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Aerodynamics: SPOKE FINS

Unmatched Crosswind Efficiency and Stability

SPOKE FINS™ convert the round spoke into an optimal aero spoke that automatically adjusts to the immediate crosswind component by swiveling like a wind vane around the spoke, constantly adjusting orientation as the wheel rotates. SPOKE FINS maintain laminar flow over the round spoke—and particularly over the critical spoke nipple—regardless of the crosswind or wind speed. By maintaining laminar flow even at higher vehicle speeds, vehicle propulsive efficiency is dramatically increased.

Moreover, flow across either bladed or round spokes becomes turbulent as wind speed increases, and even so eventually against a direct headwind. Since wind speed at the top of the wheel in any modest headwind easily exceeds twice the vehicle speed, flow across even a bladed aero spoke quickly becomes turbulent at higher wind speeds, regardless of the crosswise directional component. Thus, no spoke is truly aero at more extreme wind speeds, where laminar flow cannot be maintained.

SPOKE FINS reduce the drag coefficient of a round spoke by up to a factor of five, maintaining laminar flow far longer than any aero spoke as wind speeds increase. Thus, for high speed wind conditions—such as cycling fast downhill or with substantial headwind and crosswind exposureAEROCROSS™ wheels with SPOKE FINS are simply superior in overall performance, boosting both speed and stability of the cyclist.

Similarly, simply reduced exposure of the critical spoke surfaces to these higher wind speeds near the top of the wheel can also improve propulsive efficiency. And road testing has shown that use of a somewhat wider profile rim can provide some natural wind shielding just inside the rim at the top of the wheel, shadowing the critical uppermost drag-inducing spoke nipple. However, road testing has further shown that this minor wind shielding provided by a wider shallow rim is only effective against direct headwinds. Any crosswind present tends to obviate this spoke wind-shielding advantage of a wider rim over the use of SPOKE FINS for streamlining the spoke.

For this reason, and only under directly headwind conditions, some shallow wide or alternatively much deeper profile rims can offer similar performance to our AEROCROSS relatively shallow rims with SPOKE FINS. However, with any substantial crosswind present, SPOKE FINS maintain laminar flow with lower drag across the spoke, thereby providing enhanced aero efficiency over more costly aero wheels. And AEROCROSS wheels with SPOKE FINS are simply unmatched in crosswind stability, offering much greater cycling safety with the accompanying increased pedaling efficiency.

The only wheel having tested somewhat faster in downhill coasting tests compared to our AEROCROSS shallow rims with SPOKE FINS is a very expensive, quite deep rim with hidden nipples. (Aero wheels with exposed nipples have not tested faster than our wheels.) However, these downhill coasting comparison tests were problematic and cut short due to the unmanageable instability of these deep rims in windy conditions. So for general use in windy conditions, our AEROCROSS shallow rims with SPOKE FINS are the preferred wheel set for both optimal drag reduction together with more pedaling-efficient crosswind stability. Deep rims in these windy conditions are generally unmanageable, and therefore not safe for general use.

Indeed, many pro cyclists have stated that the maximum rim depth they will consider for general use is around 35 mm. Any greater rim depth presents too many safety compromises for general cycling outdoors, without offering substantially increased vehicle drag reduction.

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