United States Golf AssociationandR&A Rules Limited
Interim Procedure for Measuring the Coefficient ofRestitution of an Iron Clubhead Relative to a BaselinePlateRevision 1.3
January 1, 2006

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1.0 Summary
This document outlines the interim procedure used to determine the Coefficient ofRestitution (e) of an iron clubhead relative to a baseline plate.
2.0 Preparation of Clubhead
2.1 Assign an official log number to the iron clubhead upon submission.
2.2 If the club is shafted, remove the shaft from the clubhead.
2.3 Measure and record the mass of the clubhead in grams.
2.4 Mark the clubface for mapping.
2.4.1 Using a marker, mark a vertical gridline on the face of the club at
the geometric center of the clubface, mark additional vertical
gridlines at 1/8-inch spacing across the face. The actual grooves
designed into the clubface, if they exist, are used for the horizontal
gridlines. If there are no horizontal grooves, then mark horizontal
gridlines in the same manner as the vertical gridlines.
2.5 Label the x-coordinate alphabetically from left to right and the y-coordinatenumerically from top to bottom.
3.0 Baseline Plate Calibration
The baseline calibration plate has a membrane thickness of 0.125-in and a mass of
190.0 grams. The plate is constructed of Ti-6Al-4V alloy, ASTM B348 GR. 5 tothe dimensions specified in Figure 3.0.
3.1 Set the launching device to produce a ball velocity of 133+ 0.5 fps
3.2 Attach carbon paper to the face of plate and position the plate on the support
pins of the plate support fixture.
3.3 Fire the ball into the plate and examine the position of ball imprint relative tothe target circle.
3.4 Adjust the position of the plate and repeat Step 3.3 until the imprint iscompletely within the target circle.
3.5 Record the incoming ball velocity, outbound ball velocity and velocity ratio(outbound velocity/ incoming velocity, (Vrat))
3.6Repeat steps 3.2 through 3.5 until 10 shots within the target circle are obtained.
3.7 From the 10 measured data points obtained, discard the data points with thehighest and lowest velocity ratios.
3.8 Calculate the mean of the remaining eight data points. This value is thebaseline velocity ratio.
3.9 Substituting the baseline velocity ratio and plate mass into Equation (3.9-1),
calculate the baseline coefficient of restitution.
ebaseline = [(Vrat )(M + m ) + m] M
where: M is the plate mass andm is the ball mass

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scribed target circle

Ø1.000"Ø4.000"±0.001

R 0.010"±0.001

0.125"±0.001

1320.318" Ref.
Machine as required for total
plate mass of 190 g

Date: January 2, 2002
Revision: 1

Drawing No.: CP125-190
Material: Ti-6AL-4V

Velocity Ratio Test 0.125" Calibration Plate

United States Golf Association, Far Hills, New Jersey

132 chamfer

132

3.000"±0.005

4.000"±0.005

4.0 Clubhead Mapping to Determine Sweet SpotTo expedite the mapping process, determine the approximate location of the sweetspot using a center of gravity balance indicator.
4.1 Prepare Apparatus and Club for Mapping
4.1.1 Set the launching device to produce a ball velocity of 133+ 0.5
fps
4.1.2 Attach carbon paper to the face of the clubhead in order toobtain an imprint of the area where the ball impacts theclubface.
4.1.3 Mount the clubhead (sole down) on the support pedestal, using
the clubface square to insure that the impact will be normal tothe clubface.
4.1.4 Fire the ball into the clubhead and determine the alphanumericcoordinate of the center of the imprint area.
4.1.5 Make gross adjustments in the positioning of the support baseand repeat step 4.1.2 through 4.1.4, as necessary, in order toobtain impacts with centers at the approximate location of thesweet spot coordinate identified in step 4.0.
4.2 Map the Clubface
4.2.1 Mount the clubhead (sole down) on the support pedestal, using
the clubface square to insure that the impact will be normal tothe clubface and that the sweet spot coordinate of the club andthe center of the clubface square are in alignment.
4.2.2 Fire the golf ball and record the incoming ball velocity, out
going ball velocity, ball identification number, andalphanumeric coordinate of imprint center.
4.2.3 Replace the carbon paper and repeat steps 4.2.1 and 4.2.2 for 2
additional shots.
4.2.4 Repeat steps 4.2.1 to 4.2.2 moving the position of the sweet
spot coordinate of the iron by 0.25 inch left, 0.25 inch right,
above and below (as permitted) the center of the clubface
square.
4.2.5 From the mapping data, determine the coordinates of theimpacted area (interpolating as necessary) with the highest ratioof outbound velocity to incoming velocity.
4.2.6 Using the impact area coordinates determined in step 4.2.5 as
the center, mark a 30-mm diameter circle on the clubface.
5.0 COR Test of the Clubhead
It is important that balls from the same dozen(s) used in the baseline platecalibration be used in the COR testing.
5.0.1 Set the launching device to produce a ball velocity of 133+ 0.5
fps.
5.0.2 Attach carbon paper to the face of the clubhead and position theclubhead (sole down) on the support cup.
5.0.3 Fire the ball into the clubhead and examine the position of ball
imprint relative to the target circle.
5.0.4 Adjust the position of the clubhead and repeat Steps 5.0.2 and5.0.3 until the imprint area is completely within the target circle.
5.0.5 Record the incoming ball velocity, outbound ball velocity andvelocity ratio (outbound velocity/ incoming velocity, (Vrat))
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5.0.6

Repeat steps 5.0.2 through 5.0.5 until 10 data points are
obtained
5.1 Calculation of the Clubhead COR
5.1.1 From the 10 measured data points obtained in step 5.0, discard
the data points with the highest and lowest velocity ratios.
5.1.2 Calculate the mean of the remaining eight data points. Thisvalue is the iron clubhead velocity ratio.
5.1.3 Substituting the iron velocity ratio and iron clubhead mass into
Equation (5.1.3-1), calculate the iron coefficient of restitution.
eiron = [(Vrat )(M + m ) + m] M

(5.1.3-1)

where: M is the iron clubhead mass andm is the ball mass
6.0 Clubhead Conformance Assessment
6.1
Subtract the baseline value calculated in 3.9 from the clubhead coefficientof restitution calculated in 5.1.3. This is the clubhead coefficient ofrestitution conformance differential.
6.1.1 If the adjusted clubhead coefficient of restitution conformancedifferential calculated in step 6.1 is less than or equal to 0.008, thenthe clubhead conforms.
6.1.2 If the adjusted clubhead coefficient of restitution conformancedifferential calculated in step 6.1 is greater than 0.008, then theclubhead does not conform.

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