If you are going wheel shopping, be prepared to describe your vehicle's bolt pattern. Close inspection of a wheel reveals that the bolts are arranged along the circumference of an imaginary circle. This bolt circle is usually described by the length of the bolt circle diameter (BCD), or pitch circle diameter (PCD). The bolt circle diameter is a function of the number of bolts in the pattern and the distance between the centres of two adjacent bolts. Knowledge of your wheel's BCD and the number of bolts in the bolt pattern will help take the guesswork out of wheel purchasing.
- Skill level:
- Moderately Easy
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Things you need
Count the number of bolts in the bolt pattern of the wheel. Let the number of bolts be represented by n.
Measure the distance from the centre of one bolt head to the centre of an adjacent bolt head. Let this bolt separation distance be represented by d.
Find the sine of (180/n) and let this result be represented by m. For example, if the wheel has a five-bolt pattern, then m = sin (180/5) = 0.588. Make sure your calculator is set for degrees and not radians when doing this calculation.
Determine the BCD using the relationship BCD = d/m. For example, for a wheel with a five-bolt pattern and a bolt-separation distance (d) of 30.0cm, the BCD = d/m = 30.0/0.588 = 51.0cm.
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