Shimano UN54 Bottom Bracket

...ones. From personal experience the Shimano ones look to be better made, and a bit more durable. Truvativ ones seem to have a habit of failing suddenly - they still give a long service life though.

'''Overall'''

Not the lightest, stiffest, or most sexy, but yesterdays technology delivers a cheap, highly durable, abuse tolerant, fit and forget item.  ... more

What's this?

It is a JIS square tapered BB. And in english...

BB - bottom bracket. Screwed into the frame and serves as a mounting point for your cranks.

Square taper - Axle shaft of the bottom bracket is tapered. The shaft is wider at the base than it is at the tip, so it ensures a very tight fit when the crank is tightened up. It fits square holed cranks only.

JIS - Japanese International Standard. This is a measure of the degree of taper. There is also the ISO standard used by European manufacturers, such as Campagnola. The vast majority of mountain bike cranks are JIS. An ISO crank may or may not fit a JIS square taper BB depending on the tolerances and amount of wear.

Evolution or Devolution?

In the last few years BB's have undergone something of a revolution. Octalink from Shimano, ISIS from the others, and now the latest big thing are external bottom brackets. X-type from Race Face, Hollowtech II from Shimano, and GXP from Truvativ. While they have different names they are all interchangeable, as they are all made to similar specifications and similar dimensions.

Each "improvement" has led to stiffer and lighter BB's, with a marked decrease in durability. The latest BB's can last as little as 3 months on a mountain bike, if ridden in wet and muddy conditions. ISIS are particularly bad in this regard, but the new external BB's are in a class all of their own. Mounting the BB outside of the frame, makes them extremely vulnerable to water penetration and early failure.

The UN54 will give you an average service life of around 18 months to 2 years, and this reason alone may be enough to persuade you to stick with square taper. My last one a UN53 square taper got changed after 14 months, because it had become a bit gritty. I did however regularly ride it through rivers and mud, and then pressure washed it afterwards. It could have easily gone on for several more months, but square taper BB's are so cheap I changed it.

The downsides

The threaded spacer ring is made from plastic, and has a tendency to split when you are tightening it up. Always save the old one, when you are replacing your BB.

The other major downside is poor choice of cranks. All the money and development is in lightweight cranks with BB's made from cheese, so your choice is limited. If you want sexy, then your only real choice is Middleburn, but these are expensive at around £130 just for a pair of cranks without rings.

How does the UN54 compare?

The only BB I have to compare it to are Truvativ ones. From personal experience the Shimano ones look to be better made, and a bit more durable. Truvativ ones seem to have a habit of failing suddenly - they still give a long service life though.

Overall

Not the lightest, stiffest, or most sexy, but yesterdays technology delivers a cheap, highly durable, abuse tolerant, fit and forget item. 

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