No bicycle is complete without a stem. Stem size and shape have a large influence on the aerodynamics and handling of your bike, so picking the right one is essential.Bicycle Stem Function
A bicycle stem is an essential part of the steering system. It connects the handlebars to the front fork, so that when the handlebars are turned the front wheel also turns. It also has a second important function, which is to influence the rider’s sitting position. The shape of the stem helps to determine the height and the angle of the handlebars, which in turn determines how far the rider has to lean forward. This has a huge impact on the rider’s comfort and aerodynamic profile.Quill and Threadless Stems
Quill or threaded stems have traditionally been the most popular option for bicycles. These fit into the steerer tube and are kept in place using a bolt that screws into the steerer’s internal wedge. Adjusting the wedge height allows the height of the handlebars to be changed quickly. Modern performance bikes often come with threadless headsets instead. A threadless stem clamps onto the outside of the steerer. This makes height adjustment much more difficult, but reduces weight and makes fork changes easier. Using a threadless stem makes it especially important to get the right stem size and angle in order to ensure correct handlebar position.Stem Types
As with other bicycle parts, the type of stem required depends on the type of riding that you intend to do. Competitive road bike stems are typically quite long and sit at a low angle to promote an aerodynamic forward lean. In order to keep weight low, road stems are usually made from aluminium or, in the case of more expensive stems, carbon fibre. Typical lengths range from 90mm to 130mm depending on the rider’s height, and on how much discomfort they will put up with for faster times. Mountain bikes typically have shorter stems than road bikes, typically between 50mm and 80mm, in order to allow good handling on both ascents and descents. An over-long stem can make the bike too front-heavy for downhills, but too short makes it difficult to manoeuvre. Cross country stems tend to be a bit longer than those found on all-mountain bikes. BMX bike stems are even shorter, usually around 50mm, partly due to the overall smaller size of BMX bikes.Stem Sizing
Stem length and angle aren’t the only considerations to be made when sizing a stem. A stem fits into or onto the steerer tube at one end and attaches to the handlebars at the other. Both steerer tubes and handlebars vary in size, so selecting a stem that is the right shape is essential to get the right fit.