At first, measuring a bike stem can be confusing because there are a number of factors to consider and more dimensions than you may realize. But once you understand all the variations, it’s pretty straightforward—or angular, depending on your riding preference.
In this post we’ll cover how to measure a bike stem and the tools you’ll need. We’ll talk about the difference between threadless and quill stems, how stem length affects how your bike handles, and which kind of stem works best for your style of riding.
How to measure a bike stem
All you need to measure your bike stem is a flexible tape measure. Avoid straight-edge rulers for more accurate results.
It helps if your bike is secure and won’t move, slip or fall. Leaning it against something is fine—you don’t need an entire bike stand for this.
Bike stems are measured in millimeters. To get an accurate measurement, there are actually several parts of the bike stem you need to measure.
Stem extension is the basic measurement in determining your bike stem’s length. It is the length of the stem, but instead of measuring from one end to the other, standard practice is to measure from the center of the steering bore to the center of the handlebar clamp.
Reach is a similar measurement to the extension. It’s taken from the same points, the center of the steering bore and the center of the handlebar, but it does not account for rise. In other words, it is the horizontal extension of your stem only.
Angle is usually broken down to Angle A and Angle B.
Angle A is the angle between an imaginary horizontal line and your stem. If your stem goes straight out, perpendicular from your steering column, then this angle is zero. If your stem slopes slightly upward, then you’ll have an angle here.
Angle B is the full angle between the steering column and your stem. It is usually referred to as the stem angle. Again, in a bike where the stem sticks straight out perpendicular from the steering column, Angle B would simply be 90 degrees. But if the stem has a slight rise, then you can add Angle A to 90 degrees and the sum will be Angle B.
The height that is gained as a function of the stem’s angle and length is the rise. It’s a vertical measurement that would be measured from the center of the handlebar clamp to an imaginary horizontal line that extends perpendicular from the central meeting point of the extension and the vertical part of the stem.
This is the overall length of the vertical portion of the stem.
You might also hear terms such as attachment diameter, steering column diameter or handlebar diameter.
Steering column diameter is the inside diameter of the steering column on your bike frame when you’re talking about a quill stem, or the hole in the stem clamp itself if you’re talking about a threadless stem (below). We’ll define quill stems and threadless stems in a moment.
Likewise, the handlebar diameter is the diameter of the hole in the stem through which the handlebar will pass.
Both the steering column diameter and the handlebar diameter are known as attachment diameters, because these are the areas where the stem attaches to the bike’s other parts.
Bike stem types
Now that you know what measurements to look for, it’s good to know how stem types differ so you can make sure you get one that’s compatible with your bike.
The two most common stem types are threadless and quill stems. The stem required for your bike must be compatible with your headset and fork.
A threadless stem clamps around the steering column and is held in place by tightening the pinch bolts. If you are replacing your bike stem look for a modern 4-bolt design. They are stronger, stiffer and creak less than their predecessor 2-bolt face plates.
Standard threadless steerers come in 1-inch, 1 1/8-inch and 1 1/2-inch sizes, with 1 1/8-inch being the most common. But beware of shifting trends.
A quill stem attaches to the inside of the steering column through compression by inserting the stem and tightening a bolt on the top that pulls the stem up.
Threaded quill steerers normally come in one-inch to 1 1/8-inch diameters. They are sized down for the internal diameter of the tube. Industry standard for quill stems has been one inch for decades.
Structurally, they differ from threadless stems so they’re measured a little differently. The measurements are taken perpendicularly from the center line of the steerer tube to the center of the handlebar clamp.
Attachment to handlebars
Threadless and quill bike stems still need to be compatible with your handlebar diameter. They both slide on and are fastened in place by bolts. There are two sizes of handlebar diameters: 31. 8 mm on newer bikes and 25.4 mm on older, retro and less common bicycles.
How stem length affects handling
Stem length shouldn’t be taken lightly because it’s one of the most important factors in how your bike handles. Stems must work in tandem with the other components of your bicycle. Specifically, the length of your top tube will be a factor in how short your bike stem can be before it negatively impacts your handling.
Stem length affects how responsive your bike is. The longer the stem, the slower the bike responds, while a shorter stem helps speed up the response time.
Mountain bikes and road bikes usually have quite different stems. If you spend most of your riding time a mountain bike, a longer stem will make you feel more stable while climbing, especially if you’re using narrower handlebars. The recommended mountain bike stem length is 50-80 mm.
Your bike’s stem length impacts how far you have to bend at the waist and how far forward you have to reach to grasp your bars. Ensuring you have the right stem length for your body means you can breathe easier, your weight is distributed more evenly, and you can maintain a more comfortable position for your neck and back.
Road bikes have drop bars because they keep the rider in a downward position, improving aerodynamics and ensuring more power behind each pedal rotation. This dropped, stretched-out position is typically initialized by a lower angle road bike stem.
It’s worth getting the right stem fit
Bike stem length and angle affect your riding position so much that even the slightest adjustment can greatly impact how your bike handles.
I learned firsthand when I started working at a bike shop and my co-worker noticed I was over-stretching my arms to reach my handlebars. Fortunately, my bike was equipped with an adjustable stem. So with one swift turn of an Allen key, he angled the bar towards me, putting me in an upright position and enabling me to relax my shoulders. This also helped me better distribute my weight. Suddenly, I could enjoy every ride without putting too much exertion on my arms.
If you’re just starting out and are unsure what bike stem is right for you, rely on the folks at your local bike shop who know how to measure a bike stem and get you the proper bike fit. Or you can buy an adjustable bike stem, try out a few positions and figure out very quickly what works best for you.