Revealing the Science Behind Aerodynamic Rim Design: A Deep Dive into Rim Shapes and Technologies
Aerodynamics has been the most important innovation in racing bikes in recent years, from the frame to the wheelset. We are familiar with the pneumatics of the frame, but we still use the frame height to judge the pneumatics of the wheelset, which is not the case in reality. Aerodynamic wheels have evolved over three generations, from the original V-shape to the U-shape to the UV-shape; the V-shape was abolished because of its lower resistance to frontal winds, and the U-shape is the so-called "fat rim", which is more stable in terms of sailing effect and crosswind stability compared to the V-shape. Compared to the V-shaped wheel, the U-shaped wheel is an advance in terms of "sailing effect" and crosswind stability. However, the pursuit of aerodynamics is endless, the wheel shape has evolved into the UV shape, of course the UV shape is just a metaphor, the cross-sectional shape of each major brand's wheels is not quite the same and represents different design concepts. However, all concepts are based on extensive CFD calculations and are checked in wind tunnel tests. The aerodynamic performance of the new UV-shaped wheels can even exceed that of the U-shaped wheels with larger frame height, so it is no longer correct to judge aerodynamics based on frame height.
ICAN AERO 52 WD disc
The ICAN Aero Disc series is ICAN's most cost-effective and high-performance wheel series. Building on the success of the previous Fast and Light series, the Aero wheels utilize an advanced carbon fiber layup and aerodynamic design to deliver unrivaled performance. With an external width of 30 mm and an internal width of 21 mm, the rims are designed for minimal air resistance. Their 25-40mm tubeless compatibility supports even the fastest tires.
Shimano Dura-Ace wheelset
The latest generation of Shimano wheelsets is no small step forward. To find the rim shape for each model that minimizes air resistance and is less susceptible to crosswinds, Shimano uses CFD for analysis and AI for rim design. The AI looks for wheel shapes to analyze at scale, including shapes a human designer would never think of. This reminds us of the recently popular ChatGPT: Wouldn't it be possible to design a super aerodynamic wheelset using deep learning and training?
The C36, C50 and C60 all have the same inner (21mm) and outer (28mm) rim widths, so the same tires fit in the same way. The cross-section of the rims looks like a normal "UV" shape, but this has been achieved through AI-assisted design that takes both aerodynamics and stability into account.
Bontrager Aeolus RSL wheelset
The new generation of Bontrager Aeolus RSL wheelsets have been developed to a very high level. Until now, the optimization process at Bontrager was carried out in 2D mode. The 3D CFD method used in the development of the Aeolus RSL allows the precise calculation of the subtle pulling forces that the wheelset generates in different directions, at different levels and in different parts of the wheelset. With this new approach, Bontrager was able to better replicate real-world conditions, adapting the design accordingly and optimizing aerodynamics and rolling resistance. The result is the Aeolus RSL, the fastest wheelset Bontrager has ever built.
The Aeolus RSL 51, 62 and 75 have the same inner (23mm) and outer (31mm) rim width, with the larger inner width allowing a more rounded expansion of the tire, providing a better aerodynamic connection to the tire and greater comfort.The Aeolus RSL 37 has a different inner (21mm) and outer (28mm) rim width optimized for a lower frame height. The Aeolus RSL wheels have a "pointier" looking cross-section, which is due to the larger external width.
Zipp Hookless tubeless tire wheelset
Unlike other brands, Zipp relies on TSE (Total System Efficiency), which takes into account not only wind resistance, but also gravity, rolling resistance and vibration losses. That's why Zipp relies on low-pressure tires, hookless tubeless tires and a wider inner rim width. Zipp also has a "unique" technology, the Bionic Wave rim, which combines the aerodynamics of a high frame wheelset with the stability of a low frame wheelset (the 454 NSW with the Wave rim saves about 6 watts of power). The golf ball's potholes create an aerodynamic rim effect by delaying vortex shedding from the rim surface.
The 454 NSW and 404 Firecrest rims are 23mm wide on the inside and 28mm on the outside (excluding lip hooks), so they are significantly narrower than the Bontrager Aeolus RSL rims with the same internal width, so the 454 NSW and 404 Firecrest rims are in cross-section don't look quite as "pointy". The cross section of the 454 NSW and 404 Firecrest wheels looks less "pointy".
