BryanD wrote:
http://aerogeeks.com/2018/02/06/zipp-858-nsw-carbon-clincher-review/
One more thing worth mentioning about the design… if you compare the 858 with an 808 NSW, you’ll quickly notice that the 858 is thinner—significantly so. An 808 is 26.4mm (front and back), while the 858 has a maximum width of 24.4mm at the front and 23.7mm for the rear wheel. According to Zipp, the reasoning behind the width difference as to reduce aerodynamic drag. In the absence of a crosswind, a narrower wheel is always going to be aerodynamically faster.
One of the reasons aero rims have grown wider (an innovation that was first made by Zipp with their Firecrest wheels) is because, with a conventional rim, wider rims are less affected by crosswinds. With the Sawtooth rim design, Zipp has improved crosswind performance so much that they were able to go to a narrower rim and improve the aerodynamics even more. In short, they are able to get the best of both worlds—a wheel that is aerodynamically faster and more stable in a crosswind than a wide conventional rim.
At least two separate issues in there.
Rims that are wider in the middle than at the brake track have been around since the 90s - I have a HED CX that is a good 3mm more and even my early 90s 24" front is 1-2mm. Same principle as lenticular vs flat disc. HED broke with many years of convention that narrower was better with the C2 - slightly wider tyres on much wider rims for a better tyre stance - comfort, grip and aero improvements. Firecrest, SCT, Aeolus etc followed on from there with blunter less "teardrop" shape profiles that provide a greater aerodynamic balance of the front and rear (reversed) profiles of the rim, for better crosswind behaviour.
Low drag of a deep rim relies on generating sidethrust, so there is no perfect solution, merely minimizing unwanted non-linear aerodynamic properties.