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Discussion Starter · #1 ·
What happens if you use a longer spindle? I have a 113mm BB shell on my Titus ML, and was looking at upgrading the cranks. I found a good deal on Shimano XT cranks, but the description says they need a 121 mm bottom bracket (K- version). Does anyone know if you can just run a normal BB? I always thought spindle length was determined by the BB, and an octalink (or isis) type crank would just bolt up to whatever was installed.

Thanks for any help!
 

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ups and downs
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Cranks do have different profiles at the BB spindle interface. For example some of the newer Race Face ISIS cranks used a 108mm ISIS BB instead of the older 113mm BB. Often the offer various versions to allow the crank to fit around frame clutter at the BB shell, or provide a different Q factor, and still maintain a 47.5-50mm chainline (after all, the chainrings need to be in much the same place on all bikes).

If you use a 113mm spindle on a crank designed for a 121mm spindle, your chainline will be waaaay inboard (43.5-46mm), and the large ring may hit the chainstay and the granny ring may touch the frame, or at the very least may be too far inboard for the front derailleur to drop the chain onto the granny ring.
 

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You have a misconception about your bb shell, which isn't 113mm, but rather 68 or 73 mm. Spindle length is a separate measurement. Frames aren't designed for a spindle length whatsoever (in the case of 68 or 73mm shells at least; downhill bikes with wider bb shells I suppose you could say are...). Your crankset primarily determines which bb spindle length to use; your bb shell is a factor for which bb width to buy, or which spacers to use in the case of the newer external bb style cranks.
 

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Discussion Starter · #5 ·
Brain flatuence!

Thanks Bikeinfool...

I was typing that while during a meeting and kind of spaced out that little fact. I am too used to looking for a 73x113 mm BB that I kind of spaced it.

So, if I can find a 121mm octalink BB with the right splines then I should be good to go? That would be a 73x121mm, I gather?
 

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If it's a FC-M752K you'll need a 121MM to make your chainline 47.5mm. I made the mistake of buying a BB-ES73-118 and it was too narrow for the K crank. This is the normal 50mm chainline BB for the non-K version. I complained to Cambria bike and they seemed to have updated their website to tell buyers what correct spindle length is needed now.

You'll have to buy a BB-ES51 to get the 121mm spindle I believe.

http://www.cambriabike.com/shopexd.asp?id=28922

glenk
 

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Greetings,

I wanted to rehash this thread as I'm in a similar position as the OP. The deal sounds good, but is there a difference in performance of a longer spindle BB/crankset? It just seems like these are priced much lower than other XT cranks. I'm looking for a good crank that won't break the bank to put on my Karate Monkey build up.
 
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