Dimensions, Volume and Moment of Inertia
Appendix II, 4b is divided into three categories – woods, irons and putters.
| When the club is in a 60 degree lie angle, the dimensions of the clubhead must be such that:
(a) the distance from the heel to the toe of the clubhead is greater than the distance from the face to the back;
(b) the distance from the heel to the toe of the clubhead is not greater than 5 inches (127mm);
(c) the distance from the sole to the crown of the clubhead, including any permitted features, is not greater than 2.8 inches (71.12mm)
The Rule goes on to clearly explain and illustrate where these measurements should be made and the official test protocol has been published. When performing these measurements in the field, the best method would be to use a pair of callipers. For the heel to toe measurement, a rigid, straight edge (e.g. a ruler) should be held upright against the extremity of the toe end.
| The volume of the clubhead must not exceed 460 cubic centimetres (28.06 cubic inches), plus a tolerance of 10 cubic centimetres (0.61 cubic inches).
In practice, many of the larger headed clubheads in the market place have a marking somewhere on the head indicating the approximate volume of the head (this is the "cc" value).
Fortunately, for clubs where there is no indication of volume, there is a fairly simple method of determining the actual volume of a clubhead in the field and it is broadly based on Archimedes’ Principle and the displacement of water. All that is needed is a large measuring container, half filled with water. The measure of clubhead volume would be the amount by which the water level rises once the clubhead has been submerged into the water. Therefore, if the container is filled with 1 litre of water and the level rises to 1450 millilitres when the head is submerged to the base of the hosel – this would mean that the head has a volume of 450 cubic centimetres.
The official test protocol for measuring volume is a more accurate method, but not that much more complicated, and it requires a similar container of water placed on a set of digital weighing scales.
Archimedes’ Principle states that the buoyant force on a submerged object is equal to the weight of the fluid that is displaced by the object – and since water has a specific gravity of 1.0, this means that 1 cubic centimetre of water has a mass of 1 gram. Therefore, the container of water should be placed on the scales and the weight should be set to zero. When the head is submerged in the water, the weight displayed on the scales (in grams) is equivalent to the volume of the head (in cubic centimetres). In situations where a club is marked with a "cc" value which is in excess of the Rule (i.e. above 460 cubic centimetres), The R&A’s policy is to rule that the club is non-conforming – regardless of the real volume measurement. This is to avoid confusion in the marketplace.
Prior to measuring the volume of a clubhead, the head should be inspected for cavities. All cavities on the crown should be filled with waterproof clay or other similar material to create a ‘straight line’ which connects the edges of the cavity. The ‘straight line filling technique’ does not follow the taper or curvature of the surface of the head, rather the cavity is filled so that it becomes a flat surface which adjoins the outer edges.
Only significant concavities on the sole will be filled, for example any cavity or series of cavities which have a collective volume of greater than 15cc.
Moment of Inertia (MOI):
| When the club is in a 60 degree lie angle, the moment of inertia component around the vertical axis through the clubhead’s centre of gravity must not exceed 5900 g cm2 (32.359 oz in2), plus a test tolerance of 100 g cm2 (0.547 oz in2).
The MOI test is a measurement of a club head's resistance to twisting and, therefore, it is an indication of its 'forgiveness.'
The measurement of MOI is one of the few limits within the Rules which cannot easily be performed in the field. This is because the testing equipment is very specialised and it can only be measured by de-shafting the head (the hosel remains on the head for the purpose of the test). However, high MOI is only associated with modern, hollow, high volume driver heads and, due to the publication of the Conforming Driver Head List (see Section 8 - Spring Effect & Dynamic Properties), most of these clubs are now routinely submitted to the Governing Bodies for a ruling – so that they can be included on this List.
As the MOI of a driver head is affected by a change in its weight and the position of the centre of gravity, a driver which is designed to be adjustable for weight must still, of course, conform to the Rules in all configurations (see Section 5 - Adjustability). Moreover, when adding additional weight to a driver (e.g. with lead tape), the player must be certain that the club is still within the limit. To assist golfers with this determination, The R&A has developed a policy whereby if a driver head is submitted for a ruling and it is measured to have an MOI which is close to the limit, the manufacturer will be encouraged to advise its customers that the addition of any other weights to this model (including lead tape), other than the weights which were supplied by the manufacturer, is not permitted as it would likely render the club non-conforming. However, the manufacturer must take care over its claims and must not advertise that the product is over the limit for MOI.
| When the clubhead is in its normal position the dimensions of the head must be such that the distance from the heel to the toe is greater than the distance from the face to the back.
In practice, due to the shape and size of iron heads, this Rule is rarely encroached. It is retained in the Rules, in part, to help maintain the traditional shape by which irons are recognised. However, whilst most irons are still relatively narrow from front to back, the emergence of hybrid clubs might mean that this Rule will have greater utility in the future.
| When the clubhead is in its normal address position, the dimensions of the head must be such that:
• the distance from the heel to the toe is greater than the distance from the face to the back;
• the distance from the heel to the toe of the head is less than or equal to 7 inches (177.8mm);
• the distance from the heel to the toe of the head is greater than or equal to two thirds of the distance from the face to the back of the head;
• the distance from the heel to the toe of the face is greater than or equal to half of the distance from the heel to the toe of the head; and
• the distance from the sole to the top of the head, including any permitted features, is less than or equal to 2.5 inches (63.5mm).
Note: There is an illustration in the Rule book which tries to put each of these individual limits into context.
The Rule goes on to clearly explain where these measurements should be made and to state that, for unusually shaped putters, the toe to heel dimension may be made at the face. Given all of the dimensional restrictions listed above, which help us define the size and shape of a putter head, this additional clause is rarely, if ever, applied. However, it is retained as a safety net.
It is important to note that appendages are not permitted if their only purpose is to make the clubhead meet the dimensional specifications described in this Rule (see Section 8 - Plain in Shape(iv) and Figure 22 above).
There are no Rules regulating the volume or MOI of irons and putter heads.