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I'm planning a new build and for the first time, I'm working with a frame that requires an Italian bottom bracket. My question isn't specific to the build, but a general question about chainline.

Given the same crankset, say Shimano FC-6400, the corresponding English BB would have a 113 mm symmetric spindle (i.e. 3H or D-3H), while the corresponding Italian BB would have a 115 symmetric spindle (i.e. 5H or D-5H).

Why would the spindle lengths be different at all? Why wouldn't the Italian BB spindle have the bearing shoulders further apart by 2 mm while keeping the 113 mm length? Would the chainline not increase by 1 mm in this case? If so, how should this be accounted for?

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Sheldon Brown makes a point about different spindle lengths being to do with seat tube diameter, when listing Shimano BB sizes. He says,

Shimano sometimes specs two different lengths for the same crankset. This usually relates to the diameter of the seat tube. The shorter dimension is normally preferred, but bikes with fat seat tubes may require the longer size to accommodate the swing of the front derailer.

Because of the wider BB Shell, Shimano may expect the frame's seat tube will also be wider and therefore recommend a slightly longer spindle to allow for front derailleur clearance on a wider seat tube. The 1mm difference in chainline due to a 2mm difference in spindle length is almost negligible for most riding, particularly for bikes with a derailleur.

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I would have suspected both BB spindles to have JIS taper, as they are for the same crank. Thus, the crank taper would meet the spindle taper at the same position from the end, no? –  fideli Jan 6 '13 at 6:05
    
Ah, your update makes sense to me. I definitely missed that line when reading that page. Thanks! –  fideli Jan 6 '13 at 23:53
    
@fideli I removed the discussion about longer tapers, since you are right, the JIS taper should be the same length. –  Jason S Jan 7 '13 at 22:07
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Were you aware that the shell width on Italian BBs is 70mm, whereas on English threaded BBs it is 68mm?

References:

Sheldon Brown BB Crib Sheet

Sheldon Brown on BB Sizes

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Fully aware, yes. To paraphrase the question, why would the overall spindle length need to change because of the 2 mm difference in BB width, and not just the spacing of the bearing shoulders inside the BB shell? I'm asserting that the spindle length should be dependent solely on the crankset, with the intention of someone proving that assertion wrong and providing an explanation. –  fideli Jan 5 '13 at 7:47
    
There's no law of science which requires a 113mm spindle with an English BB, so far as I know. I would think they went with 113mm because it worked, i.e., it was long enough to get the job done without adding unnecessary length (and weight), but would 114 mm or 115 mm have worked, as well? I'd wager so. In other words, I believe there is no compelling logic behind the choice. Maybe the Italians like nice round numbers, so 115mm instead of 113mm. –  Zippy The Pinhead Jan 5 '13 at 17:54
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Why would the spindle lengths be different at all? Why wouldn't the Italian BB spindle have the bearing shoulders further apart by 2 mm while keeping the 113 mm length?

No idea about this. The great thing about bicycle standards, is there's so many to choose from.

I would guess it's to keep the same amount of spindle sticking out of the bottom bracket shell on both widths.

Would the chainline not increase by 1 mm in this case?

Yes

If so, how should this be accounted for?

There's two of ways to deal with this, I can think of off the top of my head.

You could ignore the 1mm difference, on a derailler system a 1mm chainline difference isn't that huge. You're lowest gears might be a bit more crooked, but on an 8 or 9 speed system, I think you'd get away with it.

Or, why not get a 70x113 bottom bracket, a quick google turned up a number of these.

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As you have mentioned, the point of having a variety of spindle lengths is to allow selection of a BB which creates the proper chain line, while ensuring proper clearance from your frame. (Typically 51 mm from BB center to the center of the chain ring set.

The assumption that using the Italian BB mandates a wider BB spindle is not correct. You should have no issue using a 70 mm x 113 mm Italian BB.

That said, you are unlikely to notice any difference in shifting performance from a 1 mm difference in chain line, unless your original BB was already significantly offset (incorrect). It's just not enough difference to matter. So if you already have the 70 x 115, use it. You'll be fine. If you still need to buy the BB, order a 70 x 113.

To directly answer what I think is your main question, There is no reason for the spindle length to change, except the convenience of using parts which are already on hand.

I hope that helps.

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51 mm chainline may be a bit high for the road double that I plan to build. I think what has thrown me off is that the same BB, Shimano BB-6400, comes as 68 x 113 for English, but 70 x 115 for Italian. Why would Shimano not have just released a 70 x 113 mm to begin with? –  fideli Jan 6 '13 at 13:08
    
51mm is Shimano's idealized specification. They don't release every size in every quality level. Look for something of a higher or lower quality level, where the correct size was released. –  zenbike Jan 6 '13 at 17:29
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