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Bottom Brackets: Who’d have ‘em!
Quick links…
The bottom bracket is the part that connects your two crank-arms together, and  secures them within the bottom of the bike frame.
That's the part of the bike shown in the photo on the left. Notice the screwthreads either side. The bracket width, or shell size, would be measured from one face of this to the other.
The bottom bracket (or BB) has a spindle inside, which the crankset is attached to. There are bearings around the spindle, to allow it (and the attached crankset – one crank either side) to rotate freely. The chainrings and the pedals are fitted to the cranks.
So, the threaded sleeve, the spindle, and the bearings:
This is the typical 3-piece bottom bracket.
Some people use the word axle and some use the word spindle. Since an axle is generally fixed and the movable piece turns around it – we are using the word spindle… (Which generally – hopefully – does move!;-).
The BB has threads around the outside that screw into the hole in the frame. These can be either Italian (Metric) or British (Imperial) - and they come in various sizes. The photo - on the right - shows an Italian BB (not just .jpg) because of the Campagnolo name!;-). Notice the '36 x 24T' marked: This means 36mm, with 24 teeth per inch.
(Yeah! It can still be confusing! Why mix them?;-)
The spindle may have a square tapered fitting (like the photo), or cotter pin, or a toothed ring (splined bottom bracket), or even a threaded bolt. The bottom bracket type, or interface, must match the crank type: For example, a crank with a square tapered hole must be mounted on a bottom bracket with the square receiver.
Bottom brackets come in different widths, and with different spindle widths: the bracket width should match the frame, and the spindle width should put the cranks out away from the frame, and allow the front derailleur to transfer the chain onto the smallest ring without hitting the bike frame. The bottom bracket will usually have its size printed on the cartridge, but you may have to remove it to read the numbers. For example, a 68x116 bottom bracket is 68 mm wide as it fits into the frame (the bracket width, or shell size), and the spindle is 116 mm long. If you wanted the pedals a bit closer together, you might buy, for example, a 68x112.5 BB.
So, the spindle interface could be one of the following:
Cottered - with a cotter pin securing it. This is the oldest form of fitting and is now – more or less – obsolete. Two problems with this method:
1. was that difficult to remove the crank, and
2. the cotter pin (being soft steel) was generally damaged in the removal and impossible to re-use.
Square taper – or ‘cotterless’. This replaced the cotter pin version and is now one of the most popular methods. It has the advantage of making a very secure, efficient, and simple fitting. Of course, not all square tapers are the same and not all square taper cranks and bottom brackets are compatible. 
Look at the photo on the left:
It's a Campagnolo square taper;
It's the RECORD range;
it's English (1.370 x 24T);
it shows that that is the 'right' side;
it shows its spindle size is 102mm.
(See! They are *still* mixing the sizes!;-)
Oh and - Yes! - it shows it is made in Italy!;-)
It does not show what shell size it is for - that would be on the box it came in. Mostly - these days - the shell size is 68mm, but 73mm is still used. You can find 100mm shell size, but that's getting rare.
There have been many variants, both of spindle (hollow or solid, steel or other materials, etc) and the bearings (inside or outside the bottom bracket shell). These changes were designed to produce a stronger, or lighter, or more reliable bottom bracket. Shimano have – to some extent – moved away from the square taper and now use a splined attachment called Octalink. Campagnolo have gone of a design called Ultra-Torque, which uses splined interface between spindle halves.
Splined - eg ISIS – the International Splinded Interface Standard. This is an open-standard splined specification for the interface between the bottom bracket spindle and the crankset. It was created by King Cycle Group, Truvativ, and Race Face.  If everyone used it, people would be able to use any bottom bracket with any crankset. But not everyone uses it. Shimano uses an 8-spline bottom bracket for the Octalink, which is not compatible with ISIS (which uses 10-spline).
Shown - on the right - is an Ergomo unit (notice the leads connected) for use with Octalink (notice the 8 splines).
The ISIS bottom bracket tends to have a shorter bearing life than the square taper, so – in an effort to overcome this – there has been developments using external bearings.
Integrated – Shimano (called Hollowtech II) and FSA (called MegaExo) are 2  of the companies producing this type of bottom bracket.
The photo (left) shows an FSA MegaExo BB, notice it's English (1.37" x 24T).
With this type of BB, the bottom bracket 'cups' only are threaded into the frame; the integrated BB (spindle and crank arm combined) fits thru this.
This arrangement means the spindle is larger and the integrated assembly makes for a more robust and durable product. This is probably the strongest and lightest setup you can currently get for your crank/bottom bracket interface.
What do I need to find for my choice of powermeter:
Ergomo:
Firstly you need to find if your bike has an English (1.37" x 24t) or Italian (36 * 24t) thread and choose this thread type.
Secondly you need to choose the crank fitting type from, square taper, Octalink or ISIS drive
If you want to use your existing cranks then you need to choose the right version to match your current cranks
If you are taking advantage of our special offer with free cranks then this much match the crank type available i.e. ISIS.
SRM:
You need to either:
Match the crank type to your existing bottom bracket - square taper, Octalink, Hollowtech II or Mega Exo
or
Buy a new bottom bracket to fit with your chosen system
A selection of manufacturers (with website links):
From their website:
“American Classic’s ISIS bottom bracket is light because of the unique split ring design (patent applied). The new and improved split ring and cups support the bearings and keep them in place. The bottom bracket axle is designed to distribute the stress loads applied when riding.
Replaceable stainless steel bearings.”
Campagnolo BBs include VELOCE BB, ULTRA-TORQUE BB overboard cups, etc.
FSA do a wide range of BBs: Mega EXO (including ceramic), ISIS, and square taper.
Race Face have X-TYPE (External bearings - variants for road racing, time-trials, cross-country, etc, etc.), SRX (cross-country), and EVOLVE (for freeride, cross-country, etc.)
Shimano do a variety of BB, including Hollowtech II. Octalink, and various sealed cartridge assemblies.
Truvativ have HOWITZER, GXP, GIGAPIPE, and POWERSPLINE for a variety of disciplines eg road racing, downhill, cross-country, etc. Some in CrMo, steel, or alloy.
Finding and fixing faulty BBs
You may have a bottom bracket that is a sealed cartridge unit, as most are nowadays. These cannot be serviced or adjusted. If it makes a noise, or starts wobbling, or catching, etc it has to be replaced as a unit. Just remove the old cartridge and replace it.
To test a bottom bracket…
Take the chain off the chainrings, and put it on the frame next to the bottom bracket. Clamp bike in a workstand, shove the base of the crank-arms side-to-side to detect motion in the bottom bracket. Next try to pull the cranks away from - then back towards - the bike to detect wobble. Turn the crank and listen for noises within the bottom bracket. These tests should help you to detect a damaged bottom bracket.
If you do need to replace the bottom bracket, the best idea may be – Don’t!
Get a bike shop to do it.
There are a number of reasons for this:
1. The cranks may need a special tool to remove or re-fit them – just to get at the bottom bracket.
2. The bottom bracket itself may also need a special tool. These are different for ISIS 8-spline and for 10-spline Shimano etc.
3. If you try removing the above without the special tools, you are liable to damage the parts, or the bike frame itself. You could end up with an un-usable, unrideable frame!
Here's a small selection of the tools you might need:
4. Most frames have reverse threads on the drive side of the bike: so it spins the opposite way from most other fittings. Trying to remove it the wrong way can damage and ruin your bike.
5. You have been warned!
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