Bottom Bracket Size Needed for Correct Chainline?

[Rosco]

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?

 

[Zog The Undenia]

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.

 

[David L. Johnso]

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. (_)/ (_) |

 

[Rosco]

> 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?

 

[David L. Johnso]

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 (_)/ (_) |

 

[Rosco]

> 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.