Originally Posted by
due ruote
I am installing a 600 crankset from an Arabesque group. The original BB spec is, I believe, 116mm. I'm not sure that's pertinent, but I know someone will ask.
I had a 118mm UN-55 BB laying around so I did a test installation. I didn't measure the chainline, but I can tell you it's too far outboard. I have 7mm between the inner ring and the stay, and, given the gearing and the combinations that will get the most riding time, I would like to have a clearance of 3 or 4 mm.
The 118mm unit has 21.5mm of spindle on the driveside, measured from the outer edge of the mounting flange. So my question is, does someone know which UN-55 length would have a drive side length of around 18mm?
This is a very long thread for such a simple problem to solve.
You started with a 118 but did not measure chainline so we don't have a true measure of how far off you are. If you had, it would be very straightforward to do the math and buy the right size. So let's extrapolate and use your "clearance" measure. You say it's 7mm and you want to bring it in by 4mm, which will give you 3mm of clearance to your chainstay. The Barnett manual says a minimum of 2mm so we'll say the 3mm clearance you want is good.
So the 118 is 4mm too long on one side, which is 8mm too long total. The math says you need a 110, but that is the oddball that is asymmetrical. Look at the chart and take a look at the measurements for a 113mm. That's most likely the one you need.
For what it's worth, my LBS stocks a variety of UN55s and if they don't have one they get it overnight. So when I am changing a crankset and BB, I let them do it. If the first one they try doesn't give a good chainline, they can measure from there and put the right one in. Plus they have a torque wrench and I don't. That's the only thing I pay someone else to do and it's so rare that I don't mind.
There is generally one and only one Shimano size that is correct for your bike and crankset combination. Notice the increments - there is one for your bike that will get you very close to the correct chainline.
All this talk about optimizing it for a certain gear range is silly. If you're optimizing for a certain gear range, and that's NOT approximately within the center of your chainline, I would take a look at your gearing. Maybe you need higher ranges. Your gearing should be set up so that your most-used range is mostly in the middle of the chainline. I make sure that for my riding, I'm using the "middle" of my rear cluster most. Since it's optimized for the center of the chainline, it works smoothly up and down, since everything is within spec. Look at your cassette and your chainring size if you find that your most-used range is anywhere but in the middle.
I understand the OP is not building a bike but when you do build one, you start with this very thing - the BB and crankset, and getting a proper chainline. Then you build from there. Rear wheel dishing and cassette combinations and even front derailleurs have a chainline spec.