When selecting a stem you must consider not just the length, but the rise in degrees, and the correct diameter for your handlebar. For instance if you are seeking a more upright position you may only be able to raise the stem a few centimeters on the fork steerer tube, but a stem rise of 25 degrees will allow a significantly higher position. Most stems may be "flipped" to allow either a higher or lower position. Stems are generally made from aluminum alloy or carbon fiber, with carbon being lighter but not necessarily stronger. When installing your stem be sure to torque the bolts to the factory settings to avoid damaging your handlebar. This is especially important in the case of carbon fiber handlebars. MTB stems are more robust and designed for the rigors of off road riding. Do not use a road stem for a mountain bike unless it is specified for off road use.