Chainrings and Guides

Are you planning on replacing or upgrading the chainrings on your bike? If so, please be aware that you’ve gotta match the new chainrings to your existing cranks. Chainrings come in several sizes. Their dimensions are measured via their bolt-center diameter (BCD). Chainrings that have different BCDs are not compatible, regardless of whether the chainring has the same number of teeth.
Further, chainrings are available in both 4 arm or 5 arm configurations to match a variety of crankarms. Your bike’s crankarms and chainrings must match. The image illustration below shows a 4-arm chainring.

How to calculate your bolt-center diameter (BCD)

BCD is equivalent to the diameter of an imaginary circle drawn through the center of the bolts which hold the chainring to the cranks (dimensions are in millimeters). It doesn’t matter if the crank has 4 arms or 5 arms -- the BCD is still measured the same way. Be aware: the BCD is not simply the distance between two bolt holes.

If you’re measuring a double crankset, the same BCD is used for both the inner and outer chainrings. When measuring a triple crankset, please measure the BCD of both the inner chainring and the middle/outer chainrings (their dimensions are different).

Pairings common sizes

Mountain bicycles and other bicycles with triple cranks have two different BCDs per bike. The smallest (inner) chainring has it's own BCD; the middle and outer chainrings have the same BCD. Two different sets of BCDs are commonly found on mountain bicycles: 58mm (inner) and 94mm (middle, outer) setup. Other types of mountain bicycles use 74mm (inner) and 110mm (middle, outer) in their setup.

When it comes to road bikes and other bikes with double cranks, the inner and outer chainring have the same BCD. The most frequently-encountered BCD on double cranks is 130mm. To replace chanrings, just match the BCD measurements you’ve just taken with catalog items with the same dimensions.