Bottom brackets are one of those things that many people find confusing, and rightly so – there are a ton of different ‘standards,’ each of them different. To call any of them ‘standard’ is misleading, though there are some sizes and designs that are more common than others. While things got a little crazy for a while about ten years ago, many manufacturers are now converging on the same or similar designs, making things a little easier.
So I’m here today to help shed some light on the more common bottom bracket (BB) standards you might find on a modern mountain bike. I’ll explain how to identify the BB you might need for your bike, and also define some commonly used jargon. Hopefully I’ll also dispell some bottom bracket misconceptions.
What is a bottom bracket?
Let’s start from the bottom (pun not intended), the bottom bracket, or BB for short. It’s a rarely-seen piece of kit that sits between your cranks and your frame and houses a set of bearings that allow the cranks to spin. The BB can almost be thought of as an adapter between cranks and frame, connecting a crank with one standard, to a frame with another standard, and is usually threaded or pressed into the frame. In this way, bottom brackets are replaceable.
If we focus on modern two-piece cranksets, then breaking things down further, the two vital parts that the BB connects are the bottom bracket shell and the crank spindle. The bottom bracket shell is the part of the frame that the BB fits into – on a traditional frame it’s usually a piece of tube welded into the bottom part of the frame front triangle; on carbon bikes it can simply be a hole in the construction of the frame. The crank spindle is the axle between the two crank arms; it’s the axis around which the cranks rotate.
On a two-piece crankset, the spindle is usually permanently connected to one crank arm. However on older styles of cranks, the spindle is a part of the BB (for example square taper, Octalink, Power Spline). This article will only cover the more common modern mountain bike standards where the bottom bracket is separate from the spindle.
Common BB standards
Now that we know what a bottom bracket is, let’s look at some common standards. Starting with the bottom bracket shell, there are two common types, usually referred to as “threaded” and “press-fit”. These refer to a BSA or British Threaded BB shell, and a PF92 shell respectively, and while these two are most common, neither is the only “threaded” or “press-fit” option. So be careful when looking for one of these.
The standards below describe the diameter of the BB shell and fitment (pressfit or threaded) as well as thread pitch etc, but each of them is typically available in different widths to suit road, mountain, DH bike, fat bike etc. For example, BSA comes in 68mm (usually road bikes), 73mm (usually mountain bikes), 83mm (usually DH bikes), and 100mm (usually fat bikes) widths. These are to accommodate different rear hub spacings and therefore different chainlines.
Common BB shell standards
- BSA Threaded (1.37 in x 24 TPI, 33.6-33.9mm ID)
- PF86/92 (41mm ID)
- T47 Threaded (M47 x 1 thread, approx 46mm ID)
- BB30 (42mm ID)
- PF30 (46mm ID)
In terms of crank spindle, there are a similar number of standards, if not more. Again, most of these standards describe the diameter of the crank spindle and have multiple options in terms of length for different Q-factors (crank width).
Common MTB crank spindle diameter standards
- Shimano Hollowtech II (24mm)
- SRAM DUB (28.9mm)
- SRAM GXP (24-22mm, stepped)
- Praxis M30 (30-28mm, stepped)
- FSA MegaExo (24mm+ a bit)
- Race Face EXI (24mm)
- 30mm is widely used by many brands including Race Face, Cane Creek, Praxis, Rotor, and SRAM
As you can see, there are a few brands using similar or the same standards like 24mm and 30mm. While you can mix and match crank/BB brands, you may run into spacing issues and need to use spacers to make them work. Running the same brand BB as the crank usually leads to fewer compatibility issues.
How to determine the correct BB for your frame and crankset
So how do you figure out which BB you need? To be absolutely sure, the answer is to measure the BB shell and crank spindle, but we can make life easier for ourselves by starting with just looking at the crank/BB, or by taking to the internet. You need to know two things: What is the BB shell standard on the frame? What is the crank spindle standard? When you know these two things, you can find the correct BB.
If you start on the internet, frame manufacturers will often state the BB shell in their frame spec list. Unfortunately they don’t often list the actual item in their component spec lists, however if you bought a complete build, it’s worth a look, and you may very well be able to figure out the crank used. If you already have the BB out of the frame, then simply measuring the BB shell is the easiest way to be sure. Using a caliper to measure helps, though a tape measure works too, since you just need a closest millimeter. For example 68mm and 73mm threaded BBs are pretty far apart.
Measure the width of the BB shell and also the inside diameter, taking note of whether the shell is threaded or press-fit, then compare to the chart linked here to find your BB shell standard.
To find the crank spindle standard, start by looking at the crank. Often the standard will be printed on it – for example most SRAM DUB cranks and BBs will say DUB on them somewhere. Alternatively you can measure the crank spindle diameter using a caliper, making note of whether or not there’s a step in the spindle at one end to bring the diameter down (SRAM GXP and Praxis M30 spindles are stepped).
Also pay attention to the length of the spindle. Some bikes — like fat bikes or those with Super Boost spacing — may use a longer spindle. The length is often printed on the spindle, for example DUB Wide. Again, check out the chart here to compare your measurements against various crank standards.
While we’re focussing on common mountain bike crank spindle standards, there is one current outlier in that some lower end builds, for example the GT Sensor I recently tested, use a SRAM Power Spline crank/BB. Power Spline houses the spindle in the BB rather than the crank. You know you have a Power Spline crank when you have an 8mm allen bolt head in the centre of each crank arm in place of there there would usually be a self-extracting bolt cap that says “DUB” on it.
Once you know your specs you can start looking for the correct BB you need, for example BSA Cinch, or PF92 DUB. Sticking to the crank manufacturer can be advantageous since you know it’s going to work, for example SRAM DUB bottom brackets are good enough and come with all the documentation and are simple to figure out, as are Race Face Cinch BBs.
However some riders prefer a higher quality bearing, in which case you could look for an option from Cane Creek, Wheels Manufacturing, Chris King, Enduro, etc. For a little extra help, check out this useful BB finder tool over at Wheels MFG.
Personally I’m usually happy to stick to the stock BB unless it’s failed prematurely. However a little preventative maintenance goes a long way in keeping your BB alive. Stay tuned for a future article about BB servicing and replacement.
