I am building a single speed bicycle, and I have most of the drivetrain parts:
- Rear hub
- 18T rear cog (40-40.5mm ideal rear chainline -- wheel undished)
- 1/8" chain
- crankset with 40T chainring
I used the formulae in the answers of How do I calculate the diameter of a chainring from the number of teeth? and calculated that a 32T or smaller front chainring is needed to maintain the chainline for an undished rear wheel (otherwise the chainring would rub the chain stay of my specific frame, and I will be forced to increase the chainline, adjust spacers on the rear hub, and dish the rear wheel).
But 32/18 is too low of a gear ratio, so I made a compromise and chose a 40T chainring which I intend to place as close as possible to the chain stay, in order to have a front chainline as close as possible to 40mm, for a rear wheel as undished as possible (because it will be stronger, from what I've read).
The last part that I need is a bottom bracket cartridge with the proper spindle length, that will dictate the front chainline and how close the chainring seats to the chain stay. For the proper spindle length I need to take into consideration (and maybe other factors as well):
- JIS vs ISO square taper types -- I think I need to add 4.5mm to the chainline just to leave room for a non-ideal combination of the 2 standards that may result now, or with future replacements; But using this knowledge: How can I tell the difference between a J.I.S. and ISO square taper bottom bracket? I think I can avoid mixing them wrong.
- Flexing of the steel chain stay and frame while riding which may cause the chainring to hit;
- After being used for some time, the crank hole might get larger, thus needing to be screwed in farther on the spindle, closer to the frame;
- The chainring might slightly bend with time. I don't want the lightest hit to cause it to rub.
No crankset chainline info is specified. But I have an old spindle, I will install the crankset on it, make some measurements, then calculate the proper spindle length and symmetry relative to this one that I have.
How should I proceed with the measurements? What would be the optimal chainring - chain stay distance to aim for? I don't have enough experience to know how much each of the above mentioned factors weighs into the equation.
UPDATE: It turns out that in order to have the 40T chainring placed at 4-5mm away from the chain stay, the rear wheel needs a 4mm dish. How much will 4mm dish weaken the rear wheel?