Join Date: Oct 2004
Location: Chattanooga, TN
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BB Madness un72, un73???
I've practically memorized Sheldon Brown's chainline article and have searched and searched through the forum archives, but am still confused about finding the right BB to get the chainline I need for my FG conversion.
After doing all the math, I figured out that I needed a BB with spindle length of 109 mm (68 English/ISO) to get that elusive, perfect 42 mm chainline in the front. I'm running an IRO fixed/free rear hub with an EAI 15t 3/32" track cog. Rear chainline measures out at almost exactly 42 mm, as far as I can tell.
So, I go to the LBS, and they don't have a 109, but they do have a 110 in the Shimano un73. So, I get it back home take it out of the box, and realize that 1) the left/right orientation is the opposite of the sealed BBs I'm used to seeing---that is, the fixed cup was on the right and the other cup on the left---and that 2) the driveside spindle end is much shorter than the non-driveside spindle end. The measurements I had made were based on the driveside and non-driveside spindle ends being of equal length. Actually, I've determined that what I REALLY need is a measurement of about 25.5 mm on the driveside spindle end (the portion of the spindle that protrudes past the fully tightened cup on the driveside)
I decide to go ahead and try to thread it in anyway, just to see how it worked. I could not get it to thread in properly, either on the right or left side.
SO, my questions are:
1) Are certain BB models made to thread into the driveside and some in the non-driveside?
2) If so, which of the Shimano models will work on my bike (possibly the un72?---what's the difference in these models?)?
3) Given that the driveside and non-driveside spindle end lengths apparently vary, how do I find a BB that will give me a driveside spindle end length of 25.5?
I would greatly appreciate any help. I'm so close to completing my fixie build, and this last damn detail is just taunting me and making me insane.