Chainline calculation

Question

I need to replace my completely worn cheap cranks (not sure how those are called, but the chainring and right crank are one piece). The chainline of my singlespeed freewheel is approx. 43 mm. I have a brand new Sugino RD2 crankset lying about (listed as 45 mm chainline with ring in outer position when used in combination with a 103 mm JIS spindle). I pulled my left crank and it revealed a Campagnolo bottom bracket (looks like centaur or veloce). I measured the width of my bottom bracket frame tube (standard 68 mm) and the spindle protrudes for 22 mm on the left side (measured from the edge of the bottom bracket tube), so I concluded that the spindle length is probably 111 mm. First question, is that a safe guess? Or should I really pull the right crank too? Quick inspection gave me confidence that the combination of the ISO bb and the JIS crank won't be a problem. According to the late Sheldon Brown, this would cause the chainring to sit 4.5 mm more inboard, so the combination should give me a chainline of 44 mm with ring in outer position. Is this right?

Answer 1

You're taking the right approach, although 2(34+22)=112. Measuring half of the spindle as you did is fine as long as you're 100% confident it's a symmetrical spindle. In practice it's something I might do with the knowledge I have to double check later and fix things if by chance they didn't work out.

Even if Sheldon's 4.5mm number is right on, this would give you 45mm front chainline, which would work despite not being ideal. Personally I don't know if it's just 4.5mm cut and dry like that in all cases. But it's probably worth a try.