How to choose your MTB chainrings?

MTB chainrings are certainly one of the most complicated parts to replace when you want to fit something other than what the manufacturer intended. Let us try to lay out the basics in order to choose this part without making a mistake.

The standards

The number of speeds: 9, 10, 11 or 12
The fixing diameter
The chainring type: indexed or single chainring
The tooth profile
The mounting standard

Chainring standards and types

All the components of an MTB drivetrain are designed to work under a speed standard, so you cannot pair a chainring designed for 9-speed with a 10-speed drivetrain, for example. Today, most MTBs are 11 or 12-speed, but 10-speed can still be found easily, and 9-speed more rarely.

The fixing diameter of a chainring, also known as BCD*, is the diameter of the circle that could be drawn passing through the centre of the chainring bolts. To measure it, on a 4-arm MTB crankset you can measure the centre-to-centre* distance between 2 fixing holes directly opposite each other; for a 5-arm crankset, measure the distance between 2 consecutive fixing holes and refer to the diagram.

A (mm)	B (mm)
58	34,1
64	45,3
74	43,5
88	62,2
92	53,3
94	55,3
96	-
102	-
104	-
110	64,7
120	-
130	76,4
135	79,4
144	84,6

There are 2 chainring types: those that are indexed a for gear shifting and therefore designed for a crankset with 2 or 3 chainrings, or those with no indexing b and therefore designed for single chainring use only.

REMINDER: the indexing is different for each manufacturer, so all the chainrings on a crankset must be at least from the same brand.

The tooth profile of chainrings works in the opposite way to cassettes: the fewer teeth a chainring has, the less effort will be required.

Finally, the point that causes the most problems: the mounting standard. Today cranksets are increasingly optimised with specific assemblies between chainrings and cranksets (such as the Directmount standard with direct fixing). Even if the fixing diameter is often standardised, the position and/or method of fixing the chainring can differ. It is important to correctly identify the specifications when fitting or adapting a new chainring.

Which chainring for my crankset?

When you want to replace a chainring, you may not necessarily have a choice: the simplest and most reliable option will generally be to take the chainring from the crankset brand that is specific to the model you have.

If however you opt for an aftermarket option, be aware that you can find chainrings with particular technologies to improve the reliability of gear changes. In the case of a single chainring, many brands now offer models with a Narrow Wide* tooth profile that literally holds the chain on the crankset, allowing you to do without a chain guide most of the time.

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