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I'm trying to make my indoor bike smarter by fitting a crank arm power meter. The bottom bracket is a Tange bc 1.37x24t. Apparently it is of the English kind which is the most common. Since there are many different models (Shimano, Cannondale, etc...) for the same kind of crank arm power meter, I would like to know which is more appropriate for my bottom bracket.

Thank you

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It is correct that you've got an 'English' or 'ISO' threaded cartridge bottom bracket; this refers to the interface between the BB and the frame. But you also need to consider the interface between the BB and the cranks. This has what is called a "square taper" spindle, and because the bike world is filled with competing standards, there is more than one standard for square tapers (which determine the angle of the taper and how wide it is at the seat). It looks like Tange BBs use a JIS square taper, which is widely used. So you would look for cranks that have a square broach in the middle, and that specify a JIS square taper.

But here's where we run into trouble. This kind of bottom bracket, where the spindle is separate from the cranks, is an arrangement called a "three-piece crank" and was used on all midrange and high-end bikes up until about 20 years ago. Today, most high-end cranks are based on a "two-piece crank" design, where the spindle is permanently bonded to the drive-side crank (this is lighter, and because these use larger spindles, they're also stiffer, although that may not provide any real-world benefit to most of us). As far as I can tell, all power-meter cranks are based on two-piece cranks, so you can't use this bottom bracket with one.

So if you are set on your plan of using a crank-based power meter, you'll need to replace the bottom bracket with an English BB designed for two-piece cranks, which is just the bearing cups and bearings. Something like this:

outboard bb

Note that you'll need to make sure that the BB fits the cranks. There are different spindle diameters used in two-piece cranks.

The easy alternative would be to use a pedal-based power meter.

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  • Made a small edit as 1.37"x24tpi threads are known as 'English' not 'British' - 'English' and 'British' are not synonymous. – Argenti Apparatus Sep 23 '20 at 14:25
  • Thanks, @ArgentiApparatus – Adam Rice Sep 23 '20 at 14:31
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    As an expat, pointing out 'English' and 'British' are not synonymous is a service I've been providing to the residents of the United States for 25 years now – Argenti Apparatus Sep 23 '20 at 14:33
  • I'm aware of the difference. Although now that I check, I only see references to "British" BBs, and not "English" BBs. I guess Scotland and Wales use them too. – Adam Rice Sep 23 '20 at 14:36
  • British is fine, it has many names – Swifty Sep 23 '20 at 14:37

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