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Discussion Starter · #1 ·
I'm building up a new SS, and I'm having some chainline woes.

Frame = Karate Monkey
Crankset = FMF BMX style five arm, 110 BCD (sprocket will only mount to one side of crank). Nice look to the part... less is more.
Front sprocket = Steel Surly 34T
Bottom bracket = FSA ISIS 73x118mm (Part # CR4999)

Rear Hub = Surly Disc / SS (Nicely built by me with alex 29er rims, 36H..)
Rear cassette = ACS Claw

Parts went on okay... Front chainline center = 50mm from bike centerline
Rear Hub chainline center = 54.5mm

Big Ooops. I haven't put chain on bike yet, as these numbers seem too far off. According to Sheldon Brown a mountain bike SS chainline is 52mm. Great... I'm in the ball park, but its hard to manage a bunt when you are standing in center field. When you eyeball this from the back of the bike the offset looks huge.

Now what do I do?

-- Buy a new ISIS 73 x 128mm bottom bracket set? Will I notice the extra width on a 128mm bottom bracket when I'm pedaling? I see that universalcycles has a 73 x 123mm and a 73 x 128mm downhill BB for sale, (128mm backordered... ouch.) These are downhill parts with smaller bore drilling and are bound to add some weight.

-- Look for five arm 110 ISIS crankset on ebay and pray that it works... ? Fat chance. Sell the FMFcrankset and buy somewhere else new? Every thing I see in the value chain tends to be four arm cranksets.

-- Shim the bottom bracket towards the right? (I don't think I dare go more than 1 or 2 mm...Does anybody know the limiting factor here?)

-- Heck with it, just use these parts as they are? Unfortunately with the snow on the ground, I don't think I will really be able to test this combo right til spring... sigh...

What would you do?

Why the heck doesn't anybody post these numbers up front so you can verify chainline before you purchase parts? Has anybody built up a database on dimensions for hubs, cassettes, cranksets? I've seen notes on how to modify an existing, but nothing for folks starting out from scratch.

many thanks in advance,

3,927 Posts
I'd start by shimming the BB to the right. The limits are the left crank striking the chainstay or your heel doing the same.
Next a wider BB.

(other options)

Sell the wheel and use a cassette-cog system - a cog with spacers can match nearly any chainline.
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