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?

110mm

Going to 110mm, as suggested, from 118mm will move both sides in half the difference or 4mm.

Thanks. Makes logical sense but I saw one post where someone said he sized up but all the additional length was on the NDS. Which makes zero sense to me.

Not all bottom brackets are symmetrical. I just measured a 115 mm BB-UN72 and the DS flange to spindle distance was also 21.5 mm.

Try this old thread:

https://www.bikeforums.net/bicycle-me...bracket-4.html


#82 has a chart from Tange, good BB's

Well, I ordered the 110mm UN-55, so I hope it's right. Will report back in a few days.

If one plans to work on their (and friend's/other's) bikes over the years it isn't wrong to have on hand a few parts that were thought to be the "correct" size but turned out not to be so. Some repair in the future and you'll be using that part for either sizing or for actual service. Andy

+1 That's why I still have that 115 mm UN72 in my parts box although I haven't had the crank it was specified for (8-speed Ultegra double) for many years. Who knows when one of those or something else it fits will come along.

That's the single most important measurement on your bicycle. Do you know what it's supposed to be? You always go by that, not the distance to your chainstays or anything else. Chainline is everything.

I live where it's quite flat and tend to do 90% of my riding in a couple gear combinations. I try to set up the drivetrain so that I have an optimal chainline in those gears. To me that makes more sense than going by what's dictated in a manual, although in practice I doubt I am very far from spec.

I am intrigued though by your statement that chainline is the most important measurement on the bike. Can you elaborate? Are you including fit measurements?

I agree and I have a number of options on hand, just not the one I need. A common plight.

Read this: All About Bicycle Chainline

There is nothing about "fit measurements" when you are talking about chainline. It's a spec, not a "preference".

Chainline determines everything else, from your wheel dish and cassette combination to your crankset and front derailleur. Get that wrong from the start and you will be fighting drivetrain issues. Staying within your preferred narrow gear range to compensate is hardly correct.

Chainline basically sets the measurements for the rest of the drivetrain. Get as close to that as you can, and build the rest of the drivetrain from there.

Does this help?

Notice the 110 is a real oddball?

OP isn't BUILDING A BIKE!
The OP is changing a crank/BB and wants their chain to run as straight as possible for the rings/cogs they use the most.
I do exactly the same thing! I use the longest BB that the FDER will span.
Both my bikes are 3X9 that I use as a 1X9 about 98% the time on the middle ring.
Even then, we're talking about the thickness of about 1.5 cogs.

I wrote in general about chainline here:
Bicycle chainline - Cycle Gremlin

What the risk of putting crankset too close (and too far) from the frame might be. Could be useful info.
Having said all that, it often is trial and error.

I cannot count the bikes I've seen where somebody just couldn't seem to get their shifting quite right, no matter what they did. It's fine in the lower gears but something rubs or doesn't shift in the higher gears, or vice versa, or somethings just not quite working perfectly as it was designed to do from the factory.

Assuming no damage (bent derailleur hanger, etc.) first thing to check is chainline and many times someone has installed the wrong bottom bracket. Shimano BB sizes are spaced closely enough that you can always come within a millimeter of spec. But, you have to know what it should be for your bike, and actually measure.

If you start with a good chainline and everything else is also within spec, your bike will shift properly and smoothly as it did when it came from the factory. You won't have to keep it in any certain gear range.

Apparently, you missed the entire point.

Well, maybe I did. The OP is trying to figure out what size bottom bracket to install in his bike.

He is going about it wrong, using the wrong measurements. You don't use the measurement from the inner chainring to the stays. I believe the Barnett manual specifies a minimum of 2mm clearance there, but that's not your primary measurement.

So I thought the point was "How can I tell what size bottom bracket is right for my bike?" and I provided some direction along with links to further reading on the topic. Not what he's looking for?

I wouldn't take issue with anything you have said, if I was setting up a bike to sell, or for an unknown rider on unknown terrain, who might use the full gear range. But that's not me. I have been riding for over 40 years, I don't have major hills to deal with, I keep a pretty even cadence and know which gear combinations I use the most. On most rides I only use two or three combinations and am almost never outside the 65-80 gear inch range. I am most definitely not limiting my shifting to compensate for an improper drivetrain setup. My drivetrain will shift well into any gear combination on the bike, so like I said previously, I am probably not far out of spec. I am simply trying to achieve the best (i.e. straightest) line in the gears I ride in.

Btw I understand that chainline has nothing to do with fit. I was simply responding to your statement that chainline is the single most important measurement on the bike, and wondering what other measurements you were considering.

That's helpful and yes, the 110 is the odd asymmetrical length. I wonder if that was produced just for a certain type of crankset that would compensate for the offset somehow.

One measurement that I've started taking is the distance from the pedal crankarm to the downtube, when you have those lined up. There is variation there. Even if you have a good chainline on the right side, the left arm may not be symmetrical to the bicycle body.

Now that I see that chart (assuming UN-54 and UN-55 have the same specs) I might have been better off with the 114. Hopefully the 110 will work; maybe with a spacer.

(Edit) Looking at that chart again, I am not sure how to interpret the numbers. As I said in the initial post, my 118mm by measures 20.5mm on the drive side, not 25mm like it says on the chart. So either the specs have changed or they aren't measuring the same way I am. Perhaps their measurement is the exposed end of the spindle, rather than the distance from the flange.

You probably didn't measure from the frame (end of BB shell in the frame), but just measure the spindle length protruding on the side measured.

mmm did I explain that well? :/

I think there is a reference point on the square where it is a certain dimension. Thus the "stub" could be longer or shorter (to some degree) but the crank doesn't know better.

Maybe you can figure something out from this. look at page 9-
https://law.resource.org/pub/in/bis/....1131.2006.pdf

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.