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Unsure about BB - cranks not centered on frame

Old 03-21-17, 10:03 PM
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Unsure about BB - cranks not centered on frame

I have a 80s lightweight steel bike with a 53/42 suntour superbe pro crankset, with a 144 mm BCD. Since I moved to a much more hilly area I'm attempting to drop down to compact gearing. I found a old Sugino double crankset with a 110 mm BCD so I can use 50/34 rings.

I had already replaced the original BB with a shimano cartridge BB (68x113), and I was super careful to get the chainline just perfect (~43.5). However, after installation I noticed the NDS crank arm passed ~4mm closer to the seat tube than the DS crank arm. I don't really understand why this is, it almost makes more sense for it to be the other way, no? Also not sure if this is even worth worrying about...?

So now I'm replacing the crankset, I stuck the new cranks on that same BB and noted the chainrings moved inwards by 3 mm relative to the old ones. So a 118 spindle should be pretty good for the chainline, however, the NDS crank is still 4mm closer to the centerline than the DS crank. I'm wondering if I can fix this? The UN55 BB has a flange on the fixed DS end that prevents it from screwing any further into the shell. Is there different BB model that I can screw in 2mm further to even out the crank arms? Or should I just stop being a perfectionist?
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Old 03-21-17, 10:13 PM
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Back in the day both BB spindles and crank arms were not symmetrical. meaning that a spindle might have different drive side and non drive side lengths (as measured from the shell centerline). Also the two sides of the crank arms could be different in their spindle fit to their pedal fit as referenced from the frame centerline.


A good shop would have a lot of different spindles in their draws and try a likely one then refit the nest choice based of the first's in place dimensions. Today much, but not all, of this has been made vastly easier. Still there are the same rules, the best choice is the one that in place meets the need. Andy
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Old 03-22-17, 02:19 AM
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"if it ain't broke, don't fix it".


I did plenty of miles on my first drop bar bike with an offset like that. Never noticed it while riding.
Next bike was symmetrical.
My eyes noticed, my legs didn't.
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Old 03-22-17, 08:53 AM
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the crank arm clears the frame by a CM ?
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Old 03-22-17, 08:58 AM
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Ok, so I'm willing to let the asymmetry slide I suppose. I can always adjust the cleats on my shoes laterally to compensate if I want, and riding on platforms my feet should naturally adjust.

However, I am wondering if every length of the shimano UN BB's are symmetrical? For example, is the 68x113 22.5mm on each side, and the 68x118 is 25mm on each side? Or are the odd numbered lengths longer on the drive side or something strange? Does anyone know of a diagram with dimensions for different length shimano UN BB's?
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Old 03-22-17, 09:53 AM
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Crank designs have changed over the years..

FWIW Phil wood can be set up asymmetrical, but being a USA small shop produced component

they are more than the made in the millions shimano .
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Old 03-22-17, 10:25 AM
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Originally Posted by fietsbob
Crank designs have changed over the years..

FWIW Phil wood can be set up asymmetrical, but being a USA small shop produced component

they are more than the made in the millions shimano .
Ahh I see, looks like it has two removable "cups" that screw in rather than the DS having a fixed flange like on the shimano. >$100 probably isn't worth it though.
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Old 03-23-17, 02:00 PM
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So in case anyone's interested, I popped on an Sunlite cartridge BB 68x118 I had lying around and weirdly, my chain line is off by about 2.5 mm (too far in), but both cranks are now exactly the same distance from the frame! So strangely, the UN55 68x113 BB must be asymmetrical, with the DS being a bit longer than the other. It's kinda tricky to measure this accurately, but this is the only explanation. The asymmetry had nothing to do with the cranks. OR maybe the Sunlite is asymmetric with the DS a bit shorter. Who knows. I might just call it good, chainline be damned.
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