Bottom Bracket Sizing

[EigerSA]

Hi all. I apologise if my terminology is wrong, but I'm wanting to find out the the size of the 'bottom bracket'. I need the internal diameter, internal diameter as well as length. Oh and the thread size.

 

Also, can you purchase then separately? Any help appreciated.

 

Thanks

 

[BenDL]

Unfortunately there is no standard in BB sizing.

If you take a look at:

http://www.chainreactioncycles.com/Models.aspx?ModelID=908

you will see there there are a number of variants just for this one type of BB.

 

Best to remove the BB & try and identify it. Or take it to the LBS & say "I want one of these, please."

 

Good luck!

 

[Eugene]

As far as I know most BB's have a standard size (except certain Cannondale models). It is the thread of the BB that causes the problem.

 

You get English and Italian threads. There are different types of BB's, mostly to do with the method of attaching the cranks.

 

You get square taper, ISIS and Octa something or other. The best is to do what Builder says and take it in. The new ones have outboard bearings, which will still fit (assuming the thread is right).

[splat]

Just dont answer this question: 'Does my Bottom Bracket look big in this?'

[Johan Bornman]

There are very firm standards when it comes to BB sizing.

 

The two that are most popular are the co-called English and Italian thread.

English thread BB shells are 68mm wide and Italian sheels 70mm. The shell is the part attached to the frame.

 

Further, the British thread BB uses  a left hand thread on the right side of the bike. This means it is lefty tighty instead of lefty loosy. The other side is standard. This is to prevent loosening cups due to a phenomena called precession.

 

On Italian bikes (except new Bianci's I'm told), you'll find an Italian thread BB. Not only is the BB shell 2mm wider but the thread diameter is a bit bigger and both sides are standard right hand thread.

 

On new hollow cranks with external BB  bearings, the BB width is determined by the crank's axle. These are all standard but the difference and compensation for unusual chainline is made by inserting or not inserting one or more of three possible spacers on either the left or right or both sides is various combinations.

 

On older BBs where the cranks are separate, you also get different length axles, from about 100mm (I think) to about 118 mm. This affects both the chainline and Q-factor.

 

It is best to just replace your BB with one similar to what came out. The figures to look for are:

 

1) BB axle width. This will be something like 113mm.

2) English or Italian thread. English thread is 1.375 x 24 TPI. Translated, this means the diameter of the cup is 1.375 inches and it is threaded with a fine thread of 24 threads per inch. Italian BBs are slightly bigger and as I said, right hand thread both sides.

3) BB axle width - not relevant with two-piece (hollow axle) cranks.

4) Offset. Some BB axles are offset to one side to compensate for double or triple chainrings.

 

The BB axle standards are also in place. Currently in use is square taper, Octalink one and Octalink two, ISIS and now hollowtech and BB30, a new standard started by Cannondale. It requires a much larger BB shell on the frame.

 

 

[EigerSA]

Thanks Johan, that will help quite a bit. I was going to machine my own, or even look at making carbon fiber testers, but maybe I'll just buy them outright.