The Mountain Bike stem is a device that makes the link between the fork pivot and the handlebar.
Function of a MTB stem:
Depending on the use, a MTB stem can have a different angle and length, the stem of a downhill mountain bike will be short to allow rapid changes of trajectories while the stem of a cross country or freeride mountain bike will be longer for the search for stability. A short and or inclined stem makes it possible to obtain a comfortable raised position while a long stem makes it possible to obtain a more aerodynamic position.
Criteria for choosing a MTB stem:
1 / ergonomy:
A stem has two key characteristics: its length and its angle.
The shorter it is, the more responsive your bike’s steering will be, but its stability will be reduced.
Its angle allows the handlebar to be raised.
There is no formula for determining the best compromise between stability, responsiveness and comfort. Determining its best position requires testing in different configurations (descent, ascent, flat …) several lengths and angles of stem.
2 / technic:
- fork pivot mounting diameter and mounting type (plunger or headset)
- diameter of the fixing interface with the handlebar
- length between axes (from the axis of the fork pivot to the handlebar fixing axis)
- angle if needed
Assembly of a MTB stem :
MTB stem standards have evolved, initially fitted with a plunger for mounting in the fork tube, it is now attached to the fork tube (headset system). The mounting diameter on the fork tube was 1 “(25.4 mm), it is now 1” 1/8 (28.6 mm), some brands like Cannondale use a non-standard dimension (1.5 “= 31.8 mm). There are two diameters for fixing the handlebars: 25.4 mm and 28.6 mm (oversize).
The MTB stem can be made of aluminum alloy or carbon, this material makes it possible to reduce the weight and increases the rigidity but it is not recyclable.
No maintenance of this component is needed, cleaning and regular visual inspection are sufficient to detect any deformations which could be abnormal. Since this component is a safety item, it will be necessary to replace it in the event of a proven defect.
Dismantling and then replacing a stem on an MTB may require a set of allen keys, it is a simple and quick operation but it is necessary to respect the values of the tightening torques recommended by the manufacturers in order to not damage the threads and particularly sensitive threads in aluminum or carbon.