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How to choose a bottom bracket spindle length for new cranks on a new frame?

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How to choose a bottom bracket spindle length for new cranks on a new frame?

Old 06-30-20, 01:54 AM
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Funktopus
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How to choose a bottom bracket spindle length for new cranks on a new frame?

I'm putting some Stronglight 200lx cranks on a Vitus 787 frame. Both are new to me, so there's no precedent to compare to. I know a need a threaded 68mm square taperered bottom bracket, but I just can't figure out the spindle length I need.

Googling around seems to suggest it's a case of just trying a bottom bracket and seeing how the chain line is. Is that really the case? And if so, what's the best guess I can go with?

Thanks
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Old 06-30-20, 04:43 AM
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Originally Posted by Funktopus View Post
Googling around seems to suggest it's a case of just trying a bottom bracket and seeing how the chain line is. Is that really the case? And if so, what's the best guess I can go with?

Thanks
​​​​
There might be someone here using the same crankset who can tell you the spindle length they’re using. If not, you’ll need to use the method described above. Really only need one BB to get an initial reading, then you can source the correct one. Be mindful of different tapers which can impact arm seating and measurements.
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Old 06-30-20, 04:52 AM
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My method is simply to use the same spindle length as the original and wing it. It works for me on most builds 99% of the time. I also have several junk sealed BBS, and spindles to fuss about before I order anything new. I also think most manufactures will have a recommended spindle length, but there are other ways HERE.
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Old 06-30-20, 05:32 AM
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Funktopus Having gone through this challenge lately, I learned that you can take a shortcut by finding out what the spindle length needs to be for your specific crank.
Without that knowledge I would start taking measurements. Specifically the distance between the center of the rings and the surface of the boss around the inside of the spindle opening. Next number is the distance between the center of the bike and the center of the sprocket cluster. This last number plus the crank number will give you a close target for half the length of the spindle. Oh of course you need to know how far the spindle is likely to enter the crank.

This last measurement could be taken from an existing bike with a square taper BB. Measure the distance between the washer surface and the end of the spindle and subtract from the overall length of your crank taper and add that number to the last number above. Somewhere between 108 and 118.

It is harder than determining the crank BCD (122)
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Last edited by SJX426; 06-30-20 at 05:42 AM.
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Old 06-30-20, 03:55 PM
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Originally Posted by SJX426 View Post
Funktopus Having gone through this challenge lately, I learned that you can take a shortcut by finding out what the spindle length needs to be for your specific crank.
Without that knowledge I would start taking measurements. Specifically the distance between the center of the rings and the surface of the boss around the inside of the spindle opening. Next number is the distance between the center of the bike and the center of the sprocket cluster. This last number plus the crank number will give you a close target for half the length of the spindle. Oh of course you need to know how far the spindle is likely to enter the crank.

This last measurement could be taken from an existing bike with a square taper BB. Measure the distance between the washer surface and the end of the spindle and subtract from the overall length of your crank taper and add that number to the last number above. Somewhere between 108 and 118.

It is harder than determining the crank BCD (122)
Wow interesting. I won't have a chance to do this until the weekend, but I'll give it a try. Some phrases I don't understand - what's the "boss around the inside of the spindle opening". A raised lip around the square hole on the bracket-side of the cranks? And when you say center of the bike, is that the center of the bracket opening?

Thanks!
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Old 06-30-20, 05:33 PM
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Funktopus Kinda yes. Here is a picture but you really want to measure to the edge of the tapered hole. We are trying to get close, not exact.
P1030067, on Flickr
And yes. One would hope the center of the BB shell is the center of the bike. Better location is the center of the down tube or seat tube.
https://www.sheldonbrown.com/chainline.html
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Old 06-30-20, 06:06 PM
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You might try Velobase, looking at the Stronglights to find your crank. Their database often lists the correct BB length if your crankset is there.

VeloBase.com - View Brand

Does that model Stronglight crank require special threads on a crankpuller?

Last edited by Dfrost; 07-01-20 at 01:23 PM. Reason: Added crank puller question.
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