The stem ratings were determined on three basic design variables - performance, price, and durability.
Performance of stems is primarily based on mass, with some subjective improvements to be had by increased stiffness. It could be assumed that once a stem has an "acceptable" stiffness, any additional improvements will have diminished effect. Furthermore, at this point in time the relative contribution to overall steering column stiffness of the stem is unknown. It could be speculated that the bar itself is the primary contributor to the perceived amount of flex in the overall steering system due to its length - stiffness being proportional to length cubed. However, the stem does provide the boundary condition, or "fixity-ness", for the handlebar and is therefore an important part to evaluate.
Based on these two variables of stiffness and mass and the preceding logic, a relative weighting scheme was determined to help quantify stem performance. Overall stem performance was rated using a weighting of 80% for the importance of mass and a 20% weighting for the importance of stiffness.
The range of the average of horizontal/vertical stem stiffness values (with a little buffer above and below what has already been measured) was broken up over a scale of 1 to 10 as illustrated below:
The range of stem mass was also divided up on a scale of 1 to 10 as indicated below:
The price rating was determined in a similar manner as the core stiffness and mass rating values. Reasonable limits for pricing were assigned to ratings of 1 and 10. The values in between were determined on a linear basis.
A minimum value of 1 was assigned to a stem that survives what one might consider to be a maximal loading condition (approximate 200 lb horizontal load) for 25,000 cycles. It should be remembered that since this is a comparative rating system, this value of 1 does not necessarily mean that the product should be considered unsafe. The author has been riding a stem that ranks in the 3-4 range for nearly 100,000 kilometers. The upper end of the durability scale was determined by the capacity of my compressor/air cylinder system which is in the 900 lb range.