hoyc

On my 198? Nishiki Continental, I am going to use the existing triple crankset as a double. The current bb is shot and I'm going to replace with a modern cartridge unit.

According to velobase my spindle length for a triple should be 121. (https://velobase.com/ViewComponent.as...2a018&Enum=115)

Would this measurement be for the current assymetric spindle?

If so, how would I convert this to modern, symmetric sizing?

Also, how much do I decrease spindle length to account for going to a double?

If it helps, the road double versions of the same crank use either 116 or 119, although I'm not sure what that refers to. (https://velobase.com/ViewComponent.as...6e371&Enum=115).

jonwvara 

Someone else will likely have a better answer, but when converting from a double to a triple--which is what I usually do--I figure on adding 6 mm to the drive-side length of the spindle, so in your case I'd subtract 6 mm. Sheldon Brown's bottom bracket size database has some useful information when it comes to figuring out those kinds of conversion--it's here: https://sheldonbrown.com/bbsize.html.
Based on the database and my "subtract 6" rule of thumb, I'd try a symmetrical 116 cartridge, and figure on adding a 1 or 2 mm spacer on the drive side if I needed to adjust the chainline a little.

wrk101

Spindle length not affected by assymetrical as long as you use a spacer under the drive side of the cartridge to mimic it. I always just use a 2mm spacer.