MTB crankset and bottom bracket

The MTB crankset and bottom bracket assembly transmits the power supplied by the cyclist in the bicycle drivetrain and then to the rear wheel.

Use of an MTB crankset :

A MTB crankset consists of two cranks on which the pedals are fixed, on one of them, the MTB drivetrain chainrings are also fixed. A MTB bottom bracket containing the bearings is attached to the frame of the bike and with the crankset.

In order to match the morphology of each, there are several crank lengths: 170 mm, 172.5 mm, or 175 mm (for taller cyclists) are standard values.

Compatibility and choice of a MTB crankset and bottom bracket:

There are several types of MTB crankset assembly, in fact it can be made up of two cranks and of a MTB bottom bracket provided with a axle or for the most recent models of two cranks, one of which has a fixed axle and a bottom bracket containing the bearings only.

Here are some examples of standards assembly:

MTB cranks and bottom bracket:

the square axle which corresponds to an old type of assembly, still available on certain ranges Shimano, Stronglight, ...
the octalink axle is fluted, it was designed by Shimano
case in which the axle is part of one of the crank: hollowtech of shimano or X-type or Megaexo designed an axle diameter 24 mm, GXP of Sram, BB30 for an axle diameter 30 mm, Sram Dub for an axis diameter 28.99 mm and Paxis M30

MTB bottom bracket and frame:

screwed by BSC standard thread (also called BSA): 1.37x24
press fit in the frame: press fit

The bearings of the MTB bottom bracket can be steel or ceramic, this material is used to reduce friction and maintenance and increase service life. For best performance it is necessary that the balls and guide rings are made of ceramic.

Maintenance of an MTB crankset :

The pedals and MTB bottom bracket are subject to many stresse form environment such as mechanical and sealing constraints.

It is essential to regularly clean the cranks and the chainrings in order to remove the residues of dried grease, mud, dust, ... which could be accumulate and limit the functionnality of the MTB drivetrain.

The MTB bottom bracket bearings are greased for life, the balls and rings are not accessible and cannot be adjusted, greased or disassembled.

A wide movement in the bearings is the sign of the end of life of the MTB bottom bracket.

Advice: please respect the tightening torque values of the items screwed on carbon cranks, in fact, aluminum inserts are glued, they are used for the interface of tight items.