Your bike's stem will affect the way your bicycle handles and the way your upper body feels the road. While being one of the most important bike parts for comfort and handling, it is also very customizable and upgradeable.
When purchasing a stem, you should consider several factors. First you must look at the steerer type (threaded or not). Then you must match it to a steerer tube diameter and a handlebar diameter. Finally, you must decide what length and how much rise or drop you want your stem to give you. Once you have decided on those characteristics, you must decide on the material you'd like and consider its price.
The two main types of bike stems are "threadless" and "quill." The type of stem required is determined by the headset and fork installed on your bicycle--either threaded or threadless (the vast majority of today's forks use a threadless steerer tube). Once you have determined which headset style your bicycle requires, you must determine the different compatibility factors: steerer tube diameter and handlebar diameter.
Threadless steerer tubes come in four standard diameters: 1-inch, 1-1/8-inch and 1-1/2-inch. These sizes correspond to matching stem sizes. 1-1/8-inch headsets are currently the most common. However, with new trends always surfacing and old trends always resurging, this could change in the future.
Threaded steerers typically come in 1-inch and 1-1/8-inch diameters with corresponding quill stems sized down for the internal diameter of the tube. 1-inch threaded steerers were the industry standard for decades and remain very popular in the vintage and retro bike world. They remain the most widely used and readily available.
Steerer Tube Diameter
Stems attach to the bike via the steerer tube. Threadless stems clamp around the steerer, while quill stems use compression to attach to the inside of the steerer.
Handlebars come in many shapes and sizes. It is very important that your stem's handlebar clamp be compatible with your handlebar. Different brands have minor variations to the standard sizes. Typical HB Clamp sizes are 25.4mm (ISO and MTB), 26.0mm (road standard) and 31.8mm (oversized road and MTB).
Now you have the two main factors down. Once you determine the clamp size for a threadless stem or the steerer tube inner diameter for the quill stem and the appropriate clamp size for you handlebars, you can decide what length and drop are necessary for your ideal bike fit. You can then decide what material and price to look for in your stem.
Stem length and rise/drop are determined based upon fit requirements. Different types of bicycles and riding styles require different stem dimensions. Your stem purchase should be based on your required fit, not the other way around.
Length or reach is the horizontal measurement of your bike stem. It is measured from the center of your steer tube to the center of your handlebar clamp along the central axis of your stem. Stem length typically ranges from 60mm to 150mm with some outliers in the extremes.
Road stems for racing bikes are typically on the longer side of the scale, giving the rider a more stretched-out riding position, with mountain bike stems and recreational bicycle stems on the shorter side, providing a more upright position. BMX stems are an exception and are in a class by themselves. Quill stems are generally measured perpendicularly from the center line of the steerer tube to the center of the handlebar clamp.
Stem length can affect the responsiveness of the bicycle. A longer stem will cause slower bike response, while a shorter stem will react much more quickly.
Rise and drop are measured by the degree of difference from 0 or 90 degrees from the steerer tube. Quill stems are set with a positive or negative (rise or drop) or a 0-degree rise. A threadless stem is usually measured from 0, and often can be flipped to give an equal rise or drop, depending on orientation. For example, a threadless stem with a +6-degree rise will have a -6-degree drop. A few manufacturers may also list no rise as 90-degrees and go up or down from there.
Road stems for racing bikes are often in the drop position because this gives the rider a dropped and stretched out stance, while MTB and recreational bikes typically have a rise orientation to allow the rider a more comfortable upright position.
The vast majority of bike stems on the market are made from an aluminum alloy or from carbon fiber. Some stems are built with an alloy structure wrapped in carbon fiber for aesthetic and comfort reasons. Alloy stems are typically more affordable than the carbon variety, though some high-end aluminum stems made of the highest grade alloys can be lighter and more responsive than their carbon counterparts, making them cost considerably more than the most affordable stems on the market.
Carbon stems are built for weight and comfort on the bike. Road vibrations travel up the bike, through the fork and stem and handlebars into the rider's hands. Carbon fiber has great vibration-dampening properties and aids in removing a large part of this "road static." In order to make the carbon strong enough to be a safe stem material, they overbuild it to add strength. This is why the highest quality alloy stems can be lighter and stronger than many carbon versions.
Some boutique bicycle companies offer titanium stems that have their own benefits. They are comfortable, look great with a Ti bike and are lighter than the less expensive alloy versions.
Stem price is determined by several factors, namely material and weight. It's easy to make a cheap alloy stem and put it on the market. This stem is going to be heavy and lack aesthetic value. Likewise, it is easy to build an inexpensive carbon stem. This stem will offer little benefit. It will be built with such thick carbon that it will transfer more road static than other carbon models and will be heavier than even moderately priced alloy stems.
All bike stems, when properly chosen, are going to function identically. They are going to be safe. They will hold your bars, your steerer tube and will steer your bike. When you look at the upper price range, you are looking at more engineering and more research money put into that stem that the company is trying to recoup. Carbon and high-end alloy stems are going to top the charts alongside titanium, with cheap carbon and other alloy stems filling in the lower price ranges.
Some Extra Tips For Choosing The Right Stem
When using Aero-bars, cruiser bars or others that place added torque on the bar, be sure to use a stem that is able to withstand the increased torque.
Most quill stems and some threadless stems secure the handlebar with a single binder bolt securing the lower portion of a wrap-around bar clamp. This requires you to remove any shifters or levers and grips or bar tape from at least one side of your bar to do a stem swap. More common today is a detachable faceplate with 1-4 securing bolts. The faceplate can be removed, alloying the bar to fall free. This is a much more convenient setup that is just as secure as the older version, though less aesthetically pleasing in some people's eyes.