Chain line/axle length question

[Ben Micklem]

Hello,

I have a hybrid from 1997 that I have converted into a cyclocross bike. It
has an STX triple drive at the moment. The rear hub is 135mm. I guess the
chain line is 47.5mm.

I want to put in a new BB (old one has had a hard 10 years of use), and was
thinking of putting on a cheap single chainset, e.g. ebay item 130001399527.
The description of this item says it is OK for derailleur chains, and would
be ideal for a nexus geared bike. From Sheldon's site, I see Nexus has a
chainline of 45mm. Does this mean that the axle I use for this chainset
should be roughly 2.5mm longer than the Nexus one (116mm) to bring the chain
line out to the 47.5mm that the cassette is most happy with? So I should
look for a 119 or 118mm bottom bracket?

Or am I confused?

Ben

 

[Nate Knutson]

On Jul 3, 10:33 am, Ben Micklem <[email protected]> wrote:
> Hello,
>
> I have a hybrid from 1997 that I have converted into a cyclocross bike. It
> has an STX triple drive at the moment. The rear hub is 135mm. I guess the
> chain line is 47.5mm.
>
> I want to put in a new BB (old one has had a hard 10 years of use), and was
> thinking of putting on a cheap single chainset, e.g. ebay item 130001399527.
> The description of this item says it is OK for derailleur chains, and would
> be ideal for a nexus geared bike. From Sheldon's site, I see Nexus has a
> chainline of 45mm. Does this mean that the axle I use for this chainset
> should be roughly 2.5mm longer than the Nexus one (116mm) to bring the chain
> line out to the 47.5mm that the cassette is most happy with? So I should
> look for a 119 or 118mm bottom bracket?
>
> Or am I confused?
>
> Ben


No, you can't make any of those assumptions and there's no real way of
figuring it out confidently, because you don't know for sure what
chainline this crank will give for a given length spindle. You can do
this kind of thing when you do have that information, though.

One might be able to make the case that it's a somewhat decent GUESS
that the chainline of this crank will be in the same ballpark as the
Nexus. I wouldn't go buying stuff based on that. But if you were to
assume it is, and you were to assume that all the other numbers
involved here are accurate, which they're probably really not, then
for your 47.5 setup in back you'd ideally want a spindle length that's
5mm longer than what what works to put it's chainline at 45. That
ideal spindle would be 121, and the closest you can get is 122,
putting the ring .5mm further out than ideal.

 

[A Muzi]

Ben Micklem wrote:
> I have a hybrid from 1997 that I have converted into a cyclocross bike. It
> has an STX triple drive at the moment. The rear hub is 135mm. I guess the
> chain line is 47.5mm.
>
> I want to put in a new BB (old one has had a hard 10 years of use), and was
> thinking of putting on a cheap single chainset, e.g. ebay item 130001399527.
> The description of this item says it is OK for derailleur chains, and would
> be ideal for a nexus geared bike. From Sheldon's site, I see Nexus has a
> chainline of 45mm. Does this mean that the axle I use for this chainset
> should be roughly 2.5mm longer than the Nexus one (116mm) to bring the chain
> line out to the 47.5mm that the cassette is most happy with? So I should
> look for a 119 or 118mm bottom bracket?

It's not at all intuitive.
If you look at a few different brand/model cranks, you'll see the crank
designer has wide latitude in the relationship of taper to arm to ring
to pedal.

I can't tell what crank it is from the photo but if you place it on your
present spindle and measure, it shouldn't be too hard to determine the
correct spindle length.
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971