PCD (Pitch Circle Diameter) Defined

PCD, or Pitch Circle Diameter, is the diameter of an imaginary circle drawn through the fixing holes of a chainring. Since it's not possible to measure this directly, you can instead measure the distance between the centres of consecutive fixing holes and convert to PCD using this table:

Distance between fixing holes	PCD
34.1mm	58mm
43.6mm	74mm
50.6mm	86mm
55.3mm	94mm
64.7mm	110mm
71.7mm	122mm
76.5mm	130mm
79.4mm	135mm
84.7mm	144mm

Chris Juden of the CTC offers this additional information in the June-July edition of Cycle:

Chainrings are sold by number of bolts and PCD or BCD, which is the same thing and stands for Pitch Circle Diameter or Bolt Circle Diameter. Easier than diameter to measure is the centre-to-centre distance between two adjacent bolts. That can then be multiplied by the appropriate geometrical factor to get the PCD: in the case of 4 bolts x1.414 or 5 bolts x1.701.

2008-05-15 10:46:38 santosh kumar singh says: "I think the definition could be more generalised."
2007-06-06 00:47:07 Deepak says: "The PCD - distance co-relation is for 5 equispaced circles."