ENVE 4th Generation SES wheelset
ENVE also relies on hookless tubeless tires. ENVE's hallmark is the Smart ENVE System (SES) series, based on the ENVE philosophy of practical, fast design. The airflow at the front of the bike is different to the airflow at the back of the bike, and all SES series wheelsets have frame heights and shapes specifically designed for the flow field in which they ride. The front frame height is smaller than the rear and has a rounded profile to maximize crosswind stability and control, while the rear frame height is taller and more streamlined to maximize power transfer and minimize drag. The SES wheelsets were designed and tested for speeds of 48km/h and 32km/h respectively, resulting in rim shapes that offer best-in-class performance at both real-world speeds.
Taking the SES 4.5 as an example: Although the frame heights of the front and rear wheels are different, they have the same internal width (25mm) and external width (32mm), so it can be said that the internal width is very aggressive and only can be used with tires over 28c, which of course can easily reach the maximum tire pressure of 73 psi for hookless tubeless tires. ENVE's rims have thicker sidewalls that are more stable and resistant to snake bites, so the outer width can reach 32 mm without the tire lip getting stuck. The ENVE rims are thicker on the sidewalls, stronger and more resistant to snake bites, so the external width can still reach 32mm without the tire lip hooks, making the front wheels look thicker and the rear wheels look thinner.
DT Swiss ARC 1100 DICUT wheelset
The ARC 1100 DICUT is the aerodynamic top wheelset from DT Swiss at the highest technological level. ARC stands for Aerodynamic, Race, Carbon and 1100 for the use of 180 DICUT hubs, SINC ceramic bearings and the Ratchet EXP ratchet system. The DICUT is the lightest, stiffest and most aerodynamic hub and spoke system from DT Swiss and is aimed at road and track riders. Road and track racing where speed is of primary importance and very high tensions can be maintained using special spoke heads.
Since 2016, DT has been developing wheelsets together with the aerodynamics specialist Swiss Side that are characterized by both their aerodynamics and their stability. The rims of the ARC 1100 DICUT DB 80, 62 and 50 are designed very differently, with an internal width of 20mm, but external widths of 32mm, 27mm and 26. The rims of the ARC 1100 DICUT DB 80, 62 and 50 are designed very differently, with an internal width of 20mm, but an external width of 32mm, 27mm and 26.5mm.
Roval Rapide CLX wheelset
The Roval Rapide CLX is a wheelset with a difference. Blitz engineers have used an out-of-the-box design with the aim of ensuring stability and speed in a high frame aerodynamic wheelset. The Rapide CLX has an "amazing" front wheel external width of 35mm and a frame height of 51mm, with a significantly shortened (and blunted) profile and 25% more stability in crosswinds than the CLX50, the most stable wheelset on the market over 50mm Frame height. The Rapide CLX front wheel is an astonishing 35mm wide and 51mm high, with a significantly shorter (and blunter) profile that is 25% more stable in crosswinds than the CLX50, and is the most stable wheelset on the market with a frame height of over 50 mm. The rear wheel is 60 mm higher and has a "pointer" profile optimized for wind resistance. The Rapide CLX is the best interpretation of the aerodynamic concept and does everything to optimize the design , the rim's sidewalls are thicker and it is hollowed out to be lighter. The first generation of the Rapide CLX did not recommend the use of tubeless tires to ensure reliability. The Rapide CLX II was designed with a carbon fiber tire. Redesigned stack design that dramatically improves testing standards, and although the weight has increased slightly, it is still very light at 1500 grams.
Despite the large external width, the Roval Rapide CLX wheels are only 21 mm wide internally, allowing 25 or 26 mm tires to fit. The front wheel is obviously thicker, but if you scale it down, the frame height is about 37.5mm, if the external width is 28mm, think of the Bontrager Aeolus RSL 37, maybe its crosswind stability is similar. BX Jun has the Roval Rapide CLX used for a long time, and it is driven even on a gusty day, and the stability is really impressive.
Today we all know better that the most technical part of a wheelset is the rim, and the big brands invest a lot of resources and time in research and development, and each has its own focus. It's time to shift the focus of the wheelset from weight to aerodynamics, because a hundred grams less is not nearly as good as a more aerodynamic and stable experience.