I was recently working on a new Schwinn Racer single-speed, The specs on line are 46 tooth chainwheel, 18 tooth cog. It actually has a 44 tooth chainwheel and 16 tooth cog. How common is such a variation? In any case it is geared too high and I want to substitute an 18 tooth cog. Is this a standard part available in LBS? I have changed cogs on old Raleigh 3-speeds.