A non circular chainring is completely specified by giving four characteristics: the number of teeth, the ovality, BCD and number of crank arms.
Number of teeth (e.g. 52 teeth or e.g. 38 teeth)
Install preferably oval chainrings with the same number of teeth as the circular ones mounted on your racing bike. The average gear ratio is identical, but the efficiency with PowerOval® is very much better.
Is the ratio of the largest to the smallest diameter of the PowerOval®.
This is e.g. 1.25. Usually written as 25 %.
Strive for the highest possible ovality to reach maximal efficiency gains.
But, the oblique course of the derailleur chain (misalignment) with 10- and 11 speed drivetrains, combined with the use of pronounced non-circular chainrings mounted on particular bike configurations and when cycling in rough conditions, may cause a chain drop off the outer ring when shifting towards the smaller cogs. Indeed in these circumstances the limits of the technical possibilites are reached.
For this reason the ovality of the PowerOval® has been limited to 25%
and the shape of the PowerOval® is optimized for “zero-chain -drop-off”
Indicates the Bolt-Circle-Diameter of the crank.
This is the diameter of the circle going through the centers of the bolts of the crank.
BCD is e.g. 130 mm or 110 mm (Compact).
It is also possible to derive the BCD by measuring the distance (say X) between two consecutive mounting holes for the bolts. See the figures.
Shimano ( -compatible) cranks:
X = 76.4 mm (5-arm crank) then BCD = 130 mm.
X = 64.7 mm (5-arm crank) then BCD = 110 mm
X = 90 mm / 63 mm (rectangular) (4-arm crank) then BCD = 110 mm
Campagnolo (only made to order)
X = 79.4 mm (5-arm crank), then BCD = 135 mm
X = 64.7 mm (5-arm crank), then BCD = 110 mm
The choice of the BCD also determines the ovality and the (minimum) number of teeth on the inner ring. Conversely the ovality and the number of teeth determine which BCD is possible.
As an example, a 42 teeth chainring with 20% ovality is possible with BCD 110 and with BCD 130. However with an ovality of 30% the 42 teeth oval can only be manufactured with BCD 110.