Chainwheel rubbing chainstay problem

Probably best to actually work out what the chainline measurement is at the rear first.

Measure the rear hub if you don't know its width (120 / 130 / 135 etc). Measure the distance between the inside of the dropout (drive side) to the middle of the teeth on the sprocket. To get the rear chainline measurement you just do:

(hub/2) - dropout_to_sprocket.

You now need to work out what size BB to put it based on the previous measurement. First, measure from the centre of the seattube to the middle of the teeth on the chainring. The difference between this distance and the rear chainline measurement is how far out your chainline is.

Suppose that the rear measurement was say 48mm, and the front was 31mm then you know you need to find 9mm -- which would equate to getting a BB that was 18mm longer than your current one.

Hope that helps somewhat.

I am putting together my first build. I have a wheel with a Miche hub. The frame came with a bottom bracket for a cottered chainwheel and I thought, for the sake of convenience and expense, I would stick with that BB and buy a new 46t chainwheel.

I have only handtightened the chainwheel but it looks like it will rub against the chainstays (see red ring in photo below). This may have been a problem for the previous owner as the paintwork is marked at this point.

It seems to me that I am probably going to need to get a new BB (the chainline is not so great with the present set up either) so my two questions are:

• does anyone have an alternative suggestion for solving this?
  
• does anyone have any good advice for finding a suitable BB?