Bold Circle Diameter
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The virtual generated circle of the chainring screw holes is called bold circle. And the diameter of this circle is the bold circle diameter ( BDC ). The BDC serves as a measurement for the specification of the appropriate chainring.
Measuring of the BCD:
The measuring of the hole distance is easier than the measuring of the bold circle. For this the distance between the chainring screw holes gets measured. The measured distance gets multiplied by 1,7 for 5-arm cranksets and by 1,4 for 4-arm cranksets. The result is the actual bold circle diameter.