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Bottom bracket, old crankset, chain line question

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Bottom bracket, old crankset, chain line question

Old 07-14-13, 09:53 PM
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jankdc
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Bottom bracket, old crankset, chain line question

Hi, I found some cool old cranks for my fixed gear conversion. They are stamped Bridgstone and are made by SK. I think that they are from the late 70's (118 BCD). In my effort to try to figure out the bottom bracket spindle size, I tried putting them on several bikes. The only one that they would fit on were on my wife's old Taiwanese Terry with a 122mm spindle (and a triple crankset). The measurement from the center of the seatpost to the center of the chainring was 44.75mm. So I am calculating that if I want a 42mm chainline (per Sheldon Brown), I am off by 2.75mm and I need to double that and subtract 5-6mm and that I need 117 to 118 spindle to run single/fixed.


Question 1:
Am I correct in that I need a 117-118 spindle?

Question 2:
This seems like a long spindle for a single/fixie. Am I missing something? Does this indicate that there is too much wear?

Thanks!
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Old 07-14-13, 10:02 PM
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The only thing you're missing is that not all spindles are symmetrical (most aren't), so you need to care about the right side, not double it and subtract.

As for 117 seeming long, understand that spindles have been steadily getting shorter since the sixties. Cranks aren't moving in, but more of the offset is being built into the arms themselves, so they can shorten the spindles to make the entire system stiffer and lighter.
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Old 07-14-13, 10:13 PM
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Originally Posted by FBinNY View Post
The only thing you're missing is that not all spindles are symmetrical (most aren't), so you need to care about the right side, not double it and subtract.

As for 117 seeming long, understand that spindles have been steadily getting shorter since the sixties. Cranks aren't moving in, but more of the offset is being built into the arms themselves, so they can shorten the spindles to make the entire system stiffer and lighter.
I was going to put in a symmetrical bottom bracket, so that's why I was thinking 117-118 spindle. Should I be considering an asymmetrical one? I just checked and the spindles that I want are only available in 116mm and 119mm.

Also it seems that you aren't worried about needing a long spindle.
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Old 07-14-13, 10:30 PM
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I can't answer directly because you don't say whether the existing spindle is symmetric or not. My point is that you have to concern yourself with the change to the right side length, while also keeping an eye on left crank end chainstay clearance (rarely the issue, but I've seen people get surprised).

If it's any help to you, here's a short list of some common spindles. Note that most 68mm BBs use spindles with 52mm center distance, so you can take that as a given, then take some measurements of the existing spindle, and do some simple arithmetic to determine the right side length, then shop accordingly.
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Old 07-17-13, 07:49 AM
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jankdc
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Originally Posted by FBinNY View Post
I can't answer directly because you don't say whether the existing spindle is symmetric or not. My point is that you have to concern yourself with the change to the right side length, while also keeping an eye on left crank end chainstay clearance (rarely the issue, but I've seen people get surprised).

If it's any help to you, here's a short list of some common spindles. Note that most 68mm BBs use spindles with 52mm center distance, so you can take that as a given, then take some measurements of the existing spindle, and do some simple arithmetic to determine the right side length, then shop accordingly.

Ok, so I took new measurements and got new results. I'm going to have a friend look at it and finalize measurements. What I would like is confirmation that my logic is accurate to decide on spindle size.

The measurement of the center of the bottom bracket to the end of the seat tube is 57mm. The measurement of the chain line is 45 mm.

My chain line is off by 3mm. 57-3= 54 mm. I would need a 108mm spindle.

Does this make sense?
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