Which Bottom Bracket and Cranks Do I Need?

 

Author: Tim   Date Posted:8 March 2016 

https://www.mountainbikesdirect.com.au/blog/which-bottom-bracket-and-cranks-do-i-need/

Bottom Bracket and Crank Standards are often a source of mass confusion. So let’s jump in and figure out what’s going on down there!

 

Bottom Bracket or BB as they are commonly known, refers to the bearing system that your cranks spin on. Bottom Bracket Shell refers to the part of your frame that your bottom bracket is installed into. There is a multitude of different designs that are employed in the hope of creating a stiffer,lighter and stronger frame. In this article, we're going to stick to the commonly used current BB standards.

 

Types of Bottom Bracket Shells

Threaded (BSA) - As the name implies, the frame is threaded, and a cup screws into the frame. With the newer 2 piece style cranks, these are often called external bottom brackets as the bearing is housed outside the frame. Originally designed for either 24mm spindles or GXP spindles, however you can now get BBs to suit 30mm spindles too.

 

Pros: Very simple to maintain and install. Can be done with very basic tools found here.

Cons: Slightly heavier and slightly more expensive to manufacture than pressfit systems.

 

Press Fit BB92 - Uses the same sized bearings as a threaded BB, but the bearings are housed in a plastic cup that is pushed into the frame. The bearings sit inside the frame. Originally designed for either 24mm spindles or GXP spindles, however BB's are now available to suit 30mm spindles too.

 

Pros: Lightweight, BB Shell is wider so downtubes can be made wider and potentially stiffer. 

Cons: More expensive press tools are required to install like this. To remove the BB it's not uncommon for it to be damaged and require replacement. If the frame tolerances aren't tight enough the cups can creak in the frame.

 

Press Fit 30 (PF30) - Uses the same sized bearings as the superseeded BB30 design, but the bearings are housed in a plastic cup that is pressed into the frame. Originally designed for 30mm spindles, however adaptors are available to suit 24mm and GXP spindles.

 

Pros: Easily fits all spindle types, still very light

Cons: More expensive press tools are required to install like these. To remove the BB it's not uncommon for it to be damaged and require replacement. If the frame tolerances aren't tight enough the cups can creak in the frame.

 

 

 

Cranks, Crankset or Crank Arms refer to the arms that your pedals screw into.

 

Types of Cranks

 

 

 

One Piece: Very old design, only used on very cheap bikes. Once piece refers to the crank arms and axle are all made from one piece.

Two Piece: The current designs. Light and Strong. 2 piece refers to the one of the cranks and spindle being together as one piece and a separate arm that bolts to it.

Three Piece Design: An older design. Doesn't have the reliability of two piece designs. Called Three piece as there is 2 crank arms and a separate spindle, so 3 components.

 

In this post, we’re just going to focus on the current 2 piece crank systems.


Types of Crank Spindles (Axles):

24mm Spindle - This is the most common type made popular by Shimano’s Hollowtech 2 system over 10 years ago. These use a straight steel spindle that is 24mm in diameter. Predominately used by Shimano, RaceFace

 

 

 

GXP Spindle - These run a 24mm Steel Spindle that steps down to 22mm at the non-driveside bearing. Predominately used by Truvativ / Sram.


 

 

30mm Spindle - A straight aluminium spindle 30mm in diameter. Used in Sram's BB30/PF30 Cranks, Race Face's Cinch Cranks, Rotor Cranks and Hope Cranks.

 

 

With 30mm crank spindles it's important to note that they do come in different lengths. For instance SRAM make spindles that are either 95mm or 101.5mm from the tension adjuster to the end. The short spindles will only work in a BB30/PF30 BB Shell.