From: brash@pubff1.triumf.ca (BRASH, ED)
Subject: Re: Why is face of a wood curved, but irons are flat?
News-Software: VAX/VMS VNEWS 1.41    
Organization: TRIUMF: Tri-University Meson Facility
Date: Tue, 18 Aug 1992 00:12:00 GMT

In article <Bt52Kv.6Bx@well.sf.ca.us>, tmurphy@well.sf.ca.us (Thomas Murphy) writes...
>In article <1992Aug17.140602.846@kong.gsfc.nasa.gov> sheldon@kong.gsfc.nasa.gov (Roger Sheldon - AISS) writes:
>>An iron blade is flat, but the face of any wood is curved.  Why?
>>
>>(I started wondering this because I can hit my irons pretty good, but I tend
>>to slice my woods.)
>>
>>Thanks,
>>
>>Roger
> 
> 
>A wood club has bulge and roll to it to create some of the same effects as a
>cavity backed iron.  This effect is called the gear effect and is discussed
>in a very nice manner in the Maltby book on club design and repair.  I
>suggest that if you are hitting slices with your woods that you are either
>hitting somehting of the same nature with irons but don't see the effect in
>a pronounced manner or are gripping the club preventing release and trying
>to really "hit" the ball with your woods but are more relaxed with the
>irons.
> 
>tom

Notwithstanding Tom's comments above, the shape of the face of 'wood'
clubs has its roots in the physics of angular momentum.  Suppose that we
are looking at the flight of the clubhead from above for a right handed
golfer.  If the golfer hits the ball on the toe of the club, this will
cause the golf club to want to turn clockwise in the golfers hands.  The
entire system of golf club plus ball had zero angular momentum before the
collision (neither were spinning), and since angular momentum must be
conserved (usually) then that means that the golf ball will leave the
club spinning in the opposite direction to the club, that is, in a
COUNTER-CLOCKWISE direction.  This is the type of spin that causes a 
draw/hook/right to left type of ball flight.  The curved surface of
the club serves to counteract this effect by starting the ball along a 
flight path right of target so that when it goes left because of the
spin imparted at contact it still ends up in the center of the fairway.

Now of course, in real life things are a lot more difficult than 
indicated in the above paragraph.  All of this has assumed that the
club was aiming straight down the center of the fairway at impact,
and that there was no friction between the club and ball during impact.
Such friction would mean that angular momentum would, in fact, NOT be
conserved and this counteractive effect of the curved face would
be much less effective.  This friction would be most likely to occur
for hits far from the center of the club.  In my opinion, most slices
are caused by 'cutting across' the ball at impact and using the friction
between ball and club to impart a clockwise spin to the ball and causing 
the all too familiar left to right ball flight. 

The main point of this is that with a wood, everything is amplified compared
to an iron because the club is heavier and bigger.  This means that woods
have much higher rotational moments of inertia than irons, and if the club
turns in your hands, or even twists the shaft of the club in a tortional way,
then the ball will have to be spinning much faster than
for a similar type of situation with an iron.  Believe me, if the clubface
of a wood were not curved you would be hitting wild slices and hooks for all
but the most on-center hits.  With irons the problem is not as severe, and
so one opts for the flatter face so that one starts the ball along the correct
flight path for a wider range of contact positions along the club-face.

I hope that this was not too technical.  

Later,
Ed.