RAM makes killer mounts. Excellent quality with hundreds of customizable options (they have custom cradles for many GPS and cell phonemodels). But their selection can sometimes be confusing. We wrote this post to help explain how their numbering (and lettering) system works. Some of of this information can be found on their web site if you know where to find it. But as a large reseller of their mounts, we thought it would be helpful to provide a brief explanation.
RAM’s system is a simple ball and socket design. You need to make sure the ball size matches the socket. The majority of RAM mounts sold for GPS, satellite radio, HAM radio are one-inch balls (RAM-B or RAP-B). Most use the the metal (the M in RAM-B) but to save a few dollars, some will opt for the plastic (the P in RAP-B).
Arms typically come in several sizes too. Again most common is the one inch ball, and there are three arm sizes (1.75, 3 and 6 inch sizes). Most of our mount listings have an option to choose an arm size.
Here is RAM’s high level numbering system and arm sizes in detail:
A size ball = .56″ diameter (seldom used)
B size ball = 1″ diameter (by far the most common)
C size ball = 1.5″ diameter (used for heavier items like marine electronics)
D size ball = 2.25″ diameter (very seldom used)
E size ball = 3.38″ diameter (very rarely used)