Anatomy of a square-taper bottom bracket

Remember that Bino, my folding bike, had a three-piece crank as stock. I’ve since swapped it out for a Shimano FC-R565 two-piece unit and an SM-BBR60 Hollowtech II bottom bracket. Ever wonder what happened to the stock square-taper bottom bracket?

Here it is in all its greasy glory.

Mushrooming out of each side are the crank bolts, which I screwed into the ends of the spindles just so they won’t get lost. Otherwise, that’s how the bottom bracket would look like had it still remained inside the Dahon Vitesse frame’s bottom bracket shell, with the two bearing cups screwed in. One of the cups is fixed, on the drive side, while the other non-drive side cup is adjustable.

I’m holding the bottom bracket here by the cups. You’ll notice it’s actually pretty slender in the middle.

Moving the adjustable bearing cup outward reveals the actual ball bearings themselves, contained in a cage. They work with the spindle by suspending it by its raised flange in the middle, acting as a bearing race, with the rest of the support handled by the bearing cups. As you can see, this is pretty much the guts of any bottom bracket – the ball bearings that let the spindle rotate freely.

Over on the drive side, it’s a similar affair, except that the fixed bearing cup itself also acts as the cage for the ball bearings. The whole point of having an adjustable cup is to be able to tighten the entire bottom bracket and add pre-tension to the bearings so that they can still spin freely, but with zero play or looseness at the spindle.

As far as square-taper bottom brackets go, this CH unit is about as simple as it gets. It’s much simpler than, say, a Shimano BB-UN55 sealed cartridge unit.

Even with the relatively unsophisticated construction, however, this bottom bracket still does its job and spins smoothly. It’s easy to knock square-taper bottom brackets, but their design is sound, and thousands of bikes around the world still pedal on with this humble bottom bracket providing kilometer upon kilometer of reliable service.

I hope you found this interesting.

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