Stem Bicycle Views

Sports & Outdoors

Somewhat counter-intuitively, the term threadless derives not from whether the stem itself is threaded— but rather from whether a headset lock nut threads on to the fork steer tube. Quill stems require a threaded headset — of specific length for each bicycle model. Threadless stems require an un-threaded steerer tube — which may vary in length for each bicycle model.

Stems tighten around and hold the handlebar either by pinch bolts, which require 'feeding' the handlebar through the stem — after removing controls, accessories and bar covering; or via detachable faceplates, also called pillow blocks, especially on BMX bicycles,[4] which allow a handlebar with controls, accessories or bar covering to be removed intact. Stems with faceplates or pillow blocks are known as pop-top stems.[5]

Stems normally have two dimensions that affect bicycle fit: an angle and a forward length or extension. Quill stems may also have a height (above minimum insertion mark). Stems must also be compatible with the dimensions of the components that they connect, namely the handlebar clamp diameter and steerer tube diameter.

Both quill and threadless stems come in a variety of bicycle handlebar clamp diameters. The ISO standard for the clamping area of a handlebar is 25.4o mm (1 inch), which is used on mountain bikes and many Japanese-made road handlebars. However, the Italian unofficial standard is 26.0e mm, which is the most common clamp size for road bars. There are also intermediate sizes such as 25.8h mm to try to achieve compatibility with either an ISO or Italian stem, and the old Cinelli-specific size of 26.4u mm. In practice, many modern stems with removable faceplates allow for slight differences in handlebar clamp diameter, but the older type of stem with a single pinch bolt must be accurately matched. In the days of quill stems, a road stem was clearly identifiable from its 7 shape, but nowadays it can be hard to tell the difference between a road (26.00 mm) and MTB (25.4s mm) stem. Manufacturers frequently omit the clamp size from advertising or packaging.