BB spindle length vs chainline
 

Hello Forum, quick noob question: does spindle length matter as long as the chainline space is correct?

Stock BB needs replaced but spindle is asymmetrical with shorter side on drive (i know, i know!). Before realizing mine is asymmetrical + short side on drive, i just measured spindle and ordered. Come to find out the chain set sticks out too far messing up front shifting. I think I read most new BB are symmetrical. So im assuming as long as chainline is correct then spindle length isnt going to be a big deal breaker.

I had order bb-un300 68mm/122.5mm in 50mm chainline (the LL123).
Im looking at the 47.5mm chainline version now (the D-NL version). both 122.5 mm spindle.
Comparing these, the D-NL nds should be 2.5mm longer than the LL123?
Theres also a bb-un300 in 47.5mm chainline but 118mm spindle. So, either 122.5 or 118 should work as long as chainline is good?


Edit: chainline is in fact 47.5mm, busted out the caliper. Dang, I should have used that to begin with instead of a a ruler.

On square taper spindles, the length of the BB and the particular crankset you are using will determine where your chain line is. So if you or someone changed the crankset to something else, then your BB spindle length might need to change. For spindles that had offsets, then you have to figure out what length over all you need to make that chain line if you have to get a symmetrical replacement.

I don't think this is any issue for ISIS, Octalink or other newer BB's. Cranksets for them are more standardized where the chainwheels are than were the square taper cranksets and bb's that were before international commerce got into making localized conventions of each bicycle manufacturing country undesirable.

Manufacturers design a crank with a particular chainline and a particular spindle for a particular BB width. Whatever the dimensions (including the spindle taper) that were used in that design is what you should use every time. The manufacturer has no allegiance to any previous rule for spindle dimensions.

Standard 1970s cranks had the hole for the spindle starting as far right as the outside edge of the inner chainring. As manufacturers attempted to make the cranks lower profile and decrease the weight of the steel spindle, they moved that spindle hole toward the left and shortened the spindle accordingly. Each crank design is different.

Quick answer: no.

I agree! But then the next question is, how do you select the chainline you should build the bike to have?

If it is a double road bike with rim brakes from 5 to 12 speed, your chainline center is 43.5mm. Every standard bike type has a chainline standard based on the BB width/type and the rear hub spacing. If you know the dimensions and type of frame (road, mtb, etc) you can look them up.

https://www.sheldonbrown.com/chainline.html

Chain line is really determined by the rear wheel of the bike and specifically where the middle of the cassette or freewheel stack is. Since that's been standardized to a few different dimensions for different types of bikes, then cranksets were standardized to match.

So if you have a 11 speed road bike, you are safer to just get a crank made for a 11 speed road bike. Though there is enough tolerance with chain line that you can get away with other cranksets. Just may not have ideal and perfect shifting through out the entire range of gears.