MTB Bottom Bracket – These Dimensions Matter for Mountain Bike Inner Bearings

The mtb bottom bracket, refers to the two bearings that sit in the bearing shell of a mountain bike frame and in which the crankset turns. As the bearings are constantly in motion when pedaling, and are also exposed to bumps and knocks as well as the stress of dirt and water, the bottom brackets on mountain bikes are one of the wearing parts that often cause problems. It is therefore not surprising that they inevitably have to be replaced over time. Even so, there are some different standards for cranksets for mountain bikes and e-mountain bikes, so there are also different inner bearings. Which inner bearings you need depends on the diameter of the crankshaft (axle), the prescribed distance of the two bearings to each other (shell width) and the bearing seat in the frame. 

• BSA (Hollowtech II / GXP / M24): Widely implemented with Shimano’s Hollowtech II cranks, this is the basis for many modern standard mtb bottom brackets. Other manufacturers use the term GXP or M24. The spindle diameter can be either 24 mm in diameter on its full length (Hollowtech II / M24) or 24 tapered to 22 mm at the end. The bearings are housed inside the bottom bracket cups, while sitting outside the frame shell, that have a left-hand thread on the right and a right-hand thread on the left, labelled as 1.37” x 24 TPI. The shell width can be 68, 73, 83 or 100 millimetres on a mountain bike.
• Pressfit/BB92/GXP Pressfit: This bottom bracket version is also mainly based on crank spindles with a diameter of 24 millimetres (e.g. Hollowtech II). However, the bearings are housed in plastic shells directly mounted in the bb frame shell. The shell width is 89.5 or 92 millimetres on an mtb, the outer diameter of the bearings is 41 mm.
• BB30: An open standard with an axle diameter of 30 millimetres. The bearings, without a shell or cup, and with an outer diameter of 42 mm, sit directly in the bottom bracket shell of the frame, which requires particularly lower manufacturing tolerances. The bearings’ outer diameter is 42 mm on the BB30 standard.
• PF30/Pressfit T 30: This bottom bracket standard is an adaptation of the BB30 standard (30 crank spindle) However, these types of mtb bb's are not pressed directly into the frame, while sitting in a plastic shell – so that manufacturing tolerances of the frame can be better compensated. The outer diameter is 46 mm.
• Special cases – Trek BB90/95: This frame manufacturer deviates slightly from the common Pressfit standard but can usually be adapted with spacers or adaptors. For this special case, the manufacturer’s specifications must be followed to the letter. 

Admittedly, the bottom bracket thing is complex. But once you have found the right MTB bottom bracket, you can of course change it yourself. NB: always follow the assembly instructions for the respective crankset for which you have to replace the inner bearings. The manufacturer’s instructions will also tell you whether you need any special tools and which one. 
BTW: At BIKE24 you’ll always find a wide range of the right bicycle tools. So that your bottom bracket change is a perfect success.