The Bottom Bracket That Was Too Good

When I built up this bike in '03 I used a UN series BB

A while back, a reader here asked a question in the comments which has inspired this post. It is about one of my favorite bicycle components of all time - the UN series of bottom brackets by Shimano. 

The Background:

First of all, it may be a good thing to consider the context of the times before Shimano created the cartridge bottom bracket. This was when all bicycles in bike shops generally had some sort of square taper spindle bottom bracket which was "fully serviceable". 

What I mean by the term "fully serviceable" is that the component could be torn apart, inspected, have parts replaced, re-lubricated, and reassembled making the potential for the component to last a long time fairly high.

Diagram showing a cup & cone serviceable bottom bracket

While there were variants on the theme, a serviceable bottom bracket was typically made up of a hardened steel spindle, loose ball bearings, also in steel, two "cups", and usually some sort of lock ring. 

In this sort of arrangement it was absolutely critical for the frame to have clean, straight threads inside the bottom bracket shell and that the faces where the bottom bracket tube faced outward on either side be 100% parallel to each other. If these things weren't true, the bottom bracket would wear prematurely, or be unusable due to poor adjustment capabilities. 

The cup and cone set up of this type of bottom bracket required skill to set correctly or the bearings would wear out, along with the cups and spindle, before they should. This adjustment required various different spanners, specific to certain types of bottom brackets. Additionally, this sort of bottom bracket was prone to contamination from wet weather riding, dust, and dirt. 

Take these things and add the dozens of variants (differing spindle lengths and thread types) and a repair shop could quite literally have a machinist's cabinet full of spares to service these bottom brackets. Not to mention the time required to install and adjust these bottom brackets.

The Cartridge Bottom Bracket: 

Along about 1992 Shimano introduced a new bottom bracket which was sealed up inside a metal tube so you could not see the bearings. They called it a "cartridge" type bottom bracket. While the first cartridge style bottom brackets looked very similar to what you can still buy today, there were a couple of weird quirks regarding the originals.  

Originally the plastic cup was on the drive side.

Oddly enough, the plastic non-driveside cup you might be familiar with on a cartridge style Shimano bottom bracket was actually on the driveside originally. Obviously these are left-hand thread and are exceedingly rare. Because the plastic wasn't quite up to the rigors of drive side pressure, Shimano quickly switched the arrangement to what is currently available today. 

Another oddity was the original cartridge style bottom brackets were serviceable. You could take the bearings out, re-grease the unit, and the cones were adjustable via a specialized tool sold by Shimano. I happen to have the tool, oddly enough! 

Additionally, the cartridges were not specific to bottom bracket shell length originally. This difference between 68mm and 73mm shells was accounted for by the shoulder in the inside diameter of the plastic cup. A wider shoulder was for 73mm shells and a narrow shoulder was for the 68mm shells. You could use a cartridge unit in either shell as long as you had the proper non-driveside cup. This reduced the number of variants Shimano had to make since every cartridge worked with both bottom bracket shell widths.

Finally, and most importantly for mechanics, the Shimano cartridge system reduced the number of bottom bracket variants by a very significant amount. There were less spindle length variants and you didn't have to worry about bearing race variations between similar width spindles either. This made stocking bottom brackets easier for shops and easier for manufacturers as well, since Shimano only offered certain spindle lengths. In fact, now you pretty much only have three spindle lengths which cover almost every application.

The original UN series was split up into three levels matching the old DX/LX/XT hierarchy for MTB and the 105/Ultegra/Dura Ace on the road side. XT, and eventually XTR, along with Dura Ace had metal non-driveside cups instead of plastic and were UN-90 series. The mid-level was UN-70 series, and the original entry level cartridges were UN-50 series. Later on lower series cartridge bottom bracket variants were offered to accommodate entry level MTB/Hybrid applications, and some road bike/touring bike applications as well. 

Image from a recent eBay listing for a UN-71 cartridge bottom bracket

Just Too Good!

As I said, these were made so well they were too good! Shimano sealed the units so well they resisted contamination to a great degree, making the units last far longer. While the UN-70 and 90 series were lighter and had better machining, the UN-50 series was so much better than previous serviceable bottom brackets, and went for such ridiculously low prices, everyone used these when they could. Of course, Shimano eventually made less expensive, lower-tier cartridge bottom brackets and those - while possessing the long-wearing traits of the upper end units - were far less quality in terms of bearings and free-movement. 

Eventually Shimano moved on to pipe spindle type bottom brackets around the year 2000. The square taper bottom brackets fell out of favor as crank sets became two-piece with spindles and bearings were housed in separate cups. This was when I started hoarding square taper UN-50 series bottom brackets. 

I've run the same UN-52 BB in this bike since it was built in '07

I ended up with several since I was a mechanic and when folks upgraded, or thought these were bad, I had first crack at owning them. I used a few on bikes I still ride today like my Karate Monkey, my OS Bikes Blackbuck, and my Pofahl Signature. 

I had to replace the one in the Karate Monkey after 4 Winters of abuse, That one was frozen in the frame! I had to remove it piecemeal. 

I maybe have had to replace one other of my used bottom brackets due to it being worn out. 

So, you perhaps can see why Shimano moved on to stiffer pipe spindle bottom brackets and different designs, but there is absolutely nothing wrong with a square taper crank and a Shimano cartridge style bottom bracket. You can purchase brand new UN-55's yet, and used UN-70 series bottom brackets are out there for right around $35.00 - $40.00 each. 

If you have any further questions about these or other components, let me know in the comments. Thanks for reading Guitar Ted Productions!