Bottom Bracket Size Needed for Correct Chainline?

Just got a Raceface Turbine LP crank (26-36-48) with ISIS bottom bracket to
put on my touring bike. The rear spacing is for 130mm hubs. The shop sold
me a bottom bracket with 113mm spacing. The bottom bracket instructions say
the ideal chainline for this equipment is 48.5 to 49.5mm.

I measured the existing chainline before taking apart the bike, and found it
to be about 46mm (to the center of middle chainring). Was this bottom
bracket sold to me thinking that I had a 135mm rear spaced setup? Would a
108mm bottom bracket work better for my setup?

rosco wrote:

Just got a Raceface Turbine LP crank (26-36-48) with ISIS bottom bracket to
put on my touring bike. The rear spacing is for 130mm hubs. The shop sold
me a bottom bracket with 113mm spacing. The bottom bracket instructions say
the ideal chainline for this equipment is 48.5 to 49.5mm.

I measured the existing chainline before taking apart the bike, and found it
to be about 46mm (to the center of middle chainring). Was this bottom
bracket sold to me thinking that I had a 135mm rear spaced setup? Would a
108mm bottom bracket work better for my setup?

Try measuring the existing chainline at front and rear (with the new
cranks and a middle ring fitted) using the instructions at
http://www.parktool.com/repair_help/clg2.shtml

If there's a discrepancy you'll then know what size BB you need to
correct it.

On Sun, 02 Nov 2003 18:20:54 +0000, rosco wrote:

Just got a Raceface Turbine LP crank (26-36-48) with ISIS bottom bracket
to put on my touring bike. The rear spacing is for 130mm hubs. The shop
sold me a bottom bracket with 113mm spacing. The bottom bracket
instructions say the ideal chainline for this equipment is 48.5 to 49.5mm.

I measured the existing chainline before taking apart the bike, and found
it to be about 46mm (to the center of middle chainring). Was this bottom
bracket sold to me thinking that I had a 135mm rear spaced setup?

Yes, it probably was meant for 135mm spacing, and you might benefit from a
shorter bottom bracket. I use mountain bike cranks on my road bike, and
did go for a shorter-than-recommended bottom bracket. It works fine, even
when I put the granny on (which I use only for tours -- once per Summer).

Measuring the chainline with what you have is a good idea.

--

David L. Johnson

__o | The lottery is a tax on those who fail to understand
_`\(,_ | mathematics.
(_)/ (_) |

Yes, it probably was meant for 135mm spacing, and you might benefit from a
shorter bottom bracket. I use mountain bike cranks on my road bike, and
did go for a shorter-than-recommended bottom bracket. It works fine, even
when I put the granny on (which I use only for tours -- once per Summer).

Measuring the chainline with what you have is a good idea.

--


Thank you.

I really want to get this right without using the trial and error method.
If I install the BB, I doubt it could then be returned if it's not right. I
assembled the BB and crank off the bike and measured the distance between
the middle chainring and the outer edge of the ISIS drive side cup (which
according to instructions should sit 1mm outbound from a 68mm BB shell).
This will definitely put the chainline out around 49 to 50mm.

The second thing I've noticed is that MTB front deraileur's seem to spec a
47.5mm chainline. If I end up with an ~46mm chainline, will the deraileur
be my next problem?

On Sun, 02 Nov 2003 22:39:43 +0000, rosco wrote:

The second thing I've noticed is that MTB front deraileur's seem to spec a
47.5mm chainline. If I end up with an ~46mm chainline, will the deraileur
be my next problem?

I must have missed this. You have a mountain bike front derailleur? But
130mm rear dropouts? The change in derailleur adjustment should be
doable, after all it's only 1.5mm, but I can't say I have done it. I*have
a road derailleur on the front.

--

David L. Johnson

__o | There is always an easy solution to every human problem - neat,
_`\(,_ | plausible, and wrong. --H.L. Mencken
(_)/ (_) |

I must have missed this. You have a mountain bike front derailleur? But
130mm rear dropouts? The change in derailleur adjustment should be
doable, after all it's only 1.5mm, but I can't say I have done it. I have
a road derailleur on the front.


It's a 13 or 14 year old Bruce Gordon Rock 'n Road frame. I believe back
then, 130mm was the MTB rear spacing.