Originally Posted by
TallRider
I'm a tall guy, and use 180mm cranks when I'm able.
I plan to use a Shimano "deerhead" triple crank (FC-6206) as a compact double. I am trying to calculate how much shorter a bottom bracket spindle I should use to get proper chainline. I calculate 8mm (4mm shorter on each side). Can anyone verify?
My calculation (assuming 130mm rear spacing for 8-11 speed road drivetrains):
Triple crank chainline is 45mm, measured to the center of the middle chainring.
Double crank chainline is 43.5mm, measured to the midpoint between the two chainrings.
Chainring spacing is approximately 5mm.
Moving from 45 to 43.5mm subtracts 1.5mm from the drive side
Moving the measuring spot from the middle ring to the midpoint of middle and outer ring subtracts another 2.5mm from the drive side.
To keep equidistant pedal spacing, 4mm should also be removed from the non-drive side.
Hence 8mm shorter BB spindle. (any asymmetry in the original spindle would still apply)
Let me know if my math/reasoning is off.
That sounds about right. It's not life-or-death precision, and it wouldn't hurt to measure your current chainline to see if it's even close to Sheldon's ideal before ordering parts.
Another option would be to use the middle and granny rings as an "ultra compact" double, in which case you're already so close it wouldn't be worth changing the BB.