear this out. That's because the swingweight calculators don't use these rules of thumb -- these approximations. They actually calculate swingweight and/or MOI from first principles -- or at least get a lot closer to it than the rules of thumb. How good are the approximations you're using? Well, you can look at my tables in http://www.tutelman.com/golfclubs/DesignNotes/swingwt2.php?ref=#table From the numbers there, you can see that, over a set of clubs: * Points per inch varies from 5 to 7. 6 is a good rule of thumb (which is what you used), but don't expect it to be 6.000. It's somewhere between 5 and 7. * Points per headweight gram varies from 1.7 to 2.3. 2 grams per point is a good rule of thumb, but... Get the idea? * With the precision you're trying to read into this, remember that each inch longer gives more shaft weight, by 2-3 grams. That's another .2 to .3 swingweight points. When you use the actual numbers and not the rules of thumb: * At the long-iron end of the scale, a 7-gram 1/2-inch set of increments gives a slope of about a quarter point per club. That's less than a half point. You'll be sorta' matching halfway between swingweight and MOI. That's not a bad thing, but it's not a true MOI match. * At the short-iron end of the scale, a 7-gram 1/2-inch set of increments gives a slope of about a half point per club IN THE OTHER DIRECTION. That is, it is trending AWAY from an MOI match. Over a whole set of irons, it will balance out to about a swingweight match, not an MOI match. But even that swingweight match will likely be off by up to a point -- that is, the swingweight will vary within about a one-point band. On to other approximations of approximations... At 02:54 PM 4/19/2005, Scott Stephens wrote: >Someone will probably beat me to this, but if you check Dave Tutelman's >site, you will find that using a spacing of 0.4" per club rather than 0.5" >will also approximate a MOI match. . and ... At 04:05 PM 4/19/2005, Colin Dick wrote: >The problem with 0.4" is it isn't a definate mark on the ruler. >I haven't gone MOI yet, but I did note that 1 cm= 0.3937008", >really close to Dave T's 0.4". It would be worth your while to buy a ruler >with cms. marked on it for MOI matching. This would be better than 1/4" >which is only .25", or staying with .5" which is 25% more than 0.40" By now you should be able to guess what I'm going to say. But let me say it anyway. "It's an approximation of an approximation!" Let's not obsess about something too small -- AND INACCURATE -- to matter. OK, a few facts: (1) The exact number isn't 0.40000". It's a little less than 0.4". Rules of thumb work best when they are simple numbers (like 0.4"), as long as people realize it's a rule of thumb and not the gospel handed down from the deity. (2) Even if you use the exact number, you'll still get some variation, just as there is swingweight variation even if you use exactly seven-gram and half-inch increments. Turns out the MOI variation from this method is a little bigger than the swingweight variation. Still useful, just not exact. So Colin's remark about using a meter stick and increments of 1cm is very reasonable -- as a CONVENIENCE. But I doubt it is measurably more ACCURATE than using your existing ruler and a 3/8" (0.375") increment. Hey, ALL the numbers that we're talking about here (.4", 1cm, 3/8") are within 2.5 hundredths of an inch of each other. That's closer than you routinely measure when cutting clubs to length. Arnie claims to be able to measure this close (and I believe him), but I know I can't and I doubt you can either. Nor that you would want to. Nor that it will make any difference in the performance of the club. Guys, let's not polish turds. ;-) DaveT -- No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.308 / Virus Database: 266.9.17 - Release Date: 4/19/2005 -- No virus found in this incoming message. Checked by AVG Anti-Virus. Version: 7.0.308 / Virus Database: 266.10.2 - Release Date: 4/21/2005 -- No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.308 / Virus Database: 266.10.2 - Release Date: 4/21/2005
