How to Choose the Right Stem
Information about bicycle stems will generally fall into a category of compatibility, handling characteristics or fit.
Under the category of compatibility, one will want to look at things such as the clamp diameter: 2-bolt vs. 4-bolt faceplate designs, and quill style vs. threadless models. You need to know the diameter of your handlebar in millimeters. If it is a road bike handlebar, it will be either 26.0mm or 31.8mm, the later being common on a newer bike. A mountain bike handlebar diameter will be either 25.4mm or the newer 31.8mm size. The bicycle industry has essentially made 31.8mm the new handlebar diameter standard. 2-bolt designs are generally not as stiff as 4-bolt designs, and they often do not distribute the clamping load on the bar as evenly, which is especially a concern for those with carbon fiber handlebars. 2-bolt designs exist because they can still offer reasonable stiffness and by design, can often shave some weight. Quill style stems drop into the steerer tube of your bicycle fork and have an expanding wedge at the end. Generally seen on older bikes, both road and mountain, they are still used on modern comfort bikes, because they allow easy, quick adjustment. They come in two sizes: 1 inch and 1 1/8 inch. Threadless style stems are newer designs, found on road and mountain bikes in either 1 inch or 1 1/8 steerer tube sizes. There is also a 1.5 inch size, although quite rare.
Handling characteristics of a bike are often changed drastically by changing the stem length. A 100mm length is considered an average length. A shorter stem will quicken the steering speed, allow a rider to shift his/her weight further behind the saddle during a descent, and provide a more upright position. These characteristics make shorter stems popular with all mountain, downhill and aggressive trail style riding disciplines. A shorter stem keeps the body compact and allows the rider to not only transfer weight rearward, but allow the mountain rider to lift and roll the handlebar forward as they launch over trail obstacles. On the flipside, a longer stem will slow down the steering speed, open up the chest area for better breathing, apply more weight to the front end, and increase stability while climbing. Longer stems become preferable when a rider is climbing. The steeper the hill, the more you lean forward; and when you climb standing, your body position is much more forward. A longer stem will allow you transfer your weight forward for this task and remain stable. If your knees currently hit your handlebar during the steepest standing climbs, that is a sign you need a longer stem. If you feel your front wheel wandering or wish for more weight over the front end on a steep mountain climb, try out a longer stem.
Changing the stem rise or length will alter the fit on your bike. When a bicycle is being fit to you, there are three main steps. The first is to establish the correct saddle height. Second, one must establish optimal fore/aft saddle position. Much has been written and discussed about fore/aft saddle position. Often, one will see a plumb line placed below the kneecap, checking for position over the crank arm. This practice is an OK place to start, but actually has little science behind it. Riders on recumbents, mountain, road and time trial/triathlon have very different knee-over-pedal relationships. Fore/aft saddle position is all about the balance point. In reference to a traditional road or mountain bike, riders whom have stronger core/back muscles and lower upper torso weights can run the saddle more forward and get more power out of the pedal stroke, because they have more of their body weight directly over their legs, opposing the upward force from pedaling. Moving the saddle back will relieve pressure from your wrists, forearms and shoulders, by shifting that balance behind the bottom bracket a bit more. Think of the relationship between your body over the seat tube, like that of a balance beam. Only after you have your saddle at the optimal height and your saddle at the optimal fore/aft position, should you then consider the various dimensions of the stem.
In regard to fit, the rise measurement of the stem will determine how high or low the handlebar will be placed relative to the height of saddle. More rise (+15 degrees vs. +5 degrees) will bring the handlebar higher. This can be confusing, but a 90 degree stem is the same as a zero rise stem. A 95 degree stem is the same as a +5 degree rise. Flip the stem upside down (yes, it’s OK to do this), and now you have an 85 degree stem or -5 degree rise. As you raise the height of your handlebar, you relieve pressure from your wrists and forearms, but you increase pressure on your sit bones, soft tissue areas and spine. As you lower the height of the handlebar, you decrease the frontal area, which reduces wind resistance. You place more weight on the front wheel, and lower your center of gravity. Again, it is about the balance point, finding an optimal saddle to handlebar height that allows you to ride longer, faster and with more comfort. A good place to start is to choose a stem rise dimension that will place your handlebars a couple inches lower than your saddle height. For super long rides and maximum comfort, aim to get the saddle and handlebar the same height.
Once again, the stem length comes into play with bicycle fit. On a road bike, where we are talking about a seated pedaling position, we are looking for a stem length that allows the arm/shoulder to be at a 90 degree angle to the back. The angle of the back is ideal at 45 degrees (relative to ground) for balanced weight distribution and comfort, but with different style of bikes, we will see this angle change drastically. As you can see, changing one area often changes another area, and this whole search for the ideal stem becomes more complex. The most experienced bicycle fit technicians in the world can only get you somewhat close to what you will ultimately find as ideal. The final result will come only through understanding the basics and then experimenting. So get out there and ride your bike. Try experimenting a little bit with different stems and find what works for you.
Performance Bicycle Store Associate