The bottom bracket is an important part of the bike, to which the drive sprockets are attached, the movement of which makes the wheels spin. This unit is located in the carriage glass of the frame. The carriage for a bicycle in modern models is a cartridge (closed type), a shaft with industrial bearings, which does not require maintenance, works longer and must be completely replaced after wear. Cartridges are of different types: wedge (can be used for road bikes); under a square (for all types of bicycles, a more reliable mount); Octalink (Shimanooman) - cranks are attached to the cone, they are more rigid and lighter; Hollowtech (Shimano) low weight system, right side mounted, like the previous type, designed for professional use. To buy a cartridge for a bicycle, you need to know the dimensions of your carriage. Often these dimensions are indicated in the markings. The bearings in the bicycle cartridge are closed, they are more protected from moisture, dust and dirt. Balls can spill out in open carriage assemblies, which does not happen in the cartridge.