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Wedge bolt
Conventional handlebar stems use a long bolt running down the middle of the shaft to clamp the stem to the inside of the steerer. At the bottom of the stem shaft, this bolt screws into a special shaped nut. These nuts are of two different types:
- Wedge-type stems have the bottom of the shaft cut off at an angle, and the nut is cylindrical with an angled top surface to match the angle of the bottom of the stem shaft. As the bolt is tightened, the wedge-shaped nut is pulled sideways so that it presses against one side of the steerer, and the shaft presses against the other side of the steerer. Most newer stems are of this type.
- Expander-type stems have square-cut shaft bottoms, but have one or two slots cut up from the bottom of the shaft. The nut is in the shape of a truncated cone. As the bolt is tightened, the conical nut spreads the bottom edge of the stem shaft outward, pressing it against the inside of the steerer.
Each system has advantages and disadvantages:
- Wedge-type stems:
- Provide a more positive lock against the steerer (which may be an advantage or a disadvantage...if the stem is too tight there is a greater risk of bending the handlebars in a crash.)
- Are easier to adjust. In most cases, just loosening the wedge bolt will permit the bars to be raised, lowered or straightened.
- Are cheaper to manufacture.
- Expander-type stems:
- Are less likely to damage the steerer if overtightened.
- Do not require quite as deep an insertion into the steerer.
- Are harder to adjust, because it is usually necessary to knock the wedge loose by striking it with a mallet after loosening up the bolt.
Note: Steerers are butted at the bottom, so the hole in the steerer is constant-diameter until near the bottom, then the walls taper inward in the butted section.
It is vitally important that the stem extender (or stem) not be inserted so far that the wedge is installed where the steerer is narrowing, or it could come loose unpredictably.
When this happens, only the edge/corner of the quill or wedge contacts the steerer, and it is trying to "grab" a slanted surface.
This is sometimes a problem on smaller frames if you try to insert the stem or a stem riser too far down into the steerer.
Illustration by Nicholas Flower
