Adjusting chainline with BB

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

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. 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.

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.

Help please.

--
Ted Bennett Portland OR

 

Ted Bennett wrote:

> 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.

> 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.

> 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:

http://sheldonbrown.com/chainline

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.

> 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.

> 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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>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 wrote:
>>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!

Carapace Completed Umber Princeton, New Jersey
+---------------------------------------------------------------+
| Anyone who can't use a slide rule is a cultural illiterate, | and should not be allowed to vote.
| --Robert A. Heinlein |
+---------------------------------------------------------------+

 

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.

Peter Chisholm Vecchio's Bicicletteria 1833 Pearl St. Boulder, CO, 80302
(303)440-3535 http://www.vecchios.com "Ruote convenzionali costruite eccezionalmente bene"