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I need to replace the bottom bracket bearing on my fixie. I plan to get a Shimano UN26, but I note that there are a number of different sizes. The sizes seem to be expressed as 68-110, where 68 is the shell width and the second number varies according to the table below, from Shimano's tech specs.

enter image description here

I assume the second number relates to the width somehow.

How do I measure my existing bearing? Is it length from end to end of the axle? Given that the only tool I have for this is a ruler (no calipers) what's the best way of measuring my existing bottom bracket to ensure I get the right size?

Update

I removed the bottom bracket and it had "103" stamped on the axle. I also measured and it was exactly 103mm from end to end. So it looks like the measure is the length. I went to my LBS and they assured me a 107mm (coincidently the shortest one they carried) would be fine. I installed it and my chainline is marginally out (I guess by 2mm). So I'm going to try to find a 103mm from somewhere else.

Thanks!

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  • I know it's not always easy. The two measurements, IIRC, are the total shaft length and the offset. I think that for a fixie the offset is normally zero. And, I guess, since you have a Shimano spline and not a square tapered shaft, the shaft length should be reasonably easy to measure. But I don't know if you measure tip-to-tip, shoulder-to-shoulder, or where.
    – Daniel R Hicks
    Commented Feb 1, 2012 at 2:54
  • OK, that one is tapered square, so you have the complication of where to measure.
    – Daniel R Hicks
    Commented Feb 1, 2012 at 4:17
  • This may help.
    – Daniel R Hicks
    Commented Feb 1, 2012 at 4:21

3 Answers 3

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The numbers 113, 115, etc. mean the total axle length in milimeters.

On a singlespeed (fixie, internal-geared-hub), the important measure is how much the right side of the axle "shows out" (not always perfectly symmetric), and how much the crank can "sink" into the tapered square, which may vary from model to model even with the same nominal spindle length, and with how much torque you apply to the bolt when installing.

I have two single-speeds (fixie and IGH), which required some "fine tuning" to get a straight chainline:

  • One had a 73mm bottombracket tube on the frame, so I had to use a slightly shorter spindle, because it is installed off-center (2.5mm to the right because of extra "frame width" on the right side, the nut on the other side didn't have a lip and could be inserted fully inside the bb thread);
  • Other was regular 68mm, but two models with same spindle length had different offsets on the crank (because of taper, lip width of the shell, and some other obscure factors) and I had to install a cheaper model (generic instead of Shimano), it was the only one that fitted, and I'd rather have an aligned chain than a slightly better bottombracket.
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  • What is an IGH?
    – Darth Egregious
    Commented Feb 1, 2012 at 14:20
  • IGH would stand for Internal Geared Hub (sheldonbrown.com/internal-gears.html). Which is not strictly a single speed, but kind of looks like one as it doesn't have a derailleur.
    – Kibbee
    Commented Feb 1, 2012 at 15:36
  • How did you figure out which one fit? Did you use trial and error or some other system of measurement?
    – Mac
    Commented Feb 2, 2012 at 2:08
  • @Mac unfortunately, some trial and error was necessary after a previous narrowing-down of the available sizes at the shop. But this only because I needed (wanted) a "perfectly aligned" chainline.
    – heltonbiker
    Commented Feb 6, 2012 at 11:55
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Most modern square taper BB's are symmetrical. The 2nd number refers to the overall spindle length in mm. Measure the BB on your bike (with the cranks removed) and replace it with one of the same spindle length. Calipers are the best way, go buy a cheap pair at a discount tool shop near you. The spindle length is measurement A in this photo: http://branfordbike.com/articles/bottom-brackets-pg59.htm

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  • My bike (a roughly 15-year-old Novara with a triple) has an asymmetric BB. The right side is 2-3 mm longer than the left side, IIRC. If you look at the Sheldon Brown page I referenced above you'll see that to replace one with a symmetrical BB you need a slightly longer unit.
    – Daniel R Hicks
    Commented Feb 1, 2012 at 12:33
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    One problem with Shimano square BB cartridges is that one end tends to be tapered differently from the other, and there's no clear reference point on the cartridge body for "center", so determining offset (or even true overall length) is a problem.
    – Daniel R Hicks
    Commented Feb 1, 2012 at 12:38
  • Great answer, thanks. I'm going to accept the other one though because it contains a bit more information about asymmetrical cranks too. I wish I could accept them both!
    – Mac
    Commented Feb 17, 2012 at 3:30
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Typically you will want to replace you bottom bracket with the exact model that is currently in the bike assuming that the one in the bike is the correct BB, or the original. Yes there is the measurements on the BB, 103 for you to be exact, but in order to get the ideal performance a lot of crank sets are designed to go with a specific Bottom Bracket so even though a bottom bracket may fit in your bike, that doesnt mean it will work optimally. If you are looking to dind a bottom bracket that is a higher quality where it wouldn't be the same as whats in your bike your best bet is to take it to a shop

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  • 2
    What do you mean by "work optimally"? Quite apart from anything else, the vast majority of bikes are at the lower end of the market where, honestly, nothing really "works optimally". And, even on higher quality bikes, upgrading parts (especially bearings) is a thing and, the very definition of upgrading is that you're replacing an item with something better, so the original can't have been optimal.
    – David Richerby
    Commented Oct 27, 2018 at 19:49

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