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