We’re often asked to compare the diameters of rims from different makers. Whether a rim measures 17.5 or 18 mm depends on where it’s measured. There’s no “right place” to measure a rim and unless the maker tells you where a rim has been measured it’s impossible to compare.
The Lawson rim shown above was measured at a point .025″ down from its high point (crown). That’s where the “705” number comes from. We think that’s a less-than-ideal place to take a measurement because it’s in the middle of the curve.
Osmun mouthpieces are measured .050″ down from the crown because at that point the rim blends into the cup. So, in our measuring system the diameter would be .683″. That’s .022″ (.56mm) different.
Other makers may use different measuring points and unless they tell you where they’re measuring, it’s impossible to compare. So, take rim measurements with a large grain of salt.