For individuals who play golf on a purely social level, there are very few restrictions in regards to the types of clubs that can be used. In particular, many social golfers purchase golf drivers that are considered illegal for use in certain amateur and professional events. Different governing bodies in the golf world have varying regulations for what constitutes an illegal driver and publish an illegal driver list to prevent confusion. Several manufacturers make these illegal drivers available to the general public, although the drivers are banned from use in official competitions.
Common factors that make an illegal driver unusable during an official competition include a club face that has a high coefficient of restitution (COR), violates certain depth shape requirements, or uses materials in the club face or shaft that have been banned. For example, some golf associations ban the use of an illegal driver that has any opening toward the back of the club face. The reasoning behind this ruling is that the open design alters the weight and aerodynamics of the club face and provides an unfair advantage to those who use it. Another variation of this rule, enforced by certain associations, states that a club head cannot have multiple cavities toward the rear of the club.
Most golf associations are interested in a level playing field that ensures fair competition amongst golf amateurs and professionals. Similar to the club shape requirement, some organizations ban drivers with a high COR. This attribute typically increases the elasticity of the club, meaning that when the face of the driver hits the ball, the reaction is more elastic and more force is applied to the ball. The result is an increase in drive distance, which is clearly advantageous to the individual using that particular driver.
Another common factor that makes certain golf drivers illegal is the shape of the front of the club face. Many golf associations have minimum requirements for the angle of the club face. If the face is perfectly flat, a golfer may have an advantage, because his or her drives will typically be straighter. There are other similar club face attributes that could dampen a golfer’s deficiencies and therefore provide an unfair advantage over the competition. For example, many governing bodies prohibit the use of drivers that have club faces over a certain size, weight, or volume.
Although less common, another attribute that has resulted in certain drivers being placed on an illegal driver list is the composition of the shaft. Most manufacturers use steel or graphite shafts, but some groups disallow the use of other materials or composites. Likewise, some groups do not allow the core of the club head to contain certain materials. Many drivers have titanium, wood or steel cores, and some associations do not allow the use of any other materials.