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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 Feb 1 '12 at 2:54
    
OK, that one is tapered square, so you have the complication of where to measure. –  Daniel R Hicks Feb 1 '12 at 4:17
    
This may help. –  Daniel R Hicks Feb 1 '12 at 4:21

2 Answers 2

up vote 3 down vote accepted

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? –  user973810 Feb 1 '12 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 Feb 1 '12 at 15:36
    
How did you figure out which one fit? Did you use trial and error or some other system of measurement? –  Mac Feb 2 '12 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 Feb 6 '12 at 11:55

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 Feb 1 '12 at 12:33
1  
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 Feb 1 '12 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 Feb 17 '12 at 3:30

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