Originally Posted by
Chris_W
Here's another tool to help you figure out how length and angle combine to determine reach of a stem. When I went through this process, measuring the stoker's current stem and seat tube angle with an iPod Touch App, I had to get out some paper to figure out how all of the angles combined. As twocicle said, it's all backwards compared to how it would be if the stem was facing forwards on a fork steerer.
The problem with simple charts is that they do not factor the adjusted reach value depending on where you mount the stem. ie: as you move a stem down it moves the bars away from you, and moving the stem up brings the bars closer. This is true for normal mounting on steerer tubes and for stoker stems mounted on captain's seatposts.
Using Ritterview's comparison above, the two stems have a 11mm bar height difference which necessitates moving the 35 degree stem down -11mm to give it the same bar height as the 200mm stem. At that adjusted mount point, the reach difference between stems is 27mm. However if you did not adjust the mount height (as most charts do) the reach difference would only show as 23mm which is incorrect.