Golf Grooves - What do they do?

I was reading an article about the recently changed rules regarding grooves on golf clubs and it made me think about what the change in the size of the grooves actually does. 

The general reasons stated for the rule changes were to reduce the spin that a golfer can achieve when hitting out of rough. 

Ok, that sounds realistic.

The reason for this is to counter the recent trend of hitting a long, but not necessarily accurate, drive and being able to make a controlled recovery shot from the rough. The proper thinking being, when you hit onto the rough, you should suffer some sort of consequence.

So, what is the story about grooves? 

I always thought they were there to impart backspin which doesn’t really tally with the reason for the rule change. 

Time to do some experiments.

What would happen if you hit a ball with a club with no grooves?

I couldn't find a golf club with a smooth face (so no grooves) so I could not make my own comparison. However, I did find a book (Search for the Perfect Swing by Cochran and Stobbs) which discusses grooves.

Their argument is that the grooves do little to increase backspin. Indeed, a ball hit with a smooth faced club will fly with similar spin and distance compared to a grooved club.

How can this be? 

Collision theory is the culprit.

When the club makes contact with the ball, the ball starts to skid up the club face but soon the effect of friction between the ball and clubface takes over and the ball starts to spin.

Because of the forces involved at impact, (and at impact the ball is only in contact with the clubface for .45 milliseconds) the ball will, by force of nature, be spinning when it leaves the club face.

A study by the USGA indicated that square grooves give a negligible spin increase when a ball is hit from a dry fairway but give substantial spin increase from wet rough. In fact, spin rates increased 25%-50%.

So grooves are not there to impart spin they are there to get rid of any liquid which gets trapped between the ball and clubface (e.g. wet grass). Just like the tread on car tyres is there to disperse water, not to give grip.

A wedge hit from the fairway will perform similarly, with or without grooves.

A wedge with no grooves hit from the rough would, the theory goes, give virtually no control over the shot whereas one with grooves will provide more control. Essentially making a difficult shot easier.

Grooves do provide a more consistent hit and also the psychological positives of feeling that there is more control, but too rough a face can actually reduce the spin. This assertion is complex and frankly beyond me but Cochran and Stobbs outline the physics and maths for those who are interested.

Just to prove this, note that some drivers have no grooves in the sweet spot area but you still achieve spin. Interesting.

My conclusion is, therefore, that we do not need to become fixated on the shape or depth or distance between the grooves but that, if like me you live in the rough, you should always make sure you grooves are clean!