Originally Posted by
Chris_W
I believe there is some variation between different stem manufacturers as to how stem length is measured. Although it is certainly measured center-to-center, which line they measure could be:
(1) Along the length of the stem, ignoring the angle.
(2) The extension perpendicular to the steering column; so that only stems with zero degree rise would have the same value for methods (1) and (2).
(3) As the horizontal extension level with the ground; so that only stems with -17 degrees rise would have the same value for methods (1) and (3).
If someone could put me straight on what method is normally used and whether some manufacturers do use different methods, then that would be great. Hopefully that will then also answer the questions as to how stems with adjustable angles are measured.
Over my 40 years selling stems in the bike industry, I've never seen one that wasn't measured on centers along the axis of the stem. I exclude hinged stems, but the ones I've seen were measured the same way while straight. There's a reason it's done this way, in that any other way would depend on the orientation of the stem requiring assuming some unknowable information to make a measurement.
Imagine a basic 100mm 17° stem. Mounted angle, down the axis length and horizontal reach would be equal, but with the same stem inverted, they'd be very different. The same applies to height which changes with orientation of angled (non 90°) stems.
So we have two sets of dimensions, angle and length, which are inherent properties of the stem, and reach and height which depend on the mounted position, and become properties of the bike as a whole. Obviously stem makers want to publish absolute measurement specs, and they do so.