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Old 04-24-11, 08:35 PM   #1
nicoth
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How to replace an asymmetrical bottom bracket?

Hi. The old bottom bracket in my bicycle (mid 80's trek) has a spindle that's about 3mm longer on the drive side. My impression is that the Shimano UN-54s are the same length on both sides. I put in a replacement of the same overall length (118mm) and now my chainline is a little bit closer to the frame. So what size do I use the get the same chainline? Thanks.

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Old 04-24-11, 09:10 PM   #2
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I don't think it will hurt but the next size up is a 122.5 and then 127.5
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Old 04-24-11, 09:33 PM   #3
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Ok. I think maybe it's not as complicated as I suspected. A symmetrical spindle would increase the spacing equally on each side, so if I want +3mm on the drive side, I would want a +6mm spindle, which means 124mm total, right?
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Old 04-24-11, 09:39 PM   #4
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My impression is that the Shimano UN-54s are the same length on both sides.
Not necessarily. There is asymmetry that varies between the various sizes. I think the asymmetry of Shimano cartridge BBs is the same as for Shimano loose ball BBs. However, this may not match other brand spindles.

What cranks do you have?
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Old 04-24-11, 10:06 PM   #5
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My cranks are are a mystery: SR SXC-200 according to old trek brochures. Sheldon Brown's BB database doesn't have any info on it.
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Old 04-24-11, 10:26 PM   #6
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Priced upwards, Phil Wood, is adjustable in offset, get the length right

and the spindle is a press fit, so can pressed to change the offset.

+ the mounting rings are entirely in the shell, no fixed cup edge.
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Old 04-24-11, 10:38 PM   #7
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Sheldon says that a particular washer sort of thing can be used to add space; maybe I'll try that. Everyone says that Phil Woods BBs are good, but I'm not sure they're a good solution since they are worth about half as much as my bike.

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Old 04-25-11, 05:34 AM   #8
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Ok. I think maybe it's not as complicated as I suspected. A symmetrical spindle would increase the spacing equally on each side, so if I want +3mm on the drive side, I would want a +6mm spindle, which means 124mm total, right?
No, he simply needs spindle 3mm longer. He had 57.5 nds/ 60.5 ds = 118. He needs the long side of 60.5 x 2 = 121. FWIW, 122.5 should fit and shift just fine.
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Old 04-25-11, 07:12 AM   #9
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No, he simply needs spindle 3mm longer....
6mm is right, he needs to add 3mm to the drive side, but the new spindle is symetrical so he needs to double it. In real life the 122 or 127 will work, I usually go for the wider spindle just to be safe.
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Old 04-25-11, 12:05 PM   #10
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6mm is right, he needs to add 3mm to the drive side, but the new spindle is symetrical so he needs to double it. In real life the 122 or 127 will work, I usually go for the wider spindle just to be safe.
Wrong. He needs to add 1.5mm to the each side relative to his symmetrical 118 that he bought. His asymmetric 118mm spindle is 3mm longer on the drive side. To achieve symmetry, he just needs 3mm on the non drive side.

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Old 04-25-11, 12:30 PM   #11
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No, he simply needs spindle 3mm longer. He had 57.5 nds/ 60.5 ds = 118. He needs the long side of 60.5 x 2 = 121. FWIW, 122.5 should fit and shift just fine.
+1
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Old 04-25-11, 03:38 PM   #12
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Wrong. He needs to add 1.5mm to the each side relative to his symmetrical 118 that he bought. His asymmetric 118mm spindle is 3mm longer on the drive side....
If he goes with 121, his drive side will be 1.5mm closer than his stock asymetric, not enough to matter most of the time, but without pictures and not knowing how far the old crank will install in the spindle, I'd go for a little extra to be safe.
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Old 04-25-11, 06:12 PM   #13
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If he goes with 121, his drive side will be 1.5mm closer than his stock asymetric,
If he can find 121, the cranks will be the same distance as stock on DS and 3mm further out on NDS.
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Old 04-25-11, 09:12 PM   #14
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My cranks have plenty of room, by the way; I'm just interested in getting optimal chainline. 121 mm sounds right. My error originally was that I thought the symmetrical spindle would be 3mm shorter on the drive side, when it would actually only be 1.5 mm shorter.
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Old 04-26-11, 05:17 AM   #15
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I'm just interested in getting optimal chainline.
If that's what you want rather than the same chainline (as first posted) I'd work toward narrower rather than wider on a road bike. On the road, riders typically spend most time in the outer chain rings. The problem you may encounter would be that the FD does not retract enough to drop the chain onto the inner most chain ring. FWIW, 1.5mm is not much so I'd try to set up what you bought.
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