Re: ShopTalk: MOI Matching

[Alan Brooks]

Thanks, Mark, your MOI formula assumes a couple of things that may, or may not, introduce errors.  First, the MOI of the grip and the clubhead about its own c.g. are ignored (a reasonable thing to do - the grip only adds about 0.1% to the MOI and the MOI of the clubhead about its own c.g. less than that), Second, it assumes the shaft is a slender rod with the c.g. in the middle (this is pretty good for most shafts, a notable exception would be Tom's COP shaft), and third it assumes the c.g. of the clubhead is the same distance from the butt as the tip of the shaft.  This last one can be significant.  Because of the length squared dependence of MOI small errors in the distance from the butt (the axis of rotation about which you are determining MOI) can produce significant MOI errors.  A 1% error in this distance will produce a roughly 2% error in this component of the MOI determination.  Since the clubhead contributes something like 85% of the total MOI this can be significant. 

The good news is that (as I think either Dave T or Ed R pointed out) if you are using the same clubhead (which you typically are) the error you introduce will be roughly the same for all the clubs.  This means that the set may be 1% or 2% higher or lower than you expected, but they will still be MOI matched.  Because utility irons and woods may have different c.g. locations than your iron heads there may be a difference between your MOI matched irons and the MOI of the utility irons/woods you think you have matched to them. 

What's not clear at this point is how much error in MOI matching is significant.  Tom W. says 1%, which correlates to about 1 SW point, but the MOI variation in a SW matched set of steel shafted irons and graphite shafted woods is about 10% - and we've been using those and calling it good for years.  If you believe that MOI matching is the way to go (and I do) a couple of percent error in MOI matching is way better than what we've been calling good.  This is particularly interesting when you consider all the emphasis on matching swingweight to 1 SW point throughout a set of clubs.  Interesting times are upon us.

Regards,

Alan Brooks



At 03:51 PM 4/17/2005 -0400, you wrote:

Alan,

The formula I have is SW=(L*(H+S/2)-14*(H+S)-10*G)/50  and  MOI=(L*L)*(H+S/3).

Doing it the way I had it calculated the sw difference between the 6 and 9 iron would be 1.8 points.

Thanks,

Mark