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Replacing a BB spindle -- 1970s Motobecane

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Replacing a BB spindle -- 1970s Motobecane

Old 07-12-13, 03:19 PM
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majining
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Replacing a BB spindle -- 1970s Motobecane

Hi everyone,

I have a late-1970s Motobecane road bike, and I want to replace the BB spindle/axle so that I can get rid of the cottered cranks and replace them with newer, cotterless ones. Due to threading issues, I want to re-use the existing cups, not replace the entire BB.

So my question is this:

What measurements do I need in order to ensure that a new axle will work with the old cups/bearings/etc.?

I have not picked out the crank-set yet, so the only measurements I need are to ensure compatibility with the rest of the bottom bracket.

Also, I am open to recommendation on brands and online stores for purchasing said axle. I don't need anything fancy, just functional.

Thanks in advance!
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Old 07-12-13, 03:38 PM
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The most important measurement for compatibility with your BB and cups will be the ridge-to-ridge middle length. (if you go to Sheldon Brown's site on spindles: https://sheldonbrown.com/bbsize.html, you'll find a table). On Sheldon's table, this is measurement "B". In order for you to fit your cups, you'll need this to be close on the new spindle. It can't be shorter than your original spindle. But it may be a tad longer, like half mm, because you could use slightly smaller loose bearings in the BB and this should all the cups to squeeze ever so closer and the lockring and left side cup should allow for this.

Next, you also need the right side measurement ("C") to be such that it gives you the same chainline. Assuming it was correct before with the cottered cranks, you may be able to chose a replacement crank, look up the suggested BB width (a reference for modern symmetric cartridge bearing BBs) and pick a BB spindle that is a couple mm shorter on the "C" side since your freewheel is likely to be less outboard than more modern bikes. And you'll know if it's going to work because the clearance the new crank will have with the right chainstay will be roughly the same as before.

This can be tricky and may be prone to trial and error, which is easier done at the LBS where they may have a bunch of spindles to try and already in stock. The non-drive side is probably going to be okay for the most part as long as you have 1cm or more clearance between crank and chainstay near the pedals.
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Old 07-12-13, 05:29 PM
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You can't really measure a cottered crank to get an equivalent cotterless, but there's no need for trial and error either. But the very first thing you need to do is verify that the cups are in good shape, without pitting or uneven wear. Any spindle will be compatible with the cups - essentially all are designed to work with 1/4" balls. As for dimensions you need first to determine what rear wheel and cassette/freewheel you are going to use. If you are keeping the OE it's fairly straightforward, but takes a few measurements.

1. Do the "string test" just to make sure your rear triangle is centered. Tie a string to the lower part of one dropout, run it through the other dropout, around the head tube, back to the first dropout and then tie off or hold under tension. The string should be the same distance from each side of the seat tube. If not things get complicated, but let's assume it's OK (w/in 2mm is good enough)

2. Measure the distance between the inside of the rear dropouts and divide by two.

3. With the wheel mounted measure the distance from the inside of the right dropout to the middle cog on your freewheel. Subtract from the result of step 2.

You now have your "chainline" and all you have to do is get a crankset (BB and crankarms) that have that chainline, or very close. Again, possible to make some adjustments but no reason to go there unless you hit a problem.

DO NOT uses smaller loose bearings, but rather the standard 11 1/4" balls on each side, and the middle distance should not be an issue, as you have a standard 68mm BB shell width.
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Last edited by cny-bikeman; 07-12-13 at 05:37 PM.
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