The Effects of Miss Hits

Most of the literature you read regarding Golf's Ball Flight (D-Plane geometry, Impact Plane geometry, etc.) offer explanations of ball flight based on centred (sweet spot) contact. I think it's very important to understand how that relationship works, but also think that being aware of all the effects of non-centred contact has on a golf shot is fairly important, given that a fair number of any average golfer's shots are not hit on the Sweet Spot.

When we look at analyzing miss hit shots, we'll consider shots missing the sweet spot in 4 directions: towards the toe, towards the heel, high on the face, and low on the face. Of course, you can combine these influences should you want to look at a high-toe miss, for example.

When the clubface impacts with the ball, the surface of the ball is in contact with the face for mere milliseconds, but this is when all the influence happens. When the ball is not hit on the centre of percussion (sweet spot), the face will rotate towards the direction of contact, even as the ball is still compressed against it. Since the face will be more open at separation (when the ball leaves the face) than it was at impact, the Effective Face angles (the ones we use in D-Plane calculations) will come from where they were in the middle of the Impact Interval (also the time when the ball is at maximum compression). Also, the twisting open of the face has the effect of spinning the ball in the opposite direction. This is called Gear Effect.

Image taken from: http://www.physics.usyd.edu.au/~cross/GOLF/GOLF.htm

If we break the golf ball's flight down to it's initial components, we'll have: Ball Speed, Vertical Launch, Horizontal Launch, Spin Rate, and the Spin Axis (indicates how sideways a ball is spinning). This list takes care of pretty much everything that the golf ball will do upon being launched. We'll look at how the different miss hits affect each component.

Smash Factor: Not too surprisingly, when we hit the ball away from the sweet spot we see a lower Smash Factor, which is the ratio of ball speed to club speed (Smash Factor = Ball Speed / Club Head Speed, eg. 147mph Ball Speed / 100mph Club Head Speed = 1.47 Smash Factor). The higher the Smash Factor, the higher the resulting Ball Speed (Driver shots have much higher smash factors than wedges). Shots hit towards the heel will see a lower ball speed than sweet spot contact. Shots hit towards the toe will also see a reduction in ball speed, but less so since the face at the toe of the club is actually moving slightly faster than the face at the sweet spot (since the face is rotating around the hosel). As a side note: higher smash factors are a result of higher COR and lower Spin Loft numbers, but that's for another blog...

Toe hits: When the ball impacts the face towards the toe, the face opens immediately, and this opening is what causes the following effects:
Smash Factor/Ball Speed - lower, but higher than heel hits
Launch - more right*
Spin Rate - slightly higher
Spin Axis - tilted more left

Heel hits: face rotates closed and:
Smash Factor/Ball Speed - lower
Launch - more left*
Spin Rate - slightly higher
Spin Axis - tilted more right

Hit high on the face: face rotates upwards and:
Smash Factor/Ball Speed - lower
Launch - higher
Spin Rate - lower
Spin Axis - unchanged

Hit low on the face: face rotates downwards more than normally:
Smash Factor/Ball Speed - lower
Launch - lower
Spin Rate - higher
Spin Axis - unchanged

*I've put asterisks on two effects that Trackman (a leader in this area of research) disagrees with me on. They still maintain that the effects of off centre contact do not include a change in horizontal launch (Trackman Newsletter #5, pg. 3).  I believe that Trackman would agree that low-face contact will result in a lower launch, and that high-face contact will result in a higher launch.  It would be due to the same effect that happens in the horizontal frame, so why the difference?  I think it has something to do with the way that Trackman calculates and presents their Face numbers, likely displaying the Effective Face angles from the middle of the impact interval (but that has twisted from its "delivered" condition).  
They could consider showing two sets of numbers for the Face - "delivered" (by the player) and "effective" (post-twisting).  A small difference in the numbers would indicate better contact.
Dave Pelz found that the effects of heel impact on putts sent the initial launch of the ball to the left (RH golfers) and that toe hits sent the ball to the right (Dave Pelz's Putting Bible, pg 354).  I think it's fair to assume that the same principles apply to the initial launch of a golf ball in putting, as well as full shots.

Interestingly enough, the combination of hitting a drive off the high-toe is not necessarily a bad miss hit for a lot of people. The combined effects of more draw spin (leftward tilted spin axis), higher launch, and less spin can actually help certain people hit it further and straighter than they normally would.

Side Note: Clubs are now being designed with higher Moment of Inertia (MOI), which will increase the club head's resistance to twisting. Less twisting means smaller effects on off centre hits, in terms of spin and launch.  Club with different Centres of Gravity (COG) will experience different degrees of twisting and Gear Effect; the further behind the face that the COG sits (like in fairway woods and drivers), the more twisting that will occur on those off-centre hits, and the more exaggerated these effects will be.

So, if you're watching ball flight on the range, and are trying to figure out what the club is doing to cause that flight, you might want to consider non-centred contact (it happens a lot).

Thanks for reading (sorry for all the brackets)!

-Mark
@StrongerGolf

References:

Jorgensen, T. (1994). The Physics of Golf. AIS Press

Cross, R. (2006). Physics of Golf.
Retrieved from http://www.physics.usyd.edu.au/~cross/GOLF/GOLF.htm