Introduction
When it comes to modern wheel design, rim depth is one of the most important factors shaping a wheel’s aerodynamic performance. Whether you’re a pro cyclist, an amateur, or a beginner, aerodynamics affects you. It matters whether you are riding on the flats, downhill, or even uphill. Rim depth can affect, aerodynamics, speed, weight, and comfort. In this article, we will explore what rim depth is, why it matters, and which is right for you.
What Is Rim Depth?
Rim depth refers to the vertical distance between the outer edge of the rim (where the tire met the rim) and the spoke bed, where the spokes meet the rim. In simple terms, it’s how “deep” or “tall” the rim profile is.
Shallow Rims
These are typically between 20–35mm. Everything else being equal, these are the lightest rims and are best suited for those who want to have a better climbing wheel or just want to have a lightweight bike. Most alloy (aluminum) rims fit into this category. Though this rim height is not the norm for carbon rims, though some ultra lightweight wheels like the GCX30 Ultralight and RCX33 Ultralight fit into this category, weighing just 1020 grams and 1030 grams for the pair respectively.
Mid-Depth Rims
These rims typically range from 40-55mm deep. They are the most common rim depths for carbon wheels, but are extremely rare among alloy rims due to a very high increase in weight. Carbon wheels can be made much lighter, especially at higher rim depths as compared to alloy wheels. Mid-depth rims offer a good balance between aerodynamics, weight, and control. The most popular rim depth is 50mm, since it is relatively aero and can still be easy to manage in crosswinds if the rim profile is designed to be so. At Particle, all of our rims feature either U-shaped or UV-shaped profiles, significantly improving crosswind stability.
Deep Rims
These rims are typically 60mm or deeper. They are designed for maximum aero benefits. They are ideal for the flats. Some extreme examples of deep section rims are those used in time trials and in triathlons. In these races, drafting is impossible, so having the most aerodynamic bike possible is important, especially since most TT and triathlon courses are relatively flat. While these wheels offer great performance in terms of aerodynamics, they are more difficult to control when there are crosswinds or when descending at high speed.
How Deep Section Rims Improve Aerodynamics
Airflow Around a Rim
When air hits a rim at speed, it tries to flow smoothly along the rim surface. If the rim is too shallow or boxy, the airflow can’t stay attached for long. It “separates” (breaks away), creating turbulence and vortices (swirling eddies of air). This increases drag.
If the rim is deeper and shaped well, it guides the air more gradually around its curve. This helps the airflow remain attached for longer before separating, which reduces turbulence and lowers drag.
This is similar to how an airplane wing or a golf ball dimple works, keeping airflow orderly reduces resistance.
Rim Depth and Leading Edge
Think of a deep rim as extending the surface the air has to follow. A shallow rim is like a sharp corner. The air gets “confused” and separates quickly. A deeper rim is like a longer curve so that air stays attached longer, smoothing the transition around the wheel.
That’s why modern aero rims like the GCX50 and RCX50 are often U-shaped or toroidal rather than V-shaped: the rounder shape lets air reattach more easily even when it hits from an angle (crosswind).
About Yaw Angles
Cyclists almost never ride in perfectly straight headwinds. Winds usually hit at a yaw angle (a few degrees off to the side). Deeper rims present a shape that can “catch” the angled airflow and redirect it smoothly. Instead of separating, the air flows along the rim, reducing drag.
Why Not Just Choose the Deepest Rims?
If depth keeps air attached longer, why not 100mm rims or full-disc wheels everywhere? The simple answer is crosswinds. Crosswinds make it harder to control deep section wheels because they generate side force when air hits them, disrupting a wheel’s equilibrium. This can make riding even a bit dangerous in windy conditions or when riding fast, such as on a downhill descent.
The Trade-Offs
Aerodynamics isn’t the only consideration. Rim depth also affects handling, weight, and ride quality. These are all important considerations when considering a wheelset.
Crosswinds
Deeper rims catch more side wind, which can cause instability. While things like weight and stiffness can affect performance, a wheelset that is difficult to control can affect one’s feeling of safety. Modern “U-shaped” rims reduce this effect but don’t completely eliminate the issue.
Weight
All else being equal, a deeper section rim generally weighs more, which affects climbing and acceleration. Though it is important to understand that not all rims are created equal. Our RCX60 Ultralight for example, weighs just 1200 grams, making it lighter than most climbing wheels.
Stiffness and Ride Feel
In general, deep rims are laterally stiff. What this means is that they are good for sprinting and improved power transfer, but the ride can feel harsher. However, if you want to improve the ride quality, the first place to look is tires. Wider rims such as the 25mm internal rim width GCX series wheels can be used with wider road and even gravel tires, allowing for much improved comfort. Another consideration is spokes. While steel spokes tend to be more compliant and carbon spokes tend to be harsher, new 4th generation carbon spokes offer the best of both worlds.
Road vs. Gravel
Road Riding
Since road cyclists typically ride at higher speeds and on relatively flat terrain, the importance of an aerodynamic wheelset is more apparent. Narrower and lower rolling resistance tires, smoother tarmac, and a more aerodynamic bike and position inevitably make road bikes faster, meaning you will go faster, and at faster speeds, aerodynamics is the primary force preventing you from riding faster as opposed to climbing, where weight becomes more important. The best for most riders is 40-55mm deep rims.
Gravel Cycling
Speeds on gravel are usually significantly lower, reducing the importance of aerodynamics. Another factor that reduces the speed of gravel riding is wider tires, which disrupt airflow and increase CdA, both increasing drag. We recommend 30-40mm rims for most gravel riders.
Conclusion
If there is anything that should be taken away from this article, it is that what rim depth is best for you is not necessarily what is best for others. If you ride at high speeds, especially on the flats, a deeper rim would be very beneficial. If you want to focus more on climbing, then shallow rims are for you. For those who want a bit of both, then mid-section rims would be ideal.
What is important is that you have a wheelset that can matches your riding style and is in align with what goals you may have. Deeper rims are more aerodynamic, especially at high speeds. At Particle, we recommend mid-depth wheels to most riders. There are super light wheels in this rim depth range that can climb well due to advancements in material design and components used.