Adjusting chainline with BB

Ted Bennett said:

How does one go about moving the chainline inwards in a predictable way?

Here's the situation: I'm building a fixed gear using a track hub, which means that the chainline
must be at 42.5mm from the bike's centerline.

Actually most are a bit closer in than that.

Quoted message said:

To economize, I will be using road, rather than track cranks.

Shimano BB spindles are symmetrical, so if you change the length, the chainline changes by half the
difference.

F'rinstance, if you go from a 118 to a 113, chainline moves in by 5 mm.

Quoted message said:

Using the recommended BB for the road cranks will place the chainline at
43.5mm, so with the 5mm chainring seperation, the rings would be at 41 and 46. Thus the ring is at
the wrong place at either location.

41 is pretty close. Hub chainline varies more than you might expect. I've just put up a new article
on this topic, see:

sheldonbrown.comchainlineOpen ↗

You will find there a chart of hubs and another chart of sprockets and freewheels. With this
information you can determine the actual chainline.

Also, you can move the rear chainline inward, if needed, by flipping the cog over. In practice,
however, it's almost never necessary to move it farther in in back.

Quoted message said:

I'd like to use the outer location because it looks better, so that means I need a BB that is
3.5mm shorter on the right side than the recommended one.

That would want a 7 mm shorter spindle, which may or may not exist.

Quoted message said:

The cranks are Shimano, but Shimano's website is not very forthcoming with technical details such
as right and left spindle lengths for its various cartridges. I'd be happy to use some other
brand. For example Phil Wood has a nice list at www.philwood.com/webcatolog/page11.htm but I still
have no idea what to specify to get the resulting chainline right.

Phil bbs are particularly nice for this because they are adjustable for chainline.

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Quoted message said:

F'rinstance, if you go from a 118 to a 113, chainline moves in by 5 mm.

isn't 2.5 half of 5 Mr Brown but if l am wrong it wouldn't be the first time

stu said:

Quoted message said:

F'rinstance, if you go from a 118 to a 113, chainline moves in by 5 mm.

isn't 2.5 half of 5 Mr Brown but if l am wrong it wouldn't be the first time

That bonehead always did have trouble with basic arithmetic!

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ted-<< Here's the situation: I'm building a fixed gear using a track hub, which

means that the chainline must be at 42.5mm from the bike's centerline.

Not really-the ring you use must be aligned with the cog/hub you use. make the line parallel to
the top tube.

I wouldn't fret about that number-adjust the hub so that the rear cog is aligned with the ring.

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