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Discussion Starter · #1 ·
Hello,

Seeking advice, I am putting together a 2x9 drivetrain. Using a crankset with 42t and 30t rings. I want to use a FSA ISIS road 68x108 bottom bracket. However, my bb shell width is 73mm. Should I run 2.5mm spacers or use cup(s) off a 73mm bb.:confused:

Thanks in Advance
 

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You should get the proper BB. How would you use spacers to make the BB shell narrower?
Will the 108mm spindle even give you a "proper" chainline?
 

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shiggy said:
You should get the proper BB. How would you use spacers to make the BB shell narrower?
Will the 108mm spindle even give you a "proper" chainline?
108 will be fine. i'm using a road octalink which cannondale spec'd to use on their 2x9 crankset for mountain bikes.
 

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108 mm should work fine. The reckoning point is on the right side of the BB. A 108 mm BB in a 73 mm shell should be equivalent to a 112 or 113 mm bb chainline on a regular 68mm shell. The problem is on the non drive side of the shell-- the cup shouldn't have any flanges, which will allow it to go deeper into the shell to cup the BB properly.I havent' tried using a 73mm cup, but it should work

I used a shimano 107 mm UN52 on a 73mm shell on my old bike, and had calculated it to be equivalent to a 112 mm BB on a 68 mm shell.
 

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Without knowing what crankset you're using hard to advise anything (or did you mean to say you're using an FSA ISIS road crankset?). I'm not that familiar with FSA crankset/bb requirements in any case. Like shiggy sez otherwise...
 

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Broseph said:
108 will be fine. i'm using a road octalink which cannondale spec'd to use on their 2x9 crankset for mountain bikes.
You have no way to know without knowing what spindle length the crank was designed to use and whether the rings are in the outer/middle or middle/inner positions on the spider.
 

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Discussion Starter · #7 ·
Thanks for all the advice, I am building up a uber light rigid 29er. I am using a Race Face Turbine 42/32/22 crankset. My plan is use the 42t and install a 30t middle ring and forego the small 22t ring.. As stated, I want to use a road bb to save a few grams and have a straighter chainline. However, as you know, road bb's have 68 mm shells and my Mt Tam has a 73 mm shell. As chrisg_ph pondered that a 73mm cup might work-that is what I was banking on. Any input on why that wont work?

Thanks a bunch
 

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edwood said:
Thanks for all the advice, I am building up a uber light rigid 29er. I am using a Race Face Turbine 42/32/22 crankset. My plan is use the 42t and install a 30t middle ring and forego the small 22t ring.. As stated, I want to use a road bb to save a few grams and have a straighter chainline. However, as you know, road bb's have 68 mm shells and my Mt Tam has a 73 mm shell. As chrisg_ph pondered that a 73mm cup might work-that is what I was banking on. Any input on why that wont work?

Thanks a bunch
I would be concerned about the left side crank arm clearing the frame.
 

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shiggy said:
I would be concerned about the left side crank arm clearing the frame.
Same here. You're going 5mm narrower, so you should be paying close attention to present clearance and frame flex (and flex of other components).

I wonder how that would feel, with the BB being offset to one side like that ..... :confused: :skep: :eekster:
 

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AndrewTO said:
Same here. You're going 5mm narrower, so you should be paying close attention to present clearance and frame flex (and flex of other components).

I wonder how that would feel, with the BB being offset to one side like that ..... :confused: :skep: :eekster:
It is the spindle that is making the cranks narrower, not the BB shell (we still do not know what the original length "should" be). The BB shell width difference moves the crank an additional 2.5mm to the right.

Most cranks are not centered on the frame and I have ridden ones offset as much as 10mm. Not really noticeable.
 
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