setting chainline question

Quoted post said:

Originally posted by RBS
hi all

i'm building up a new bike from parts on an older bike that i have. i'm going from a triple to a double crank because i got a good deal on a beautiful aluminum shimano ultegra crank. compared to the cheaper triple with steel rings there's no contest. shimano specs a 113mm bb for road double cranks and 115mm for road doubles with an overly large seat tube. my seat tube is 1.25" (not sure if this is large or not). shimano spec says the set the chain line at 43.5 mm from the centerline of the seat tube to the the the middle of the space between the two rings.

here's the question. the bb spindle in now 118mm. so, realizing that i need a new bb, i mounted the double crank on the existing triple bb. i then measured and calculated the chain line distance with this setup as 46.75mm. my reasoning now suggests that if a 118mm bb gives a chainline of 46.75, then a bb of 115 will give a chainline of 43.75 (close enuf for jazz). does this logic work in this case? do you experts suggest i go for the 115 or just put in a 113 as shimano says to do? does it even matter?

thanks to all

bob

Go for the 113 as Shimano says.
The length of the spindle is from end-to-end. Each BB has it's own offset or lack there of. Your math didn't allow for some shortening of the axle on each side.

"RBS" <[email hidden]> wrote in message "]news:[email hidden]...
| hi all
|
| i'm building up a new bike from parts on an older bike that i have. i'm going from a triple to a
| double crank because i got a good deal on a beautiful aluminum shimano ultegra crank. compared to
| the cheaper triple with steel rings there's no contest. shimano specs a 113mm bb for road double
| cranks and 115mm for road doubles with an overly large seat tube. my seat tube is 1.25" (not sure
| if this is large or not). shimano spec says the set the chain line at 43.5 mm from the centerline
| of the seat tube to the the the middle of the space between the two rings.
|
| here's the question. the bb spindle in now 118mm. so, realizing that i need a new bb, i mounted
| the double crank on the existing triple bb. i then measured and calculated the chain line distance
| with this setup as 46.75mm. my reasoning now suggests that if a 118mm bb gives a chainline of
| 46.75, then a bb of 115 will give a chainline of 43.75 (close enuf for jazz). does this logic work
| in this case? do you experts suggest i go for the 115 or just put in a 113 as shimano says to do?
| does it even matter?

This reminds me of a related question. I built up a Salsa Campeon road frame and it has a massive
(by my past experiences) 31.6 seat tube. I installed a DA 109.5 BB and a braze on DA front
derailleur with a shimano 31.6 clamp.

I doubt if this is giving me the proper chainline. I notice the chain light touching the big ring if
I shift any farther down than the 4 largest gears on the 9 speed rear. I've considered a shim on the
drive side of the BB as there is about 2-3mm of thread showing on the non drive side. Is there a
better fitting BB for this?

John Rees

Quoted post said:

Originally posted by daveornee
Go for the 113 as Shimano says.
The length of the spindle is from end-to-end. Each BB has it's own offset or lack there of. Your math didn't allow for some shortening of the axle on each side.

thanks david

i suspected something of the sort but wasn't sure and couldn't find any reference to an offset.

later

bob

RBS said:

i'm building up a new bike from parts on an older bike that i have. i'm going from a triple to a
double crank because i got a good deal on a beautiful aluminum shimano ultegra crank. compared to
the cheaper triple with steel rings there's no contest. shimano specs a 113mm bb for road double
cranks and 115mm for road doubles with an overly large seat tube. my seat tube is 1.25" (not sure
if this is large or not). shimano spec says the set the chain line at 43.5 mm from the centerline
of the seat tube to the the the middle of the space between the two rings.

here's the question. the bb spindle in now 118mm. so, realizing that i need a new bb, i mounted
the double crank on the existing triple bb. i then measured and calculated the chain line distance
with this setup as 46.75mm. my reasoning now suggests that if a 118mm bb gives a chainline of
46.75, then a bb of 115 will give a chainline of 43.75 (close enuf for jazz). does this logic work
in this case? do you experts suggest i go for the 115 or just put in a 113 as shimano says to do?
does it even matter?

Uh, if you have 46.75 with a 118, you'll have
(46.75-((118-115)/2))=45.25 with a 115.

You'll achieve (46.75-((118-113)/2))=44.25 with the 113. That's better. Shimano asks for more
chainline ( crank farther from the centerline) than would otherwise work well but their
changers seem to like that also ( Many Shimano fronts cannot shift a close chainline to the
inside very well )

Did you torque the crank to the suggested torque with a lubricated fastener? Lubricated tapers are
IMHO a good idea but some will argue it; No reasonable person torques a dry fastener. I ask because
your numbers seem wide.

Did you sight the chainline? How much is actually needed, aside from Shimano's request? Get you head
between the BB and front hub. Sight across the inside face of the outer chainring. You should see
the center cog. It's easy and accurate to see how much off it really is by sighting in this way.

And if your existing spindle is asymmetric you might remove it, flip it over and see if that isn't
close. Fortuitously, flipping an old spindle often gets very close with a new crank. You lose any
potential Q improvement, and in fact sometimes gain it, but you save the new BB expense.
--
Andrew Muzi www.yellowjersey.org Open every day since 1 April, 1971