Convexo Convex. This barbaric name self explains the physical phenomenon Paul Lew, forerunner in bicycle wheels world, relied on for his newest invention. This time-trial disc wheel differs from the others by its shape, which converges on the rear, and features the swirl lip generator reducing the crosswind sensivity.
The tests performed in a windtunnel, proved this double convex disc wheel to optimize the aerodynamic of the rider plus bike system: it reduces the drag, so the power needed to ride at a given speed.
This wheel pushes the limit of the disc design to a new standard. Where the flat sided disc wheels create a lot of turbulences, a lens shaped disc wheel, by its curved shape on the non drive side, reduces the drag by making easier the perturbated air flow.
Nevertheless, according to the manufacturer, this construction sees its limits: the wheel behaves as an aircraft wing because its curved shape on a side, and flat shape on the other creates a pressure difference on each side, meaning extra drag.
"For a minimal drag, the perturbated air flow must converge behind the rider", Paul Lew told us.
This is here this wheel differs from the others, the Convexo Convex is curved on both sides. This special shape helps the air flow, perturbated by the presence of the rider and its bike, to stick the wheel and leave the system clearly. Obviously the speed, the bike shape, and the rider position significantly modify these drawings.
On the right, the flat sided ovoïd rim disc wheel, such as a Zipp Sub9, has the airflow perturbated behind it because of the flat flanges.
On the left, the airflow of the Convexo Convex wheel converges and leaves the system as a clear flow, synonym of low drag, so better aerodynamic properties.
The tests performed at Mooresville, North Carolina, USA, recorded thousands datas. The wheels claimed to be the fastest of the market were tested on the bench.
The drags are recorded for the wheels only, and more interesting, for the wheels installed on the bike, including a rider. The position of the rider is kept thanks to several markers.
The results are summarized in the table below:
Regarding the wheels only, Zipp is indeed the fastest: 12% less drag for the front wheel, and 53% less drag for the rear wheel! The difference is significant and corroborates the datas claimed by Zipp, constantly improving the aerodynamics on its wheels.
Yet, testing a wheel alone is not really accurate to predict the results of a complete system rider/bike. Lew Racing thus pushed the tests further to know what happens with a world class rider and its Cervelo P3C.
The results are very different, and sometimes advantage the Lew configuration, sometimes the Zipp:
A few points are important here: with a yaw angle of 10°, the Zipp404/Zipp Sub9 configuration is as fast as the Lew Pro VT-1/Convexo Convex, taking into account the measure errors.
When comparing the configuration 1 and 2, only the rear wheels change. The fastest configuration is the one mixing the 404 front, and the Convexo Convex rear. The fastest rear wheel at 10° is the Lew.
At yaw 20°, the configurations "full" Zipp or "full" Lew are as fast, when taking into account the errors. Weirdly the fastest configuration at 10° is the slowest at 20°.
At this angle, the configuration 1 is faster than the number 2. So the Zipp Sub 9 is faster than the Convexo Convex. However, the configuration 3, which is the "full" Lew, is as fast as the "full" Zipp configuration!
It shows how difficult it is to predict the aerodynamics of a system. It also means testing a wheel only is not the best idea for a complete system...
Only Zipp wheels, among the current fastest wheels have face the Lew in this test.
The results can be downloaded here.
What's on the scale?
Of course, as every products coming from the Lew factories, the components are extremely light. The carbon and boron wheel, lens shape, is 780g. As comparizon, the Lightweight Disc, flat sided, is 800g, the Zipp Sub9, flat sided with ovoid rim, are 1000g, the Corima flat sided are 1060g, and finally, the Mavic Comete flat sided/lens shape is 1285g.