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 holesPCD
34.1mm58mm
43.6mm74mm
50.6mm86mm
55.3mm94mm
64.7mm110mm
71.7mm122mm
76.5mm130mm
79.4mm135mm
84.7mm144mm
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.


3 Comments

by santosh kumar singh on 15 May 2008 Reply
I think the definition could be more generalised.
by bob on 11 January 2008 Reply
this is really hard :@
by Deepak on 05 June 2007 Reply
The PCD - distance co-relation is for 5 equispaced circles.

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