A bolt circle is a pattern comprising the set of bolts that hold a wheel in place. Like any other circle, the bolt circle is described by its primary dimension, the diameter of the circle. In the wheel industry, this primary dimension is called the pitch circle diameter or the bolt circle diameter. The diameter of the bolt circle depends on the symmetry of the pattern. The number of bolts in the pattern, and the position of each bolt determine the dimensions of a wheel bolt circle.
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: 24.95cm, 25.05cm, 25.00cm, 25.10cm, 25.00cm and 24.90cm. The average inter-bolt distance (s) = (24.95 + 25.05 + 25.00 + 25.10 + 25.00 + 24.90)/6 = 150.00/6 = 25.00cm.
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 25.00cm, will have a PCD or BCD = s/w = = 25.00/(sin(180/6))= 25.0/0.5 = 50.00cm. The calculated bolt circle dimension in the example is 50.00 centimetres.