xenologer

That Data is Very Useful
Where did you find it??

Whenever I want to order a BB, the parts catalogs never specify the details of DS vs NDS actual lengths, only overall lengths.

Bike Gremlin

Generally, but not always the case. Some crankset chainring comibnations are crazy expensive. Same goes for some cassettes. For example, a 14-32 cassette is very hard to find in 7, 8 or more speed at a low price. The 14 T starting ones are usually double the price of an 11 starting cassette with the same number of sprockets.
Same goes for triple and double chainrings that vary from some often sold.

So one needs to make some compromises often.

HillRider

AFAIK, Shimano offers cassettes starting with a 14, 15 or 16T smallest cog only at the Ultegra level and these are aimed at the Junior racing market which has roll-out restrictions that preclude the use of an 11, 12T or even 13T smallest cog. They are also only made with a 25 or 27T largest cog. So, these have limited sales and are priced accordingly. They aren't even advertised to the general market.

wphamilton

I also question his premise that the center of the cassette is necessarily where the parallel chainline must be.

Logically, when the chain line is perfect for one gear, it is less than perfect for other gears, no matter which gear you choose to be optimal. It is reasonable to optimize for the range of gears you use most, rather than arbitrarily picking the center. Someone might seldom use his largest cog for example, but still needs it. It might even be the largest two cogs. Why would you not set the chainline to the center+1 cog in that case? Another person with different habits and different terrain might reverse that, seldom using an 11 or 12 cog but feeling they are necessary.

due ruote 

This is what I was getting at. I literally never use the small cog. It's nothing more than a placeholder. So why shouldn't I center the rings on the remaining 5 cogs of a 6-speed?
Although I must admit Jeff Neese's way is easier.

wphamilton

In his defense he did say "approximately" in the center, and the exact center of 6-speeds is between number 3 and number 4. Approximately would include the chain on one or the other. If you center on the remaining 5 cogs that would still be on the 3rd one, so you'd both be in general agreement.

I was reacting to his saying that it's "silly" to adjust for a gear range. It's perfectly logical in some cases.

Bike Gremlin

Exactly. Link in my post #16 gives explanation of what happens if you miss the center line by too much.

SquidPuppet

A member here, ThermonicScott did the measuring and put it together. It has helped more than a couple people get their chain line right in the Single Speed and Fixed Gear forum.

I am fortunate that the 110mm BB is not symmetrical. I have an old bike that has a BB shell that is almost 4mms off center to the right, so the Shimano UN55 110 lines everything up perfectly.

Lucky.

Jeff Neese

I'll change my term from "silly" to "incorrect".

You optimize right down the middle so things work just as smoothly both up and down. The assumption is you are using your bike's gearing to it's fullest potential and you choose your cassette and chainring combination so that you are riding in the middle most of the time. That's the way you're supposed to do it, not the other way around. "Correct" vs. "Incorrect".

There's never any downside to doing something "by the book", and in this case it's fairly simple measurements. Of course anyone is free to set up their bike any way they want. You don't have to follow the specs or even best practices. It's your bike.

due ruote 

Thought it might be useful to post my results with the 110mm UN-55 unit. It worked out great.
Again, I don't know how they are measuring these things for the chart posted earlier, but the 118mm unit I tried initially had 21.5mm of driveside spindle, as measured from the outer edge of the flange. The 110mm unit is 17.5mm measured the same way, so just what I wanted (though perhaps not what would be expected based on the chart).

I am using a 13-24 freewheel (6-speed) and 39/48 rings. With the 110mm BB, the 48 ring is centered between the 17 and 19 cogs, which is right where I wanted it, and, as it turns out, the chainline is 42.5mm, so only slightly under spec.

Bill Kapaun

Sheldon Brown's Bottom Bracket Size Database
Scroll down just a bit-
You can see the measurement is taken from the bearing race to the end.
Now I'm pretty sure the "end" isn't the actual exact point, but a reference point where the square taper is exactly a certain dimension that may or may not be exactly at the end.

trailangel

Good deal. That's a good chainline.