Most road wheelsets have an inner width of either 21mm or 23mm. However, the dominant external width is still being determined. We can be confident that the external width of wheelsets is increasing alongside the inner width. This increase in external width improves aerodynamics and stability, resulting from a better understanding of the wheelset's aerodynamics.
Reason 1 For Increased External Width: Improved Aerodynamics
The initial design of the pneumatic wheelset is shaped like a knife blade, with a very flat V-shape and a narrow outer width rim. This is done to minimize the frontal windward area, reducing air resistance when the wind blows at a 0° angle. However, in the real world, wind typically comes from multiple angles. The rim shape is improved to optimize the wheelset's performance, and the external width is increased to enhance its "sail effect." This way, the wheelset can perform extremely well even when the wind blows at angles of up to ±15° and even achieve a negative wind resistance effect. As a result, major brands have increased the external width of their wheelsets to 28mm (with an inner width of 21mm) or 30mm (with an inner width of 23mm) or more.
Reason 2 For Increased External Width: To Improve Stability
The design of bicycle wheelsets has always aimed to balance aerodynamics and stability. Initially, high-frame pneumatic wheelsets were created solely to be more aerodynamic, but they proved challenging to use on windy days. As gusts of wind can be unpredictable, it became necessary to improve the stability of wheelsets. For amateur cyclists, stability is sometimes more important than aerodynamics, especially when faced with strong wind gusts.
There are many ways to improve stability, such as using a wavy rim, but increasing the external width of the rim is more effective. The duller the rim, the more stable it is, so the only way to improve stability is to keep increasing the external width until you get to a very exaggerated 35mm, which of course, only needs to be used in the front, where the force on the front wheel affects the steering rudder, and the Roval Rapide CLX is a head start, with an "astonishing" 35mm external width in the front, and a frame height of 51mm, which is a significant truncation. The (very blunt) profile provides a 25% improvement in crosswind stability over the CLX50.
Reason 3 For Increased External Width: Wide Tire Fitment
Currently, the most commonly used tire sizes are 25/26mm and 28mm, which are mounted on wheelsets with inner widths of 21mm or 23mm. The actual tire width can range between 27mm to 30mm. For better aerodynamic performance, it is recommended that the difference between the tire width and the rim width should be within ±5%. Some argue that wider tires relative to the rim are better, while others prefer slightly narrower tires. In any case, the external width of the rim is also an important factor to consider. Considering the inner width of the rim and the thickness of the tire lip hook, the external width of a rim with a 21mm inner width is 28mm, while the external width of a rim with a 23mm inner width is 30mm. The external width can also vary based on the rim's design, with some having a more pronounced bulge in the middle for increased width.
28mm External Width Genre
The Shimano Dura-Ace wheelset is a popular choice among cyclists, with an internal width of 21mm and an external width of 28mm. Shimano uses Computational Fluid Dynamics (CFD) and Artificial Intelligence (AI) to analyze and design the wheelset. The AI searches for wheel shapes that can be analyzed on a large scale, including shapes that a human designer may not think of. Interestingly, the AI looks for wheel shapes that appear 'ordinary,' but in reality, the Dura-Ace wheelset's performance is anything but ordinary.
Hookless rims do not have a lip hook, so the sidewalls are straight, and the 23mm inner width is 28mm external width, as typified by the Zipp 404/454. Since hookless rims use at least 28mm tubeless tires, the actual width of the tires will be slightly wider than that of the rims.
The new Bora Ultra WTO wheelset has an inner width of 23mm and an external width of only 28.4mm, resulting in a slim rim sidewall with 21 G3 spokes and a lightweight design. The wheelset is a Campagnolo product, and while you may disagree with its design, the technology used in the wheelset, including the hubs and rims, is undeniable.
30/31mm External Width Genre
Typical of the Bontrager Aeolus RSL, the rims are 23mm in inner width and 31mm in external width and are designed to be both aerodynamic and stable. The front and rear wheels have the same profile, resulting in a more aerodynamic rim and a smoother tire-rim connection, which reduces aerodynamic drag and saves power at high speeds.
The Vision Metron 45 SL has an inner width of 21mm but a maximum external width of 31mm and a bulbous center rim. The key to the Vision wheelset design is airflow modeling and wind tunnel testing, which is best achieved with a 25-28mm external tire, and the Vision Metron 60 SL uses the same design concept, with a maximum external width of 33mm due to the 60mm frame height. The Vision Metron 60 SL uses the same design concept, with a maximum external width of 33mm due to the 60mm frame height.
Different Designs For Front And Rear Wheels
The Roval Rapide CLX is known for its unique design, which prioritizes stability and speed in a high-frame aerodynamic wheelset. Specialized engineers have gone to great lengths to optimize the design, resulting in the best interpretation of the aerodynamic concept. The front wheel is impressively external, with a width of 35mm, a thick rim sidewall, and a hollow design that ensures lightweight construction.
ENVE SES wheelsets are designed differently at the front and rear because the airflow at the front of the bike is different from the airflow at the rear of the bike, and all SES wheelsets have frame heights and shapes specifically designed for the flow field in which they are mounted. The front frame height is smaller than the rear and has a more rounded profile to maximize crosswind stability and control, while the rear frame height is taller, sharper, and more streamlined to maximize power transfer and reduce drag. Despite the different wheel designs and frame heights, the external widths are the same. ENVE SES 3.4 and SES 4.5 are 25mm inner width and 32mm external width.
Conclusion
There is no standard pattern for wheelset external width, and designs range from traditional to radical to pie-in-the-sky. However, the goal is all about aerodynamics and stability. Broadly speaking, 28mm external width is the starting point, 30mm external width is the mainstream, and how wide you can go depends on how bold the manufacturer is.