BB Spindle Lengths
 

I have recently replaced my caged bearing BB with a Shimano UN-55 square taper. I also replaced my three ring crankset with a compact 34T-50T two ring. In the rear I run a Shimano 13T-26T seven speed cassette.

The old caged bearing spindle measured 127mm so I ordered Shimano's 127 UN-55. It installed great and everything in the drive train seems to be getting along well.

Now I would like to move the 50T chain ring more inboard. It is now at 56mm from the center of the seat tube. This was probably OK with the triple ring crank set, but I live using my 5th, 6th, and 7th rear cogs and would like a better chain line from the 50T chain ring to them.

So, if I replace the 127mm UN-55 with a 113mm UN-55 will the difference spindle length of 14mm be from the right side of the spindle? If so then that would position my 50T chain ring about 42~43mm from the center of my seat tube. This would be great.

Suggestions will be welcomed because I don't want the order several BB's trying to reposition my crankset. Large 'thanks' for advise........

Attachment 595207

Attachment 595208

The crank manufacturer has the info on spindle length.

My understanding is that the UN series of BBs are symmetrical and any changes in length would be evenly shared so a 14mm-shorter spindle would take 7mm from both ends. You might want to check your crank-to-chainstay clearances to be sure you do not create an interference.

Since not all cranksets use the same BB length, it's very important to find out what BB length your crankset needs. The manufacturer should provide this info without any need for you to go off measuring and guessing -- which crankset did you buy? ;)

davidad,

+1

Sometimes I find myself 'wrapped-around-the-axle' or in this case 'spindle'.... Gee's, all I had to do was go to the Shimano site, type in my Shimano Compact crankset and Voila there was all the information I needed.

Looks as a UN-55 MM110 will do the trick.

Should have remembered this when I was doing all my research on my front and rear Shimano hubs.

Oh well,,, good folks like yourself will get a grin from my, well whatever, and bail me out.

Thanks to all....

Attachment 595212

Sweet! :thumb:

I don't understand how this was helpful but then it just may be me? But no, the crank manufacturer does not dictate the length of the bottom bracket spindle, though they may give a recommendation. One of the "benefits" of the old square taper cartridge type bottom brackets was the ability to adjust the chainline, which was essentially what the OP is asking.

As to length, while the overall reduction in width between a 127 mm and 113 mm BB is 14 mm, this is split evenly between the drive side (D) and non-drive side (NDS). So your chainline would be reduced to 49 mm (56 - 7). The typical chain line for a road double crankset is 43.5 mm, as measured between the center of the seat tube and a point halfway between the two rings. Note that your method of measuring chainine is not standard (see
https://www.sheldonbrown.com/chainline.html).

Before you buy a new bottom bracket, measure the space between the chain stay and the existing small and large chain rings. I don't know of a minimum clearance but you don't want to install a new bottom bracket only to find your chain rings hit the chain stay.

Karl

Maybe some help here:

https://www.sheldonbrown.com/bbsize.html

Bottom brackets aren't necessarily symmetrical. Some are about the same length, but made for a triple or a double, and are a bit different in left side/right side.

https://www.sheldonbrown.com/images/spindle.jpg

D is how they list them.. if cartridge bearing,, A&C may be symmetrical to end of cartridge,
shown is loose ball type..

I second, try Sheldon's Browns website. Pretty much the Bible of Bike repair around here. MTB' bikes have that right chain stay that swings out. I doubt a 28-38-48 Tooth, trekking crank set would fit. That's a pretty common crank set on hybrid,, city bikes today. I see what you're trying. You want a larger range of gears on a fat tire bike. cheers

Here's a chart for Tange cartridge BB's, very good quality square BB.

110mm or 108mm symmetrical.

Follow-up to Post
 

Thank you all for the input! Using Shimano's chart for the crankset it recommended their UN-26 or UN-55 BB with an axle length of 110mm.

I ordered and received it. Then compared it to my existing UN-55 127mm BB. The left side axle on both the 127mm and 110mm BB are the same, but the right side of the 110mm BB is much shorter. So, Shimano takes the reduction off the left side not equal amounts from each side of the axle. I can supply photographs for any doubters out there.

After the 110mm BB was installed my seat post to center of the two chain rings was a wonderful 43mm, as best I could measure. The clearance at the frame was good and my chain line is spot on.

During my next morning's 20 mile ride I swear that my chain noise was much less throughout all the gears and shifting seemed crisper. Maybe my chain will last longer.

Again, thanks to all for the advise.