Count the wheel bolts, and assign the variable b to represent the number of bolts.
Calculate the value of the function w = sin (180/b). For example, if a wheel has 6 bolts, then w = sin (180/b) = sin (180/6) = 0.5.
Measure the distance between adjacent bolts. Use the ruler to measure from the middle of one bolt to the middle of the next bolt. Repeat this measurement for each inter-bolt distance on the wheel. Find the average inter-bolt spacing by adding all the measurements, and dividing by the total number of measurements. Let the average inter-bolt distance be represented by s. For example, for a wheel with 6 bolts, there are 6 inter-bolt distances to measure: 24.95 cm, 25.05 cm, 25.00 cm, 25.10 cm, 25.00 cm and 24.90 cm. The average inter-bolt distance (s) = (24.95 + 25.05 + 25.00 + 25.10 + 25.00 + 24.90)/6 = 150.00/6 = 25.00 cm.
Calculate the primary dimension of the pattern of bolts, also known as the pitch circle diameter (PCD) or the bolt circle diameter (BCD). The PCD or BCD = (inter-bolt distance)/(sin (180/b)) = s/w. For example, a wheel with 6 bolts in the bolt pattern and an inter-bolt distance (s) of 25.00 cm, will have a PCD or BCD = s/w = = 25.00/(sin(180/6))= 25.0/0.5 = 50.00 cm. The calculated bolt circle dimension in the example is 50.00 centimeters.