Why did the 48 cm require a setback seat post and the 50 cm didn't? The relationship between your ass and your feet did not change so why would setback change. Cannondale does not change crank size between 48 and 50 cm. If this was just to feel comfortable with the reach to the handlebars, then the 48 cm was really too small! Frame sizing and component sizing should always be based on physiology not on whether or not it was a "good deal."